420 Stereo Power Amplifier
Contents
1. se trouve a l interieur aucune piece pourvant entre repar e l usager S adresser a un reparateur comp tent The lightning flash vvith arrovvhead symbol vvithin an equilateral triangle is intended to alert the user of the presence of uninsulated dangerous voltage within the product s enclosure that may be of sufficient magnitude to constitute a risk of electric shock to persons The exclamation point within an equilateral triangle is intended to alert the user of the presence of important operating and maintenance servicing instructions in the literature accompanying the product i e this manual Caution To prevent electric shock do not use the polarized plug supplied with the unit with any extension cord receptacle or other outlet unless the blades can be fully inserted Terms Several notational conventions are used in this manual Some paragraphs may use Note Caution or Warning as a heading Certain typefaces and capitalization are used to identify certain words These are Note Identifies information that needs extra emphasis A Note generally supplies extra information to help you to better use the 420 Identifies information that if not heeded may cause damage to the 420 or other equipment in your system Identifies information that if ignored may be hazardous to your health or that of others Controls switches or other markings on the 420 s chassis Strong emphasis Caution Warning C2. 6 LOW FROM waar OUT 4 TERMINAL STRIP 4 HIGH NOT USED amp wy GROUND FROM NON TRANSFORMER ELECTRONIC BALANCED OUTPUT TYPICAL OF SYMETRIX PRODUCTS TO UNBALANCED IN FEMALE XLR hee MALE TS PLUG PIN 1 GROUND LOW nia TIP HIGH PIN 2 HIGH 0 SLEEVE GROUND PIN 3 NOT USED 2 LOW REV B Figure 8 1 Input and output connector wiring These diagrams represent the majority of connectors used in modern audio equipment at line or microphone level Locate the source connector in the left column and match it up with the destination connector in the right column Wire your cable according to the diagrams 18 Chapter 9 Troubleshooting Troubleshooting Chart SYMPTOM PROBABLE CAUSE No output Check cables and connections Are inputs driven by outputs and outputs driving speakers or headphones Are all of the cables good Is there signal coming out of the source Are the loudspeakers or headphones OK Is the front panel SPEAKER MUTE switch out Check for AC power presence Power LED on Check output by plugging headphones into front panel headphone jack Is the fuse OK No output in bridge mono Are you using the CHANNEL 1 INPUTS2 SPEAKER MUTE switch out Outputs out of phase Mono bridge switch engaged Hum or buzz in output Check input connector wiring refer to Figure 8 1 Ground loop Check related system equipment grounding Are all system components on the same AC ground
3. UNBALANCED UNBALANCED N 2 P 6 ee I ee 222 BALANCED SERIAL NUMBER AC Power INPUT FUSE OUTPUTS INPUT 2 Input 1 Mono BRIDGE switch Please send in the completed warranty card IEC power connector Connect only to appropriate AC power source Refer to rear panel marking for correct AC source value 1Ampere Slow blow fuse Replace only with same type of fuse 117V ac 250V slow blowing Bussman type MDL 1 230V AC 0 5A 250V ac slow blowing Bussman type GDC 500ma 6 screw terminals This is the output of the 420 These terminals may be turned on off via the front panel SPEAKER MUTE switch For stereo or 2 channel applications connect one load to the channel 1 output and connect the other load to the channel 2 output The minimum load impedance is 4 ohms For mono bridge applications connect the positive load connection to the channel 1 output terminal connect the negative load connection to the channel 2 output terminal The minimum load impedance is 8 ohms XLR female pin 2 hot paralleled with TRS phone jack XLR female pin 2 hot paralleled with TRS phone jack Use this input for mono bridge applications Located internally Chapter 5 420 Basics Although power amplifiers are one of the basic building blocks of any audio system they still remain one of the least understood Why is minimum load impedance important What really happens when you mono bridge
4. t too serious a limitation unless you are into recording orchestras or other large groups live using one mix Figure 7 2 shows a typical distribution box In a typical studio there might be four or five of these boxes sprinkled hither and yon throughout the studio Each box has four headphone jacks on it you re welcome to add more and interconnects to the headphone system using mic cables 3 e A CN e 2221 7 6 y Symetrix V J R HEADPHONE R IMPEDANCE 2 600 2000 100 ohms 2 watts 120 500 47 ohms 2 watts 100 or less 22 ohms 5 watts Figure 7 2 A simple headphone distribution box We use mic cables for this application because there are usually more mic cables hanging around the average studio than anything else TRS plugs or stereo phone plugs short their tip and ring connections momentarily as they are mated or unmated This can be harmful to the 420 most other amplifiers too Figure 7 3 shows the connections needed at the amplifier end Notice that all connectors are simply wired in parallel Take special pains to ensure that pin 1 of the connectors remains isolated from the system ground except via the 420 RIGHT man RIGHT COMMON ee LETC COMMON ADDITIONAL CONNECTIONS AS REQUIRED 77 420 AMPLIFIER __ j o 9 REV A Fi
5. A good rule of thumb is DON T operate any amplifier below its rated minimum load impedance The minimum load impedance of the 420 is 4 ohms in stereo or dual mono mode and 8 ohms in mono bridged mode Mono Bridged Operation Mono bridged operation is acommon method of increasing the power delivered to the load Bridging requires two amplifier channels and for many amplifiers but not the 420 translates into either four times the power of a single channel into the same impedance or twice the single channel power into twice the impedance Using two amplifier channels to drive both sides of the load simultaneously usually one side of the load is grounded increases the voltage developed across the load Paralleling the inputs and connecting the load across the two output channels doesn t work because both ends of the load are driven in the same direction there is no voltage across the load Leaving the output connec tions alone and inverting the polarity of one amplifier channel causes one end of the load to swing positively the other end swings negatively Thus the voltage across the load is twice what it would be if the load had one side grounded which results in four times the power since the power increases in proportion to the square of the voltage As far as the amplifier is concerned each channel drives an equivalent load equal to one half of the load impedance An 8 ohm load connected across the two amplifier channels results i
