Saturday, 25 April 2020

Synthesizer Build part-29: VCA (Yusynth design).

A good working and easy to build VCA for use as extra VCA or as the endstage and line out for your modular set-up.

This module is a great solution for all your VCA needs. I needed a new VCA module for the second stage of my synthesizer and I wanted to upgrade from the first one I built, although it functions fine I hasten to add. This VCA is a more luxurious version of the first one you could say. And this one doesn't invert the signal like the first one did.

The function of a VCA:
Before I get ahead of myself, let me explain what a VCA does for all the people who are new to DIY synth building. A VCA or Voltage Controlled Amplifier, is used to stop your synthesizer from continuously making sound and to only produce sound when you press a key on your keyboard. So it's like a volume knob that is only opened up if it receives a signal from the Envelope Generator. Now the higher the voltage from the envelope generator (or ADSR)  is, the louder the output of the VCA will be. VCA's can of course be used for other things, like in drum machines, as a sort of Gate, to let through short pulses of noise to create percussion sounds but this is not what we're going to be dealing with here. This VCA is primary used as the last stage in your synthesizer from where the audio goes to the HiFi amplifier and the speakers. But the output must be further attenuated to make it suitable for the line in of a HiFi amplifier. 
So to sum up: The VCA lets through audio when it receives a signal from the Envelope Generator and it stops audio from passing through when there's no envelope signal present on the ADSR input of the VCA. Opening up the 'Gain' control will enable you to bypass the Envelope Generator Input and output sound/audio without pressing any key on the keyboard and therefor without any Envelope Signal being present but normally Gain is set to zero.
I hope that's clear but you can always ask me things in the comments if you need more info.

The VCA is capable of handling input signals of + and - 10V and outputs them at the same level if the audio and ADSR potmeters are fully opened up and if you use an Envelope Generator that outputs an envelope of  +10Vmax. This VCA is perfect therefore to pair with the Yusynth 7555 ADSR from article 24.
If you need to output the signal to a HiFi audio amplifier or mixing desk then you must use extra attenuation. You can easily do this with a resistor voltage divider that divides the amplitude of the audio to one tenth of the input amplitude. If you then turn the audio and ADSR potmeter back, the signal will be low enough to go into the Line-In of a HiFi amplifier or mixer. There's an extra additional layout below to show you how you can do that.
As to running this VCA on a dual 12 Volt power supply; I don't think there will be any problems with that. After all the opamps and transistors all work fine on 12V so I think it's just a matter adjusting the trimmers. You could have an issue where one of the trimmers is turned all the way to its limit because of the lower voltage but I have no data on this so I can't be sure. The first VCA I describe in chapter 10 is purpose built for +/-12V so you can always build that one if you're not sure. I've build 3 of those so far and they work very well. As you can see on the layout below, this design has one extra potmeter compared to the earlier VCA I built, and that's the audio level control potmeter on the input, which is always a good thing to have.
The two BC547 transistors Q1 and Q2 must be a matched pair. It's enough to just stick them in the Hfe meter of your multimeter and select a pair that have the same Hfe value. When you measure them let the transistors cool a little after you touched them because their values will change with temperature.

Before we start with the layouts I need to tell you these are updated layouts. Before the 16th of August 2021 there was a little mistake in the layout, a cut in a strip was placed to the wrong side of a wirebridge. I placed the wirebridge closer to the input opamp now. The picture below here shows the correction I made. Sorry if you tried to build this before and it didn't work.



Here's the verified stripboard layout. Wiring diagram:


(Last revised: 16-Aug-2021: corrected a mistake at the input opamp.)

Print only. Btw, the green wire-bridges indicate connections to ground:



This is the schematic from which I made the layout. In my build I have the 10µF electrolytic capacitors on the input as is shown in the schematic below. That means this VCA is an AC version meaning it can not handle signals from a Low Frequency Oscillator (LFO). If you need a VCA capable of handling very low frequency signals (a DC version) then leave out the 10µF caps on the input but for normal audio use LEAVE THEM IN! In my own build I also added a 10µF cap on the output (+ towards the opamp) as an extra failsafe against DC offset voltages on the output but you don't have to replicate that. (It's not in the schematic but it's in the layout as extra option.)



Bill of Materials. The logarithmic potmeters are noted as linear types in the BOM. It hardly makes any difference it's just that a logarithmic taper sounds more natural to the ear but you can use whichever you prefer.



