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Single transistor VCA circuit
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synthmonger



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PostPosted: Thu Mar 26, 2009 12:58 pm    Post subject: Single transistor VCA circuit
Subject description: Korg used one right?
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Having trouble finding a schematic for it. I can't remember exactly what Korg circuit used it. Any help would be greatly appreciated! ;D
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Rykhaard



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PostPosted: Thu Mar 26, 2009 1:28 pm    Post subject: Reply with quote  Mark this post and the followings unread

Rough memory but weren't it the MS-20?
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slacker



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PostPosted: Thu Mar 26, 2009 2:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

Juergen Haible talks about it on his PolyKorg pagehttp://www.jhaible.de/polykorg/jh_polykorg_clone.html. I think it's the bit around the 2SC945 transistor in this schematic http://www.jhaible.de/polykorg/jh_3200_gates_2of2.gif.
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ChrisR



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PostPosted: Thu Mar 26, 2009 2:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

I was going to post the same reply as slacker Smile

I also looked at the schematics of the MS-10, and there seems to be a similar circuit in there. Would be nice if i could isolate it since i have a use for a simple vca.

Chris
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slacker



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PostPosted: Thu Mar 26, 2009 2:33 pm    Post subject: Reply with quote  Mark this post and the followings unread

The WP20 from Music From Outer Space has basically the same thing as its VCA if that's any help. I think you can sub virtually any opamp for the LM3900.
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synthmonger



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PostPosted: Thu Mar 26, 2009 2:44 pm    Post subject: Reply with quote  Mark this post and the followings unread

Ah! Neat that looks like it'll work. I'll BB it and see how it operates.
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richardc64



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PostPosted: Thu Mar 26, 2009 3:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

I've been playing around with the VCA from Roland's DR-110 percussion unit. It seems very promising. Basically a transistor, diode and a couple of resistors. R2 is 33k. For a.c. coupled input R1 is typically 1Meg. For a d.c. signal, such as from a cmos output, R1 is instead a 100k in series with the base. Envelopes are around +5V.


110vca.gif
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110vca.gif



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Rykhaard



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PostPosted: Thu Mar 26, 2009 3:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

Excellent! Thankee for posting that Richard! I'll be testing it out in my Lunetta machine!! Very Happy If it works well, I'll be building a bunch of them. (Not that I'm THAT concerned with audio quality in that machine, mind you. Wink )
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andrewF



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PostPosted: Sun Mar 29, 2009 6:32 am    Post subject: Reply with quote  Mark this post and the followings unread

slacker wrote:
The WP20 from Music From Outer Space has basically the same thing as its VCA if that's any help. I think you can sub virtually any opamp for the LM3900.

The 3900 is a Norton op-amp - current differencing rather than voltage.
A very special chip, Serge Tcherepnin probably exploited it more than anybody when it came to synth circuits.
can be sub'd with the LM2900 or MC3401 but not regular opamps
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richardc64



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PostPosted: Sun Mar 29, 2009 7:02 am    Post subject: Reply with quote  Mark this post and the followings unread

Yes, but I've seen the same basic idea used with other op amps. The scrap below, using a TLO72, is from the TR909 clone rim shot. I've also seen an LM324 used.


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v-un-v
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PostPosted: Sun Mar 29, 2009 12:07 pm    Post subject: Reply with quote  Mark this post and the followings unread

Richard, can you provide a link to the circuit that uses the LM324? Thanks.

Tom

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richardc64



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PostPosted: Sun Mar 29, 2009 5:53 pm    Post subject: Reply with quote  Mark this post and the followings unread

No can do. I can't even find it on my hard drive now. I'm pretty sure it was in some percussion unit.

Here's the MS-20(?) I found somewhere or other.


ms20vca_156.jpg
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Clack



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PostPosted: Fri Jul 31, 2009 4:41 am    Post subject: Reply with quote  Mark this post and the followings unread

hey gonna just bring this thread back up, because I want to discuss reasons for vca designs.

im trying to build a stackable (CV can control more that one channel) vc-mixer with the most minimal but most readily low cost available parts (ssm parts would be amazing if they weren't so rare)

I want to know what are the reasons for using a transconductance op-amp or other desgin over just a simple transistor/op-amp vca design?

im guessing its because of

lower input impedance?
is there less of a linear response or the response is less manipulative?
lower dynamic range?
more susceptible to offset and therefore distortion?

are these assumption correct or relevant?

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richardc64



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PostPosted: Fri Jul 31, 2009 5:44 am    Post subject: Reply with quote  Mark this post and the followings unread

I don't know about your assumptions, but my assumption is that one advantage of using an OTA is that it can be D.C. coupled. With no capacitor in the signal path it can operate with lower losses at low frequencies, down to slow-moving envelopes and LFO ranges. At higher audio frequencies there would be none of the phasing effects of capacitor coupling -- unless that would be what you wanted.

All the simpler designs discussed above generally need a capacitor somewhere in the signal path, at input, output or both, like the MS-20 or the WP-20.

I'm sure someone here can correct my simplistic reasoning.

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JovianPyx



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PostPosted: Fri Jul 31, 2009 7:49 am    Post subject: Reply with quote  Mark this post and the followings unread

One reason for using an OTA type VCA over a single transistor is that with the single transistor VCA, the output's voltage will always be on one side of zero volts, either above or below depending on the circuit. You can see this with the drawing above that shows input and output waveforms.

An OTA will give an output that looks like the envelope is copied on both sides of zero volts.

The main effect of this is that the control voltage will feed through to the output with the single transistor VCA and will not feed through to the output with the OTA VCA.