6. Is the hum or buzz coming out of the preceding unit Distortion Check input signal Is it too hot or is it already distorted Are the input gain controls turned down too far causing clipping in the source because it doesn t have enough output level to overcome the loss in the input gain controls Are the Cur LEDs indicating clipping Check output loading Should be above 4 ohms in stereo 2 channel mode or 8 ohms in mono bridge mode Is something else clipping Noise hiss Check input signal levels and level control settings Check gain settings on upstream equipment Is the input signal already noisy Blows fuses Are the speaker connections shorted Disconnect speakers disconnect inputs replace fuse turn unit on If the fuse blows immediately call the factory The clip LEDs do not indicate clipping caused by the activation of the amplifier s internal current limiting circuitry 19 Symetrix Specifications Chapter 10 420 Specifications Input Output Physical Inputs XLR Female Paralleled with TRS 4 jacks Size hwd 1 72 19 x 7 25 inches 4 37 x 48 26 x 18 415 centimeters 10 kilohms Line Level Balanced Bridging Weight 7 6 Ibs 3 5kg net 10 Ibs 4 6 kg shipping Balanced CMRR greater than 55 dB 1 kHz Outputs Two 4 Ohms minimum impedance 6 screw terminals Electrical Maximum Input Level 21 dBu Power Requirements 120V nominal 108 to 132V AC Maximum Output stereo 2 channel 20 watt
7. an amplifier Do you really need a BIG amplifier to run head phones in your studio VVe ll try to answer these questions and more in this chapter What is a Power Amplifier A power amplifier is one capable of delivering significant amounts of power to its load Power is a measure of energy which is the ability to do work like move a loudspeaker cone or heat up dummy load resistors Power is measured in watts which is defined as the product of voltage times amperage One watt is one watt regardless if it is the result of a current flow of one volt and one ampere amp or 10 volts and 0 1 ampere P IE where P power in watts 1 current in amperes amps and E electromotive force in volts Why is this important Only because loudspeakers are relatively inefficient devices and as such require significant amounts of power to make them operate Thus an 8 ohm loudspeaker will require 2 82 volts at 0 35 amps to drive it with a one watt input Amplifier Output Limitations All amplifiers have limitations to their output Since output power is determined by the voltage and current that the amplifier can deliver to the load power is volts times amps the amplifier s power supply voltage determines the maximum voltage that the amplifier can deliver to any load and the power supply current capacity combined with amplifier circuit limitations determines the maximum current that can be delivered to any load The relationships between vol
8. fallen into the apparatus the apparatus has been exposed to rain or moisture does not operate normally or has been dropped Chapter 3 Fast Setup Fast Setup Follow these instructions to get your 420 up and running as quickly as possible The intent of this section is fast setup Refer to later chapters for explanation of the 420 s controls and functions Connections Connect your input source to the either the XLR connectors or to the TRS phone jacks Connect the 420 s output to the loudspeakers or other load using the barrier strip connector located at the rear of the unit If the 420 is driving a headphone distribution system you Il get better results read louder if you use the screw terminals on the rear of the unit The front panel headphone jack is really only intended to drive one pair of phones INPUT 2 INPUT 1 OUTPUTS CHANNEL CHANNEL UNBALANCED UNBALANCED BALANCED BALANCED BALANCED OHMS CLASS 2 WIRING Figure 3 1 420 generic hookup diagram Connect the AC input to an AC power source of the proper voltage and frequency as marked on the rear of the unit Caution Failure to connect the 420 to the proper AC mains voltage may cause fire and or internal damage There are no user serviceable parts inside the chassis Refer all service to qualified service personnel or to the factory Warning Lethal voltages are present insi
9. 00 millivolts I O Impedances The 420 is designed to interface into almost any recording studio or sound reinforcement applica tion This includes e 600 ohm systems where input and output impedances are matched e Unbalanced semiprofessional equipment applications e Modern bridging systems where inputs bridge and outputs low source impedances voltage transmission systems The 420 s input impedance is 10 kilohms balanced and 10 kilohms unbalanced The inputs may be driven from any source balanced or unbalanced capable of delivering at least 10 dBu into the aforementioned impedances The 420 s output is intended to drive loudspeaker or headphone loads The minimum load imped ance is 4 ohms in stereo dual channel mode and 8 ohms in mono bridged mode Of course there is no problem driving high impedance loads the 420 can drive a 600 ohm load to 27 dBm This might be useful for instance if you needed a no fooling distribution amplifier capable of driving a large even ridiculous number of loads simultaneously Polarity Convention The 420 uses the international standard polarity convention of pin 2 hot Therefore if your system uses balanced inputs and outputs and uses the 420 this way then the polarity convention is unimportant If your system is both balanced and unbalanced then you must pay attention to this especially when going in and coming out through different connector types like input on an XLR out
10. 420 Stereo Power Amplifier User s Guide Symetrix Symetrix Table of Contents Chapter 1 Introduction Chapter 2 Operator Safety Summary Chapter 3 Fast Setup Chapter 4 Front amp Rear Panel Overview Chapter 5 420 Basics Chapter 6 Using the 420 Chapter 7 Applications Chapter 6 Technical Tutorial Chapter 9 Troubleshooting Chapter 10 Specifications Chapter 11 Warranty amp Service Appendix A Disassembly Instructions amp Bridged Mono Mode Switch Rev A 00 25 February 1999 Symetrix part number 53420 0A00 Subject to change without notice 1999 Symetrix Inc All right reserved Symetrix is a registered trademark of Symetrix Inc Mention of third party products is for informational purposes only and constitutes neither an endorsement nor a recommendation Symetrix assumes no responsibility with regard to the performance or use of these products Under copyright laws no part of this manual may be reproduced or transmitted in any form or by any means electronic or mechanical including photocopying scanning recording or by any information storage and retrieval system without permission in writing from Symetrix Inc NN W N 10 13 16 19 20 21 23 Symetrix 6408 216th St SW Mountlake Terrace WA 98043 USA Tel 425 778 7728 Fax 425 778 7727 Email symetrix symetrixaudio com Chapter 1 Introduction The Symetrix 420 is a two channel power amplifier intended for use in prof
