[sdiy] Slightly OT: High quality VC attenuator

[Henri Kovalainen]

Hello,

I'm afraid that I'll have to postpone all my (already pending) DIY
projects, for I have been struck by a more urgent project.

What I basically need is a very high quality preamp, that would be capable
of switching in a number of inputs. This would be located between a player
and a pair of active speakers. Actually it will be sort of modular with
the internal in/out cards plugged into a "motherboard". So the exact
number of inputs/outputs (and possibly other modules) will depend on the
application or needs. At least five channels of audio will be supported.
The inputs will be switched on/off the main bus by a reed relay. I will
not bore you with the exact detailed description, although I'm gladly
willing to (maybe privately) give one if somebody wants it.

The trick here is that I want it to be controller by a uC, which means
that the attenuator, in other words the volume control, has to be
voltage/current controlled. And due to various design aspects it
naturally has to be an attenuator.

What I have been tinkering with more or less from the very beginning is
H11Fx optocouplers (by the specs released, H11F3 would do here). This
would be hooked with a "regular" resistor to form a simple voltage
divider, the H11F3 resistor being on the input signal side so that no
current on the coupler equals maximum attenuation. This would be buffered
by a high quality FET opamp (low current noise, I had the Burr-Brown
OPA134 series in mind). Actually, the buffer will already be the
non-inverting side driver of the balanced output. With unity gain (in
straightforward follower configuration) I don't think the performance of
the opamp will be an issue in this case.

However, I have been unable to find any real performance specifications of
the resisting element in H11F3. The data sheet states it performs like an
ideal FET designed for distortion-free control of signals. In this case, I
consider distortion-free a subjective notion (unless it really does defy
the laws of physics).

So, I guess what I'm really asking is if anybody has ever tried one in a
similar application and formed an idea of its performance? I would also be
very happy to hear any comments or possibly suggestions.

Oh and one more thing. I'm striving for strict quality here. The whole
unit will be over-engineered with a passion, so there is no solution too
impractical or work demanding to be discarded. =)

Thank you in advance.

| henri kovalainen   | peltokatu 19 b51   | hekovala at kovalainen.fi
| phone: 0400-683750   90120 oulu finland   hekovala at paju.oulu.fi

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