At 06:52 PM 5/7/98 -0400, you wrote:
>On Fri, 8 May 1998 08:13:06 +1000 Dennis Plunkett
><dennis@RDD.NECA.NEC.COM.AU> writes:
>>At 11:22 AM 7/05/98 +0100, you wrote:
>>>Hello all
>>>        I'm thinking about making an attenuator ('volume control')
>>for a
>>>hifi preamp. I don't want to use semiconductor devices for this
>[...]
>
>You could use the standard "Pi" or "Tee" attenuator networks used in RF
>work.  A relay would either switch each attenuator in or bypass it.  With
>attenuators for 1, 2, 4, 8, etc. dB then the overall attenuation could
>vary from 0 to 2^(n-1) dB.  For each stage you need 3 resistors and a
>DPDT relay.  You can also buy multi-stage attenuators already built, but
>they have a 50 ohm impedance and are expensive.  One nice feature of this
>sort of setup is the input and output impedances are constant regardless
>of the attenuation setting.  This is essential for most RF circuits and
>unnecessary for most audio circuits.  But it may help sell it to the
>"hi-fi types."

I may not be seeing something here, but why don't you just use a stepper
motor connected to a log pot? It is much more cumbersome than the digital
log pot, but it requires less power, board space, and cost than N relays
for 2^N steps.

Sean


+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
Save lives, please look at
http://www.all.org

Personal page: http://www.people.cornell.edu/pages/shb7
mailto:shb7@cornell.edu
Phone(USA): (607) 253-0315