VCAs, and Noise reduction for NE571

[The Dark force of dance]

Y-ellow Juergen 'n' y'all.

At 03:30 PM 07/28/99 +0200, Haible Juergen wrote:
>	>From there it was only a hop step
>	>and jump to using an OTA instead of the one built into the 571 and
>the
>	>whole thing was redundant. 
>
>I fully agree with that. Once built a compressor around the 570 (or 571),
>and found no way to get a fast attack / slow decay operation.
>
>	>Believe me a 3080 is a whole lot quieter than
>	>the 571.
>
>Are you sure this is not just because of the shitty internal opamp ?
>There is a recommendation to replace it with an external opamp and
>only use the variable gain cell. This is mainly to get a reasonable
>slew rate, but probably also to improove the SNR. Don't know, it's
>too long since I've played with these.

I tried this. I don't recall the exact details but it was a failure.
Something about the variable impedance of the gain cell. It might be
workable but the whole chip goes skew once you pass out of the recommended
operating procedures. It's just a whole lot easier to use something else.

As I said. The NE572 is a whole other ball game. It's quite and well
behaved. Everything the 571 should have been.

Now I can't remember how I found this out but the Gain cell and the
supposed op-amp are really just an OTA. Similar in concept to a 13600. It's
actually a current controlled device. You can actually drive it through the
rectifier-cap pin or even the THD trim pin. Likewise you can derive a
control current from either of these pins. The problem, as Paul Perry
suggest, with using steering diodes is that there is only one node where
you could connect such a thing. Unlike the 572 with which you can conjure
some quite complex schemes. If you use this node to derive a control
voltage you can't practically use it to feed this back to the gain cell.

The so called op-amp also has some weird biasing arrangement as I recall.
When you try to do anything outside the norm with this you have to be
mindful of this. The Gain cell expects to see this biasing. Whilst I
haven't re-visited these chips in 10 years, I recall that it was all too hard.

When I designed my micro-noise-gates I ended up using OTAs and 324s.
Besides which I could put a transistor and a LED on each channel and I have
to say I miss seeing 22 channels of noise gate flashing in the dark.

I worked with the 571s because at the time, they were the only thing
practically available to do the job. But they were a bugger of a thing to
work with. I've pretty much assigned these things to history. And now of
course, Noise reduction isn't big on anyone's agenda. But if you wished to
build a simple and effective NR for an already noisy process, and you can
get a cheap source of 571s, then they might be your answer. But if you can
get 572s I can recommend the latter.

Hope this helps.

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