Low capacitance audio coax
 

Don Pearce wrote:
On Tue, 01 Dec 2009 10:32:20 +0000 (GMT), Jim Lesurf
wrote:

In article , 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.
I've been trying to recall the type of the UHF cable I've tended to use for
general purpose audio coax. I bought a drum 25 years ago because it was
c60pF/m and worked OK. Single solid inner, foamed spaced, sparce braid
outer. Fairly high diameter but works nicely for long runs. I'd recommend
it if I could remember the type number! :-)

Slainte,

Jim

RG63 comes in at 10pF per foot; it's an air-spaced polyethylene, so
well suited to an installation like this.

d


That's pretty good for a 10mm dia cable. Thanks

Ian

Low capacitance audio coax
 

"Serge Auckland" wrote in message
...

"Eiron" wrote in message
...
Dave Plowman (News) wrote:
In article ,
David Looser wrote:
"Dave Plowman (News)" wrote in message
...
In article ,
David Looser wrote:
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.

Hence the DIN idea of using a low input impedance at the other end of
the cable. A pretty crap idea (IMO) but as a way of reducing the HF
loss from cable capacitance it worked.
But only if you have an even lower source impedance. ;-)
Easy to do with transistors. With valves you need an extra stage.


Err.. no. The DIN system worked with high output impedences and low
input impedances. There was, of course, significant signal attenuation
in so doing with consequent S/N ratio implications, but it did reduce
the HF loss due to cable capacitance.

You've got me confused there. Thought low out high in was the rule. What
make would I be looking at for this? I had a Quad 3 series that used DIN
connectors throughout - but that was all low(ish) out high(ish) in.

Most domestic equipment in the sixties and seventies used DIN connectors
without following the DIN electrical standards.

That's right. DIN is both a standard for the connector and the electrical
signal. The electrical signal was effectively a current drive, going from
a relatively high output impedance (100k or thereabouts if I remember
correctly) into a relatively low (2kohm again from memory) input
impedance. Very few manufacturers applied this, Grundig and Philips comes
to mind...I had a Philips receiver with the tape ins and outs to the DIN
signal standard as well as connectors. Other manufacturers including Quad,
A&R Cambridge and Naim used the connectors but in a conventional low-out,
high-in fashion.

I get it now. The low impedance load was effectively in parallel with the
high output impedance source, and became a effectively low source impdeance
for the interconnect. The minus was loss of gain (which was made up
elsewhere) but the plus was an effectively low source impedance and improved
frequency response over what you might presume from the 100K output
impedance of the source.

Low capacitance audio coax
 

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

Low capacitance audio coax
 

"Jim Lesurf" wrote in message
...
In article , Dave Plowman (News)
wrote:

Given that they were making up a new 'standard' I assume they could have
done that, but it would have meant defining a cable standard as well
as ones for source and load. Hardly rocket science, though!...

They weren't really "making up a new 'standard'", they were making up a
quick fix to solve a particular problem.

David.

Low capacitance audio coax
 

David Looser wrote:
"Ian Bell" wrote
I don't find comments like '50K is stupidly high' and the later comment
using the same 'stupid' word to be at all constructive.

Well OK. But I don't find comments like "You assume too much" constructive
either, and you were the one asking for help. IME if someone is asking for a
solution it helps to explain as much as possible about the background to the
requirement.


That is itself a double edged sword. Not long ago I posted on another
technical group asking for help and I explained the problem in detail.
The result was a quote ' are we ever getting to the point?' by one
respondent.

Seems I am wrong if I am succinct and wrong if I ain't.

Cheers

Ian
A bit of lateral thinking may provide a better solution than
the one the OP has in mind.

David.

Low capacitance audio coax
 

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

Low capacitance audio coax
 

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.

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info
Filtering everything posted from googlegroups to kill spam.

Low capacitance audio coax
 

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

Low capacitance audio coax
 

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

Low capacitance audio coax
 

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