11. APITALS Boldface Important Safety Instructions Please read and keep these instructions Heed and follow all warnings and instructions Install in accordance with the manufacturer s instructions Power Source This product is intended to operate from a power source that does not apply more than 250V rms between the power supply conductors or between either power supply conductor and ground A protective ground Chapter 2 connection by way of the grounding conductor in the power cord is essential for safe operation Grounding The chassis of this product is grounded through the grounding conductor of the power cord To avoid electric shock plug the power cord into a properly wired receptacle before making any connections to the product A protective ground connection by way of the grounding conductor in the power cord is essential for safe operation Do not defeat the safety purpose of the grounding plug The grounding plug has two blades and a third grounding prong The third prong is provided for your safety When the provided plug does not fit your outlet consult an electrician for replacement of the obsolete outlet Danger from Loss of Ground If the protective ground connection is lost all accessible conduc tive parts including knobs and controls that may appear to be insulated can render an electric shock Proper Power Cord Use only the power cord and connector specified for the product and your operating local
12. This is important only for ancient audio systems power amplifiers and RF Technically speaking the reason is power transfer which reaches a maximum when source and load are matched Modern audio systems are voltage transmission systems and source and load matching is not only unnecessary but undesirable as well e Ancient audio systems operate at 600 ohms or some other impedance value and must be matched both at their inputs and at their outputs Generally speaking if you are dealing with equipment that uses vacuum tubes or was designed prior to 1970 you should be concerned about matching These units were designed when audio systems were based on maximum power transfer hence the need for input output matching e Power amplifiers are fussy because an abnormally low load impedance generally means a visit to the amp hospital Thus it s important to know what the total impedance of the pile of speakers connected to the amplifier really is e systems are matched because we really are concerned with maximum power transfer and with matching the impedance of the transmission line keeps nasty things from happening Video signals composite baseband or otherwise should be treated like RF Some folks seem to believe that balanced unbalanced lines and impedances are related or even worse that they are associated with a particular type of connector Not so Unbalanced signals are not necessarily high impedance and balanced signals line
13. assis Perform all service work with the unit disconnected from all AC power Top Cover Removal 1 Ensure that the 420 is disconnected from the AC power source Disconnect any input or speaker cables Remove four 6 32 x 1 2 inch screws from the rear panel Remove two 6 32 x 1 2 inch screws from each side of the chassis Remove one 6 32 x 1 2 inch screw from the top middle of the front panel Use caution to prevent the screwdriver from slipping and marring the front panel Remove two 6 32 flat head screws from the top cover Slide the top cover inch towards the rear of the unit then lift the top cover free of the chassis Circuit Board Removal N A Ensure that the 420 is disconnected from the AC power source Remove the top cover using the procedure described previously Unplug the two transformer connectors one for each printed circuit board Remove one screw from each of the four corners of the front panel and one screw in the bottom center of the front panel Remove the knobs from the front panel Remove the front panel and set it aside Remove four 6 32 x 1 4 inch SEMS machine screws from the main printed circuit board Remove the two 6 32 x 1 2 inch machine screws that fasten the IEC connector to the chassis Remove two 6 32 SEMS screws that fasten the power supply PCB to the chassis Disconnect the green safety ground wire at the chassis screw terminal by removing the nut Ensu
14. at the Buyer gives Symetrix prompt notice of any defect or failure and satisfactory proof thereof Products may be returned by Buyer only after a Return Authorization number RA has been obtained from Symetrix Buyer will prepay all freight charges to return the product to the Symetrix factory Symetrix reserves the right to inspect any products which may be the subject of any warranty claim before repair or replacement is carried out Symetrix may at its option require proof of the original date of purchase dated copy of original retail dealer s invoice Final determination of warranty coverage lies solely with Symetrix Products repaired under warranty will be returned freight prepaid by Symetrix via United Parcel Service surface to any location within the Continental United States At Buyer s request the shipment may be returned via airfreight at Buyer s expense Outside the Continental United States products will be returned freight collect The foregoing warranties are in lieu of all other warranties whether oral written express implied or statutory Symetrix Inc expressly disclaims any IMPLIED warranties including fitness for a particular purpose or merchantability Symetrix s warranty obligation and buyer s remedies hereunder are SOLELY and exclusively as stated herein This Symetrix product is designed and manufactured for use in professional and studio audio systems and is not intended for other usage With respect to products pur
15. chased by consum ers for personal family or household use Symetrix expressly disclaims all implied warran ties including but not limited to warranties of merchantability and fitness for a particular purpose This limited warranty with all terms conditions and disclaimers set forth herein shall extend to the original purchaser and anyone who purchases the product within the specified warranty period Warranty Registration must be completed and mailed to Symetrix within thirty 30 days of the date of purchase Symetrix does not authorize any third party including any dealer or sales representative to assume any liability or make any additional warranties or representation regarding this product information on behalf of Symetrix This limited warranty gives the buyer certain rights You may have additional rights provided by applicable law Limitation of Liability The total liability of Symetrix on any claim whether in contract tort including negligence or otherwise arising out of connected with or resulting from the manufacture sale delivery resale repair replacement or use of any product will not exceed the price allocable to the product or any part thereof which gives rise to the claim In no event will Symetrix be liable for any incidental or consequential damages including but not limited to damage for loss of revenue cost of capital claims of customers for service interruptions or failure to supply and costs and ex
