First Watt model F4 Operation and Service Manual
Contents
1. externally or not as you please LEFT CHANNEL LOUDSPEAKER 2 OHMS F4 AMPLIFIER RED OUTPUTS WIRED TOGETHER OR DUAL INPUT CABLES OR DUAL CABLES JUMP XLR PINS 2 3 RIGHT CHANNEL LOUDSPEAKER 2 OHMS PREAMP LEFT CHANNEL RCA OUTPUT y yv K PREAMP RIGHT CHANNEL RCA OUTPUT F4 AMPLIFIER yY y F4 PARALLEL MONO CONNECTIONS With everything connected up and the source equipment powered up first you can proceed to turn on the power switch to the amplifier Turn on and turn off thumps and noise are small in this amplifier and should not present any hazard to delicate drivers At this point you should be able to listen to music This amplifier has less gain than most but at 15 watts rating it should not need it If you need to turn the gain up on your preamp then do so If you can t get enough gain then you are probably using either the wrong speaker or the wrong amplifier Talk to your dealer if this is the case The power supply of the amplifier is isolated from the chassis and AC earth ground by a thermistor which connects the circuit ground to the chassis and earth ground This helps to prevent ground loops but the thermistor stands by to conduct AC line voltage to ground until the fuse blows in case of transformer or other such failure The input impedance is 48 Kohms and the input capacitance is very low so you should find it easy to drive with tube equipment if you like The a2. good objective performance gives us a superb sounding amplifier The low distortion bandwidth extension and high damping results in midrange clarity treble detail and control on the bottom end While these are available from most good solid state amplifiers the F4 also brings depth imaging midrange warmth and top end sweetness Overall it is one of the best sounding amplifiers and if you can live with unity voltage gain in your amplifier it is possibly your best choice The amplifier is designed for fairly unusual applications 1 You can drive it directly with a line source such as a preamp Using this amplifier presupposes that you have a preamplifier or other source with adequate voltage and or loudspeakers sufficiently sensitive to be able to work with the voltage and gain you have You would think offhand that this might be a real problem but it is my experience that this is often not the case First you will find that the more sensitive drivers deliver good volume levels at quite low wattage My Lowther DX55 s 93 dB watt mounted in Alerion enclosures adequately fill my 11 000 cubic foot listening room with about 5 volts the equivalent of a 4 watt amplifier can achieve good levels with headroom to spare with my phono stage or DAC feeding an X1 preamp which has a maximum of 14 dB gain If lived in an apartment could get complaints from my neighbors Thanks to the dutiful efforts of John Atkinson at Stereo
3. watts Fuse 3AG slow blow type 2 5 Amp for 120VAC 1 25 Amp for 240 VAC Warranty Parts and labor for 3 years not covering shipping costs or consequential damages Copyright 2007 General Amplifier General Amplifier Inc PO BOX 7607 RENO NV 89510 7607 2SK37 R11 150 IRFP240 IRFP240 47 3W R12 ae re IRFP240 47 3W KA LED1 2N4736 47 3W IRFP9240 2SJ108 R14 47 3W IRFP9240 7 NINAN R5 FIRST WATT F4 rg 6747907 CHANNEL SCHEMATIC Notes Output device bias is approximately 43 amps per device which measures 200 mV across the 47 ohm source resistors All devices are matched for Vgs to 02V Cold bias is adjusted low at 130 mV and readjusted as the amplifier warms up The proper value is achieved after 1 hour with the heat sinks at about 50 deg C P2 is adjusted for minimal DC offset at output FIRST WATT PS R1 POWER SUPPLY COPYRIGHT 2005 GENERAL AMPLIFIER RESISTORS ARE 47 OHM 3W CAPACITORS ARE 150 0UF 25V THERMISTORS ARE CL6Q TYPE 35A 200V z AC LINE UNLESS NOTED i 120 R 9 c3_ 4c4 _ 4 c7_ 4c8 ae e_ TH 25A SLOW 18VAC e SWITCH y y P H 120 VOLT T 122 18vVAC 35A R1 200V z AAN ZAVAN S R2 Wi x Oe ty 1 25A SLOW ni R
4. 3 SAG 4 k R4 2 240 VOLT fe CHASSIS GROUND POWER SUPPLY SCHEMATIC Notes Thermistors in series with the AC line are also used to select line voltages at 120 VAC or 240 VAC as seen on the front edge of the PC board layout FIRST WATT F4R LEFT CH HO LHOW ONtd LIVM 1590 L d R25 LE FIRST WATT F4R LEFT CH Gm FP240 Q3 L HO LHOWW OY LLYM ISH je _ R25 1q BOTTOM TOP PN PN NF N HY DN PN PN NING PN PN NI NI 1 UN 4 zi 1 Ic RIF UT AT wy t EB en EI r ae Rem W K ll HER HE8 9 59 Aa Ia 1 SP UI S
