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 Forum index » DIY Hardware and Software » Developers' Corner
A few VCA questions
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Cfish



Joined: Feb 24, 2016
Posts: 477
Location: Indiana

PostPosted: Sun May 01, 2016 7:32 am    Post subject: A few VCA questions Reply with quote  Mark this post and the followings unread

my attempt to get a much firmer grasp on synth circuits has had me playing with several different VCA circuits.

I have a few questions I'm hoping you folks of far greater experience and knowledge can give me a little guidance on.

While working on a VCA schematic I found online from Polly fusion I had some problems. Never did get it to work. However it did help my understanding of the OTA a lot.

So the first schematic is something I put together as a simple OTA VCA

It works beautifully for what I am needing in a simple project to gate a signal.

After reading for hours everything I can find on OTAs (all of it above my pay grade) and about half of it going over my head. I decided to ask if you all see anything wrong with using this circuit as drawn.

It will be between a triangle VCO and a basic low pass VCF?

What risks do I run not having a buffer on the output?

Do any values look out of whack for +- 15 v ?

I'm getting a little lost in the 3080 dattasheet

Was trying to understand any current and voltage limitations that need to be honored with respect to pin 5


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Cfish



Joined: Feb 24, 2016
Posts: 477
Location: Indiana

PostPosted: Sun May 01, 2016 7:52 am    Post subject: Reply with quote  Mark this post and the followings unread

Next I have a PAIA schematic.

I have a little better gasp on what is going on in this schematic, but it is quite noisy. ( transistor click on key press) . I'm sure I could filter most of it out. But I'm wondering if that is a characteristic of this circuit, or am I missing something.

I am using 2n3904 transistors and a TL071 opamp, it calls for a 748 opamp with frequency compinsation. Not an opamp I'm fermilior with.

I am currently testing it on +- 15v as I don't have a supply built for my PAIA items yet. Calls for +-9v

I started trying to figure out resistor values for 15 volts and realized I was in over my head.

Most simple PAIA schematics I have used were of a much better sound quality than I'm getting from this one. May be I'm trying to squeeze apple juice from an orange.


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etheory



Joined: Feb 20, 2013
Posts: 13
Location: Sydney

PostPosted: Mon May 02, 2016 8:22 pm    Post subject: Re: A few VCA questions Reply with quote  Mark this post and the followings unread

Having dealt quite a lot with circuits of this description, I'll attempt to help.

Cfish wrote:
I have a few questions I'm hoping you folks of far greater experience and knowledge can give me a little guidance on.

It will be between a triangle VCO and a basic low pass VCF?


I don't think the circuit, as presented, will work as you intend it to.

If it's meant to be a VCA, then connect the output 10K and 100pF in parallel from the OTA output to ground. The 10K will set the output voltage as a function of the bias current and Gm, and the 100pF will act as a low pass filter.

If it's meant to be a VCF, then connect the 100pF directly to ground without the 10K resistor, then buffer the output, the tie the buffered output back to the -ve input via a 100K resistor. That will make a unity-gain low pass VCF whose centre frequency is controlled via the pin 5 bias current.

Cfish wrote:
What risks do I run not having a buffer on the output?


The CA3080 is an Operational Transconductance Amplifier, and hence it has a current output.

A current output expects, in general, one of two things:

1.) To be shorted directly to a virtual ground.
2.) To be connected to a resistor-to-ground, then connected to a high impedance buffer.

Many other configurations are possible, and sometimes used in older circuit designs, but one of the two above will give you the best theoretical result with respect to noise and output swing.

Remember that since the 3080 is a current output, as the resistive load increases (the resistor value decreases), due to V=IR, the output voltage will decrease as R decreases (the load increases) for a constant I (constant I is ensured with a current output).

Output swing is limited to 1.2V within +V and -V. This is due to the wilson current mirrors it uses at the output. Each of these current mirrors drop 2 diode drops (0.6V * 2) to operate. Hence the circuit needs 1.2V from each rail to work properly. Outside this limit it will hard clip.

