MOTM-440 User Manual
Contents
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4. 1 Philips screwdriver Fingernail brush for washing off the organic flux Old towel for blotting dry board Small amount of heat sink compound not supplied For more information of tools used and suggestions see the MOTM FAQ and Tutorial pages at http www synthtech com You also may find it useful to purchase heat shrink tubing 1 8 diameter 2 1 shrink ratio and solder Just be sure it 15 NOT rosin based flux HOW TO FOLLOW THE DIRECTIONS Please read the entire instruction before proceeding There may be valuable information at the end of the instruction Each instruction has a check box next to it After you complete the instruction check the box This way you can keep track of where you are in the process VERIFY THE PARTS LIST g Verify that all of the parts are in the kit as shown on the parts list SYNTHESIS TECHNOLOGY PAGE 4 MOTM 440 ASSEMBLY 3 25 00 WWW SYNTHTECH COM A WORD ON SOLDERING There are 2 very different types of solder used in the kit Most of the soldering uses Organic Flux solder This is strictly for use on the pc board and is NOT to be used on the front panel wiring In order for solder to stick to the copper a chemical called flux is embedded in the solder The flux leaves a residue on the pc board that should be cleaned with warm water DO NOT USE SOAP OR OTHER CLEANSERS Most of the parts in the kits are waterproof and can be washed in the sink The flux is OSHA
5. 2 pad and the Orange wire into the 3 pad Find the long red white black wire It solders into VR5 Solder the White wire into pad 1 the Black wire into pad 2 and the Red wire into pad 8 Find the 2 red black twisted wires They go into J2 and J3 Solder the Red wire into pad 1 and the Black wire into pad 2 Find the 3 orange white twisted wires They solder into J1 J7 and SW1 In all cases solder the White wire in pad 1 and the Orange wire into pad 2 Find the black TEMPCO resistor It goes on top of U14 You will notice the 2 large holes on the ends of U14 the CA3086 But first you will need to apply a very small amount of heat sink compound to the top of U14 Heat sink compound can be found at Radio Shack Allied Mouser DigiKey Maplin Farnell and most other electronic supply stores The key is not to use too much The TEMPCO resistor will squish the heat sink compound as it is lowered to rest flat on top of U14 Once the TEMPCO is resting flat on U14 with the heat sink compund solder the 2 leads of R57 YOU ARE NOW FINISHED WITH THE PC BOARD WORK BREAK TIME PART 7 FRONT PANEL PREPARATION You will now attach components to the front panel It is HIGHLY recommended that you use a set of hollow shaft nut drivers NOT PLIERS to tighten the nuts This prevents scratching NOTE all references to part orientation is from the REAR of the panel Locate the 8 Switchcraft jacks Notice that from the rear ther
6. 7 inches red black white lea Power Cable 20 Hardware bag containing 4ea 8 32 x 3 8 black screws for mounting module to rack 4 6 32 1 2 zinc screws for attaching pc board to bracket 4ea 1 4 inch aluminum spacers 6ea 6 KEPS nuts 2 for attaching bracket to front panel 4 for board 6ea small tie wraps Organic Solder No clean Solder PC Board MOTM 440 SYNTHESIS TECHNOLOGY PAGE 3 MOTM 440 ASSEMBLY 3 25 00 WWW SYNTHTECH COM GENERAL INFORMATION Thank you for purchasing the MOTM 440 Discrete OTA Lowpass Filter If you have any issues concerning the building or use of the kit please contact us at 817 498 3782 or by email synth1 airmail net This kit should take the average builder between 4 to 5 hours The VCF kit contains many different resistors and special parts However please remember this is NOT a speed contest it is an accuracy contest There is no rule that you have to complete the entire kit in one day as long as you wash the flux off Successful kit building relies on having the proper tools Here is a list of what you will need to build your MOTM 440 Soldering iron 5 0W max power Needle nose or chain nose pliers Diagonal cutters Allen key set for securing the knobs 1 16 or 1 6mm Magnifying glass to read the capacitor codes and to inspect solder joints Lead bending tool optional but makes the job go much faster DVM Digital Volt Meter or oscilloscope to check the output
