Interconnect length
 

In article , Don Pearce
wrote:
On Tue, 18 Jul 2006 10:03:54 +0100, Jim Lesurf
wrote:


In theory this *always* happens. :-)


In theory, theory and practice are the same. In practice, they aren't.

Actually I used to think this, then I found out that I just hadn't got
sufficient theory yet.

You then discovered that theory generally does not agree with theory,
either... :-)

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

Interconnect length
 

Jim Lesurf wrote:

The rounding is in the form of an exponential. So if the change in
voltage from top to bottom of the square wave is V then the shape of
the rounded edge is

V*(1 - Exp(- t/Tau))

where t is the time *measured from the instant when the transition
starts*.

Hence at the time when t = Tau after the transition start, the change
will be
(1 - Exp(-1)) i.e. (1- 1/e) ...or as engineers say, "about two
thirds of the way." ;-

eyes calculator...and decides not to


So, set up a square wave and adjust its size and the scope display so
that the peak-top-peak size is, say, three vertical divisions on the
scope graticule. Then alter them time-base and read off using the
time scale how long it takes for the rounded edge to get two thirds
of the way from top to bottom of the square wave. That tells you Tau.
Knowing the C you used, you can then work out R. :-)

Which I did with the function generator last night, and was out by a factor
of only 1000....

Just measured the preamp. I had the scope set to fit the square wave to a
vertical 6cm, and looked to get as short a timebase as I could for a given
frequency (did 1, 2, 4, 10, 15, and 20KHz). I left everything set this way
and only adjusted timebase and X position for taking measurements.

At 1 and 2KHz, I got 10us, and 8us from 4KHz to 20KHz. I'll go with 8us. I
also measured the cap and got 103.4nF. So, 8000 / 103.4 = 77.37 ohms o/p Z
of a Cambridge C75 preamp. There ya go. :-)


In practice, you need to check that the rounding is negligable when
you *haven't* shunted the output with a capacitance, otherwise the C
value you have used won't be the only significant contribution.

Yup, did that with both the generator itself and the preamp. The generator
was near perfect, while the preamp showed a little overshoot (very little -
one thin little spike less than 10% of the amplitude, and maybe a couple of
tiny blips following it).


You
also need to ensure the square wave is low enough in frequency that
it still shows clear 'flat bits' at the top and bottom after the
rounding. Otherwise the display isn't showing the full transition
amplitude and you may not be measuring 23/rds of the right amount.

No problem there for up to 10KHz, lost some of the waveform at 15 and 20,
but I didn't twiddle anything - just moved the wave x-wise so it crossed at
the 2/3rds point, and then read back to the sharp angle to measure.


The above should be OK for most pre-amps and signal amps. I *don't*
recommend it for power amps in general, though, for various reasons!

Sounds like a recipe for caps that go bang in the night... :-)


Now that I have the magic number for my preamp, what sort of cable
capacitance can I get away with over a 10m run? My preferred approach is to
use balanced cable wired unbalanced (perhaps with the two signal conductors
commonned). If noise turns out to be a problem, I can convert to a balanced
set up without having 20m of useless interconnect left over (or have to buy
another 20m of coax to change to balanced - and have twice as much cable
snaking around).

So far, my searches for such a cable have left me with higher capacitance
than normal coax (found that RG59 and RG179 were in the 60-70pF/m range).
The cable I'd like is some VanDamme stuff which is nice and flexy, kink
resistant, and comes in pretty colours (the blue is nice). The datasheet on
RS says that the signal-to-screen capacitance is about 150pF, and
signal-to-signal about 100pF. Any good, or do I have to go lower?


--
Wally
www.wally.myby.co.uk

Interconnect length
 

Jim Lesurf wrote:
In article , Rob
wrote:
Jim Lesurf wrote:


FWIW I would not particularly recommend the sparse-braid for such
lengths as it is easy enough these days to get more complete braid,
etc. I just tried it, and then found it worked in my systems as well
as anything else I have tried out since.
Could you recommend a suitable easily available cable (Maplins etc), 5m
phono-phono line level?

