one would have to introduce a true balanced phono circuit internally.
While the idea of duplicated, bridged, or fully balanced (whichever term you like) is sexy, the idea that this is the only advantage is really not correct. Professional gear has been using balanced signals long before audiophiles got it into their heads to duplicate circuits. Having a differential input which is not referenced to ground has a lot of benefits for noise. And while I don't think we need full-sized XLR plugs, there are mini-XLR plugs which would work a treat for this. I've often wondered this myself. |
Phono cartridges are floating sources rather than balanced, and unless there is a low-impedance connection between phono amplifier ground and both sides of the cartridge signal coil, and/or the common mode rejection of the phono stage input stage is extremely high across a wide bandwidth, there most likely will be substantial pickup of electrical noise from the environment (i.e., connecting the phono cartridge in balanced mode will probably yield worse noise performance than an unbalanced connection).
An input transformer magnetically isolates common mode noise, and is a good solution to building a quiet balanced phono stage.
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It's common mode. By definition the currents must be matched. If they are not matched, they are not common mode, they are differential and then you cannot tell them apart from the signal. The coil in the cartridge also completes the loop. It is by analogy the isolated end of the transformer. If it is a powered microphone that has an active ground connection, power and signal, then yes, the benefit is there. The benefit is mainly in better shielding for a non powered microphone.
That
is true, as long as currents flow in the loop only. If one of the
wires go to input while the other goes to GND (unbalanced input),
currents in both wires are not even anymore. Perhaps, that's why
floating (for audio frequencies) balanced output, like transformer is
not enough and has to be connected to balanced input (not single
ended). Microphones also benefit from balanced input.
The differential input of an op-amp does not have a gain of 1. It's a current source into a differential pair, that will share current based on the transistors being perfectly matched, and it will convert that current into a voltage by the load resistors ... who's matching also impacts the common mode rejection of this stage.
As for
Instrumentation amps, yes they have laser trimmed resistors, but front
(two amp) differential section has always gain of 1 for common mode
signal independently of resistor tolerance.
That's not how it works. I think you are confusing something. For a 40db CMRR, you must have gain matching of gain matching between the two channels to +/- 0.5% (1% total), and you must have perfect transistor matching (which they never are).
If you set gain of 100 for
this section you get automatically 40dB CMRR independent of resistors
tolerance. It happens because each of two amps is referenced to input
of another (instead of the GND).
Typically you use op-amps with laser trimmed resistors and/or you calibrate. You of course need tight temperature tolerance matching which is easier at the IC level, since with low power draw, temp will be very consistent between the temperature co-efficient.
That
is the only way I can see, for truly balanced amps, to function without
converting common mode noise to normal mode signal. Otherwise matching
resistors and keeping them matched to some sensible number (like
60dB=0.1%) is not practical, while adding RIAA frequency correction and
matching capacitors to 0.1% is next to impossible.
Which would probably be pointless as you pointed out, you are not going to get capacitors to that tolerance. |
I perceive this would only be of benefit at frequencies outside the audio range, as within the audio range, effectively the cartridge completes the loop and common mode noise would be rejected. Do you have examples of commercial products that use this technique and are they measureably quieter?
An input transformer magnetically isolates common mode noise, and is a good solution to building a quiet balanced phono stage.
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Nice discussion. Ralph needs to chime in.
JCarr, You wrote, "Phono cartridges are floating sources rather than balanced, and unless
there is a low-impedance connection between phono amplifier ground and
both sides of the cartridge signal coil, and/or the common mode
rejection of the phono stage input stage is extremely high across a wide
bandwidth, there most likely will be substantial pickup of electrical
noise from the environment (i.e., connecting the phono cartridge in
balanced mode will probably yield worse noise performance than an
unbalanced connection)."
In this paragraph, you used the terms "most likely" and "probably" at very key points in your statement. Have you actually done the experiment and made measurements? Also, what would you say happens when the balanced phono circuit is floating with respect to ground, just like the cartridge itself?
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