LINE OUT:
If you want to use the VCA to feed a HiFi amplifier then you can use a little voltage divider network to further attenuate the audio output level to make it suitable for line out levels which are usually around 100mV to maximum 1V. I made a little extra layout to show you how you can add such a voltage divider to this stripboard. I used a 1M resistor and a 100K trimmer potmeter to divide the output voltage by at least a factor of 10. You can set the initial output level with the trimmer and then further adjust the level with the ADSR and Audio Level potmeters on the panel.
These extra components are not in the Bill of Materials!



Calibrating the VCA:
Before we start, do all the measurements on the 'Normal Audio Output', not the AC one. I added the AC output as an afterthought to block any DC offset voltages from coming through but normally you can use the the Normal Output. So connect your probe there for calibrating the circuit.
With trimmer A2 you set the initial bias voltage on the base of transistor Q3. This influences the working of the Gain potmeter and you should set it in such a way that with the Gain potmeter all the way closed the signal is just muted. If you turn Gain up the last played note will then become audible without pressing any keys on the keyboard. So Gain is normally closed.
I found that the audio signal initially starts at 10V and then drops to about 6V. To counteract this you need to open up the Gain potmeter a little and then trim again so the signal is muted with Gain slightly open, This will stop the voltage drop of the signal and keep it at full power all the time. You will see this soon enough if you start testing it and connect this circuit to a scope. It's easy to counteract and it is in itself not a real problem because you hardly hear the voltage drop but you know, I strive for perfection ^___^
Trimmer A1 is the Balance trimmer. You set it so that the part of the signal that is above the zero Volt line has the same level as the part below the zero Volt line. In other words, you set it so that the signal has the same amplitude in both the positive and the negative part of the wave. This is best done using a triangle or sinewave on the input, together with an oscilloscope.
For trimmer A3 I advise to use a multi turn one. With this trimmer you trim away any DC Offset voltage on the audio output. Again you absolutely need a scope to do this but a cheap 20 dollar one will do nicely here.
The circuit has a LED that indicates the presence of an audio signal on the input. It's a sort of one LED VU meter and the brightness varies with the strength of the audio signal. If you turn the audio input level potmeter up, the LED becomes brighter.

Here are some pictures of the finished product. If you look carefully you can see the electrolytic capacitor on the audio output. This is the AC audio output option to prevent any DC offset voltage on the audio output signal. If you are going to use this VCA with very low frequency signals, like from an LFO, you need to use the DC output option: (If you're a beginner then don't worry about that, use the AC output.)





You can see that I put a little white stripe alongside the counter clockwise first 20° of the throw of the Gain potmeter, to indicate to where you can turn the potmeter with the audio staying muted. If you turn it past the white, the signal becomes audible and normally the Gain should be slightly open but with in the white area.

The day after finishing this module I built two extra VCA's using the old layout and they work fine too. I wanted a few extra VCA's available but there was no need for the luxurious model. I'm going to use that as the end stage VCA in my second stage, with the output going to a HiFi mixer (at Line Out level) and use the other two VCA's for use in different patches.

Here's some pictures of the double model. I also posted these in the first VCA article:




So that's an other one done. One more and I'll have published 30 synthesizer related projects.
Okay, if you have any comments or questions please put them in the comments below and see you on the next one!

38 comments:

  1. Vraagje, in het schema staat dat je 100K log potmeters moet gebruiken. Op de veroboard layout en BOM staat LIN, is er een voorkeur ?

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    1. Het maakt niets uit Rolf. Er staat logaritmisch omdat we met audio te maken hebben maar je kunt net zo goed lineaire potmeters gebruiken. Dat heb ik ook gedaan.

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  2. Bedankt voor de altijd snelle respons, de mixer draait al. Zit nu een beetje te kijken wat als volgende te bouwen een VCA of VCF en ik heb een echt goede sequencer nodig want keyboard spelen kan ik voor geen meter.

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    1. Haha ik denk dat de meeste keyboard spelers voor synthesizers kozen omdat ze niet kunnen spelen. Geldt voor mij net zo. Als je naar die documentaire "Synth Britannia" kijkt dan hoor je dat de keyboard speler van The Human League er net zo over dacht, LOL ^___^. Btw, die sequencer die ik op mijn website heb staan werkt goed maar is niet echt je van het. Ik heb die vrij vroeg in mijn synthesizer project gebouwd en ik wil nog een nieuwe versie maken. Maar ik denk dat een variant op de "Baby-8" wel ongeveer is wat je nodig hebt. En voor wat als volgende te bouwen.... Ik zou voor een VCF gaan. Een VCA is snel genoeg gemaakt maar ik kan de Digisound 80 Lowpass filter met de AS3320 echt aanraden. Klinkt fantastisch. Als je hem bouwt doe dat dan zonder the "Frequency Fine Control". Die knop heeft totaal geen nut.

