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Old 2nd January 2003, 07:29 PM   #11
dhaen is offline dhaen  Europe
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Quote:
600 ohms impedance came from the time when we wanted to get maximium power out of a system which accurs when the load impedance is the same as the source impedance.
IMO That was not the main reason.
The main reason is that if the cable has a characteristic imedance of 600 ohms, then there is no loss as a result of the cable capacitance or inductance. In fact you can represent the cable as an infinite series of LC circuits. (Check Transmission line theory). Of course after 10Km there are losses, but these are to do with dielectric, crosstalk, and discontinuity mismatch.

For a matched system to work all three:
The source,
The cable,
The load.
Must be of the same impedance.

But it makes no sense at audio frequencies.

Just adding resistors across the conductors is nonsence.
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Old 3rd January 2003, 05:08 AM   #12
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Quote:
Originally posted by dhaen
The main reason is that if the cable has a characteristic imedance of 600 ohms,...
You know, maximum impedance is in vaccuum (as isolation) and is 377 (120*pi) ohms! There you have the theoretical limit.
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Old 3rd January 2003, 05:10 AM   #13
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Quote:
Originally posted by dhaen
But it makes no sense at audio frequencies.
I would say very, very, very little sense. Not totally nothing.
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Old 3rd January 2003, 07:30 AM   #14
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Default Re: terminated cables

Quote:
Originally posted by Fred Dieckmann
The point of terminating cables to thier characteristics impedance is for stability of the circuit driving the cable and for RF rejection.
Er... pardon? I thought that the use of cables having a given characteristic impedance was so that the source 'saw' the same impedance at the input of the cable equal to the load...... The point being that at RF, the maximum power is transferred when the source impedance equals the load impedance.

Quote:
Read the ap notes or data sheet for just about any high speed op amp you will see a section on driving capacitive loads. (such as an unterminated cable for example) I use a RC filter with the cables characteristic impedance as the resitive element and a capacitor of a few hundred picofarads. This terminates the cable
er... how does it terminate the cable? In order for the cable to be terminated, you need the characteristic impedance at both ends, not just at one end. After all, the definition of characteristic impedance is "the impedance seen looking into a cable when the cable is terminated in that impedance".

Quote:
and filters the amp at RF frequencies but is still easy the drive at audio frequencies.
Accepted... to some extent. The resistor's being the same value as the cable's impedance seems to me irrelevant though. However, at audio frequencies a piece of coaxial cable will be primarily capacitive. In many systems it is, in fact, modelled as such (a capacitor).

Quote:
A build out resistor at the driven end of the cable is a good idea also.
Why? And how do you define "build out resistor"? To me a build out resistor can either be in series with a circuit or in parallel. Which? And why? And please don't say "It sounds better". Come up with a valid reason that can be checked.

I had a look at your references. The first two talked about making high frequency op amps stable and matching (not audio op amps) and the third just talked about making op amps stable. Hmmmmm.

Quote:
In a world of cell phones and microprocessors you had better treat every circuit as a RF circuit. RF will find its way into your audio circuits.
Here we do agree.

Quote:
Shame on you Peranders for talking about this in reference to 20 KHz.
Why? I think he's right. And 20kHz is a good figure to use as it has a short wavelength for audio. Any lower frequency and the wavelength is even longer.

I think also you'd better go and look at the specs of many cables. The characteristic impedance is given for a certain frequency band. For example, I've worked with certain cables that have a characteristic impedance of 50 ohms. Fair enough, but its only above 1MHz. Below that the impedance drops away dramatically. Some cables are specified above 10kHz, others above 10MHz.

Compliments of the season to you and your's.

Cheers, Keith
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Old 3rd January 2003, 07:46 AM   #15
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Default Re: Re: terminated cables

Quote:
Originally posted by Keithj
I think also you'd better go and look at the specs of many cables. The characteristic impedance is given for a certain frequency band. For example, I've worked with certain cables that have a characteristic impedance of 50 ohms. Fair enough, but its only above 1MHz. Below that the impedance drops away dramatically. Some cables are specified above 10kHz, others above 10MHz.
It's a known fact that the speed (characteristic impedance) of a cable varies with frequency and this makes nice looking pulses to get smeared out. This can also be troublesome for video signals.
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Old 3rd January 2003, 10:28 AM   #16
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Default It's a known fact that the speed (characteristic impedance) of a cable varies with fr

Not by much........ Dielectric losses and skin effect at very high frequencies (10s to 100s of mHz) but pretty irrelevent for most video, RF, and digital signals. Build out resistor reffered to is series resistor at source. Cables and PCB traces are often terminated at one end only. This particularly true for Digital design. Cables need to be terminated at RF frequencies as line lengths for audio freqencies are too short to be considered as transmission lines.

http://www.chipcenter.com/oltu/netsim/si/termination/
http://www.sigcon.com/Pubs/edn/TransmissionLine.htm
http://www.sigcon.com/Pubs/edn/LossyLine.htm
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Old 3rd January 2003, 10:32 AM   #17
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The lack of a need of a terminating resistor must be a great relief to those on the forum that argue that the colour of a connecting wire insulation affects the sound. I mean to say, those different coloured stripes on the terminating resistor are going to play havoc with the musicality of the wire, are they not? I suppose we could get around it by using a different numbering system than base 10, therefore using a more coherent set of colours for the resistor values...
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Old 4th January 2003, 09:23 AM   #18
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The RCA handbook has many amplifier designs with performance specs.

The average input impedance of these designs is about 22K.

Signal to noise ratio measurements taken with a ten foot cable terminated at the amplifier end by 2K2 show a 10dB improvement.

I can hear 10dB.

I would not go less than 2K2 as the dynamics of most pre-amps will suffer. Just becuse your gear is rated to drive 600R doesn't mean it will sound the best doing so.
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