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Old 16th February 2006, 06:34 PM   #1
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Default Looking for Guidance - OLD parts - VCA's

I've got a collection of old stuff. As anyone who has tinkered with electronics since the 70's might understand, I have things that are clearly no longer available taking up space in my storage bins just begging to find a home somewhere.

For example: I have in mind a project using a number LM1875 chipamps (5 total) that I have lying for a PC sound card amplification system. I clearly have Home Theatre style audio sources I can provide the 5 amps though it's not something that any audiophile would love. Keep in mind, I'm just trying to use what I have on hand.

Here is where I want some guidance. I want a SINGLE main attenuation control since I can tweak each channel with the sound card before it reaches the ganged attenuator.

I'm not going to find a 5-gang attenuator or 5 gang 50K pot anywhere but... I do I have a number of MC3340 VCA's that I can use. What I'm wondering if anyone has tried doing this. That is, controlling (5) MC3340's from a single variable resistor.

I'm in the process of "doing" it... but I'm just wondering what insight others might provide.

I also have a killer 'dumpster dive' torroidal transformer that from what I can tell is a 27V CT 230VA unit. Not quite perfect, but worth a shot. (I'm willing to be dissappointed that the split supply voltage will be too low for the LM1875.


Pete Willard
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Old 17th February 2006, 12:18 PM   #2
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It all depends on what sort of quality you're after. VCAs are notorious for being noisy at one end and distorted at the other. Driving all five of your VCAs from a common reference voltage isn't a problem (although it would probably be a good idea to buffer them with a voltage follower rather than drive them directly from the pot). Making them track might be a little tricky, but the main problem will be selecting a suitable operating point.

Alternatively, you could use a five-gang switched attenuator. Not nearly as hard as you might think. If you don't fancy making it yourself, there have been some stereo attenuators on eBay at sensible prices. Just buy three, lengthen the shaft and put five wafers on one mechanism. Sorted. Tracking will be nigh on perfect, and you won't have distortion/noise problems or the complexity of making five VCA boards.
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Old 18th February 2006, 01:18 AM   #3
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I'm willing to admit. I'm an audio noob.

I'm glad I asked. I assumed tracking all the VCA's would not be a problem. I trust that the noise and distortion with the VCA's is common knowledge then... I was being rather naive I guess. I am having trouble finding ANY information on these ancient MC3340 devices.

I'm now quite inclined to take your advice and try making my own multi-gang attenuator.

Thanks.
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Old 18th February 2006, 08:38 PM   #4
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Hi,

First hit with Google: http://www.ee.washington.edu/stores/...ear/mc3340.pdf

And yes these are noisy and funny enough most at low gain settings. Have fiddled al lot with these gain cells and expensive professional discrete ones used in mixing consoles. They all tend to drift in gain control a lot over time and temperature. Nowadays there are much better PGA (programmable gain amps) available but these need a micro controller to control. If it is just for experimenting ah well ..

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