7. sure the cap is flat on the pc board Solder into C7 C8 C9 and C10 g Locate the Spectrol pots not the blue ones the other ones There are 3 type 148 9609 104 100K pots and 1 type 149 100K linear pot The type 149 pot goes into VR1 and the 3 type 148 9609 104 pots go into VR2 VR3 and VR4 Be sure they are flat and sticking out perpendicular to the pc board g Locate the 4 pieces of RG 174 black coax cable Again note that one end has longer wires stripped than the other The short ends will go in the board in locations J4 J5 J6 and J8 Look at the pc board Notice that in the coax positions there is a large hole pad lower pad and a smaller pad top hole The braided wire is soldered into the larger hole The smaller inner conductor goes in the top hole BE SURE THE SHORTER BRAIDED END GOES INTO THE PC BOARD The 1 LONG coax cable solders into J4 The 3 SHORT coax cables go into the other locations Solder each coax cable into the holes Attach a tie wrap to secure the coax cable flush to the board The tie wrap goes down into the left hole and up through the right hole Secure and trim off any excess g Find the 2 orange white gray twisted wires They are soldered into VR6 and VR7 Look at the pcb silkscreen and note that there are the numbers 1 2 and 3 beside the SYNTHESIS TECHNOLOGY PAGE 9 MOTM 440 ASSEMBLY 3 25 00 WWW SYNTHTECH COM box around the 3 pads Solder the White wire into the 1 pad the Gray wire into the
8. white silkscreen the top side NOTE some reference designators are under the resistor Be careful g We will stuff the resistors by value to make things easier The resistors and other long leaded parts are inserted on a 0 4 inch spacing The important thing is to be sure that the part is sitting all the way down on the board Push the leads in the holes push the part on the board and then bend the leads on the bottom outwards to a 45 degree angle roughly This is called cinching the leads and keeps the part from falling out From the bottom of the board solder using the organic flux applying heat to the pad for about a half second first then applying just enough solder to make a small puddle that looks like a tiny pyramid Enough solder should SYNTHESIS TECHNOLOGY PAGE 5 MOTM 440 ASSEMBLY 3 25 00 WWW SYNTHTECH COM flow in the hole such that on the top component side a small amount is on the top pad as well The rule of soldering don t use too much you can always add more Cut the leads flush with the top of the solder joint with your diagonal cutters NOTE save the large black 1K TEMPCO resistor for later Locate the 10K resistors 11pcs Solder the resistors into R10 left of VR2 R12 and R13 below R10 R20 right of 911 913 R26 right of 09 R32 above SW1 R38 left of D1 D2 R42 right of Q2 R43 above J1 R44 right of Q8 and R45 right of C7 Locate the
9. 220 ohm resistors 8 and solder into R21 below 912 R22 left of 013 R27 right of Q9 R28 left of U11 R33 right of Q7 R34 left of U8 R39 left of D1 D2 and R40 left of U6 Locate the 15K resistors 6 and solder into R18 by Q13 R24 by Q10 R30 right of C10 R36 right of Q3 R49 left of U5 and R50 by C5 Locate the 100K 1 resistors 4 and solder into R1 and R2 by U1 and R46 R47 left of U3 Locate the 150K resistors 4 and solder into R17 by 913 R23 by 910 R29 by Q7 and R35 by Q4 Locate the 1K5 resistors 4 and solder into R19 below JP1 R25 by C7 R31 by C10 and R37 by 91 Locate the 100K resistors 4 and solder into R5 R6 R7 left of VR3 and R61below SW1 Locate the resistors 8 and solder into R16 J8 and R51 R58 at top edge Locate the 51K1 1 resistors 8 and solder into R3 R4 left of U4 and R52 by C6 Locate the 68K resistors 2 and solder into R9 below VR2 and R15 below Locate the resistors 2 and solder into R11 by VR1 and R48 by SW1 Locate the 200K resistors 2 and solder into R14 by VR1 and R62 left of U9 Locate the 20K resistors 2 and solder into R41 top edge and R58 above J3 Locate the 221K 1 resistor and solder into R8 by U1 SYNTHESIS TECHNOLOGY PAGE 6 MOTM 440 ASSEMBLY 3 25 00 WWW SYNTHTECH COM Locate the 6K8 resistor and solder into R55 by Locate the 3K6 resistor and