16. de the chassis There no user serviceable parts inside the chassis Refer all service to qualified service personnel or to the factory Settings Set the controls and switches on the front and rear panel as follows Front Panel Control Setting Rear Panel Connectors Connect CHANNEL GAIN 9 00 0 clock Outputs Connect loudspeakers headphone distribution system or other load CHANNEL 2 GAIN 9 00 o clock Input 2 Connect balanced or unbalanced source here Out Stereo Input 2 Connect balanced or unbalanced source here SPEAKER MUTE Out Active POWER In On Symetrix Initial Setup The 420 s controls and switches are now set according to the preceding section All connections listed in the settings table on the previous page are now made The 420 should now pass signal The amplifier s Cuir indicators should not be illuminated Generally speaking the gain controls on a power amplifier should be operated at as high a setting as possible This ensures sufficient amplifier sensitivity that the preceding unit can drive the amplifier into clipping If the gain controls are turned down sufficiently it is possible to either clip the balanced input buffer amplifiers or to cause the preceding unit to clip With the signal level of the source at a reasonable level increase the gain of the 420 until either the Curr indicators illuminate or the acoustical output of the loudspeakers is sufficient Front am
17. e Use only a cord that is in good condition Protect the power cord from being walked on or pinched particularly at plugs convenience receptacles and the point where they exit from the apparatus Proper Fuse The user accessible fuse is a part of the IEC AC inlet connector The fuseholder accepts 5 x 20mm diameter fuses For 1 17VAC operation the correct value 18 1 0A 250VAC slow blowing For 230V AC operation the correct valueis 0 5 A 250V AC slow blowing Operating Location Do notoperate this equip mentunder any of the following conditions explosive atmospheres in vvet locations in inclement weather improper or unknown AC mains voltage or if improperly fused Do not install near any heat source such as radiators heat registers stoves or other apparatus including amplifiers that produce heat Unplug this apparatus during lightning storms or when unused for long periods of time Stay Out of the Box To avoid personal injury or worse do not remove the product covers or panels Do not operate the product without the covers and panels properly installed Only use accessories specified by the manufacturer Clean only with a damp cloth User serviceable parts There are no user serviceable parts inside the 420 In case of failure refer all servicing to the factory Servicing is required when the 420 has been damaged in any way such as when a power supply cord or plug is damaged liquid has been spilled or objects have
18. er wiring Ensure that the unit is operating in bridged mono mode You can drive 37 isolated output connec tions this way each to a level of approximately 23 5 dBm Shorting any one output has no effect on the remainder For driving long lines put the unit into bridged mono mode Drive the line balanced of course from the two plus output connections Tie the ground from the balanced line to either of the minus output connections At the other end of the line you may need to terminate the line with a resistor It may also be necessary to build out the output impedance of the unit at the sending end This depends on the nature of the line itself and whether or not there are transformers or repeat coils involved Symetrix 13 Headphone Distribution Systems The 420 is ideally suited to headphone distribution systems There are two basic strategies involved in making a loud headphone system 1 If you have low impedance phones then you need high current capability to make them loud This translates to a modest 8 ohm power rating like 20 watts 2 If you have high impedance phones then you need high voltage capability to make them loud This translates to a moderate 8 ohm power rating like 100 watts Although the low impedance phones generally get louder you can t hang as many as the high impedance guys on your headphone amplifier because sooner or later you arrive at the amp s minimum load impedance This probably isn
19. essional and commercial audio systems The 420 may be operated as a two channel amplifier with 20 watts per channel or as a single channel amplifier capable of 40 watts output mono bridged mode The amplifier is intended for use with near field monitors small speakers used for radio reference audio for video and with headphones In stereo mode the minimum load impedance is 4 ohms in mono bridged mode the minimum load impedance is 8 ohms For convenience the 420 is equipped with both 1 4 TRS tip ring sleeve and XLR connectors which accept either balanced or unbalanced signals The 420 produces full output with a 0 5V input signal balanced or unbalanced The 420 can be used as a stereo amplifier with ganged level controls as a 2 channel amplifier with independent level controls or as a single channel mono amplifier The front panel Mone switch mixes the two inputs together Broadcast and recording applications can use this feature to check the mono compatibility of their signals Commercial sound applica tions can use this feature to mix paging signals or paging and music signals In recording studios and other similar applications the 420 makes an ideal headphone amplifier You can drive one pair of phones from the front panel jack or many pairs from the rear panel terminals A front panel switch allows you to turn off the rear panel speaker terminals which is useful for near field monitor applications or for headphone drive
20. f flimsy packaging we ll have to charge you for proper shipping materials If you don t have the factory packaging materials then do yourself a favor by using an oversize carton wrap the unit in a plastic bag and surround it with bubble wrap Pack the box full of Styrofoam peanuts Be sure there is enough clearance in the carton to protect the rack ears you wouldn t believe how many units are returned with bent ears We won t return the unit in anything but Symetrix packaging for which we will have to charge you Of course if the problem turns out to be operator inflicted you ll have to pay for both parts and labor In any event if there are charges for the repair costs you will pay for the return freight All charges will be COD unless you have made other arrangements prepaid Visa or Mastercard Out of Warranty Repairs If the warranty period has passed you ll be billed for all necessary parts labor packaging materials and freight charges Please remember you must call for an RA number before sending the unit to Symetrix Appendix A Disassembly Instructions amp Bridged Mono Mode Switch Caution These servicing instructions are for use by qualified personnel only To avoid electric shock do not perform any servicing other than that contained in the operating instructions portion of this manual unless you are qualified to do so Refer all servicing to qualified service personnel Warning Lethal voltages are present inside the ch
