Tuesday, 10 December 2019

Synthesizer Build part-10: THE VCA or VOLTAGE CONTROLLED AMPLIFIER.

A very simple VCA circuit that works perfectly. Easy to build too. You do need an oscilloscope to calibrate it though.  

Please read the whole text before building this project. 
The VCA is nothing more than a voltage controlled volume knob. It turns the volume up when you press a note on the keyboard and it shuts it down after you let go of the key. This is all 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 a filter for instance 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 it's just a link in the synthesizer chain.
Here is 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:


(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.
Here's the schematic:


This is an old design and there are some updated versions out there. It's very simple but 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. But you could leave IC2 out and tap the output from pin 1 of IC1 but you must use the electrolytic capacitor on the output. The output needs to go through a 10µF Electrolytic Capacitor (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.
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.
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 (which is not necessary if you're using it for output into a HiFi amplifier) then you can change the gain of the output buffer opamp. This is not in the schematic but 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 1 Mega Ohm potmeter between pins 1 and 2 you can control the gain of the opamp with a knob on the panel. Just a thought ;)

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. The panel potmeter values are not critical in this design. Logarithmic pots would be preferable because we're dealing with audio signals but linear will work fine too. 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.


Print only:



Here's a picture of the finished VCA installed in my synthesizer:



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. 
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. 
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. 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:




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. 
Like I said, I built 3 of these and they all work fine and are absolutely quiet.

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 (ADSR):



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 new Facebook Group for this website.

22 comments:

  1. Hey Eddy, hat schon jemand die April 2020 Version gebaut mittlerweile?
    Hey Eddy, has someone already built the April 2020 version?

    Greetjes THOGRE

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    1. I've deleted that layout as of today. I'm not comfortable posting things that aren't verified.

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  2. Hi Eddy
    Will it be alright to use a 22uF on the out put instead of 10uF as I have an abundance of 22uF and no 10uF.

    Thanks

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  3. would this work with 2n3904 instead of the BC547?
    they seem similar to me.

    thanks

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    1. It might work but the 2N transistors have a lower Hfe or amplification factor which might have an influence on the working of the VCA. My advise would be to set it up on a breadboard first and test it with 2N transistors.

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  4. Thank you for the fast response.

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  5. hey eddy!
    i built this vca and it’s working! Thank you for that! But when i turn either GAIN or CV LEVEL up just a little bit it instantly jumps to max amount. i bet you have an idea what causes my problem! Please! Schöne Grüße, Matthäus

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    1. I'm sorry, I don't. You'll have to check the layout and connections, compare them with the schematic and see if you can find the problem. Are you sure you tried to trim the initial level potmeter?

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  6. hey eddy, would any values change if it were to be used on 15v?

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  7. Thank you :)
    I will build it this week
    what software you using to draw your layouts?

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  8. Hi. Any idea why my adsr in is inverted? When adsr cv goes up audio goes down, when I run cv thru external inverter it works as it should.

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    1. That is weird. The CV input is connected to the inverting side of the opamp but that is as it should be. Have you experimented with the initial level trimmer?

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  9. Yes I've been scratching my head:) and checked everything at least three times over. I'll try that initial level trimmer once more.

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    1. If you can't get rid of it maybe you can put in an extra inverting opamp stage, but this shouldn't be happening. Have you checked that connections to the minus voltage rail are not going to ground instead?

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  10. I built this today, and I'm surprised I didn't destroy it... first time I plugged it in, something on the board lit up like an LED and let out smoke, lol. Turns out I had one resistor in the wrong hole. Surprisingly it didn't fry anything... moved the resistor into the right spot and now it works perfect. :)

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    1. LOL seems to be a forgiving circuit. Glad you got it working so fast.

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