A very simple, old school, transistor based VCA circuit that's tried and tested. Easy to build and small enough for Eurorack. You do need an oscilloscope to calibrate it though. This module runs on +/-12V as standard so a very Eurorack friendly design.
There are PCBs available for this VCA. See 'PCB Service'
EDIT April 2024: There's now also a new VCA project available that's also a 3 channel mixer and has a line out option. Please go to Project 58 to check that out.
WHAT'S A VCA FOR?
The VCA is nothing more than a voltage controlled volume knob. It lets audio pass through when you press a note on the keyboard and it shuts it down after you let go of the key. (So actually the term Voltage Controlled Amplifier is a bit misleading because it does not amplify the signal, it attenuates it.) The opening and closing of the VCA is done using signals coming from the Envelope Generator or ADSR or from an LFO depending on what you use it for. Don't mistake a VCA for an Audio Amplifier. You can not hang speakers on the end of this circuit. A VCA is used to make sure your synthesizer only produces sound when you press a key on the keyboard. The keyboard produces a Gate signal that is high as long as the key is pressed down and a 1 Volt per Octave signal that tells the VCO which note to play. That Gate signal then triggers the Envelope Generator and the output from the Envelope Generator goes into the VCA together with the audio output from the VCO and as the VCA detects the Envelope signal it opens up and lets the audio pass through with the volume or amplitude depending on how high the envelope signal is in voltage. The output of the VCA must later be attenuated to audio line level if you want to feed it into a HiFi amplifier. So the VCA is just a link in the synthesizer chain.
In the picture below you see a block diagram to show you the position and function of a VCA in a synthesizer. There seems to be a lot of confusion about this with people new to (modular) synthesizers. The VCA is the white triangle marked 'Amplifier'. It does say "Output to amp/speaker" but they mean that the VCA outputs a line level signal going into a normal HiFi power amplifier. There is no line level output in my VCA layout but it's easily added with a voltage divider.
(Image taken from 'The Complete Synthesizer' PDF Book)
I used a very simple design for the VCA which I again found on the Yusynth website. Btw, this is a linear VCA.
Here's the schematic:
This is an old design and there are some updated versions out there but this is tried and tested and has served me well for over 3 years now. It's very simple and it works very well except that in my build the signal came out inverted. This isn't really an issue because it's an audio signal and they sound the same whether inverted or not but my Obsessive Compulsive Disorderly mind wants it coming out the same way it came in so I added a little opamp inverter to the output to set this straight. Afterall a VCA can also be used for Control Voltages like signals from an LFO and in that case polarity is important.
Here's the schematic:
If you want to experiment with this circuit then here is a link to a Falstad simulation I made: --- CLICK HERE ---
I put a 10µF Electrolytic Capacitor on the output (plus connected to output VCA) because I noticed a 240mV DC offset voltage on the output which I couldn't trim away with the potmeters. This may be due to a slight difference in resistance between the two 10K resistors connected to opamp U1b. Make sure you measure those and use two that have the same value resistance. In fact all resistors with the same value should be matched because this circuit depends on symmetry!! The transistor pair must be matched too.
I made an extra output on this VCA that has 1/10th the amplitude of the normal output. I though that might be handy to use as a line level output but I'm not sure if I'll need it or use it. It just consists of a 100K resistor going from the output to the 1/10th output and a 10K resistor from the 1/10th output to ground to complete the voltage divider.
Use an oscilloscope to set the trimmer potmeters. You should be able to measure a DC voltage (before the 10µF cap I mentioned earlier) and, with trimmer R18, trim away as much DC voltage on the output as you can and with the other trimmer R14 you can trim the balance of the signal. You set it so the positive part of the wave has the same amplitude as the negative part of the wave, with the zero volt line being the dividing line. If you want to use this VCA for Control Voltages then you can't put a capacitor on the output because that will block low frequency signals, which it sees as DC.
The output level may be a bit lower than the input level, even if the ADSR potmeter is fully opened up. If that's the case and you want to correct that then you can change the gain of the output buffer opamp. This is something I added myself and is not included in the schematic but only in the layout. If you change the 150K resistor between pins 1 and 2 of IC-2 for a 470K resistor, you should get 3 times gain! That should bring the level back to input level. You can experiment with this yourself. If you put a 500 KOhm potmeter between pins 1 and 2 you can control the gain of the opamp with a knob on the panel. Just a thought ;) Make sure you give it a different name though otherwise you'll have two gain potmeters on your panel :D
The output level may be a bit lower than the input level, even if the ADSR potmeter is fully opened up. If that's the case and you want to correct that then you can change the gain of the output buffer opamp. This is something I added myself and is not included in the schematic but only in the layout. If you change the 150K resistor between pins 1 and 2 of IC-2 for a 470K resistor, you should get 3 times gain! That should bring the level back to input level. You can experiment with this yourself. If you put a 500 KOhm potmeter between pins 1 and 2 you can control the gain of the opamp with a knob on the panel. Just a thought ;) Make sure you give it a different name though otherwise you'll have two gain potmeters on your panel :D
If you are interested in the functions of the transistor pair and the other components in this circuit then here's a link to a blog post that explains it in great detail: -- CLICK HERE --
LAYOUTS:
LAYOUTS:
Here's the layout I made for standard 24x55 hole stripboard using only 24x39 holes. Like I said before, I added a signal inverter in the shape of a second TL072. Only one of the two opamps in the chip is used, the other one is properly connected to ground. Make sure the Gain potmeter is linear. In fact use linear types for both pots. You can also use other values like 47K or 100K because one is an input level control and the other a voltage devider switched between 12V and ground so the values have no influence on the working of this circuit.