No particular cable to be honest. I'd tend to buy from Maplin, choose
cables with capacitances below 100pF/m, and with diameters that fit the
phono plugs I prefer. So far as I can tell, they all work much the same.
Sometimes I buy the 'CT100' types, sometimes types officially called
'audio' cables. The main advantage of the audio ones is that the come in
stereo pairs already bonded together. With the UHF cables I bind them
together with masking tape at circa 10cm intervals to make a stereo pair.
This seems to survive happily for decades. Looks quite pretty and 'hand
made'. :-)

Slainte,

Jim


Cheers Jim - do you mean this:

http://www.alertelectrical.com/produ...42&prodID=3717

I don't have any real problem with buying it, but it does seem rather
chunky/inflexible?!

Rob

Interconnect length
 

In article , Wally
wrote:
Jim Lesurf wrote:


Just measured the preamp. I had the scope set to fit the square wave to
a vertical 6cm, and looked to get as short a timebase as I could for a
given frequency (did 1, 2, 4, 10, 15, and 20KHz). I left everything set
this way and only adjusted timebase and X position for taking
measurements.

At 1 and 2KHz, I got 10us, and 8us from 4KHz to 20KHz. I'll go with 8us.
I also measured the cap and got 103.4nF. So, 8000 / 103.4 = 77.37 ohms
o/p Z of a Cambridge C75 preamp. There ya go. :-)

OK. Sounds like a reasonably low value.

In practice, you need to check that the rounding is negligable when
you *haven't* shunted the output with a capacitance, otherwise the C
value you have used won't be the only significant contribution.

Yup, did that with both the generator itself and the preamp. The
generator was near perfect, while the preamp showed a little overshoot
(very little - one thin little spike less than 10% of the amplitude, and
maybe a couple of tiny blips following it).

That implies that the impedance isn't simply a resistance, but probably
close enough to being one so you can forget the reactive parts.

[snip]

Now that I have the magic number for my preamp, what sort of cable
capacitance can I get away with over a 10m run? My preferred approach is
to use balanced cable wired unbalanced (perhaps with the two signal
conductors commonned).

Not sure about the balanced part as I've never bothered with that or seen
the need for it in a domestic system.

So far, my searches for such a cable have left me with higher
capacitance than normal coax (found that RG59 and RG179 were in the
60-70pF/m range). The cable I'd like is some VanDamme stuff which is
nice and flexy, kink resistant, and comes in pretty colours (the blue is
nice). The datasheet on RS says that the signal-to-screen capacitance is
about 150pF, and signal-to-signal about 100pF. Any good, or do I have to
go lower?

The starting point is to decide what level of HF 'roll off' you are happy
with. The output resistance and the cable capacitance act like an RC 1st
order low-pass filter. You can find the equations, etc, for that on my
website. Indeed, IIRC there is a 'Java' model you can use as well.

Alternatively, having decided how my 'roll off' you want to accept at 20kHz
you can simply try difference capacitors on the pre-amp output and find
what sort of value produces this much reduction at 20kHz. This then tells
you the total cable capacitance you want to regard as the upper limit.

The problem with some of the nicer looking and more flexible cables does
tend to be a higher capacitance/meter. Low capacitance cables tend to have
a large diameter. (They can be small, but this implies a tiny inner
conductor which can bring its own snags.)

For the sake of example, lets assume 100pF/m for 10 meters. This is 1000pF
as a capacitance. Assuming I have poked the right buttons on my guess-box
this comes out to a Tau of 77 nanoseconds. i.e a -3dB point at just over
2MHz. [1]
For obvious reasons, I doubt that would be a problem. :-)

Hence any of the cables you mention should have a low enough capacitance
not to be a worry for use with your amp.

Slainte,

Jim

[1] The -3dB point will be at a frequency of 1/(2*PI*Tau)

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

Interconnect length
 

In article , Rob
wrote:
Jim Lesurf wrote:

No particular cable to be honest. I'd tend to buy from Maplin, choose
cables with capacitances below 100pF/m, and with diameters that fit
the phono plugs I prefer. So far as I can tell, they all work much the
same. Sometimes I buy the 'CT100' types, sometimes types officially
called 'audio' cables.