10. 25 00 WWW SYNTHTECH COM 1000 ohms nom 50mv pk pk min 11V pk pk max sets initial cutoff frequency at OV input sets resonance Q or acts as RES attenuator sets gain of FM1 CV from 2 to 2 sets gain of FM2 CV from 0 to 1 sets audio input level for each IN sets NORMAL or ENHANCE bass response 15VDC 25 ma nominal 15VDC 20 ma nominal 2U x 5U 88 1mm x 221 5mm 4 375 inches 111mm PAGE 17 T Jo T 5 0002 752 v 5 queunsog ezts SSVaMOT 0 WLOW WIOW 8T8 888 O8T94L XL HLYXOM sz99 ADO IONHOSL SISSHLNAS 1 1 AST Q ust E E ASIT A sat AST sta 0 vza doe Lea 0 MOST ozz ozz ozz ozz Dossoa 022 ozz
11. and the Orange wire is lug 3 Once the wires are attached insert VR5 into the FREQ panel hole VR7 in the FM1 hole and VR6 in the FM2 hole Add a flat washer and hex nut Tighten with a nut driver Now you will solder to the BASS mode switch The SW1 White wire goes to the CENTER lug The Orange wires goes to the BOTTOM lug The TOP lug is not used Now you will solder to the 8 jacks Each jack has 3 lugs from the rear we will refer to them as LEFT TOP and BEVELED The TOP lug is for the switched contact this is a NC normally closed contact that is opened when a plug in inserted Solder the coax in J8 to the OUT jack The braid goes to the BEVELED lug The inner conductor goes to the LEFT lug This is true of ALL coax wires Solder orange white wire in J7 to the RES jack The Orange wire goes on the beveled lug The White wire goes on the LEFT lug Solder the coax in J6 to the IN 8 jack Solder the red black wire in J3 to the FM2 jack Black wire to the beveled lug red wire to the left lug Solder the coax in J5 to the IN 2 jack Solder the red black wire in J2 to the FM1 jack SYNTHESIS TECHNOLOGY PAGE 12 MOTM 440 ASSEMBLY 3 25 00 WWW SYNTHTECH COM Solder the long coax in J4 to the IN 1 jack Solder the orange white wire in J1 to the 1V OCT jack Rotate all of the front panel pots fully counter clockwise Locate the KNOBS Notice each knob has a white line on it Place the knob on the pot shaft align t
12. on the pots so that they are touching the back of the panel Again make sure each pot s nut is touching the back of the panel no gaps There will be a gap from the edge of the pc board to the panel Remove the hex nuts on VR1 amp VR4 For all of the pots first put on the flat washer then the hex nut Tighten with a nut driver DO NOT OVERTIGHTEN THE PLASTIC THREADS OF VR2 VR3 and VR4 SYNTHESIS TECHNOLOGY PAGE 11 MOTM 440 ASSEMBLY 3 25 00 WWW SYNTHTECH COM PART 9 FINISH WIRING TO THE PANEL Please read the following instructions carefully In order to neatly attach the many wires to the front panel components the wires are soldered in a specific order o 0 Find the SPDT toggle switch Insert the switch into the panel hole labeled BASS Tighten the outside hex nut but DO NOT OVERTIGHTEN Be sure the switch is oriented so the lever operates up and down and not side to side Locate the 3 blue Bourns 91A panel pots These solder to the 3 wires in VR5 VR6 and Look on the rear of the pot you will see 3 very small numbers by the solder lugs The 1 lug is on the RIGHT the 2 lug is in the CENTER and the 3 lug is on the LEFT These lugs correspond to the pad numbers on the pcb wires First solder the pot VR5 on the wires The Whites wire is lug 1 the Black wire is lug 2 and the Red wire is 3 Then solder pots VR6 and VR7 In both cases the White wire is lug 1 the Gray wire is lug 2