5. First Watt model F4 Operation and Service Manual Introduction Anyone paying attention is aware that First Watt makes unusual amplifiers with the emphasis on performance at low power levels Here s another one The F4 is Class A impedance converting amplifier having no voltage gain or feedback Its input impedance is 47 000 ohms and its output impedance is about 0 2 ohms It is suitable for driving a high sensitivity loudspeaker with the output voltage of a preamp or other line level audio source It is also useful with a less sensitive loudspeaker in a bi amped configuration where it takes input from the output of a conventional amplifier As a stereo amplifier with single ended inputs and outputs it will deliver up to 25 watts into 8 ohms with a damping factor of 40 It will do 50 watts into 4 ohms and as a mono block amplifier with parallel inputs and outputs it will do about 100 watts into 2 ohms As a mono block amplifier with balanced inputs and outputs the power output rating is 100 watts into 8 ohms at 1 and the damping factor is 20 The amplifier operates Class A to 25 watts 50 peak and the distortion is 2 4 and 3 harmonic in character rising or declining in linear proportion to the output power The amplifier has a direct coupled input and output with a 0 5 dB rolloff around 1 Hz and 200 KHz It does a clean square wave at 100 KHz The combination of a simple Class A circuit operated without feedback and the
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7. RI gt RIGHT CHANNEL WOOFER LO PASS E ACTIVE CROSSOVER F4 AMPLIFIER LEFT CHANNEL WOOFER F4 BI AMPING WITH ACTIVE CROSSOVER NETWORKS 2 You can use it with a low wattage integrated or tube amplifier in a biamplied system There are a number of popular tube integrated amplifiers on the market the so called Flea Watt amplifiers which don t have much juice but offer a warm tubey sound which is much prized These amplifiers are good candidates for operation with an F4 in a setup where the loudspeakers have two separate sets of input terminals one for the woofer and one for the midrange tweeter In such a system the F4 is used to drive the bottom end where it offers superior damping and higher current and the flea watt amplifier drives the mid and top end unburdened by the effort of driving the woofer but preserving the character of the tube amp It is particularly convenient that the F4 is close to unity gain which provides close level matching between the bottom and top In the diagram below it is assumed that crossover networks are incorporated into the woofer and mid high end speakers RIGHT CHANNEL MID HIGH RIGHT CHANNEL WOOFER FLEA WATT INTEGRATED F4 AMPLIFIER LEFT CHANNEL MID HIGH LEFT CHANNEL WOOFER F4 BI AMPING WITH FLEA WATT INTEGRATED AMPS You can take advantage of the same arrangement if you have drivers and you want to make your own custom passive crossove
8. eft channel as seen from the front has the positive output and the right channel has the negative The bottom black banded connectors are signal ground and are not used The positive speaker lead goes to the left red band and the negative speaker lead goes to the right red band LEFT CHANNEL LOUDSPEAKER 8 OHMS yY yY LEFT CHANNEL BALANCED OUTPUT PREAMP F4 AMPLIFIER XLR OR RCA INPUT CONNECTORS jas RIGHT CHANNEL LOUDSPEAKER 8 OHMS RIGHT CHANNEL BALANCED OUTPUT PREAMP F4 AMPLIFIER y yY F4 BALANCED MONO CONNECTIONS Mono Parallel Operation In mono parallel operation you can achieve up to 100 watts output into a 2 ohm load The amplifier is driven by a single ended source through the two RCA inputs in parallel The easiest way to do this is through a male RCA cable going into one of the RCA inputs with a jumper between pins 2 and 3 of the XLR input connector which parallels the two inputs This jumper is supplied with the amplifier If you look carefully at the connector you will see tiny labels next to the pin holes For parallel operation of the two channels as one you must connect the outputs left positive to right positive either at the connectors themselves or by bi wiring the loudspeaker with two sets of cables Remember that the positive outputs have a red band The black banded output connectors are already connected together to ground so you can make this connection