21. graph s worth of common sense Even for a twenty watt amplifier your speaker wire should be reasonably large Ata minimum 18 gauge zip cord works just fine If you 14 gauge wire is even better Use godzilla cable if you want The biggest single thing you can do for your loudspeakers when you pick a cable for them is to minimize the wire resistance Aside from using silver which has higher conductivity than copper sheer physical size is the way to fly Remember the longer the length of speaker wire the more important the wire size becomes Driving Headphones Driving headphones is no great feat Getting them loud can be accomplished two ways money and science The money method is simple buy a big amplifier If it isn t loud enough buy a bigger amplifier Repeat as long as there is money The scientific method starts with a volume requirement and works backward towards the amplifier To be truly successful without an unlimited budget you need to consider the sensitivity of your headphones before you buy them Usually what most folks want is LOUD The way you get LOUD is to increase the amount of power delivered to the headphones There are two fundamentally different types of headphones high impedance and low impedance For the purposes of this discussion high impedance is defined as 600 ohms or higher and low impedance is defined as 200 ohms or lower Fortunately there aren t many headphones that are popular in st
22. gure 7 3 Amplifier connections for headphone distribution system The resistors shown in Figure 7 2 isolate the jacks from each other protect the phones somewhat and most importantly protect the amplifier from shorted cables and channel to channel shorts which occur whenever you plug in or unplug a set of phones Pick the resistor value according to the impedance of the headphones found in your studio The maximum number of headphones that can be driven at once depends on the headphone impedance and the amplifier s minimum load impedance The table below lists maximum quantities for the headphones shown You can add volume controls and channel switching to the box if you want but those details are beyond the scope of this short discussion Headphones Maximum MaxSPL Maximum MaxSPL 4 Ohmtotalload 4 Ohmtotalload 8 Ohmtotalload 8 Ohmtotalload K240 175 122 87 125 Pro 4X 41 135 20 138 HD424 525 118 212 121 MDR 7506 21 134 10 137 15 Symetrix Technical Tutorial Chapter 8 This section discusses a multitude of things all related to getting signals in and out of the 420 Matching Levels vs Matching Impedances In any audio equipment application the question of matching inevitably comes up Without digging a hole any deeper than absolutely necessary we offer the following discussion to hope fully clarify your understanding of the subject Over the years we have all had impedance matching pounded into our heads
23. h each channel one in series with the left channel one in series with the right channel The resistors prevent shorting the amplifier outputs together when the headphone plugs are inserted or re moved The 1 4 inch output jack on the front panel is a TRS tip ring sleeve jack wired for stereo head phones The tip connects to channel and the ring connects to channel 2 This jack is intended only for headphones plugging in a loudspeaker won t hurt anything it just won t be very loud If you are driving a headphone system multiple pairs of phones then use the rear panel screw connections It will work better FROM BALANCED OUT TO BALANCED IN FEMALE XLR MALE XLR PIN 1 GROUND PIN 1 GROUND PIN 2 HIGH PIN 2 HIGH PIN 3 LOW PIN 3 LOW TO UNBALANCED IN FROM FROM BALANCED OUT TRANSFORMER COUPLED OR TIP FLOATING BALANCED OUTPUT MALE TRS PLUG RING LOW od I 7 IPs SLEEVE GROUND RNS SEVES SLEEVE FROM UNBALANCED OUT TO BALANCED IN MALE TS PLUG gt MALE TRS PLUG TIP HIGH TIP HIGH SLEEVE GROUND RING LOW LOW x lane SLEEVE GROUND SLEEVE FROM BALANCED OUT TERMINAL STRIP H TO BALANCED IN v g TERMINAL STRIP HIGH
24. ls without distortion Screw terminals shall be provided for power amplifier outputs THD shall not be greater than 0 04 1 kHz 1 watt in 8 Ohms Signal to noise ratio shall be greater than 95 dB Frequency response shall be 20 Hz to 20 kHz 0 dB 1 dB The power amplifier shall be protected from output short circuits A front panel Dual Tracking switch shall allow the Channel 1 gain control to simultaneously control the level of both channels There shall be a 4 tip ring sleeve jack mounted on the front panel for stereo headphone output A front panel switch shall be provided to mute the rear panel output connections Another front panel switch shall be provided to sum the two input signals and route them to the two gain controls Independent clipping indicators shall be provided for each output channel The amplifier shall be capable of operating by means of its own built in power supply connected to 120V nominal AC 108 to 132V 60 Hz The unit shall be a Symetrix Incorporated model 420 Stereo Amplifier 20 Chapter 11 Warranty amp Service 420 Limited Warranty Symetrix Inc expressly warrants that the product will be free from defects in material and workman ship for one 1 year Symetrix s obligations under this warranty will be limited to repairing or replacing at Symetrix s option the part or parts of the product which prove defective in material or workmanship within one 1 year from date of purchase provided th
25. n each amplifier channel seeing the equivalent of a 4 ohm load Few users recognize the fact that neither side of the load is grounded when you mono bridge a stereo amplifier This means that if you use TS tip sleeve phone plugs for speaker connectors and lots of folks do the shell of the plug which is usually grounded is not grounded If the plug should come in contact with a microphone perhaps via its stand it is quite possible for the entire sound system to immediately oscillate usually at full power which is not healthy for loudspeakers and living things If you operate any amplifier in mono bridged mode take steps to ensure that neither of the output connections can come in contact with ground themselves or anything else Driving Loudspeakers Considering the modest output power of the 420 there isn t much to say here If you are driving multiple speakers per channel ensure that the combined impedance of all speakers is 4 ohms or greater per channel For multiple speakers parallel connection is preferable to series connection Loudspeakers connected in series do not perform the same as the same loudspeakers connected in parallel Do it if you must but only if you must A Word About Wire We re not going to debate the pros and cons of deuterium enhanced 99 999 purity saturnian copper litz wire here wider soundstage glows in the dark out of this world high end 2x10 per meter Instead how about one para
