SUPER-ENHANCED POLIVOKS VCA DIY KIT ASSEMBLY INSTRUCTIONS IF YOU ARE READING THIS, MOST PROBABLY YOU ARE ABOUT TO BUILD ERICA SYNTHS SUPER-ENHANCED POLIVOKS VCA. The Polivoks VCA has distinctive architecture two differential opamp (Russian ICs K118UD1B) based VCAs in series each for LFO and ADSR control over signal amplitude independently. We've made this VCA flexible enough to accommodate a variety of control voltages without any modification. Control voltage to attenuation behavior of this VCA is nonlinear and not logarithmic. It is kept authentic to the response of ПОЛИВОКС, and it uses same differential opamps, as used in the original ПОЛИВОКС. Most parts of the VCA have been improved to achieve higher dynamic range, better reliability, lower noise, accept higher signal levels, keep consistent behavior, self-tuning of two internal offsets (that had to be manually tuned in ПОЛИВОКС), less susceptibility to radio interference and higher linearity, though the heart of it is kept the same. We introduced +10V regulated power supply for differential opamps, as +12V is on the edge of allowed. Also we added the second output, so users can take two output signals one from the first stage only and other - both stages summed. Normally, 0..10V control voltage scheme is used, with any input below 0.5V having maximum attenuation, and being transparent (no gain or attenuation) at over 9.5V. It can also be tuned to be controlled by 0..5V scheme or -5..+5V scheme that may have transparency at +5V or 0V. This VCA is capable of gaining the input, so you can also achieve transparency at, say, 7V and +6dB gain at 10V. This may be very useful for some modulation waveforms. Nevertheless those, who like extreme experiences, can omit all added elements and use the same PCB (with few extra jumpers) to build 100% authentic Polivoks VCA.
ORIGINAL POLIVOKS VCA SCHEMATICS
ERICA SYNTHS POLIVOKS VCA SCHEMATICS
BUILDING INSTRUCTIONS IF YOU HAVE SUCCESSFULLY BUILT ERICA SYNTHS POLIVOKS VCO, YOU WILL FIND POLIVOKS VCA ASSEMBLY EASY. OTHERWISE IT S MEDIUM DIFFICULTY PROJECT. FOLLOW SIMPLE STEPS TO BUILD THIS MODULE!
BUILDING INSTRUCTIONS step➊ First decide, if you want to build the original Polivoks VCA configuration, or two separate VCAs on one board. If you go for the original configuration, install the jumper J1. If not see MODIFICATIONS chapter. step➋ Start populating the PCB solder resistors and diodes. Silkscreen has both component values and designators.
BUILDING INSTRUCTIONS step➌ Solder IC sockets and trimpots. step➍ Solder all other components.
BUILDING INSTRUCTIONS step➎ Solder potentiometers step➏ Now it s time for the first measurements. Apply +-12V power, and measure voltage between GND and +10V points on the PCB. The voltage should read around +10V, of course. If it doesn t, something has gone wrong with the soldering. Check orientation of diodes and KT315G transistor on the PCB! If it does, wash the PCB to eliminate the flux.
BUILDING INSTRUCTIONS step➐ Insert ICs step➑ Install jacks in the panel, connect GND lugs.
BUILDING INSTRUCTIONS step➒ Connect switching lugs of LFO In and ADSR In jacks when nothing plugged in, they will be normaled to +10V, so you can adjust gain of VCA stages by CV level control pots. step10 Insert the assembled PCB into the relevant holes in the panel. Fix it with pot nuts. Complete wiring. +10V out has to go to switching lugs of LFO In and ADSR In jacks. Do not forget top connect GND to jacks.
WIRING INSTRUCTIONS CONGRATULATIONS! YOU HAVE COMPLETED THE ERICA SYNTHS SUPER-ENHANCED POLIVOKS VCA!