13. red brown orange R8 lea 6K8 blue gray red R55 lea 3K6 orange blue red R54 lea 44K2 1 yellow yellow red red R56 lea 330K orange orange yellow R59 lea 2K2 red red red R60 lea 100 ohm brown black brown R63 lea 1 3300 tempco R57 MOUNTS ON TOP OF U14 g IC bag containing the following 29 parts 13ea BC550C NPN transistor Q1 013 2ea 1N4148 diodes D1 D2 lea MXL1013 dual op amp U1 TLO72ACP dual amp U2 U3 U4 5ea SSM2220 dual PNP transistor U5 U6 U8 U10 U12 4ea SSM2210 dual NPN transistor U7 U9 U11 U13 lea CA3086 NPN transistor array U14 SYNTHESIS TECHNOLOGY PAGE 2 MOTM 440 ASSEMBLY 3 25 00 WWW SYNTHTECH COM OOO Misc 1 bag containing the following 5 parts 2ea Axial ferrite beads plain gray things L1 L2 lea MTA 156 power connector JP1 lea SPDT ON ON toggle switch SW1 lea 20K Bourns trimpot TP1 Knobs 7ea ALCO PKES90B1 4 Jacks 8ea Switchcraft 112A Pots 7ea containing the following lea 100K cermet Spectrol 149 100K conductive plastic log Spectrol 148 VR2 VR3 VR4 100K conductive plastic Bourns 95A1 VR5 VR6 VR7 Front panel Mounting bracket Wire bag containing the following 13 wires RG 174 coax 4 inches lea RG 174 coax 7 inches 2 wire set 22ga 6 inches orange white 2ea 2 wire set 22ga 3 inches red black 2ea 3 wire set 22ga 4 inches white orange gray lea 3 wire set 22ga
14. silkscreen Place the transistor in the holes but do NOT press the transistor all the way to the pc board Leave about 0 1 inch of the leads exposed between the pc board and the underside of the transistor Solder into 91 013 Locate the CA3086 transistor array Solder into U14 Apply a small bit of solder to the via holes These are the small pads that allow traces to change sides of the board DO NOT SOLDER PADS FOR THE REMAINING COMPONENTS SYNTHESIS TECHNOLOGY PAGE 8 MOTM 440 ASSEMBLY 3 25 00 WWW SYNTHTECH COM PART 5 FINAL BOARD WASH amp INSPECTION g Verify all the parts are in the correct locations Make sure all of the ICs are pointing the same direction Make sure all of the transistors are facing the same direction g Inspect the solder joints Any solder shorts Too much solder Missing joints g Wash the board under warm water Scrub gently Dry THIS IS A GOOD STOPPING PLACE TO REST OR PUT THE KIT AWAY UNTIL LATER You are now finished with the Organic flux solder All soldering past this point is using the No Clean solder You do not have to wash the board anymore PART 6 FINISHING THE PCB You will now solder in the remaining parts on the pcb in preparation for wiring to the front panel USE THE NO CLEAN SOLDER BE CAREFUL g Locate the blue trim pot Solder it into TP1 Be sure it is flat on the pc board Locate the 4 1000pf polystyrene caps Carefully bend the leads into the holes and be
15. the board Wash the board again gently scrubbing both sides Use ONLY warm water PART 4 MISC and IC STUFF Almost done with the parts on the pc board This will finish up the soldering with the organic flux Save the trim pot for later o Locate the MISC 1 bag and the IC bag Locate the ferrite beads 2 They are axial parts gray colored with no markings These are non polar and are soldered into L1 and L2 by JP1 Locate the MTA 156 power connector Solder into JP1 Note that the connector has a locking tab on one side This side is the inside facing relative to the pc board Note the silkscreen symbol for JP1 has a line on one side indicating this is the side where the locking tab goes Locate the MXL1013 amp Solder into U1 Note that all ICs are pointing down towards the bottom edge of the pc board The IC will have a notch or indentation in the top by Pin 1 Locate the 1N4148 diodes Notice they have a black band near one end This is the cathode and there is a matching band on the silkscreen for D1 and D2 Solder the diodes with the banded end pointing down Locate the TLO72 op amps 8 Solder into U2 and U4 Locate the SSM2220 dual transistors 5 Solder into U5 U6 U8 U10 and U12 Locate the SSM2210 dual transistors 4 Solder into U7 U9 011 0138 Locate the BC550C transistors 13 Match the flat side of the part to the flat side shown on the