The feed through is usually something undesired. Your milage may vary.

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richardc64



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PostPosted: Fri Jul 31, 2009 8:38 am    Post subject: Reply with quote  Mark this post and the followings unread

ScottG wrote:
...with the single transistor VCA, the output's voltage will always be on one side of zero volts, either above or below depending on the circuit. You can see this with the drawing above that shows input and output waveforms.


Capacitor coupling at the output doesn't correct that? The output remains "flatline" at the top or bottom, depending?

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Clack



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PostPosted: Fri Jul 31, 2009 10:28 am    Post subject: Reply with quote  Mark this post and the followings unread

Is it because BJT's only work in one direction?

getting to basic electronics here then

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PostPosted: Fri Jul 31, 2009 12:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

richardc64 wrote:
ScottG wrote:
...with the single transistor VCA, the output's voltage will always be on one side of zero volts, either above or below depending on the circuit. You can see this with the drawing above that shows input and output waveforms.


Capacitor coupling at the output doesn't correct that? The output remains "flatline" at the top or bottom, depending?


Yes, capacitor coupling will result in the energy distribution being the same above and below zero, but there will still be a remnant of the straight line at the bottom of the envelope. That won't happen with the OTA VCA because the same envelope shape is copied to both the positive and negative halves of the signal. Also, the size of the capacitor will affect exactly what happens.

IMO, unless you have a good technical or application-based reason to use this type of VCA, I would avoid it. I realize that it's simplicity and low cost makes it attractive, but if it sacrifices what you want to hear, or adds an objectionable quality then it's value is quite a bit less. There are many reasons why you don't find this in high end gear.

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JovianPyx



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PostPosted: Fri Jul 31, 2009 12:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

Mr Clack wrote:
Is it because BJT's only work in one direction?

getting to basic electronics here then



More because it's a single ended circuit, which is easy to do with a single BJT (or FET for that matter) and a single supply.

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Clack



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PostPosted: Fri Jul 31, 2009 12:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

yes, it was just for the sake of discussion, I was just trying to make it clear to myself why a OTA is used ... how the transistor falls flat in comparason with an op-amp

besides lm13700's (£0.90) arn't that pricey for what they give

thanks for the info both of you, it made things more understandable

I also found this using CMOS MOSFETs by Osamu Hoshuyama

http://www5b.biglobe.ne.jp/~houshu/synth/VcaMos0210.GIF

of course though 'output has much distortion'

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PostPosted: Fri Jul 31, 2009 12:50 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yes, I've seen that too, he's quite a clever guy.

A word about distortion - it's not always bad. In fact, as you know, the most common "effect pedal" for electric guitar players is a distortion box. Some distortion is good, others is bad. The ear will always tell you the difference. With synths, there are many kinds of distortion, again, some sounds good, some is horrible. For example, any waveshaper is really a distortion circuit because it's output is a distorted version of it's input. That said, I wouldn't necessarily dismiss his circuit until it's breadboarded for a listen.

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Tim Servo



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PostPosted: Sat Aug 01, 2009 5:45 pm    Post subject: Reply with quote  Mark this post and the followings unread

[quote="Clack"]hey gonna just bring this thread back up, because I want to discuss reasons for vca designs.

im trying to build a stackable (CV can control more that one channel) vc-mixer with the most minimal but most readily low cost available parts (ssm parts would be amazing if they weren't so rare)

quote]

Hey Clack,

If you're trying to build a VC mixer, you should check out the SSM2164 Quad VCA chip. Four expo response VCAs on a single chip, and available fairly cheap. The VCAs have VERY wide dynamic range and are very quiet. This is a currently available chip, made by Analog Devices and CoolAudio, and available from several sources. The CV inputs have low impedance (<5k) so you'll want to drive them with an op-amp buffer, but you can use one inverting buffer to drive all four sections on the chip if you want. Use a linear pot to provide the CV and you're jamming (that's why they made the VCAs on the chip expo - so you get a correct sounding response when driving them from a linear source). The CV range is roughly +3.3V (100dB attenuation) to 0V (no attenuation). You can even go slightly negative on the CV (about -0.6V) and get a bit of gain, giving you a dynamic range of 120dB!

Tim (Mr. Chips) Servo
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CJ Miller



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PostPosted: Sun Aug 02, 2009 2:23 pm    Post subject: Reply with quote  Mark this post and the followings unread

Simple VCAs are very cool, and definately have their uses. These single-transistor types were used in Korg MS synths, and also in most drum machines. If you want ideas on how to make a complete synth with a handful of transistors, check out drum machine schematics. We're talking linear VCOs, single-supply everything. Lots of fun noises to be had.

But I need to agree with Tim. If you want simple VCAs for a mixer, you are better off with something more robust. Something with a small footprint and doesn't require much trimming. $5 for a VCA chip might not sound cheap, but you get ultra low noise and lose the CV rejection and offset problems.
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jean-louise



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PostPosted: Tue Aug 25, 2009 3:29 am    Post subject: Reply with quote  Mark this post and the followings unread

Posted Image, might have been reduced in size. Click Image to view fullscreen.

is that a NPN transistor?
thanx!
jan
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jean-louise



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PostPosted: Tue Aug 25, 2009 4:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

ok, i found it out thanks to breadboard, it works with NPN. Very Happy also an input attenuator is good to avoid/provide some dirtiness.

anybody has a suggestion as what to use for a coupling capacitor?
the 100nF one i had at hand seems far too small - when i turn down the filter cutoff there's nothing instead of the bass frequencies.. Mad
and does it matter wether i put it at the input or the output of the vca?

jan
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