26. oudspeakers are connected in series each speaker sees the impedance of the other speaker as its source impedance Since damping factor is the load impedance divided by the source impedance in effect the damping factor seen by each speaker is unity The fact that a loudspeaker s impedance is anything but resistive only complicates matters Symetrix Using the 420 Chapter 6 This section is intended for more advanced users If you are a first time user we recommend that you start out by using the procedure found in Fast Setup Block Diagram On the following page you can find the block diagram for the 420 Please take a moment and take note of the following e The balanced input amplifiers are ahead of the Gain controls e The TRS and XLR connectors are paralleled with each other e SPEAKER MUTE switch does not affect the front panel STEREO HEADPHONES jack e front panel MONO switch is ahead of the two Gain controls e mono bridge mode switch is located internally refer to Appendix A Installation The 420 may be installed freestanding or rack mounted Multiple amplifier installations should allow breathing space between each amplifier Single amplifier installations should strive to keep the vent holes in the amplifier s top and bottom clear of obstructions by at least one rack space 1 75 inches Installation Requirements Mechanical One rack space 1 75 inches required 12 5 inche
27. p Rear Panel Overview Channel 1 Cur LED GAIN GAIN CONTROLS switch Channel 2 Cur LED GAIN Mode Mono STEREO Monitor STEREO HEADPHONES SPEAKER MUTE Power SWITCH LED Indicates the onset of clipping at the output of the 420 Controls the gain or input sensitivity of the 420 A volume control by any other name This control is active regardless of the 420 s operating mode Refer to the DUAL TRACKING INDEPENDENT switch for more information In Dua TRACKING mode the gain control for channel 1 sets the gain for both channels The inputs are still separate there s just one control for both channels Use this mode for stereo applications In INDEPENDENT mode the gain controls for both channels are separate Indicates the onset of clipping at the output of the 420 Controls the gain or input sensitivity of channel 2 of the 420 A volume control by any other name This control is active only in INDEPENDENT mode In STEREO mode both channels of the 420 are separate In Mono mode both inputs of the 420 are mixed ahead of the Gain controls in TRS jack Connect stereo headphones here This jack will drive headphones of any impedance from 4 ohms and up Mutes disconnects the rear panel 00177 terminals Turns the 420 on Indicates the presence of AC power Symetrix 100 WATTS MAXIMUM lt 420 STEREO 5 II o POWER AMPLIFIER P y INPUT 2 INPUT 1 9 RING N3
28. penses incurred in connection with labor overhead transportation installation or removal of products or substitute facilities or supply houses 21 Symetrix Servicing the 420 If you have determined that your 420 requires repair services and you live outside of the United States please contact your local Symetrix dealer or distributor for instructions on how to obtain service If you reside in the U S then proceed as follows At the Symetrix factory Symetrix will perform in warranty or out of warranty service on any product it has manufactured for a period of five years from date of manufacture Before sending anything to Symetrix contact our Customer Service Department for a return authorization RA number The telephone number is 425 778 7728 Monday through Friday 8AM 800 hours though 4 30 PM 1630 hours Pacific Time In Warranty Repairs To get your 420 repaired under the terms of the warranty 1 Call us for RA number 2 Pack the unit in its original packaging materials 3 Include your name address daytime telephone number and a brief statement of the problem 4 Write the RA number on the outside of the box Ship the unit to Symetrix freight prepaid We do not accept freight collect shipments Just do these five things and repairs made in warranty will cost you only one way freight charges We ll prepay the return surface freight If you choose to send us your product in some sort o
29. plications that the 420 lends itself to Recording Studios This is probably the primary use for the 420 Use it to drive the Auratones or the small home stereo reference speakers The 420 probably isn t powerful enough to drive most near field monitors except in very small studios You can also use the 420 to drive your headphone distribution system see below Commercial Sound In commercial sound installations use the 420 to operate a local system monitor speaker or to operate highly divided zone paging systems In a church the 420 can drive a wired hard of hearing system or simultaneous translation system Broadcast Use the 420 to drive the booth monitors or to ensure that the headphones are loud enough to draw blood No Fooling Distribution Amplifier In applications requiring driving extremely long lines or an extremely large number of outputs the 420 performs well In bridged mono mode the 420 delivers at least 27 dBm into a 600 ohm balanced load For distribution amplifier applications refer to Figure 7 1 OUTPUTS CHANNEL CHANNEL p2 alg E e 3 gt 7 62 yyy 1 UU OO D 8 0 f 150 ohms V VN CN I 150 ohms IN ADDITIONAL REV B OUTPUTS 37 TOTAL Figure 7 1 Distribution amplifi
30. position has no effect on signals at the jacks You can use the mono stereo switch three ways e mono sum a stereo input signal to check for mono compatibility e mono sum two unrelated signals like paging and music e 10 drive both amplifier channels with the same input signal 10 gt TINNVHOTONRI L IANNVHO dIL SINOHdQV3H 038315 CNNININIA INdINO 19 LAdLAO TANNVHO INdINO TANNVHO LAdLNO TANNVHO TAW 001 9004 001 L o PO O 9 901 5 SV NMOHS Od NO HOLIMS 35 0 ONON OANYS TIAN 13S4JO Le e peer PRERE gt lt O e NMOHS ILAN 0 03 1001136332 ONDOVAL SIOULNOD NIVD 1 9 40 dno NIVO TaNNVHO O _ 030315 NMOHS HOLIMS IQON NIVO LTANNVHO CAONVIVE GAONVIVENN INdNI GAONVIVd INdNI Q3ONVIVa QGJONVIVENN L INANI LAdNI Symetrix 11 Symetrix Mono Bridge Mode As described in Chapte
31. put on a phone jack XLR 1 4 Phone Signal Pin Sleeve Ground Pin2 Tip High Pin3 Ring Low Input and Output Connections Figure 8 1 on the next page illustrates how to connect the 420 to various balanced and unbal anced sources To operate the 420 from unbalanced sources run a 2 conductor shielded cable that s two conduc tors plus the shield from the source to the 420 At the source connect the low minus side to the shield these connect to the source s ground connect the high plus side to the source s signal connection At the 420 the high plus wire connects to pin 2 the low minus wire connects to pin 3 Symetrix 17 Symetrix and the shield always connects to pin 1 This is the preferred method as it makes best use of the 420 s balanced input even though the source is unbalanced The other alternative shown in Figure 8 1 converts the 420 s balanced input into an unbalanced input at the input connector This works but is more susceptible to hum and buzz than the preferred method There is no level difference between either method The rear panel output terminals accept 6 spade lugs or bare wires are intended to drive loud speakers or headphone distribution systems The minimum load impedance is 4 ohms in stereo dual mono mode and 8 ohms in mono bridge mode If you are driving a headphone distribution system ensure that each headphone jack has at least a 22 ohm 5 watt resistor in series wit