16. the panel pots switch is wired correctly g No solder shorts or missing joints SYNTHESIS TECHNOLOGY PAGE 15 MOTM 440 ASSEMBLY 3 25 00 WWW SYNTHTECH COM USE OF THE MOTM 440 VCF The MOTM 440 needs to be set to 1V oct response by adjusting TP1 a set all the IN pots to 0 b set FREQ pot to 5 straight up c set RES to 10 d set BASS to NORMAL Apply a known 1V oct control voltage into the 1V OCT jack The filter will self oscillate with an initial frequency set by the FREQ pot Using a small screwdriver set TP1 to track as close as possible Note that the setting will be approximate this is not a VCO substitute The MOTM 440 is a stand alone module that can be used for filtering just about audio source you can t plug a guitar or mic directly into it but use a direct box or preamp first The response is always a low pass filter this means most of the frequencies above the cutoff are filtered out The 24db octave slope means that frequencies 1 octave above cutoff are reduced 24db a factor of 0 06 and 2 octaves above are reduced 48dB a factor of 0 004 The 12dB oct MOTM 420 reduces frequencies 1 octave away by a factor of 0 25 Therefore a rule of thumb is a 24dB oct filter is 4 times more effective in reducing the higher harmonic content Use of the 440 is straightforward Audio signals are plugged into IN1 and are set by corresponding level controls Note that it is possible to over dr
17. when the board is sitting on the bracket Note that the bracket holes for the pc board are actually oblong This is to allow adjustment for the pc board to firmly press up against the back of the panel As a start set the 4 screws ALL THE WAY TO THE LEFT of the oblong holes Loosely tighten the 4 KEPS nuts on the bottom THIS IS A VERY IMPORTANT STEP SO PAY ATTENTION AND READ ALL OF IT BEFORE PROCEEDING Note that each of the 4 pots on the board have 2 hex nuts and flat washer Remove the first hex nut and the washer Set aside What you will do now is adjust the remaining hex nuts so that when the bracket is all the way down on the panel s threaded studs all the pot hex nuts touch the rear of the panel Screw by hand each hex nut on the pots so that it is all the way on touching the face of the pot Now pick up the pc board bracket assembly and carefully slide it over the 2 threaded studs making sure the pots are aligned in the holes Use 2 6 KEPS nuts and tighten the bracket to the panel Loosen the 4 KEPS nuts on the bottom of the bracket Slide the pcb ALL THE WAY TO THE RIGHT AS FAR AS IT WILL GO so that the 4 pot nuts are all pressing against the panel By hand put hex nuts on the outside threads of VR1 and VR4 to keep the pc board in place Now tighten the 4 KEPS nuts on the bracket The pcb and bracket should be secure with no gaps visible between the panel and the pot nuts You may need to loosen the nuts
18. D1 D2 are used to raise the op amp s output 2 Vbe drops above U5 keeping it out of saturation The control current for the VCA is just plain resistor R49 AS the VC Q sweeps from 0 5V the control current sweeps from 20ua to 650ua The audio summer is a standard inverting op amp U2a The audio is split into two paths the normal audio goes to the first filter stage but there is a resistive network 48 13 652 feeding the other input of the VC Q This allows a bass boost to occur if R48 is shorted by SW1 The additional audio feeding the first stage at the opposite polarity increases the output level at higher Q settings Normal OTA filters even the SSM2040 drop the output level as Q is increased This is an exclusive MOTM 440 feature The audio output is buffered and amplified by a factor of 10 by U4a TROUBLESHOOTING If your 440 does not work please verify ALL of the following before contacting us The following reference directions assume that you are looking at the pc board with the panel to the right and the power connector to the left g All of the ICs are pointing the same way and all notches are down Diodes D1 and D2 s bands are both pointing down All 13 transistors flat sides are facing the panel AND be SURE you didn t swap a SSM2210 and a SSM2220 g The braided wire on the coax goes to the beveled side of the jacks The parts in the right places and
