AW: [sdiy] Dual Frequency Shifter update

[jhaible at debitel.net]

> > Maybe with the predistortion before the multipliers (after the dome
> > filter? - Haven't thought this thru yet.)
> 
> That will work,

This will work?
Then it's probably _the_ solution! 
(No problem to build a matching triple (2 before mults, 1 after sum of mults)
of nonlinearities from a couple of transistor arrays, I guess. First thing
that comes to mind is just two anti-parallel diodes in the feedback of an
opamp. 3 x 2 diodes, made from 2 MAT-04s.)
And it really will work, conceptually??


> The dome filter is probably a contributor to bad S/N so you really want to
> replace that 

I don't think it contributes any noise worth mentioning.
The signal runs thru 6 opamp stages with a max. noise gain of
2 in each stage. I don't hear any noise even with cheap TL072s.
If noise is an issue, I can still choose resistors in the 600R
range and go for low noise opamps.


> A traditional compander works, yes. It is still a violation, but a smaller
> violation so it works.

The real problem with long release time constants is that you hear a tail
of modulation artefacts when the input signal decays fast. That's why
you want the release time constant not too long, and thus have to choose
a longer attack time constant.

> It seems like your damping of the loop goes out the window!

No, the attack is just too slow for the compressor to react in time!

So, I need a rather small release time (see above), and a rather 
long attack time, but with the attack time being shortened when a sudden
large upward step in level occurs. The old LM324 + 2 diodes trick.
Maybe I can try this tonight.

JH.

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