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Old 10th October 2017, 04:26 PM   #21
Mark Johnson is offline Mark Johnson  United States
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Am I correct, that the positive shunt regulator's output voltage is (VBE_Q7 * R20 / (0.5*R22)) = +14 volts?

And if Q7's VBE varies by -2.2mV per degree C, then the regulated output voltage varies by -44mV per degree C. Is that correct? So a temperature change of 10 degrees C causes a supply voltage change of 0.44 volts.

I would imagine it's a good idea to thermally couple Q7 and Q8 so their junction temperatures track each other. This would help ensure that the regulated V+ and regulated V- tracked each other.
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Old 10th October 2017, 04:27 PM   #22
kevinkr is offline kevinkr  United States
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Scott's comment about surface noise is the key, if you are below it no further practical improvement is necessary. (And still I try for that black background)

You can make this argument for all sorts of ills including hum/buzz below the medium's noise floor, but then you have to deal with audible noises when not playing records which always leaves you with the impression that things are broken.

How many of you think about the equivalent noise resistance of the cartridges you are using?

The 5534 is probably still a good device to consider where voltage noise matters most.

Bill has a great sense of humor, and a turn of phrase that is always entertaining. And his comment is oh, so true.
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Old 10th October 2017, 09:18 PM   #23
MarcelvdG is online now MarcelvdG  Netherlands
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I have often wondered whether the NE5534A was designed with moving magnet cartridges in mind; with 3.5 nV/sqrt(Hz) and 0.4 pA/sqrt(Hz) it seems to be optimized for source impedances around 3.5 nV/sqrt(Hz)/0.4 pA/sqrt(Hz) = 8.75 kohm, which is of the same order of magnitude as my 12 kohm impedance estimate.

Regarding datasheet values versus real-life values for current noise: the problem is typically the base current compensation circuit that some bipolar op-amps (but not the NE5534) have. It usually contains a scaled-down version of one of the input transistors biased at a scaled-down collector bias current. Its base current is then amplified by a scaled PNP current mirror and injected into the positive and negative inputs.

This results in two correlated noise currents being injected into the + and - inputs of the op-amp. When the scaling factor / PNP current mirror gain is large, these noise currents are much greater than the normal base shot noise of the input stage.

As the currents come from the same source and have a large correlation, they drop out of the equation when the + and - inputs are driven from exactly equal impedances - which never happens in any real-life application, but which can easily be done in a test set-up to make datasheet values look better. A typical example is the LT1028: 1 pA/sqrt(Hz) with equal impedances and 3.25 pA/sqrt(Hz) with unequal impedances.
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Old 10th October 2017, 09:35 PM   #24
MarcelvdG is online now MarcelvdG  Netherlands
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Quote:
Originally Posted by kevinkr View Post
Scott's comment about surface noise is the key, if you are below it no further practical improvement is necessary. (And still I try for that black background)
I like the noise of my amplifier to be negligible even when there is no record playing, but maybe that is a bit over the top (though not as far over the top as building an amplifier with a distortion in the ppm range to drive a loudspeaker with a distortion of the order of several percents).
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Old 10th October 2017, 11:02 PM   #25
scott wurcer is offline scott wurcer  United States
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Quote:
Originally Posted by MarcelvdG View Post
A typical example is the LT1028: 1 pA/sqrt(Hz) with equal impedances and 3.25 pA/sqrt(Hz) with unequal impedances.
I would like to see an application that really benefits from this. I'm sure you can find one looking hard enough. Besides a MM cart is a complex impedance not a resistance and a balancing act would be a real kludge.

BTW I was unable to verify the claimed degree of cancellation when I tried years ago, and I have never seen an argument in print that supports the necessary correlation coefficients (assuming the circuit presented in the JSSC).
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Old 11th October 2017, 12:28 AM   #26
Mark Johnson is offline Mark Johnson  United States
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Quote:
Originally Posted by MarcelvdG View Post
I have often wondered whether the NE5534A was designed with moving magnet cartridges in mind
I doubt it since the 1/f region of the noise spectrum is set by manufacturing process rather than design. They did a good job of working with what they had @ Signetics Grenoble, when they designed the TDA1034. Later it was renamed NE5534.
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Old 11th October 2017, 01:24 AM   #27
rjm is offline rjm  Japan
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Quote:
Originally Posted by Mark Johnson View Post
Am I correct, that the positive shunt regulator's output voltage is (VBE_Q7 * R20 / (0.5*R22)) = +14 volts?

And if Q7's VBE varies by -2.2mV per degree C, then the regulated output voltage varies by -44mV per degree C. Is that correct? So a temperature change of 10 degrees C causes a supply voltage change of 0.44 volts.

I would imagine it's a good idea to thermally couple Q7 and Q8 so their junction temperatures track each other. This would help ensure that the regulated V+ and regulated V- tracked each other.
Yes, the regulator output is referenced to the emitter diode drop of Q7,8 multiplied by the ratio of R20 or R21 over the relevant section of the trimmer R22. The output tends to be about 12-13 V.

It would be a bad design choice if a) the currents were variable and/or sufficient to raise the temperature above ambient or b) we were concerned about the exact voltage seen by the op amps.

In practice the output voltage may vary a few hundred millivolts depending on ambient room temperature, but this does not influence the performance of the circuit. Positive negative tracking is largely obtained already since the current draw is the same, and the ambient temperature also.

(Or, short answer: there is no advantage to put Q7,8 in direct thermal contact since neither transistor gets warm during operation.)
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Last edited by rjm; 11th October 2017 at 01:31 AM.
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Old 11th October 2017, 02:25 AM   #28
anatech is offline anatech  Canada
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Hi Mark,
That was interesting history about the NE5534. Thanks.

-Chris
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Old 11th October 2017, 02:41 AM   #29
rjm is offline rjm  Japan
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The current and voltage noise balance of the NE5534 is pretty much ideal for audio. Not MM specifically, though it is okay for that, but backing any kind of filter or potentiometer you find yourself typically dealing without impedances above 1k but less than 10k. Even for the typical op amp gain stage configuration, impedances seen by the inputs are so often in that range.

No surprise to learn the designers knew what they were doing but ... yeah, the designers knew what they were doing.
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Old 11th October 2017, 11:35 PM   #30
scott wurcer is offline scott wurcer  United States
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OT

rjm, Last time I was in Kyoto the traditional arts center was on its last week. Has anything replaced it?
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