On Tue, 01 Dec 2009 20:40:47 +0000, Ian Bell
wrote:

Don Pearce wrote:
On Tue, 01 Dec 2009 20:11:15 +0000, Ian Bell
wrote:

Don Pearce wrote:
On Tue, 01 Dec 2009 18:15:25 +0000, Ian Bell
wrote:

Don Pearce wrote:
On Tue, 01 Dec 2009 10:20:18 +0000, Ian Bell
wrote:

Dave Plowman (News) wrote:
In article ,
Ian Bell wrote:
I need to send an audio signal from a 50K ohm source over a distance of
a couple of feet in a screened cable. However, most audio coax seems to
be about 100pF/ft so 2ft of this and 50K will turn over just below
16KHz. So, anyone know a source of low capacitance audio coax?
Those sort of output impedances were common in valve days. Use a video or
RF coax to your requirements. Maplin sell a range by the metre.

I keep forgetting about Maplin. I checked their catalogue and they have
an AV coax by Shark that is only 65pF/metre.

Thanks Dave

Cheers

Ian
Still worth adding that inductor. Have a look at the difference it
makes, assuming 2 feet of 65pF/m cable. The green solid line is
without the inductor, the blue solid line is with.

http://www.soundthoughts.co.uk/look/highx.png

The inductor here is 56mH, connected to the wiper of the pot.

d
Looks interesting. Can you post the .asc file?

Cheers

Very easy. I just used one source, but with two cable options.

http://www.soundthoughts.co.uk/read/cable.asc

d

Thanks for that. It made me realise that the worst case source impedance
from a 100K pot is not 50K but 25K so things are only half as bad as I
thought. LOL

Cheers

Ian

Was it a 100k pot? Ok, change the inductor to 15mH or 12mH - they go
either side of optimum.

d

I need to make another tweak because there is a 150K or 220K across the
cap at the far end - will probably just worsen the Q.

Cheers

Ian

12mH will do nicely for either. It is the final cable capacitance that
really matters. Juggle the value of L for maximum flatness when you
have decided that.

d