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Input and output impedance
Input and output impedance
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Old 23rd January 2019, 08:45 PM   #21
scottjoplin is offline scottjoplin  Wales
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Is that a rhetorical question?
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Old 23rd January 2019, 08:52 PM   #22
rayma is online now rayma  United States
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Quote:
Originally Posted by Galu View Post
Is noise the same thing as signal distortion?
Once the noise is added, it cannot be removed without removing part of the signal.
The noise is not present in the input signal, so it is distortion, though uncorrelated.
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Old 23rd January 2019, 09:47 PM   #23
Galu is offline Galu  Scotland
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Quote:
Originally Posted by scottjoplin View Post
Is that a rhetorical question?
I was thinking of the distinction between picking up RF noise and the type of signal distortion referred to in earlier posts e.g. post #2.

So do we agree that, although being necessary for maximum voltage transfer, high Z inputs are inherently noisy and introduce distortion to the signal?
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Old 23rd January 2019, 11:01 PM   #24
steveu is offline steveu  United States
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Low output impedance maintains a flat frequency response when connected to cable capacitance and stub resonance. But many op-amps and discrete circuits are incapable of producing their rated voltage swing into their rated impedance, because they are current limited. The impedance is based on behavior with a small signal, and usually the result of feedback. But the output stage may not be able to deliver the current that the feedback calls for.
You may have noticed that power amps produce less distortion driving 16 Ohms than driving 4 Ohms. It's about the current in class AB amplifiers. The same thing happens in op-amps.
Also note that with a low-Z out, there is a wide range of acceptable load impedance including Y-cable connection to multiple loads.

I remember a tube stereo that my friend had, that had no output transformers because the speakers were hi-Z. But there was no exposed wiring because that meant there was high voltage on the speakers. This was not a "electrodynamic" speaker. Electrodynamic speakers used a electromagnet instead of a permanent magnet. It was usually the power supply filter choke and was used to generate a very strong magnet for better speaker performance. In those days 40Watts was "high power".

Last edited by steveu; 23rd January 2019 at 11:13 PM.
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Old 23rd January 2019, 11:35 PM   #25
scottjoplin is offline scottjoplin  Wales
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Quote:
Originally Posted by Galu View Post
I was thinking of the distinction between picking up RF noise and the type of signal distortion referred to in earlier posts e.g. post #2.

So do we agree that, although being necessary for maximum voltage transfer, high Z inputs are inherently noisy and introduce distortion to the signal?
Yes, compromises/tradeoffs everywhere in electronics, for added interest. We have to herd electrons to our will......
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Old 24th January 2019, 03:40 AM   #26
PRR is offline PRR  United States
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Originally Posted by edwtsui View Post
Why we should design the output impedance of the source ..as low as possible while the input impedance of amplifier should be as high as possible?
It is =equally= valid to design for high source impedance and _low_ load impedance.

The problem of capacitive roll-off is avoided if _either_ end is low-Z (at least for home and studio length lines).

There was a DIN standard, generally running 50k-100k sources and 1k-2k inputs, generally implemented as a mike-amp switched to line duty. Never wildly popular, even though the standard usually also combined our four (for a tape loop) RCA plugs into one neat DIN plug.

A real problem of hi-Z source low-Z load is that you can not Y-cable one source to multiple loads. A secondary issue was that the received levels were a few mV and it was hard to get really high S/N.

"Matched" interface is a real thing. It becomes routine in radio work where multiple wavelengths fit in a typical length of wire, and reflections become a problem. Also seen, not strictly, in analog telephone lines. It was common in The Old Days when amplifiers were brutally expensive and transformers were used to optimize system gain.

Distortion, of amplifiers and speakers, is a different issue.
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Old 24th January 2019, 11:41 AM   #27
DF96 is offline DF96  England
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Quote:
Originally Posted by TNT
Really... wouldn't you trade a little level for less distorsion?
But in the case we are discussing, less distortion and less attenuation are the same choice: low impedance source + high impedance load. You can alternatively have more distortion and more attenuation, but few would choose that.

Quote:
Originally Posted by scottjoplin
High impedance inputs are more sensitive to noise
Only when they don't have a low impedance source attached.

Quote:
Originally Posted by Galu
Is noise the same thing as signal distortion?
No.
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Old 24th January 2019, 11:44 AM   #28
DF96 is offline DF96  England
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Quote:
Originally Posted by Galu
So do we agree that, although being necessary for maximum voltage transfer, high Z inputs are inherently noisy and introduce distortion to the signal?
No. Unterminated high Z inputs may be noisy, but we are not usually interested in listening to 'no source attached'. High Z inputs will often cause less distortion.
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Old 24th January 2019, 12:41 PM   #29
Galu is offline Galu  Scotland
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Quote:
Originally Posted by DF96 View Post
Unterminated high Z inputs may be noisy, but we are not usually interested in listening to 'no source attached'.
Thanks for your clarification.

RCA shorting plugs are available for use with unused amplifier inputs in order to protect against the pick up of RF/EMI noise. Do you advocate their use?
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Old 24th January 2019, 04:32 PM   #30
sgrossklass is offline sgrossklass  Germany
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Quote:
Originally Posted by Galu View Post
RCA shorting plugs are available for use with unused amplifier inputs in order to protect against the pick up of RF/EMI noise. Do you advocate their use?
They should not normally be needed, as unused inputs tend to be removed from the signal path altogether. They can be quite useful for diagnostic purposes or when performing offset and idle current adjustments, however.
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