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  3. Bedankt voor de respons, die fine control schrap ik dan. Heb helaas alleen maar 200mm brede frontjes of 60, dus bouw maar gelijk twee VCF's :). Deze en die Steiner VCF

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    1. Oh de Steiner is ook een hele goeie! Klinkt fantastisch en die heeft meerdere standen in één. Lowpass, Bandpass, Highpass en Allpass. Ik gebruikt dat filter heel vaak.

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  4. Hi Eddy, Im having some trouble getting this module to work. I've checked all of voltages and connections and as far as I can tell they are fine, all except for the input of the audio signal the to tl082. I can see the voltage at one end of the electrolytic capacitor that feeds the IC but it doesn't appear on the other. I have tried replacing the cap as well as the resistor that it connects to, as well as a new chip. Do you have any ideas on how to fix this? Thanks.

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    1. THere's nothing wrong if your signal disappears at the input to the chip, so after the resistor. The opamp is wired as a virtual ground inverter. That means the chip is trying to keep the input at ground level thus inverting the signal. (Check out EEVBlog #600 on YouTube) Check if the signal is there on the output of the opamp (pin 1). Make sure the VCA gets an ADSR signal otherwise it doesn't open up to let audio through.

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  5. Interesting! I will have to check that out. If I understand correctly, I should be able to see the signal on one end of the resistor? This is not the case. Also there is no output from pin 1. I have noticed that if I disconnect the power while keeping the audio and adsr connected I can actually faintly hear it and triggering the adsr seems to have an effect on it, but when I connect the power again it goes away. Not sure if that tells you anything but I thought it was strange.

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    1. Hmmm that sounds very weird. You should be able to measure a signal between the capacitor and the resistor before it goes into the chip. All I can advise is to check your print against the layout and against the schematic methodically, connection by connection and see where you went wrong. There must be something that went wrong.

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    2. I found the issue! I was so worried about pins 1-3 that I wasn't paying attention to the other ones of that chip. It turns out that I had managed to not put in the 100k from 6-7 at all! Thanks again!

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    3. Ah, I sort of figured it had to be a forgotten component of some sort. I just know the layout works because I don't publish layouts I haven't verified. I'm so glad it's working now. Excellent! :)

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  6. Hi, does this vca only work if i connect it to an adsr?
    I ask this because at the moment I don't have an adsr and I have mounted this vca and it doesn't seem to do anything, I can't hear anything and the light doesn't come on. is this normal ?
    thanks

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    1. If you don't input an ADSR signal into the VCA it won't let any sound through. The VCA is opened up by the ADSR signal.
      This is normal and how a VCA should work.

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  7. Hello Eddy, as I am finally building the VCO with AS3340 and commenting on that page seems to be impossible, I am asking my question here: Is it necessary to use a 2N3904 for the triangle to sine converter or can I substitute it with an BC547? Also thank you for making all this great information availiable, it was a really fun build so far.

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    1. Oh is commenting not possible there? Maybe I reached a limit. I'll check it out. As for the transistors I don't think it would be wise to change them for BC547's because they have a higher hFE. It's rather a touchy circuit and needs to be accurately replicated.
      Glad you like the website Nils!

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    2. Oh yes, I remember I had to close comments for that article because I was getting too many.

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    3. Also what value is the best for Rf to make the amplitude go to -5/+5Vpp and 0/+10Vpp with a 12V PSU?

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  8. Hi Eddy, thanks for sharing your work! On your stripboard layout you have a wire going to the positive side of the cap coming from the audio level pot, but one the schematic it looks like that should connect with pin 1. Also, from the picture of you build there seems to be a couple differences from the stripboard design, such as a large electrolytic cap near the 100K trimpot. I just want to make sure I get this one right. My first attempt I designed my own stripboard from the schematic and it smoked when I plugged it in, so I think to new for my own designs just yet. Thanks

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    1. Good questions! Okay, the electrolytic cap 10µF blocks any DC voltage from entering the VCA so it is correctly placed connected to the wiper of the audio input level potmeter as this is the point where the audio comes into the VCA. You say "should be connected to pin 1" I take it you mean pin one of the potmeter? Maybe you are confused by the text "p1" on the schematic. That simply stands for potmeter 1 not pin 1.
      The large electrolytic capacitor on the output is indeed an addition of my own and is again used to block any DC offset voltage from appearing on the output. I will make a note of this in the text because it can be confusing. I hope that answered your questions.

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  9. Hi! Thanks for the reply! Sorry for being vague about pin 1. I meant pin 1 of the TL072/82. On the stripboard it appears the wire is connecting the positive side of that 10µF cap on the audio level potentiometer to the matched transistors. From the schematic I don't see that anything should be connected to the positive side other that the audio level pot. So on the output you've put a 22µF cap?

