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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 16th November 2019, 11:06 PM   #1731
syn08 is offline syn08  Canada
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Quote:
Originally Posted by stinius View Post
My question was:

is it right to compensate the peaking caused by the C seen from the low Z CFA feedback loop (at the inverting input) by adding an input L?
My answer is:

I don't know. I did not specifically compensate for the peaking caused by the C seen from the low Z CFA feedback loop (at the inverting input) by adding an input L.
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Old 16th November 2019, 11:19 PM   #1732
syn08 is offline syn08  Canada
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Originally Posted by Mark Tillotson View Post
From a consideration of skin-depth a few mm of steel is better than several cm of aluminium for screening, particularly if mains frequencies are the issue.
Set aside the machining issues, I wish it were that easy/simple; regular carbon steel has a permeability of only 100 and OTOH the resistivity of carbon steel is 5x the resistivity of aluminum. Since the skin depth varies with square root of the ratio of these, the improvement in skin depth of steel over aluminum is only 4x. That is, if 2cm of aluminum are required, then the carbon steel equivalent will need 5mm.

Pure iron is so much better, but impossible to source. The ideal magnetic shielding would be a double box of mumetal, but those are expensive hence reserved for professional use. Plus the required annealing, not something one can do at home.
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Last edited by syn08; 16th November 2019 at 11:32 PM.
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Old 17th November 2019, 02:07 AM   #1733
Mark Tillotson is offline Mark Tillotson
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Originally Posted by syn08 View Post
Set aside the machining issues, I wish it were that easy/simple; regular carbon steel has a permeability of only 100 and OTOH the resistivity of carbon steel is 5x the resistivity of aluminum. Since the skin depth varies with square root of the ratio of these, the improvement in skin depth of steel over aluminum is only 4x. That is, if 2cm of aluminum are required, then the carbon steel equivalent will need 5mm.

Pure iron is so much better, but impossible to source. The ideal magnetic shielding would be a double box of mumetal, but those are expensive hence reserved for professional use. Plus the required annealing, not something one can do at home.

Ah, yes I think I was assuming generic steel was as good as iron or laminations. Still its better than aluminium or copper for screening at audio, but at RF you also have to think of lossiness which is why its seldom used for that (skin depths are tiny anyway then).
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Old 17th November 2019, 08:30 PM   #1734
syn08 is offline syn08  Canada
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The head amp in post #375 very low frequency noise (0.2Hz-200Hz), now measured with both the head amp and the measurement amp fed from the same lead acid batteries. Overall gain is 86.8dB.

The 60Hz mains frequency component is now 5nV equivalent at the input, a sharp increase from the well shielded measurement amp (1.5nV, see the previous post). That's to be expected, since the head amp is virtually unshielded at these low frequencies. Third harmonic is also clearly visible (it is totally absent in the measurement amp, so this is coming purely from the head amp). Only 50 averages were taken, since it takes about 5 minutes per sweep (due to the time required for the A/D filter to settle, at these low frequencies).

Flat noise (input shorted) is the same 0.26-0.28nV/rtHz as measured before, but now the 1/f noise region is clearly visible. It is very encouraging that the very low frequency noise is almost purely 1/f (slope is about 12dB/decade, compared to the theoretical 10dB/decade), this is a certain plus over low noise JFETs. Also, the noise corner frequency is 2.5-3Hz, again an excellent value, much superior to JFETs (100-150Hz). The left side slight drop is due to reaching the measurement amp LF bandwidth limit (0.3Hz, as mentioned above).

Now, if we could only combine the LF noise performance of bipolars with the low input current noise of JFETs... not going to happen, so it will remain a pick your poison game.
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Last edited by syn08; 17th November 2019 at 08:39 PM.
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Old 17th November 2019, 09:43 PM   #1735
Mark Tillotson is offline Mark Tillotson
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You'll be picking up gravitational waves with that setup soon!
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Old 17th November 2019, 09:52 PM   #1736
syn08 is offline syn08  Canada
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Originally Posted by Mark Tillotson View Post
You'll be picking up gravitational waves with that setup soon!
A long way to the ~400dB gain parametric amplifier that is LIGO .
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Old 17th November 2019, 10:25 PM   #1737
stocktrader200 is offline stocktrader200  Canada
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4 NiMH AA batteries may be better then the lead Acid. 2.2AH 2.8v min and many charge more cycles. They have to be the Precharged low leakage types.
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Old 17th November 2019, 10:31 PM   #1738
Hans Polak is offline Hans Polak  Netherlands
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Quote:
Originally Posted by syn08 View Post
The head amp in post #375 very low frequency noise (0.2Hz-200Hz), now measured with both the head amp and the measurement amp fed from the same lead acid batteries. Overall gain is 86.8dB.

The 60Hz mains frequency component is now 5nV equivalent at the input, a sharp increase from the well shielded measurement amp (1.5nV, see the previous post). That's to be expected, since the head amp is virtually unshielded at these low frequencies. Third harmonic is also clearly visible (it is totally absent in the measurement amp, so this is coming purely from the head amp). Only 50 averages were taken, since it takes about 5 minutes per sweep (due to the time required for the A/D filter to settle, at these low frequencies).

Flat noise (input shorted) is the same 0.26-0.28nV/rtHz as measured before, but now the 1/f noise region is clearly visible. It is very encouraging that the very low frequency noise is almost purely 1/f (slope is about 12dB/decade, compared to the theoretical 10dB/decade), this is a certain plus over low noise JFETs. Also, the noise corner frequency is 2.5-3Hz, again an excellent value, much superior to JFETs (100-150Hz). The left side slight drop is due to reaching the measurement amp LF bandwidth limit (0.3Hz, as mentioned above).

Now, if we could only combine the LF noise performance of bipolars with the low input current noise of JFETs... not going to happen, so it will remain a pick your poison game.
I’m really surprised about the rather huge differences in mains radiation between our locations. I have no idea which of the two can be called exceptional, but I think that I must be quite happy getting a flat noise spectrum with an unshielded Head Amp producing just 0.32nV/rtHz.

Hans
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Old 18th November 2019, 09:52 PM   #1739
sgrossklass is offline sgrossklass  Germany
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Originally Posted by stocktrader200 View Post
4 NiMH AA batteries may be better then the lead Acid. 2.2AH 2.8v min and many charge more cycles. They have to be the Precharged low leakage types.
If these results from 15 years ago are any indication, they may not only be quieter, but are in fact quite likely to. (Still not as quiet as a very good regulator though, by the looks of it.)

You mean LSD NiMH cells (Low self discharge - they're not that trippy), as pioneered by Sanyo with their Eneloops in the mid-2000s. Makes sense that those with the lowest self-discharging would be the quietest.

This makes me wonder: Can you characterize cells for self-discharge by their 1/f noise, e.g. to weed out any bad apples with internal shorts? Almost seems like it. Hmm. I wonder whether a decent balanced mic preamp will do, 'cause I definitely have some of those...
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