[sdiy] Attenuators

[harry]

Hi Mike...

(looks like no one got to ya first... so here goes)  Inline

Michael Both wrote:

> Is there anything more to an attenuator (gain from 1 down to 0) than a
> pot wired up with one leg to the input signal, one leg to ground, and
> the wiper to the output?

In a simple form. NO. That's how most variable attenuators are made. Its
possible
to do the same thing with a rotary switch and fixed resistors... or a VCA
could be
a Voltage Controlled Attenuator (it usually is... we go for gain reduction
instead of
amplification). Photocells (like Vactrols etc...) can be attenuators also.

> It seems that this might introduce the
> possibility of "crackling" if audio signals are passed through,

The problem of "crackling" usually comes from DC shifts across the pot. If
there is
no DC level difference, a lot of the crackle does not occur. A worn pot WILL
still
make noise when you move it.

> not to mention the varying impedance.

Varying impedance IS an issue. But in a lot of cases... the top of the pot
is driven
by an opamp... so (lets say) a 10K is a very high impedance load compared to
the output impedance of the opamp (on the order of ohms, usually)...

Then the load on the wiper is an issue also... but with the same 10K pot,
and a 100K
resistor in series with the wiper... you can't be off in impedance by more
than 10%.

If you use an opamp to buffer the top of the attenuator... and a buffer
(voltage follower op-amp) on the wiper... now the driving impedance is maybe
10- 100 ohms
(or 0.1 to 1% impedance error) and the wiper is maybe 10e+10 ohms (with a
FET opamp, CMOS is even higher...) so impedance issues magically
"disappear".

Of course the opamps might add noise, or offset voltages, etc. But this is a
pretty
near perfect attenuator.

The Buffer driving a Pot driving another buffer is a pretty common topology.
In a lot
of cases, the buffer amp is hidden in the driving stage... and the wiper
buffer is hidden
in the following stage. In this case the pot is all you see !

>
>
> How have attenuators "classically" been implemented? I can't seem to
> find any schematics anywhere, which seems to be indicating that the  pot
> approach is taken.

Yes the pot approach is usual for manually set variable attenuators.
Electronic adjustments can be possible.

> Thanks for any help

Hope this does ...

H^) harry

>
>
> Mike