9. mplifier is largely indifferent to the source impedance of your preamp so a high source impedance is not a problem The amplifier requires about 1 hour of operation to reach normal operating temperature and this warm up time is appropriate for the most critical listening but is not otherwise an issue The amplifier s final adjustments were made after an hour and the performance difference between 5 minutes and 60 minutes is significant do not personally see a reason to run the amplifier all the time but you can do that if you want to The power supply capacitors are likely to last about 15 years or so and while they will slowly dry out just sitting there they will have a shorter life span with the amplifier running constantly Also at 180 watts it makes economic sense to shut the amplifier off if you aren t planning on using it for the rest of the day Again the heat sinks on this amplifier run fairly hot and you want to make sure that they get adequate ventilation They will run at around 25 degrees C above the ambient temperature which puts them around 50 degrees in the average listening room At this temperature you should be able to put your hand on them for about 5 to 10 seconds or so Now the following is for your protection Do not defeat the AC line Earth ground connection on the amplifier power cord It provides an extra barrier to prevent potential shock hazard Do not replace the fuse with a type other than
10. phile have been able to examine the maximum gain and output voltage of the tube and solid state preamps he has measured over the last 20 years or so www stereophile com Virtually all active line stages achieve adequate gain and voltage swing to drive the above combination Basically you need a linestage with about 12 dB gain and 10 volt maximum output This includes tube products from VTL Hovland ARC Cary McIntosh CJ EAR MF BAT Lamm and Rogue It includes solid state preamps from Pass Labs Krell McIntosh Levinson Halcro Naim Boulder Ayre Bryston Threshold McCormack Linn Sutherland and on and on If your loudspeaker is less efficient then you need more gain from your source and or more gain and voltage swing from your linestage As you go in sensitivity below 87 dB you will want to consider 100 watt balanced mono block use with a preamp capable of swinging 14 volts per balanced output and having a gain of 20 dB As your loudspeaker increases in sensitivity you need less gain and voltage swing You can also use active crossover networks to drive multiple F4 s as shown below A number of crossovers on the market incorporate voltage gain and high voltage swing a prime candidate being the Pass Labs XVR1 which can swing the full voltage capability of the F4 RIGHT CHANNEL MID HIGH HI PASS J HI PASS RIGHT li F4 AMPLIFIER LEFT CHANNEL VW Vy MID HIGH LEFT t LO PASS
11. rrangement you can also load the flea watt amp not shown here with a resistive or other load which elicits the best performance from it Setup The initial setup of the amplifier is very straight forward Place the amplifier in a well ventilated location as it draws about 160 watts during operation and requires as much opportunity to cool itself as possible You should be able to put your hands on the heat sink during operation If you can t do this for 5 seconds or so they need more ventilation On the front panel there are two blue LED lights one for each channel or each half of a mono block indicating power to the channel On the rear panel you will find a pair of RCA inputs and XLR balanced input speaker outputs a fuse holder an AC power receptacle and on off switch The label will indicate a serial number and also what AC line voltage the amplifier is set for If the voltage is 120 VAC then the fuse value will be a 3AG slow blow fuse rated at 2 5 amps If the voltage is 240 VAC then the fuse will be rated at 1 25 amps Do not substitute a larger value fuse Contact First Watt if you have any questions The amplifier can be run in three configurations There is one way to use the amplifier in stereo There are two ways you can run the amplifier as a mono block either by paralleling the inputs and outputs for more current and less distortion or by driving the two channels from a balanced source I m assuming that you kno
