[sdiy] Slightly OT: High quality VC attenuator

[harry]

The question is "how much attenuation range do you need"...

I'm guessing a LOT and that FETs will not have enough range.  I'd
think of a VCA using the standard differential pair techniques...

H^) harry

Henri Kovalainen wrote:

> 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
>
> | Grow your own dope - plant an economist.

--
Jihad Terrorism Conspiracy New World Order
Revolution Black Helicopters Freedom of
Speech First Amendment Rights: Carnivore Bait
go ahead and READ my e-mail I have nothing to
hide... how about YOU ???