Cheers Jim - do you mean this:

http://www.alertelectrical.com/produ...42&prodID=3717

The above looks similar to what I tend to buy, although the page I got from
the above didn't specify the o/d or the capacitance/length.

I can't recall the type number of the 'original' drum I had as that was 20+
years ago. However as with the 'CT100' types it has a solid inner conductor
and a relatively large o/d. An outer diameter of 7-8mm seems the norm. The
Phonos I've preferred from Maplin (the 'gold' ones with a screw-on collet
for the outer and a solder bucket for the inner) fit nicely.

These days the 'CT100' type I'd buy does have the braid-and-foil outer like
the above, but the old type simply had a sparse braid with no foil.

In each case the insulator is either foam or web to minimise the
capacitance (and dielectric losses).

I don't have any real problem with buying it, but it does seem rather
chunky/inflexible?!

Yes. :-) One of the snags of these cables is that they aren't very
'floppy' or flexible. They *do* bend, but tend to then have a fixed kink.
This would ruin their performance at RF, but doesn't matter much at audio
frequencies. Think of them as a form of 'semi-rigid coax'. ;-

The large diameter, etc, can look quite imposing, though, so if someone
wants impressive looking cables these are cheap way to get them. Binding
pairs with tape also can give a 'bespoke' look... None of which affects
the sound, though. :-)

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

Interconnect length
 

Jim Lesurf wrote:
In article , Rob
wrote:
Jim Lesurf wrote:

No particular cable to be honest. I'd tend to buy from Maplin, choose
cables with capacitances below 100pF/m, and with diameters that fit
the phono plugs I prefer. So far as I can tell, they all work much the
same. Sometimes I buy the 'CT100' types, sometimes types officially
called 'audio' cables.

Cheers Jim - do you mean this:

http://www.alertelectrical.com/produ...42&prodID=3717

The above looks similar to what I tend to buy, although the page I got from
the above didn't specify the o/d or the capacitance/length.

I can't recall the type number of the 'original' drum I had as that was 20+
years ago. However as with the 'CT100' types it has a solid inner conductor
and a relatively large o/d. An outer diameter of 7-8mm seems the norm. The
Phonos I've preferred from Maplin (the 'gold' ones with a screw-on collet
for the outer and a solder bucket for the inner) fit nicely.

These days the 'CT100' type I'd buy does have the braid-and-foil outer like
the above, but the old type simply had a sparse braid with no foil.

In each case the insulator is either foam or web to minimise the
capacitance (and dielectric losses).

I don't have any real problem with buying it, but it does seem rather
chunky/inflexible?!

Yes. :-) One of the snags of these cables is that they aren't very
'floppy' or flexible. They *do* bend, but tend to then have a fixed kink.
This would ruin their performance at RF, but doesn't matter much at audio
frequencies. Think of them as a form of 'semi-rigid coax'. ;-

The large diameter, etc, can look quite imposing, though, so if someone
wants impressive looking cables these are cheap way to get them. Binding
pairs with tape also can give a 'bespoke' look... None of which affects
the sound, though. :-)

Slainte,

Jim


That's great, thanks. Last time I made up a cable it was using some twin
core shielded 'audio' cable. It was only a cheapie Maplins job, but
seemed to work OK. The point of the twin core, IIRC, was to connect the
source to earth, and leave the component (destination) open, with the
braid/shield soldered to earth both ends (I think!). The remaining core
was connected to the centre pin of source/destination.

The issue (I'm not sure if it's a problem) is there's only one core, so
that arrangement can't be used - if it matters. It certainly helped
solve a hum problem at the time.

With all of this I'm not sure I could hear any 'cable difference' in the
unlikely event of it being there, my hearing isn't up to it for one
thing. It's just nice to know that it's properly implemented, and given
that, I'm happy.

Incidentally, plug (ahem) for Neutrik phono plugs - very well built,
nice fit to sockets, easy to grip and a doddle to solder.

Rob