"Don Pearce" wrote in message

On Wed, 21 Oct 2009 13:03:07 +0100, Ian Jackson
wrote:

In message
, Don
Pearce writes
On Tue, 20 Oct 2009 19:32:41 +0100, "Keith G"
wrote:


"Dave Plowman (News)" wrote in
message ...
In article ,
Keith G wrote:
I didn't 'leave it slamming'! I have no way of
attenuating the signal 'til I get the Alps and fix
my 'pot in a box' - is what my whole previous thread
was about!!

Make an attenuator out of fixed resistors. Only
reason to go for a posh pot is the channel balance is
likely better than a cheap one. But fixed resistors
will better them both.


I did consider that and probably have already got some
suitable/unused resistors here, but dismissed it -
certainly for long-term use - due to all the
variables. But you've given me the idea for an
interesting little fabrication - if I built a box with
rows of 'in and out' phono sockets connected to a
number of fixed resistances, what values would you
suggest?


I reckon steps of 6dB (halving the voltage each time)
would be about right. You need a resistor in series
(R1) followed by another to ground (R2). By keeping the
sum of them at about 10k, you will get the best
performance.

Att. R1 R2
6dB 5.1k 5.1k
12dB 8.2k 2.7k
18dB 8.2k 1.2k
24dB 8.9k 620 ohms

Not exact values, but the nearest that give standard
resistors.

Those are the values for insertion between a zero
impedance source and an infinite impedance load. If your
impedances are different, won't the attenuation also
differ?

Of course, but we're not engineering the input attenuator for a Fluke 4 or 5
digit meter.

This isn't for measurements, just a useful stepped
attenuator between a notionally low and high impedance.
The values won't be dead right, but near enough for
government work. The choice of 10k as a working impedance
is a reasonable loading compromise between typical input
and output impedances.

Agreed, and consistent with accepted professional practice for recording.