Input swing is about 20-30mV due to the input being directly into a transistor long tailed pair. This input arrangement has an approximately arc-tangent input distortion curve, that has a tiny linear range within about +-15mV (hence 3080 circuits often heavily attenuating their inputs to avoid distortion).

So the buffer serves to provide a very high impedance load, so that the output of the 3080 doesn't change gain with dynamic load (i.e. it separates your intended R load from the load it is driving).

This is important for constant gain at the output.

Cfish wrote:
Do any values look out of whack for +- 15 v ?


Not from a ballpark point of view, no, but clearly if your circuit was originally designed for +-9V, at 15V, with the same parts, you'll be pushing a higher current through it, which may, or may not, decrease performance.

Cfish wrote:
Was trying to understand any current and voltage limitations that need to be honored with respect to pin 5


Pin 5 is a current-mirror input. It's actually supposed to be a current-mode-input, so it sits nominally at -V, and as you increase the input current, the current through the mirror increases, and hence the bias current to the OTA core increases, increasing the gain.

The way it's configured in your schematic, the input PNP is merely a voltage buffer (that additionally raises the input voltage by 0.6V which avoids the "dead-zone" of the following PNP transistor that requires approx 0.6V to be "on"), and the second PNP is a voltage to current converter (along with the 10K/470K divider). In this configuration the voltage at the PNP emitter is "reflected" around the grounded base to the collector, and that current is presented to the current mirror at pin 5. If the PNP emitter is at ground potential, then the transistor is "off" and no current flows. As the emitter is moved above ground (specifically around 0.6V above ground), current can flow, and the current to pin 5 increases.

As a general rule, don't feed more than 500uA to pin 5, or you'll risk frying the chip at worst, or creating huge amounts of distortion as the input transistors saturate.

Hope that helps.
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LFLab



Joined: Dec 17, 2009
Posts: 497
Location: Rosmalen, Netherlands

PostPosted: Mon May 02, 2016 10:08 pm    Post subject: Reply with quote  Mark this post and the followings unread

Jurgen Bergfors did a VCA shootout And he also has some other VCA designs on his site.
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Cfish



Joined: Feb 24, 2016
Posts: 477
Location: Indiana

PostPosted: Tue May 03, 2016 5:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks Ethory. That helped a lot. Looks like if I swing the resistor to ground as you pointed out after the OTA I am pretty much ok in my application. I had placed the filter there because my keyboard circuit idles the VCO two notes above the top key, and it was bleeding through. The low pass VCF it will be hard wired in to has an input buffer, and will eliminate my need for the passive filter.
My 79L09s came today. So I can try the PAIA circuit on the correct voltage.


Thanks LFLab. Awesome link. Great information there, and some fun schematics to experiment with.
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etheory



Joined: Feb 20, 2013
Posts: 13
Location: Sydney

PostPosted: Wed May 04, 2016 2:24 am    Post subject: Reply with quote  Mark this post and the followings unread

Cfish wrote:
Thanks Etheory. That helped a lot.


You are welcome, good to hear.

Cfish wrote:
Looks like if I swing the resistor to ground as you pointed out after the OTA I am pretty much ok in my application.


Yep, and if you put the cap in parallel to ground (i.e. the OTA out goes to the resistor and cap, and both go to ground) then you get the proper output + passive filtering, all in one (should you require it).
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Cfish



Joined: Feb 24, 2016
Posts: 477
Location: Indiana

PostPosted: Wed May 04, 2016 5:14 am    Post subject: Reply with quote  Mark this post and the followings unread

Its amazing how poore my math skills got 25 years out of school.

Sorry if so many of my questions seem like a young child trying to figure things out. I know if I'm not hands on trying to relearn the math I need as I do it, I will never retain it.

As I go I have Ben trying to break schematics down to there simplest form.

I am keeping some simple LO-FI synth projects going on to keep things interesting while I learn and decide on circuits for my main modular synth project. Which is really starting to come together.

i don't have any real experience with modular synths beyond YouTube videos. I don't know anyone that ownes one. So I would be no where close to where I am without the help of the awesome people here on electromusic.com


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