12. rs RIGHT CHANNEL MID HIGH RIGHT CHANNEL F4 AMPLIFIER FLEA WATT INTEGRATED LEFT CHANNEL WOOFER LEFT CHANNEL MID HIGH F4 BI AMPING WITH FLEA WATT INTEGRATED AMPS AND PASSIVE CROSSOVER NETWORKS An alternative arrangement results when you create your own passive crossover networks using resistors as loads and feeding the results to F4 channels The networks behave more ideally driving resistors than the complex impedance of loudspeakers and the resistor values can be set over a wide range of impedances The loudspeaker drivers themselves benefit from the direct connection to the low output impedance of the amplifiers I gt gt KA RIGHT CHANNEL MIDZHIGH LEFT CHANNEL MID HIGH L geme A E Aa F4 AMPLIFIER FLEA WATT INTEGRATED K LEFT CHANNEL WOOFER F4 ALTERNATIVE BI AMPING WITH FLEA WATT INTEGRATED AMPS AND PASSIVE CROSSOVER NETWORKS 3 You can use it with a low wattage integrated or tube amplifier in a stand alone system for higher output current Dpr N N z RIGHT CHANNEL 4 OHM FLEA WATT F4 AMPLIFIER LT INTEGRATED LEFT CHANNEL 4 OHM F4 BOOSTING FLEA WATT INTEGRATED AMPS There will be occasions where the loudspeaker load is too low for optimal performance from a flea watt and in this case you can consider F4 s for the purpose of getting as much as 100 watts into 2 ohms per mono block In this a
13. specified Do not operate the amplifier outside in the weather or in and around water or anything resembling water If you spill a drink in the amplifier or if your dog cat child urinates on it turn it off immediately unplug it and do not operate it until cleaned by a qualified technician If something gets loose or rattles around inside or smells funny or if you can t touch the heat sinks for 5 seconds or so then turn it off unplug it from the wall and contact First Watt There are no user serviceable parts inside Do not open the amplifier and if you do anyway don t operate it with the cover off There are hazardous voltages inside If you need to change the operating AC voltage contact First Watt If you have a problem contact First Watt We are much happier helping you solve problems so that we can be certain that it s done properly If you are far away and don t want to ship the product for repair we will assist your technician with information and parts Contact www Firstwatt com nelson passlabs com Summary of the nominal specifications Measured at 120 V AC with a 25 ohm source and an 8 ohm load Distortion 1 watt 05 1 KHz Input Impedance 47 Kohm Damping Factor 40 Output power stereo 8 ohms 25 watts 0 5 THD 1KHz Voltage Gain 0 3 dB Maximum unclipped output 20 Volts Maximum output current 5 amps Frequency response 5 dB 0 1 Hz 200 KHz Noise 50 uV unweighted 20 20 KHz Power consumption 180
14. w how to attach the speaker cables to the 5 way output connectors provided Please make all the connections with the amplifier power switch in the OFF position Stereo Operation For two channel operation input signal is connected to the RCA inputs ignoring the XLR input Unlike some other designs from First Watt or Pass Labs there are no jumpers on the unused XLR inputs The output connections to the loudspeakers are made through the gold plated brass 5 way connectors The red top connection is positive and the black bottom is negative In this amplifier the black banded output connectors are connected directly to signal ground Mono Balanced Operation In mono balanced operation you can achieve up to 100 watts output into an 8 ohm load The amplifier is driven by a balanced source either through the two RCA inputs or the balanced XLR connector The left channel as seen from the front is presumed to have the positive phase so the positive input appears either on the left channel RCA or Pin 2 of the XLR connector The right channel gets the negative phase input on its RCA or on Pin 3 of the XLR connector Pin 1 of the XLR connector is ground Pin 2 is positive Pin 3 is negative If you look carefully at the connector you will see tiny labels next to the pin holes No jumpers or shorting plugs are necessary The output to the speakers occurs through the top red banded brass connectors on the two output 5 way binding posts The l
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