19. approved for flushing down the drain so don t worry about that A soft brush is used to gently scrub the board We recommend a fingernail brush which is about 1 x 2 and can be found for about 1 The other type of solder is called No Clean Flux because as the name implies it does not require washing This solder is used for wiring the pots switches jacks etc This solder is harder to use on the pc board because even when melted it is not very fluid about the consistency of toothpaste We will use it VERY SPARINGLY on the pc board OK let s get started on the board PART 1 SOLDERING THE RESISTORS Since there are more resistors than anything else we will start here If you do not know the resistor color code refer to the parts list Resistors are not polarity sensitive but the board will be easier to debug and look nicer if you point the first color band in the same direction for all the parts The color code is also in the README FIRST document that every customer receives with his or her first order You will start by soldering in ALL of the resistors 7 Find the RESISTOR bag g Find the MOTM 440 blank pc board There is a copy larger than actual size of the silkscreen which shows where the parts go at the end of this document It will be useful if you locate the part on the print first put the part in the board then check off the silkscreen All parts are inserted from the side of the board with the
20. e is beveled corner This corner is ALWAYS CONNECTED TO GROUND USUALLY WITH A BRAIDED CONDUCTOR Each jack has flat washer a lockwasher and hex nut Remove the nuts and washers from each jack Place aside Keep the lockwasher on the jacks Insert the 8 jacks lockwashers with the beveled corner in the upper right corner into the 8 holes Place the flat washer on the jack then the hex nut Hold the jack with one hand on the backside keeping it square Tighten the hex nut with a nut driver NOTE when tight not much of the exposed threads of the jack are exposed You are now ready to attach the pc board to the bracket and then wire up to the panel SYNTHESIS TECHNOLOGY PAGE 10 MOTM 440 ASSEMBLY 3 25 00 WWW SYNTHTECH COM PART 8 ATTACH PC BOARD TO BRACKET PANEL In the HARDWARE bag locate 4 6 32 x 1 2 screws 6 6 KEPS nuts and 4 spacers Locate the mounting bracket The pc board attaches to the bracket with the 4 screws threading from the top of the board through the spacers through the bracket and then out the bottom of the bracket The 6 KEPS nut attaches on the bottom of the bracket Note the bracket has 2 long mounting flanges with a hole in each These attach to the 2 threaded studs sticking out of the rear of the panel The 4 pots each stick in its panel hole when the bracket is screwed down on the 2 threaded posts Attach the board to the bracket The flanges will point upwards
21. he white line to the 0 tick mark and tighten the hex screw The silver part of the knob has a protective clear plastic overlay that can be removed if desired Gently rub with your fingernail across it and it will peel off Use the 2 extra wire ties to bundle the coax wires together and the wires to the blue Bourns pots together Trim off any excess CONGRATULATIONS YOU HAVE FINISHED BUILDING THE 440 kkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkk All that s left to do is test it But before we do please read the following Theory of Operation THEORY OF OPERATION The MOTM 440 uses 4 identical 1 pole VCFs in series Each individual VCF is a low pass filter which is nothing more than a simple RC section The difference is the R resistance is implemented as an OTA Operational Transconductance Amplifier which allows for voltage control of the cutoff frequency There are many OTAs that are available such as the NE5517 and LM13600 used in other MOTM modules but in this case the OTAs are done with sets of matched transistors Why We wanted to emulate the classic SSM2040 filter chip found in the Prophet 5 Rev 2 the Voyetra 8 and other early synths The MOTM 440s OTAs are similar to the OTA sections inside the SSM2040 chip An OTA is a transconductance operational amplifier The output is a current which is based on 2 things the differential input voltage a