32. r 5 mono bridge mode uses both amplifier channels to deliver 40 watts into a single 8 ohm load Some important things to remember e Both sides of the load loudspeaker are driven There is no ground as far as the loudspeaker is concerned e connectors used between the amplifier and the speaker s should be insulated so that neither side of the connector can accidentally contact ground the other conductor or any other conductor Ordinary phone plugs are especially problematic in this application since the design of the plug exposes both conductors and the two conductors are momentarily shorted together when the plug is inserted or withdrawn To avoid inadvertent operation the mono bridge switch is located internally the top cover must be removed to alter the switch setting Refer to Appendix A for setting instructions In mono bridge mode use the CHANNEL INPUT and connect the speaker across the two plus output terminals The channel 1 plus output terminal is the in phase or non inverting output connection connect it to the plus side of the speaker Only the CHANNEL GAIN control is functional If you connect a pair of headphones to the front panel STEREO HEADPHONES jack they will receive out of phase signals not harmful to anything except phase or polarity fanatics In mono bridge mode the 420 delivers 40 watts into an 8 ohm load The minimum load impedance is 8 ohms Chapter 7 Applications Here are a few ap
33. r applications We recommend that you read this manual cover to cover You will find the answers to most of your questions inside Should you have any comments or questions please do not hesitate to contact us at the numbers addresses below Your calls are always welcome Phone 425 778 7728 Fax 425 778 7727 Email symetrix symetrixaudio com Website www symetrixaudio com 4 ar O Ww 9 GAN GAN 8 2 420 we STEREO POWER AMPLIFIER A A 4 5 Y OQ OO Q 100 WATTS MAXIMUM Je 420 STEREO TYPICAL INPUTS INPUT 2 INPUT 1 5 AMPLIFIER MP 000 2 VA CHANEL reste 4 N UNBALANCED UNBALANCED SLEEVE GROUND Pe 4 GB ee FUSE BALANCED 69 BALANCED 6 ES 1 2 9 MANUFACTURED IN ING BALANCED Symetrix l 0 Rear panel Symetrix Operator Safety Summary Equipment Markings CAUTION RISK OF ELECTRIC SHOCK DO NOT OPEN TO REDUCE THE RISK OF FIRE OR WARNING ELECTRIC SHOCK DO NOT EXPOSE THIS EQUIPMENT TO RAIN OR MOISTURE a RISQUE DE CHOC ELECTRIQUE AVIS NE PAS OUVRIR SEE OWNERS MANUAL VOIR CAHIER D INSTRUCTIONS No user serviceable parts inside Refer servicing to qualified service personnel ll
34. re that this wire is reconnected when reassembling the 420 Remove two screws located on the chassis bottom that secure the heatsink Lift out both printed circuit boards taking care not to stress the two voltage regulators or the ribbon cable that joins the two circuit boards Bridged Mono Mode Switch To prevent inadvertent setting the bridged mono mode switch is located inside of the 420 To set or reset the switch use the following procedure 1 N Disconnect the 420 from the AC power source Remove the top cover using the previously described procedure Locate the switch which is situated near the input connectors Depress the switch plunger to select mono bridge mode plunger remains IN For stereo mode the switch plunger should be OUT Replace the top cover and all fasteners previously removed 23 Symetrix Symetrix Inc 6408 216th St SW Mountlake Terrace WA 98043 USA Tel 425 778 7728 Fax 425 778 7727 Website http www symetrixaudio com Email symetrix symetrixaudio com
35. s RMS 60 Hz 100 watts per channel into a 4 or 8 Ohm load Maximum Output mono bridged 40 watts RMS into an 8 Ohm load Note The maximum operating ambient temperature is 25 Minimum Load 4 Ohms stereo mode degrees C 8 Ohms monobridged mode Headphone Output 100 Ohm source impedance In the interest of continuous product improvement Symetrix Inc 19V open circuit with speakers off reserves the right to alter change or modify these specifications without prior notice Performance Data measured at 120V AC line voltage Frequency Response 20 Hz to 20 kHz 0 1 dB Crosstalk gt 60 dB 22 kHz bandwidth Sensitivity 0 dBu for 20 watts into an 8 Ohm load Maximum Gain 27 dB Signal to Noise gt 95 dB Typical Distortion lt 0 04 1 kHz 1 watt into 8 Ohms THD Noise lt 0 2 20 Hz to 20 kHz 20 watts into 8 Ohms Clip Indicators One per channel 420 Architects and Engineers Specifications The power amplifier shall be a compact two input two output high performance power amplifier It shall occupy a single rack space 1U The unit shall be capable of delivering 20 watts per channel into a minimum load of 4 Ohms The unit shall also have a mono bridge mode which delivers 40 watts into a minimum load of 8 Ohms The inputs shall be active balanced bridging designs terminated with 3 pin XLR AES IEC standard wiring and 4 TRS female The input circuitry shall incorporate RFI filters The balanced inputs shall accommodate 24 dBu signa
36. s are not necessarily low impedance Similarly although 1 4 inch jacks are typically used for things like guitars which are high imped ance and unbalanced this does not predispose them to only this usage After all 1 4 inch jacks are sometimes used for loudspeakers which are anything but high impedance Therefore the presence of 3 pin XLR connectors should not be construed to mean that the input or output is low impedance or high impedance The same applies to 1 4 inch jacks So what is really important Signal level and to a much lesser degree the impedance relation between an output signal source and the input that it connects to signal receiver Signal level is very important Mismatch causes either loss of headroom or loss of signal to noise ratio Thus microphone inputs should only see signals originating from a microphone a direct DI box or an output designated microphone level output Electrically this is in the range of approxi mately 70 to 20 dBm Line inputs should only see signals in the 10 to 24 dBm dBu range Guitars high impedance microphones and many electronic keyboards do not qualify as line level sources The impedance relation between outputs and inputs needs to be considered but only in the following way Always make sure that a device s input impedance is higher than the output source impedance of the device that drives it Some manufacturers state a relatively high impedance figure as the ou
