Ian Bell wrote:
Ian Jackson wrote:
In message , "Dave Plowman (News)"
writes
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.

75 ohm RF coax is typically 50pf/m.
Of course, you could use car radio coax (even if the inner is a
distinctly flimsy). That is even lower (say 35pF/m).
Try a Google on car+radio+coax+low+capacitance. This is one of the
more useful hits:
http://209.85.229.132/search?q=cache...fe.com/phpbb2/
viewtopic.php%3Ff%3D3%26t%3D4791+car+radio+coax+lo w+capacitance&cd=1&hl=e
n&ct=clnk&gl=uk


Yes, so far URM70 at 67pF/metre seems the cheapest alternative, but
RG62A is better at 48pF/metre and both are readily available (Maplin and
Farnel) and RG114 at 6.5pF/metre seems best if I could find a UK source.

Cheers

Ian

Correction: RG114 is 6.5pF/ft so just under 20pF/metre

Ian

mick wrote:
On Mon, 30 Nov 2009 21:09:48 +0000, 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?



Does it have to be coax? Have you tried a simple twisted pair? That isn't
a great distance - you may get away with it and the capacitance can be
lower than most coax cables.


Not really. This is inside a tube mixer and these are the feeds from
channels to the mix amp. I thought of using unscreened wire but I was
concerned about the crosstalk this would introduce especially at high
frequencies - a screen will prevent that and that is all I need it for
really. External interference is much less an issue given the relatively
high signal level and the fact the whole thing is in a screened enclosure.

Cheers

Ian

Don Pearce wrote:
On 01 Dec 2009 20:04:14 GMT, mick wrote:

On Mon, 30 Nov 2009 21:09:48 +0000, 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?


Does it have to be coax? Have you tried a simple twisted pair? That isn't
a great distance - you may get away with it and the capacitance can be
lower than most coax cables.

Twisted pair won't help much at these impedances; pickup will be
mostly electrical rather than magnetic. Shielding is the thing. On the
other hand, if the signals levels are high on this lead, that may not
be a problem either.

d


That's right, I am more concerned about crosstalk than interference.

Cheers

Ian

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

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

"Ian Bell" wrote in message

Ian Bell wrote:
Ian Jackson wrote:
In message , "Dave
Plowman (News)" writes
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.
75 ohm RF coax is typically 50pf/m.
Of course, you could use car radio coax (even if the
inner is a distinctly flimsy). That is even lower (say
35pF/m). Try a Google on car+radio+coax+low+capacitance. This is
one of the more useful hits:
http://209.85.229.132/search?q=cache...fe.com/phpbb2/
viewtopic.php%3Ff%3D3%26t%3D4791+car+radio+coax+lo w+capacitance&cd=1&hl=e
n&ct=clnk&gl=uk


Yes, so far URM70 at 67pF/metre seems the cheapest
alternative, but RG62A is better at 48pF/metre and both
are readily available (Maplin and Farnel) and RG114 at
6.5pF/metre seems best if I could find a UK source. Cheers

Ian

Correction: RG114 is 6.5pF/ft so just under 20pF/metre

http://www.awcwire.com/ProductSpec.a...-Coaxial-Cable

RG114 is also about 1 CM (0.4 inches) in diameter, so don't plan to bend it
around very much.

The center conductor is some thing like number 33 wire, which is hair-fine
so it won't be a constructor's dream to work with.

And that is how you get low capacitance per foot - big outer conductor for
lots of distance to the innner conductor, small inner conductor for small
surface area per foot.

"Ian Bell" wrote in message

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?

emitter followers, I mean cathode followers, anyone? ;-)

"Ian Bell is Bonkers"


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

** Only true is the source feeding the 100k ohm pot has near zero
impedance.

Which is obviously not the case.

Or one would use a lower value pot.



...... Phil

Arny Krueger wrote:
"Ian Bell" wrote in message

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?

emitter followers, I mean cathode followers, anyone? ;-)




Too many (40) and to noisy.

Cheers

Ian

Phil Allison wrote:
"Ian Bell is Bonkers"

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

** Only true is the source feeding the 100k ohm pot has near zero
impedance.

Which is obviously not the case.

Compared to 100K, the 800 ohm source resistance of the mu follower is as
near zero as makes no odds.

Cheers

Ian

Or one would use a lower value pot.



..... Phil