22. ive the filter with multiple VCO inputs 10V pk pk The control voltages are plugged into 1V OCT FM1 and FM2 features a reversing attenuator the pot can invert the input voltage on the 5 setting or pass it through the 5 setting The filter is designed so an increasing control voltage increases the cutoff frequency And remember increasing the cutoff means more treble passes through The reversing attenuator when set in the region does the opposite increasing voltage decreases the cutoff If no voltage is patched into the RES jack the RES pot sets the resonance At settings past about 9 the filter will self oscillate This generates a sine wave If you patch a control voltage into the RES jack the RES pot acts as an input attenuator Note that the RES control is set for a 0 to 5V range for min to max resonance The BASS switch can be set to act like a normal OTA filter which reduces output level as resonance is increased In the ENHANCE position the output level is boosted to compensate for the loss at higher RES settings SYNTHESIS TECHNOLOGY PAGE 16 MOTM 440 ASSEMBLY 3 25 00 WWW SYNTHTECH COM SPECIFICATIONS MOTM 440 DISCRETE OTA FILTER Control Voltage input levels TV to 7V Cutoff frequency range 20Hz to 8Khz Output impedance Audio input level CONTROLS FREQ RES FM1 FM2 IN1 IN3 BASS GENERAL Power Supply Size Depth behind panel SYNTHESIS TECHNOLOGY MOTM 440 ASSEMBLY 3
23. nd a control current By varying the control current the effective resistance into the filter cap changes and hence the cutoff frequency Since the filter portion is 4 identical stages in series we will only discuss the first stage The first OTA is PNP current mirror U6 differential stage U7 filter cap C10 Darlington current buffer Q2 Q3 and a 500ua current sink Q4 The audio input is first attenuated by resistor divider R42 R39 by a factor of 0 02 The input summer discussed later has an attenuation factor of 68 so the total gain reduction is 0 02 68 0 014 So a 5V input is reduced to 5 0 014 70mv Why Let s look at the differential pair of NPN transistors inside U7 Since the emitters are tied together the control current ICO is a sum of the 2 individual emitter currents If we then SYNTHESIS TECHNOLOGY PAGE 13 MOTM 440 ASSEMBLY 3 25 00 WWW SYNTHTECH COM assume high beta transistors the collector currents will equal the base currents Now we notice that the base of the left side transistor is tied to ground through R40 Labeling the transistors L for the Left and R for the Right we note VbeL VbeR 0 due to R40 Vinput and VeL VeR the emitter voltages must be equal since they are tied together Digging around in EE101 textbooks for the transistor equations we use the above facts to solve for the collector current Ic ICO 1 e Vinput Vt Where Vt is called the thermal voltage app
24. rol voltages together Ula is a special low offset low drift op amp forms a reversing attenuator The voltages are summed and fed to a divider network of TEMPCO resistor R57 and the trim pot TP1 This divider is necessary as shown in the earlier discussion we want to operate the exponential converter transistor U14e in its linear region The TEMPCO is used to counteract the Vt term Vt will change about 2mv C So we have a bad resistor that changes the divider ratio with temperature the opposite sense That is why the TEMPCO is placed in thermal contact with U14 SYNTHESIS TECHNOLOGY PAGE 14 MOTM 440 ASSEMBLY 3 25 00 WWW SYNTHTECH COM Instead of tying R57 to ground it is biased up by R58 R59 U1b to 0 85V which is just barely above 1 Vbe drop to keep Q14Ee from ever saturating Each OTA section needs a control current which needs to be a current sink The remaining NPN transistors in U14 are in parallel as 4 identical current sinks U3a and R62 form the reference current of about 75ua As the base emitter voltage of U14 changes the 4 control currents change exponentially and hence the filter s cutoff frequency changes TP1 is set to give 1V oct response The 440 has voltage controlled resonance This is done by adding a VCA stage U5 into the audio feedback path R41 C11 to modulate the feedback level into the first stage The VC Q voltage is generated by U2b which is a non inverting gain of 2 The 2 diodes