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    1. Just to clarify. I'm talking about the jumper wire just to the right of the 10µF cap.

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    2. You are absolutely right! I don't know how this happened but if you built it like I had it on the layout you wouldn't have gotten any audio out. I have updated the layouts. Thank you so much for pointing this out. I pride myself on my layouts being faultless and I don't know how this one sneeked in but it's corrected now. Thanks again!

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    3. It was a matter of one cut being on the wrong side of the wirebridge, I see now how it sneaked in but anyway, it's fixed now. I also put in the electrolytic cap in the audio output. Make sure you also cut that strip to the left of the cap, as shown in the layout.
      Right, thanks again, you're a star!

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  10. Hey! Thanks again. I had a feeling you meant it to go on the other side of the bridge. Glad I decided to ask you about it!

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    1. I'm glad you did too. This will save a lot of people from disappointment after building this module. I posted about this on the Facebook group so people are aware of it. I'm so glad you noticed the mistake.

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  11. Thank you for your excellent website - it's an amazing resource for someone like me who is delving into the DIY synth world and trying to discern which designs to try! A rather rudimental question for you please, if I may, what do you use to mark (in white) on your panels?

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    1. Thank you for that awesome compliment James! I'm so glad you find the website helpful. Actually your question is a very good one because it took me some time, when I just started out, to find the right markers. I use the ones from this link: https://www.ebay.com/itm/133472272965 Choose 'white' for color and order a few of them because they run out quite fast but they work very well.

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    2. Not at all. In a sea of lots of schematics you are a guiding light towards the good circuits and ones that are easier to build. You've added a lot of value. Thanks for the link, much appreciated - those pens look perfect!

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  12. Hi Eddy, I have another query for you on this YuSynth VCA design. Do you know why there are two signal inputs and two CV inputs? Can it control the volume of each signal independently? Or is it just mixing the signal/cv inputs together beforehand? I assume it's the later. I noticed that you didn't implement the two inputs. Seems like a function that can be achieved with a separate mixer instead.

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    1. The extra audio input just mixes the two signals together and that's why I left it out on my layout. I didn't need it. The same goes for the two CV inputs. I only used one in my layout but I did build in the extra connection, should anyone need it. Both extra's are not necessary and can be left out.

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    2. I suspected that was the case, many thanks for confirming. I think I'll use only one input & output too.

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    3. Yeah, I don't know why Yusynth included those extra inputs but you don't need them. Good luck with the build!

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    4. Thanks Eddy, just finished (breadboard) building both the YuSynth ADSR and this VCA and it's working really well so far. One issue I'm having is with the shape of the wave that is output from the VCA. I have tried a 1KHz sine wave generator (as per YuSynth's website) to calibrate. I also tried a triangle wave because I think comparing the input and output when tweaking A1 (the balance) will be easier. The triangle appears to be smoothed out somewhat towards a sine wave (not completely, but in that direction). Adjusting A1 adjusts the shape but not sufficiently. It's a bit like a low pass filter perhaps and there is an audible difference. I matched the transistors; they're a good match, but I wonder if my issue is there or perhaps elsewhere in the circuit. Any thoughts?

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    5. It's a common thing with this VCA that is does do that to triangle waves a tiny bit but it's really not that big of a deal. I use this VCA as my main one and the sound is just fine. If you're using the AC output maybe try the DC output? I don't know. I find that in normal use it's not an issue and in my VCA the distortion of the triangle wave is only in so far that it has somewhat curved sides but the top and bottom is still sharp and pointy and therefore I can't hear an audible difference in mine. It's not the transistors in any case.

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    6. Many thanks Eddy, I did some more adjustments to A1 (balance) to achieve the best shape possible. Thanks for the feedback - I think this might be a case of obsessing over an oscilloscope waveform rather than considering the real world. If it's normal for the VCA to 'colour the sound' a little then that's totally fine. Just needed to confirm I wasn't missing something. Thanks again for the great blog posts, they were extremely helpful. Now to get these off the breadboard and into modules!

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    7. Yes I've been guilty of obsessing over a waveform too. I learned that it's mostly the tops and bottom sides that produce the distinctive sounds. For instance (as an unrelated example) squarewaves; the wave goes up and at the top slopes down a bit and then goes down to the negative side of the wave. That wave will sound the same as a squarewave that is perfectly straight/horizontal at the top and bottom. Anyway, glad you got it going. It might even perform a little better once it's on a stripboard because breadboard contacts have some capacitance in them. Good luck with the build and contact me if you have more questions.

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