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?

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.

d