Audio, hi-fi and car audio - Low capacitance audio coax

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Low capacitance audio coax

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

Low capacitance audio coax

In article ,
Jim Lesurf wrote:
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! :-)

FM radio coax that you could get easily at one time was good for this -
used it with my old Quad II setup which had high output impedances to
things like a tape recorder. It was smaller than YHF stuff so just about
fitted a phono plug.

--
*Nostalgia isn't what is used to be.

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

Low capacitance audio coax

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.

David.

--
*And don't start a sentence with a conjunction *

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

Low capacitance audio coax

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.

--
Eiron.

Low capacitance audio coax

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
--
Ian

Low capacitance audio coax

"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.

--
Eiron.

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.
S.

Low capacitance audio coax

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

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.

It is now.

What
make would I be looking at for this?

Pretty much anything German made in the 50s or 60s: Grundig, Telefunken,
etc.

I had a Quad 3 series that used DIN
connectors throughout - but that was all low(ish) out high(ish) in.

Yes, but that sort of stuff used the DIN connector without using the DIN
standard.

The standard originated as a way of feeding a tape recorder from a valve
radio. In order to provide a recordable output that wasn't going to be
effected by the volume control the tape-recorder output was derived directly
from the AM detector/FM discrminator, but to minimise the loading on that it
was fed via a resistor of 100k or so. Then the "diode" input of the tape
recorder had an input impedance of around 2k. Since the tape recorders
always had adequate gain in the recording amplifier to work from a
microphone the signal loss from this form of connection wasn't a problem.

This standard dragged on in a half-hearted sort of way into the '70s when
domestic tape decks (by then often Japanese in origin) had both phono and
DIN connectors. Compared with the phono sockets the DIN had a higher output
impedance, a lower input impedance and greater input gain.

David.

Low capacitance audio coax

In article , Dave Plowman (News)
wrote:

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.

Yes. That's what has been generally adopted for tasks like domestic audio
where the idea is that it is the voltage pattern at the input terminals of
the 'load' (destination) that defined the waveform.

The idea behind the DIN 'electrical' standard was the obverse of the above.
The approach was to define the signal waveform in terms of the *current*
pattern entering the destination. Thus it reversed the approach people are
familiar with and had low input impedances combined with high source
impedances.

For short cables in both cases the cable capacitance combines with the
source and load impedances in parallel. So the outcome is similar in terms
of the primary RC low-pass effect. But in DIN 'electrical' terms you can
think of this as being a consequence of the low load resistance meaning you
don't need to significantly change the voltage on the cable. If effect, the
load resistance is so small that you aren't having to change the charge on
the cable capacitance very much so most of the current the source injects
ends up going though the load. :-)

However that meant that all you'd really done was turn around the
requirement so you now neede a low load resistance rather than a low
source impedance if you wanted to maximise the bandwidth provided when
cable capacitance was taken into effect.

So in the end if anyone had been really worried by that it would have made
more sense to use a system that was closer to being matched rather than
idealise one of the mismatch extremes like voltage or current transfer!
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!...

I had a Quad 3 series that used DIN connectors throughout - but that was
all low(ish) out high(ish) in.

Many people (including Armstrong) adopted the DIN plugs because they were
compact for stereo and we assumed they'd become the standard. But despite
adopting the physical plugs and sockets, stayed with the tradition of using
voltage transfer pattern. So used low source impedance and high load
impedances for optimum voltage transfer.

That said, the Armstrong 600s did have (without mentioning it in the
handbooks) a second 'tape out' with a high impedance to drive any recorders
made to the DIN electrical standard. Maybe Quad had a keymatic board for that,
but I can't recall off-hand. The usual trick was just to shove in large
series resistors at source to get to the defined current level.

Quite why DIN decided to adopt that approach I can't recall. Whatever their
theory, people ended up ignoring them. :-)

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

Low capacitance audio coax

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

ian