CALIBRATION DON'T WORRY, tuning of Erica Synths Polivoks VCA is easy! If the long procedure below seems too extreme for you can proceed straight to BIAS and GAIN adjustment of each stage. Connect 10V ptp sawtooth audio to the input, turn LFO and ADSR level pots to 4 o clock and connect Out1 to the mixing amplifer. Adjust VR1 to get the same signal level as input. Connect Out 2 to the amplifer and adjust VR3 to get the same signal level as on input. Adjust VR2 and VR4 for the best CV reject. There are only two variable resistors that need tuning for every VCA cascade. Improper tuning causes VCA cascades to insufficiently attenuate, not open at all or provide too much gain. First, let's tune VCA1. Control voltage input, when no jack is inserted, is default to internal 10V level, so there is no need for external control voltage sources at this point. Decide what control voltage scheme you want to use. Common scheme for VCA is 0V - maximum attenuation (silence) and 10V - transparent, unity gain (no attenuation). Set all four tuning variable resistors to approximately medium position. Apply power to the VCA module. Apply audio signal with simple waveform (sawtooth, for example) to the input, preferably 10V peak to peak. Turn VCA audio input level knob to maximum. Using an oscilloscope, ensure that undistorted audio signal is present on TP1 and TP2 test points, centered around zero and equal amplitude, second signal being inverse copy (upside down) of the first one. Check control voltage input buffers. Connect an oscilloscope and multimeter to TP5 for first cascade and turn control voltage level knob ("LFO LEVEL") on the panel - voltage should change from 0V at minimum setting to 10V at maximum setting. If control voltage signal is applied to input jack, it should have exact amplitude on TP5 point as it is on input, when level knob is set to maximum. Now reconnect oscilloscope and/or multimeter to TP6 and turn control voltage level knob for second cascade ("ADSR LEVEL") on the panel - voltage should change from 0V at minimum setting to 10V at maximum setting just like on first cascade. Important: For any measurements on VCA using an oscilloscope, use at least 1:10 probes! Using 1:1 probe, free dangling wires or improvised connections may cause certain parts of the VCA to misbehave, shiſt, become noisy, glitchy, unstable and/or oscillate, until probe or dangling wires has been removed. If your oscilloscope has voltage cursor capability, add voltage cursor marks to the peaks of the waveform, so you can later compare other signal level to this one. Aſter this point, do not touch input level or source's level or waveform controls. Everything must stay constant. If your oscilloscope has two channels, you can leave channel1 connected to TP1 and use channel2 for the connection in next step. In that case, ensure that all probe and channel settings of channel2 match the channel1. Important: For tuning, use a small isolated screwdriver made entirely of plastic with almost no metal parts except the tip - just like the probe calibration screwdriver supplied with oscilloscope probes. Connect oscilloscope (channel2, if available) to TP3. Have the voltage cursors, marks or previously memorized wave on screen (or channel1, connected to TP1). Now you can compare input and output levels of the first VCA cascade. You can have an audio amplifier connected to VCA1 OUT on the panel, but be careful with the volume. Be prepared for high output levels, and set volume of the amplifier so it has just about below normal listening level when connected directly to your audio signal source that you are now using for VCA. Turn voltage control level knob for first cascade ("LFO LEVEL") to minimum. Using multimeter, check voltage on TP8 (pin 12 OP3). Do not keep the connection longer than necessary, as it may offset the VCA control voltage or inject received AC or radio interference into the control voltage and may mislead you while tuning. As a remedy to be able to keep the connection while tuning, you can add a 500k resistor to this test point and connect multimeter through it, or just use 1:10 probe. Now adjust VR2 VCA1CTRL BIAS so that voltage on TP8 is about -0.5V. Signal level on TP3 should be very small, at least 40dB (preferably 50..60dB) less than on TP1. Turning counter-clockwise decreases the bias voltage, and turning clockwise - increases. Search for a sweet spot turning the bias to a point where it has the most attenuation, yet almost starts to rise. You can do this by first turning bias high enough to observe the signal, and then turn it down until it is no longer observable or no longer decreases. Turn voltage control level knob for first cascade ("LFO LEVEL") to maximum.
Adjust VR1 VCA1CTRL GAIN so that voltage on TP8 is in the 0..+0.5V range. Signal level on TP3 should be very loud and clear. To obtain "transparency" at 10V, you need to tune the gain so it signal levels on TP1 and TP3 match as closely as possible. Turning counter-clockwise decreases the gain voltage, and turning clockwise - increases. Now set voltage control level knob for first cascade ("LFO LEVEL") to minimum and check how much the audio signal attenuates. Adjust VCA1CTRL BIAS a bit, if necessary, to achieve maximum attenuation. Check how audio level change in response to turning the "LFO LEVEL" knob. When done, set "LFO LEVEL" back to maximum and see if cascade is still at unity gain (signal level on TP1 and TP3 matches). Adjust VCA1CTRL GAIN if necessary. You can repeat these steps for very fine tuning. Alternatively, instead of unity gain, you can also make the cascade gain higher, so it has +6dB at 10V, for example. Or be at unity gain when control voltage is 5V or 7V. All this is achievable via this tuning procedure. Just to ensure that everything works properly, check that undistorted audio signal is present on TP3 and TP4 test