37. s depth including connector allowance Rear chassis support recommended for road applications Allow at least one empty rack space above and below the unit for ventilation Electrical 117V ACnominal 60 Hz 100 Watts maximum 230V AC nominal 50 Hz 100 Watts maximum Connectors XLR 3 female for inputs Pin 2 of the XLR connectors is Hot TRS female connectors are also provided The XLR and TRS connectors are paralleled Operating Modes The 420 has several different operating modes allowing changes in how the Gain controls operate mono operation and mono bridged operation Gain Control Options The gain controls may be operated two different ways DUAL TRACKING and INDEPENDENT These modes differ as follows DuaL TRACRING The gain of both amplifier channels is controlled simultaneously by the CHANNEL 1 Gan control INDEPENDENT The gain of each amplifier channel is controlled independently by the CHANNEL 1 and CHANNEL 2 GAIN controls Monoand Stereo The Mono STEREO MODE switch located on the front panel selects between mono and stereo operation In STEREO mode the 420 operates as two independent amplifiers In mono mode the two input channels are mixed and the mix routed to both channels The proportions of the mix are determined by the input signal levels not the two gain controls The two Gain controls determine the output level of their respective outputs The Mono STEREO switch is isolated from the input jacks its
38. self immolates For solid state amplifiers operating without any load connected or with a load whose impedance is higher than the amplifier s rated minimum is harmless Of course there is a price and that price is diminished power output As the load impedance doubles output power halves this is a best case scenario and depends on actual circuit design Thus an amplifier having a 50 watt 8 ohm power rating would only deliver 25 watts into a 16 ohm load Since output power increases as load impedance decreases assuming that the voltage remains the same or nearly the same operating an amplifier into the lowest possible impedance results in the Symetrix Symetrix maximum power output Taking this to its illogical conclusion an amplifier capable of delivering 20 volts into any load impedance would deliver 50 watts into an 8 ohm load This same amplifier would deliver 800 watts into a 0 5 ohm load Obviously you can t do this to most 99 amplifiers The limiting factor is the maximum current that the amplifier can deliver At 8 ohms 50 watts corre sponds to 2 5 amps At 0 5 ohms 800 watts 20V into 0 5 ohms corresponds to 40 amps The minimum load impedance is just another way of stating an amplifier s maximum current output Operating a solid state amplifier below its rated minimum load impedance can do anything from nothing if the amplifier is well protected and or extremely underrated to triggering smoke alarms
39. tput impedance of their equipment What they really mean is that this is the minimum load impedance that they would like their gear to see In most cases seeing a output impedance figure of 10 000 10K ohms or higher from modern equipment that requires power batteries or AC is an instance of this type of rating If so then the input impedance of the succeeding input must be equal to or greater than the output impedance of the driving device 16 Symetrix equipment inputs are designed to bridge the output of whatever device drives the input that is be greater than 10 times the actual source impedance The 420 s outputs are intended to drive 4 ohm or higher impedances typically loudspeakers or headphones The two facts that you need to derive from this discussion are 1 Match signal levels for best headroom and signal to noise ratio 2 For audio impedance matching is only needed for antique equipment and power amplifier outputs In all other cases ensure that your line inputs bridge are in the range of 2 to 200 times the output source impedance your line outputs Signal Levels The 420 is designed around studio professional line levels 4 dBu or 1 23 volts The input sensi tivity is high enough that the 420 can be driven to full output with the input gain controls set to less than wide open The unit is quiet enough to operate at lower signal levels such as those found in semipro or musical instrument MI equipment 10 dBu or 3
40. ts amps impedance and watts is stated as follows E IR where electromotive force in volts R resistance or impedance in ohms I current in amperes and P power in watts E PR Using simple algebra you can manipulate these equations to solve for any combination of known and unknown variables What this means is that the impedance of a loudspeaker and the ability of an amplifier to drive it are intimately related to the amplifier s output power and the amount of voltage and current that it can deliver to a load the loudspeaker If both voltage and current are limited and they are the load resistance must remain above some minimum value the amplifier s minimum load impedance The minimum load impedance combined with the output power rating defines the maximum values of voltage and current that the amplifier can deliver to its load Forexample Make Model Watts Minimum Output Volts Output Amps Load Symetrix 420 20 4 8 94 2 23 Crown DC 300AH 295 4 34 35 8 59 Crest 8001 1400 2 52 92 26 45 Reaching an amplifier s voltage limitation results in clipping Reaching an amplifier 5 internal current limit usually results in clipping which is caused by the activation of the amplifier s internal protection circuitry Without protection circuitry or at least some sort of current limiting lower load impedances mean increasing current and upon exceeding the amplifier s maximum current rating smoke as the unit
41. udios in the in between region Now you should realize that although most of these phones say 8 ohms somewhere or another what they really mean is suitable for 8 ohm outputs or these headphones are sensitive enough to be driven from an 8 ohm output that normally drives loudspeakers The Great Gain Control Mystery The gain volume controls found on amplifiers are another object of misunderstanding For the record the gain control on an amplifier inserts an adjustable amount of loss attenuation into the signal path Given a constant input signal varying this loss varies the overall gain which varies the output signal level which amounts to changing the output power which changes the volume yes Note the phrase Given a constant input signal This means that a given amount of loss intro duced via the gain control can be overcome by increasing the input signal by the same amount This destroys the myth that reducing the gain setting somehow reduces the power output capabil ity of the amplifier The gain control reduces the gain of the amplifier or reduces the input sensitiv ity of the amplifier The maximum possible output level is the same regardless of the setting of the control as long as some signal gets through The setting of an amplifier s gain control affects only the amplification factor of the amplifier and has no bearing or effect on the amplifier s maximum output capability When two l
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