Tuesday, 10 December 2019

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

The VCA is nothing more than a voltage controlled volume knob. It turns the volume up when you press a key 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.
I used a very simple design for the VCA which I again found on the Yusynth website.
Here's the schematic:


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. 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.
Beware the output signal is about a third of the input signal. The output is attenuated to get it ready for the Line Out into an amplifier. (Further attenuation can be done with potmeters and resistors.)
If you want the same output level as the input level 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.
I'm sorry to say this layout had a little mistake in it, namely two 10K resistors went from the potmeter to pins 2 and 3 of the opamp while they should have gone from the collectors of the 2 transistors. This is now corrected and I built one of these today and it works perfectly fine. So the layout below is verified and absolutely faultless. I guarantee it.

(All potmeters viewed from the front.)



(Last revised: 09-March-2020 Designed new and more compact layout. 24-April-2020 corrected mistake with two 10K resistors going from collector to opamp pins 2 and 3.)

Print only:





EDIT: APRIL 2020
I've make a second, even more compact, stripboard design from a slightly different schematic in preparation for a double VCA I want to build for the second stage of my synthesizer. This layout is not yet verified because I haven't built it yet but I've gone over it very precisely and I can not find any mistakes in it. But because it's not verified I'm leaving the previous layout up too so you have a choise. If stripboard size is of no concern, you can build the above layout if you wish, or make your own from the schematic.
I've made the new layout for a dual 15 Volt power supply but the alterations for running it on 12 V are noted on the layout. Note that the connections for power are reversed from what I normally build!! I also left out the inverter on the output.
Here it is:



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Here's a picture of the VCA panel 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.
This VCA will bring the 10V peak-to-peak signals down to about 4Vpp on average but the audio will still need to be attenuated more before we have a line-out signal that can be input into a HiFi amplifier. This further attenuation is done by the Line Out and Effects module that is the subject of the next chapter.

Here's a picture of the double VCA that I built on April 25th and 26th of 2020. I played it safe and used the first layout in this article, not the smaller one. These VCA's work like a charm!


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

Okay, that's it for now. Any questions? Put them in the comments below and I will answer them asap.

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