points, centered around zero and equal amplitude, signal on TP4 being inverse copy (upside down) of the one on TP3. Tuning first cascade of VCA is now complete. Now let's tune the second cascade. Leave the first cascade of VCA at unity gain. From this point, do not alter signal source, VCA module input level or tuning resistors VR1 or VR2. Accidentaly touching any of these controls may require returning to the tuning of the first cascade of VCA to ensure it is at unity gain. Connect oscilloscope (channel1) to TP3, adjust voltage cursors if necessary, memorize the wave as a new reference or just leave it on channel1 if you have second oscilloscope channel available. If first cascade was well tuned, voltage cursors may need no adjustment. Reconnect oscilloscope (channel2, if available) to TP7. Have the voltage cursors, marks or previously memorized wave on screen (or channel1, connected to TP3). Now you can compare input and output levels of the second VCA cascade. Audio amplifier can be reconnected to VCA2 OUT on the panel, but still - be careful with the volume. Turn voltage control level knob for the second cascade ("ADSR LEVEL") to minimum. Using multimeter, check voltage on TP9 (pin 12 OP4). Do not keep the connection longer than necessary, as it may offset the VCA control voltage or inject received AC or radio interference into the control voltage and may mislead you while tuning. As a remedy to be able to keep the connection while tuning, you can add a 500k resistor to this test point and connect multimeter through it, or just use 1:10 probe. Now adjust VR4 VCA2CTRL BIAS so that voltage on TP9 is about -0.5V. Signal level on TP7 should be very small, at least 40dB (preferably 50..60dB) less than on TP3. Turning counter-clockwise decreases the bias voltage, and turning clockwise - increases. Just like with the first cascade, search for a sweet spot turning the bias to a point where it has the most attenuation, yet almost starts to rise. You can do this by first turning bias high enough to observe the signal, and then turn it down until it is no longer observable or no longer decreases. Turn voltage control level knob for first cascade ("ADSR LEVEL") to maximum. Adjust VR3 VCA2CTRL GAIN so that voltage on TP9 is in the 0..+0.5V range. Signal level on TP7 should be very loud and clear. To obtain "transparency" at 10V, you need to tune the gain so it signal levels on TP3 and TP7 match as closely as possible. Turning counter-clockwise decreases the gain voltage, and turning clockwise - increases. Now set voltage control level knob for the second cascade ("ADSR LEVEL") to minimum and check how much the audio signal attenuates. Adjust VCA2CTRL BIAS a bit, if necessary, to achieve maximum attenuation. Check how audio level change in response to turning the "ADSR LEVEL" knob. When done, set "ADSR LEVEL" back to maximum and see if cascade is still at unity gain (signal level on TP3 and TP7 matches). Adjust VCA2CTRL GAIN if necessary. You can repeat these steps for very fine tuning. Same as with the first cascade, you can also make the cascade gain higher, so it has +6dB at 10V, for example. Or be at unity gain when control voltage is 5V or 7V. CONGRATULATIONS! TUNING OF THE VCA IS NOW COMPLETE. If -5..+5V control voltage range is desirable, you have to provide -5V and +5V supply to the control voltage inputs for tuning. In this case, you may also adjust the VCA cascades to an arbitrary attenuation or gain at 0V (when control voltage signal level knob is set to minimum), and use the -5V on control inputs, instead of 0V, when adjusting the maximum attenuation. Similarly, use +5V or some arbitrary control voltage for adjusting the unity gain point.
MODIFICATIONS Adding R48 and R49 allows you to use small span control voltages. Adding 1k resistor will provide 2x gain to the control voltage. By adding a resistor to some bias voltage instead of ground, control voltage input can be offset. By using lower resistance values into R2 and R25 for VCA1 or R31 and R32 for VCA2 adds more gain to the audio signal in the respective cascade. However, it is advised to use this type of gain only when small amplitude audio sources are used. These resistors always have to be of matching resistance and never lower than 20k. Using higher resistance values in R8 and R9 for VCA1 or R19 and R20 for VCA2, additional gain can be obtained. These resistors always have to be of matching resistance and should be kept below 250k and above 50k. Second VCA cascade can have independent input, so the module may be used as stereo or dual VCA. To add such feature do not install jumper J1 and assemble an additional second input circuit around unused half of OP5. If your control voltage scheme is odd enough, and VCA cannot be tuned for it as-is, you can always change the control voltage input scaling circuit to suit just about anything. However, keep in mind that careless applying voltages below -1.0V or above +1.0V to the pin 12 of К118УД1Б may cause irreversible damage to this part. Just to be safe, you can add some protection diodes before changing the scaling circuit. If you are unsure about what control voltage range best suits your system, or you want it to be switchable: You can tune the circuit to the common 0..10V scheme using standard procedure, and then just place 1k resistor in R48 for VCA1 and R49 for VCA2 control input. That will make the input twice a sensitive and usable with 0..5V steering. By adding a switch in series with this resistor, range can be switched over whenever necessary. Normaling voltage for the respective input jack socket, however, should also be switched over to the appropriate voltage (5V) for the selected mode. Also you may wish to add more CV inputs. You can repeat input buffers as in schematics above or simply mix in more CV inputs aſter the buffer as in majority of DIY projects.
MODIFICATIONS