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Richard Lee's Ultra low Noise MC Head Amp
Richard Lee's Ultra low Noise MC Head Amp
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Old Today, 12:57 AM   #1641
syn08 is offline syn08  Canada
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Quote:
Originally Posted by Hans Polak View Post
Let me give it a try.
When both enclosures are connected to PE, a coupling capacitor at the far end may be used to suppress RF because the shield could act as an antenna. Although quite possible, I have never experienced any problem without this cap with a GSM or radio transmitter near the cable, so I never apply this somewhat complex cap.

Now for equipment fed from Battery or a wall wart, the interconnect shielding could very well be connected at both ends to the the enclosures, but in case of the very sensible Head Amp, I have chosen not to do, but to give it its own direct PE connection because the wall wart is producing quite some pollution that I didn’t want it to be injected in the next stage.

Without this PE connection to the Head Amp, the wall wart causes an enormous amount of very strong mains harmonics in the spectrum, so these guys are producing quite some dirt. But as shown in the above spectra, with the PE connected to the Head Amp’s analog gnd, in this very case the noise spectrum is as clean as with a battery (Power Bank)

Hans
I have to admit removing mains harmonics was always a challenging task for me. Admittedly, my lab is 6ft. away from the house 200 amp electrical panel, but I still don't understand how some people (you included) get these absolutely clean spectra? Not even a few nV which would be clearly visible in the spectra after x1000 gain?

I understand balanced connections, but your measurement preamp is single ended, how was it shielded/configured so that again not even a few nV of mains harmonics are leaking in? 10nV of 50Hz at the shorted input (-160dB) falls in a single bin and would be clearly visible against an -190dB noise floor. Mine is shielded in 12mm of aluminum and 5nV input equivalent 50Hz is about the best I can get (see #1609, 50Hz is -165dB equivalent at the input)

And I have not even looked at the PSRR yet. AD797 has an excellent 130dB at low frequencies. Subtract the input stage gain of 34dB, that leaves 96dB. Meaning that 1V power supply noise creates about 15uV of noise at the AD797 output, or about 300nV of equivalent noise at the preamp input (ignoring the other 2 gain stages PSRR). One would need less than 1mV of power supply noise (mains ripple, etc...) to be able to not see on the spectra the input equivalent noise contribution. Look with a sensitive scope at a regular power supply output and estimate the RMS noise voltage... 1mV is doable, I did it with TI RF +/- regulators, but it is far from trivial (regulator compensation, filtering, etc...).

And I am not done yet, 60dB gain @1KHz is not much for a low output MC, it is 0.2-0.5V at the RIAA preamp output, but it also means an overall 80dB gain at 20Hz. After the signal hits the 7950us pole there is still an excess gain of about 11dB gain @50Hz and 5dB @100Hz. At least the cartridge and the cartridge wiring to the TT jacks are not collecting anything?

And I'm still not done yet, but I don't want to get into balanced cables CMRR, based on the cables geometry. All I can say is that shielded twisted pairs seem to be the best, and that since contrary to what people believe it is the magnetic field that creates most problems, and not the electric field, which is attenuated by a good shielding; for regular audio shielding, the magnetic field is not attenuated at all and happily induces mains harmonics even in what appears to be a perfectly shielded cable. And unfortunately shielded twisted balanced cables are not common for audio, AFAIK. Otherwise, you'll be shocked what is the result of connecting a 1m of shielded (and shorted at the end) single ended cable at my high impedance preamp input, how do you connect your head amp to the measurement preamp for noise measurement purposes?

Figure out what this means in terms of eliminating mains harmonics contamination, my answer, until somebody enlightens me, is "I don't know how to do it".

Last edited by syn08; Today at 01:03 AM.
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Old Today, 01:22 AM   #1642
sgrossklass is offline sgrossklass  Germany
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Maybe Linear Technology appnote 159 will provide some of the desired illumination. It definitely illustrates the kind of hiccups to be encountered.

I like to think that for both electrostatic and magnetic coupling, there is a certain type of prototypical circuit that facilitates it. For the former, it is large, exposed high impedance nodes, for the latter it is large, low impedance loops. I would not be surprised if there were some sort of bizarre topological transformation from one world to the other that would show them as equivalent, but I am not a mathematician. Something like the dual transformation for filters.

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Old Today, 02:41 AM   #1643
syn08 is offline syn08  Canada
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Quote:
Originally Posted by sgrossklass View Post
Maybe Linear Technology appnote 159 will provide some of the desired illumination. It definitely illustrates the kind of hiccups to be encountered.
I know the app note; my problem is not measuring 0.3nV/rtHz (in fact I got down to 0.15nV/rtHz (when I measured my 64 parallel BF961 gain stage, results are buried somewhere on this forum), that’s easy with autocorrelation (although it takes an awful amount of time), but the mains harmonic contamination. I get an itch each time I see those crystal clear spectra after an 80dB gain, with what appears to be less than 1nV input equivalent mains harmonic components.

Per the app note, triple shielding including one layer of mu metal would certainly help; never tried that, but I will one day.

P.S. 1mV power supply harmonic noise is certainly not a problem with batteries, perhaps possible by cascading regulators (although wiring is non trivial), but with a simple wall wart (transformer, bridge, electrolytic)??? Or a switching wall wart??? Colour me blue, I am having a very hard time interpreting such a result. I am certainly missing something and I was never able to understand what.

Last edited by syn08; Today at 02:55 AM.
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Old Today, 08:59 AM   #1644
billshurv is online now billshurv  United Kingdom
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Richard Lee's Ultra low Noise MC Head Amp
I should note that at some point the cartridge itself will be picking up more mains hum than everything else, so we pretty much have a hard limit unless the whole front end is in a seperate metal box with no mains!
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Old Today, 10:06 AM   #1645
Chris Hornbeck is online now Chris Hornbeck  United States
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I wonder at what point the ground loop of the two channels' shields (if truly common on both ends) contributes.


Then, the stray capacitive coupling from mostly unshielded cartridge wiring, internal and external, to shield. Pin 1 problem, etc. Probably minor with proper floating source and proper shield termination, but you guys are talking about crazy small numbers.


For a really good Shield, I'd recommend the Smith & Wesson M&P in 9mm. About 19 ounces, so you can carry two for stereo. Arf!


All good fortune,
Chris
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Old Today, 10:09 AM   #1646
billshurv is online now billshurv  United Kingdom
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Richard Lee's Ultra low Noise MC Head Amp
Interesting point Chris, and one I have not idea about. Is it better to run 2 pair with single shield in the case? I have also assumed that a driven shield is not needed with such low impedances, although EMT used to do that.
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