The layout below is verified and absolutely faultless. I guarantee it. It has been used successfully by many people now. All potmeters seen from the back.
Stripboard only:
The Level control is for the ADSR input signal and determins the volume of the audio signal. The Gain controls the quietness of the VCA when no keys are pressed on the keyboard and it should normally be set to zero. If you turn it up, the last note you played will become audible.
You must use an oscilloscope to test the signals and trim the offset and signal levels. Put it in DC mode when testing.
The layout below is verified and absolutely faultless. I guarantee it. It has been used successfully by many people now. All potmeters seen from the back.
Here's the Bill of Materials. It says to use logarithmic potmeters but you can also use linear types. In fact for the Gain pot it's best to use a linear one:
EDIT 24-April-2025
I made a new smaller version of the VCA without any extra's. It's just what you see on the schematic drawing. This will be handy if you need a few of these behind one panel or if you're building for Eurorack. I haven't built this layout so I can't say it's verified but I double checked the connections with the schematic and I can't see any mistakes. If you do please contact me immediately.
I did switch the 10K connections to the opamp U1b in the schematic. The left BC547 (Q1) is connected to the inverting input (-) and the right BC547 (Q2) is connected to the non inverting input (+). I can not imagine that will make any difference. It doesn't in the Falstad simulator anyway.
Here's that layout, wiring:
Stripboard only:
Connect the 3 ground positions of the eurorack powerheader together by flowing extra solder to bridge the gaps and short them together.
Here's a picture of the finished VCA installed in my synthesizer:
You must use an oscilloscope to test the signals and trim the offset and signal levels. Put it in DC mode when testing.
In the picture above you see a yellow patch cable connected to the audio output. That is actually my oscilloscope probe, so I can see the output signal on my scope screen, and the audio output is connected internally, behind the panels, to the Line-Out and Effects Unit to the left. From there the audio signal goes to a stereo RCA output on the back of my synthesizer and from there to the Line-In of my HiFi Audio Amplifier.
Below is a picture of the double VCA that I built on April 25th and 26th of 2020 from the same schematic and layout. These VCA's work like a charm! They are so handy to have, I use them a lot in patches as a sort of Gate to let audio through when there's a signal on the ADSR input. When the ADSR potmeter is turned fully open (clockwise) the output signal will have the same strength as the input signal. Of course this is also influenced by the type of control voltage you feed it.
Below is a picture of the double VCA that I built on April 25th and 26th of 2020 from the same schematic and layout. These VCA's work like a charm! They are so handy to have, I use them a lot in patches as a sort of Gate to let audio through when there's a signal on the ADSR input. When the ADSR potmeter is turned fully open (clockwise) the output signal will have the same strength as the input signal. Of course this is also influenced by the type of control voltage you feed it.
With the double VCA I wired up the inputs in such a way that when there is no connection made to input 2, that input gets the same signal as is present on input 1. (Input 2 is normalled to input 1 as it is officially called.) So with one input you get two outputs. As soon as a patch cable is connected to input 2 that connection with input 1 is broken (by the socket switch of input 2) and the VCA's work as two independent VCA's.
A look behind the panel:
Eurorack module:
A look behind the panel:
Eurorack module:
EDIT 25-Januari-2022: I'm now building a Eurorack system and I needed a VCA that I can use until I buy some ready made VCA's so I took one board out of the Double VCA I built earlier and made a Eurorack sized panel for it. This worked perfectly and I just transplanted everything to the new panel and it worked fine. Here's a look at the new module. I sawed off the extra copper strips that weren't in use (where the M3 bolt went through) which left me with a 52mm deep print which I glued directly to the back of the panel next to the potmeters and sockets which are all on the right side of the panel.
Here are some pictures of the Eurorack panel:
I made an extra output on this VCA that has 1/10th the amplitude of the normal output. I though that might be handy to use as a line level output but I'm not sure if I'll need it or use it. It just consists of a 100K resistor going from the output to the 1/10th output and a 10K resistor from the 1/10th output to ground to complete the voltage divider.
Troubleshooting:
If, after building this VCA, you have trouble with noise, especially at low volume levels, then you most probably need to replace the opamp you're using. I've had people contact me about this and it turned out the opamp was the cause. An other cause of excessive noise has been bad soldering joints. So make sure your solder connections are good.
If, after building this VCA, you have trouble with noise, especially at low volume levels, then you most probably need to replace the opamp you're using. I've had people contact me about this and it turned out the opamp was the cause. An other cause of excessive noise has been bad soldering joints. So make sure your solder connections are good.
Like I said, I built 3 of these and they all work fine and are absolutely quiet.
I had some eurorack layouts here before but I'm having problems with them so I took them out again. I won't have layouts here that are not verified.
Here's an oscilloscope picture of how the VCA lets through audio (the yellow waveform) only when it gets the Control Voltage from the ADSR (the blue line). You can see how the amplitude of the audio closely follows the amplitude of the CV from the Envelope Generator:
I had some eurorack layouts here before but I'm having problems with them so I took them out again. I won't have layouts here that are not verified.
Here's a link to a Falstad circuit simulation of this VCA showing how it works.
If you want to know more about the theory behind this VCA or in effect, how this VCA works then I have a link here to a great blog post that explains the workings of this very circuit.
There's also a fantastic video by Moritz Klein which explains the technical details about the workings of a circuit like this and he also explains why the transistors need to be matched.
Okay, that's it for now. Any questions? Put them in the comments below and I will answer them asap. You can also post questions in the Facebook Group for this website.