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.
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?
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".
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".
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Yes, compromises/tradeoffs everywhere in electronics, for added interest. We have to herd electrons to our will......
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.
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.TNT said:
Only when they don't have a low impedance source attached.scottjoplin said:
No.Galu said:
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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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.
What has Oliver Heaviside got to do with analogue audio interconnects? His name gets mentioned here from time to time, usually by people who have never read and don't understand what he wrote but they may have picked up confusion from someone else and simply repeated it.
To save other people from having to do some reading, basically what Heaviside did was to develop and use a novel way to solve the differential equations for a transmission line. His solution can be found by more conventional means. He predicted that a long telephone line would benefit from added inductance. This was counter-intuitive and so was rejected by the head of the British GPO. It was accepted by the folk in America (maybe their engineers were better trained in maths and physics?) so they had good quality long-distance lines before we did. Nothing to do with audio interconnects, because they are far too short to exhibit the dispersion problem which Heaviside solved. Nothing to do with nonlinear distortion.
To save other people from having to do some reading, basically what Heaviside did was to develop and use a novel way to solve the differential equations for a transmission line. His solution can be found by more conventional means. He predicted that a long telephone line would benefit from added inductance. This was counter-intuitive and so was rejected by the head of the British GPO. It was accepted by the folk in America (maybe their engineers were better trained in maths and physics?) so they had good quality long-distance lines before we did. Nothing to do with audio interconnects, because they are far too short to exhibit the dispersion problem which Heaviside solved. Nothing to do with nonlinear distortion.
Thanks sgrossklass and DF96 for your comments re RCA shorting plugs. I also figured that, since unused inputs are not in circuit, shorting them should bring no benefit. Counterintuitively, one of my items of equipment came supplied with them!
I'd like to reinforce the fact that transmission line theory is not applicable to audio frequency interconnects like the ones between DAC and amplifier mentioned by the original poster. Transmission line theory applies in the radio frequency domain.
I'd like to reinforce the fact that transmission line theory is not applicable to audio frequency interconnects like the ones between DAC and amplifier mentioned by the original poster. Transmission line theory applies in the radio frequency domain.
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