Low capacitance audio coax
 

There used to be little two transistor gadgets made for exactly this purpose
back in the 70s. Final stage an emitter follower of course.
It is surprising though haw many outputs on hi fi are affected by capacity
in leads. It has to be poor design of the output circuit in my view. Not so
mbad these days, but it used to be a real pain.

Brian

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"Trevor Wilson" wrote in message
...

"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?


**Use a buffer first. A 50k Ohm source is stupidly high. A high quality
buffer can be assembled for a few pennies.

If you must persist in using broken equipment, you could look at a variant
of RG59/U coax. Capacitances figure range from 50pF ~ 70pF/Metre. Belden
have a cable called 1192A, which exhibits around 40pF/Metre.

Use a buffer. MUCH smarter.


--
Trevor Wilson
www.rageaudio.com.au

Low capacitance audio coax
 

David Looser wrote:
"Ian Bell" wrote in message
...
Trevor Wilson 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?

**Use a buffer first. A 50k Ohm source is stupidly high. A high quality
buffer can be assembled for a few pennies.

You assume too much.

You have supplied so little information about what it is that you are doing
that all we can do is make assumptions, and Trevor's assumption seems
perfectly reasonable to me. Unless you want to tell us why it isn't.


I was not asking for an arrogant critique of of what Trevor *supposed*
the design was. The question was simple enough as it stood.

I had been going to answer your original post to the effect that "I wouldn't
start from here" (ie, wouldn't try to send audio from a source with as high
an impedance as 50K)

No *audio* co-ax will have as low a capacitance as you think you want. Try
looking at RF co-ax instead.

David.

Low capacitance audio coax
 

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

Low capacitance audio coax
 

David Looser wrote:
"Ian Bell" wrote in message
...
Trevor Wilson 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?

**Use a buffer first. A 50k Ohm source is stupidly high. A high quality
buffer can be assembled for a few pennies.

You assume too much.


I find it curious that you ask for help, then respond like that to a
constructive response. Why should anybody bother to help you if that's the
sort of thanks they get?

David.



I don't find comments like '50K is stupidly high' and the later comment
using the same 'stupid' word to be at all constructive.

Cheers

Ian

Low capacitance audio coax
 

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

Low capacitance audio coax
 

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

--
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Low capacitance audio coax
 

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.

--
*Ever stop to think and forget to start again?

Dave Plowman London SW
To e-mail, change noise into sound.

Low capacitance audio coax
 

David Looser wrote:
"Phil Allison" wrote in message
...

** So what ???

The whole cable capacitance issue is about loss of HF response - so
testing at 1 kHz is irrelevant.

That same cart likely has a source impedance of 45 kohms at 20 kHz.


The output impedance of an MM cartridge is inductive, not resistive. The
cable capacitance creates peaking with that inductance so that the overall
frequency response is reasonably flat. You cannot do this with a resistive
output impedance, so the MM example is irrelevant.

As far as we know Ian Bell's source is 50K resistive (though it might be
nice if he'd tell us what it is, instead of expecting us to guess).

The booklet that came with a Shure cartridge claims 1550 ohms and 650
millihenries.
So it looks like everyone is right, including Phil. :-)

--
Eiron.

Low capacitance audio coax
 

"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.

David.

Low capacitance audio coax
 

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

David.