25. rox 26mv at room temperature and ICO is our filter section control current Since Vt is small we must scale Vin or the denominator term quickly gets huge At 70mv peak Ic 0 15 76 So as the audio input gets larger the collector current gets smaller If Vin exceeds say 100mv the collector current does not change much That s the reason for the input attenuation In summary you can see that the overall gain is set by ICO which is a simple linear function a VCA To form the LPF we add a cap C10 to ground on the right collector This forms a 1 pole integrator The output current is buffered by a Darlington pair Q2 Q3 In a new design this buffer is usually a single JFET But again we are trying to recreate the same OTA structure in the SSM2040 so as in that bipolar chip we use 2 transistors instead In order to swing both positive and negative on the emitter of 93 we attach a constant current sink of 500ua This is formed by resistor divider R35 R36 that places the base of Q4 at 13 6V The emitter is 1 diode drop below that or 14 2V Since R37 is tied to 15 the voltage across it is about 0 8V which makes the current 8 1500 or about 500ua To make a 4 pole filter we just connect 4 stages in series Now we need to control our filter to get the usual response of 1V octave This is done by op amp amp U38a and a CA3086 NPN transistor array First we sum all of the external cont
26. solder into R54 below L1 Locate the 44K2 1 resistor and solder into R56 by TP1 Locate the 330K resistor and solder into R59 by R56 Locate the 2K2 resistor and solder into R60 by C16 Locate the 100 ohm resistor and solder into R63 above TP1 PART 2 BOARD WASH 1 o 0 Verify all the resistors in the correct position Verify all the resistors are flat on the board Correct if needed Check solder joints Wash the board in warm water gently scrubbing both sides DO NOT USE ANY SOAP Just water Shake the board a couple of times blot dry with an old towel the leads will frazzle a good towel Let dry at least 15 minutes Take a little break You are about 1 3 of the way finished PART 3 CAPACITORS amp board wash Locate the CAPACITOR bag Set the polystyrene 1000pf caps the big silvery ones with the long thin leads aside THESE CAPS ARE SOLDERED IN AFTER THE ICs Locate the 1000pf 1N yellow box cap and solder into C16 Locate the 50V non polar caps 3 and solder into C14 and C15 Locate the 0 1 axial caps 4 and solder into C1 C2 C4 and C12 Locate the 10ufd electrolytics 4 Note that there is a stripe on the NEGATIVE terminal The pc board has a on the POSITIVE terminal Carefully stick the SYNTHESIS TECHNOLOGY PAGE 7 MOTM 440 ASSEMBLY 3 25 00 WWW SYNTHTECH COM capacitors into C5 C6 C11 and C13 with the stripe away from the pad on
27. synthesis technology MOTM 440 Discrete OTA Lowpass Filter Assembly Instructions amp Owner s Manual Synthesis Technology 6625 Quail Ridge Dr Fort Worth TX 76180 817 498 3782 www synthtech com March 25 2000 MOTM 440 PARTS LIST Please carefully check that all parts are in your kit If you have a suspected shortage please call or email If you get free extra stuff keep it for next time g Capacitor bag containing the following 16 parts 4ea 10mfd 25V or 50V Electrolytic C5 C6 C11 C13 4ea 1000pf axial polystyrene C7 C8 C9 C10 50V non polar Electrolytic C3 C14 C15 lea 1000pf yellow box marked 1N or 102 C16 4 0 1mfd marked 104 ceramic axial C1 C2 C4 C12 g Resistor bag containing the following 63 parts llea 10K brown black orange R10 R12 R13 R20 R26 R32 R38 R42 R43 R44 R45 8ea 220 ohm red red brown R21 R22 R27 R28 R33 R34 R39 R40 6ea 15K brown green orange R18 R24 R30 R36 R49 R50 4ea 100K 1 brown black black orange R1 R2 R46 R47 4ea 150K brown green yellow R17 R23 R29 R35 4ea 1K5 brown green red R19 R25 R31 R37 4ea 100K brown black yellow R5 R6 R7 R61 3ea 1K brown black red R16 R51 R53 3ea 51K1 1 green brown brown red R3 R4 R52 2ea 68K blue gray orange R9 R15 2ea 1M brown black green R11 R48 2ea 200K red black yellow R14 R62 2ea 20K red black orange R41 R58 lea 221K 1 red
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