Analog VCA's

[maylar]

Anybody know of a source (or schematic) for analog voltage controlled amplifiers?

The single-chip implementations I've found so far are very expensive.

 

[mirlo]

Analog VCAs typically depend on circuits like Gilbert cell multipliers with added distortion reduction circuitry. The operation of these circuits is heavily dependant on precise matching between transistor characteristics. It is difficult to imagine that you could obtain such matching with discrete components, almost certainly you would end up spending more than you would on a monolithic solution.

 

[Steven]

Not only Gilbert is famous for his multiplier, also Blackmer. His circuits are more related to audio. He died two years ago and many VCA's from That are based on his design. His famous gain cell is patented: 4,403,199 and you clearly see all the matched transistors that are required. Where the Gilbert gain cell requires only NPN (or PNP) transistors, for Blackmer's cell you need matched NPN's and PNP's. See picture below.

See also: http://www.thatcorp.com/vcahist.html for some background info.

Next to ICs from That Corp., maybe the VCA's from Analog Devices, who took over SSM years ago, are more cost effective, like the 4 channel VCA SSM2164. For the datasheet on the 2164 see the link.
http://www.analog.com/UploadedFiles/Data_Sheets/15183786ssm2164.pdf

Or use the VCA part of a cheap compander IC, like the Philips/Signetics NE572 (family).

Steven

 

[maylar]

Thanks very much for the Analog Devices part link - I dunno why it didn't come up for me when I searched their site. At $5.86 (DigiKey) for 4 units it's the most cost effective solution I've seen.

The THAT Corp parts are about $9 US each. And Burr-Brown has one (SSOP only) for a whopping $28.

I'm intending to try using one as a variable frequency active filter circuit. Multiple stages cascaded with a single control. At .02% THD they're not too bad.

 

[Steven]

I'm intending to try using one as a variable frequency active filter circuit. Multiple stages cascaded with a single control. At .02% THD they're not too bad.

Have a look at http://www.rane.com/sac23.html
That unit uses the 2164 as a voltage controlled filter. Schematics are given in the link.

Steven

 

[maylar]

Outstanding. Thank you, Steven.

Dave.

 

[DragonMaster]

I have a commercial synth with M5222L VCAs connected to it. They are very noisy.

They are connected to +6 and -6v and the pot controlling them is going between -6v and 0v.

What can I do with this? Bypassing is the solution provided for a camera using the same VCA. The only thing is that a synth without volume control is not great and 4 channel pots with good tracking are hard to find.


Here's the signal path:
PCM55HP DAC -> 4x 4053B Analog Mux/Demux sections -> 4x TL064 opamps sections -> Stereo M5222L VCA -> 2x TL064 sections -> Relays -> outputs

Someone told that replacing the TL064s by OPA4134s did a great improvement but that it still was bad.

 

[kASD]

Hi Maylar,

Try this Link and download the datasheet of Analog VCA LM13700 IC


http://www.national.com/ds.cgi/LM/LM13700.pdf


Hope that it helps...

K a n w a r