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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 21st July 2019, 06:36 PM   #741
Hans Polak is online now Hans Polak  Netherlands
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Originally Posted by Bonsai View Post
Hans, on the Hawking, if you run your sims but look at the collector voltage of the transistors before the DC blocking caps (with or without dampers) what are you getting? This is with the 10uF capacitors
Bonsai,

Is this what you mean, collector voltages are V(V+) and V(V-), lying exactly on top of each other.
Two Sims, one for Rs=5R gain=10 and one for Rs=1R gain=50, both with output cap 10uF, no damping.

Hans
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Old 21st July 2019, 07:13 PM   #742
Bonsai is offline Bonsai  Europe
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Are you not losing head room because the voltage on the collector before the output coupling cap is peaking at LF?

Looking at this situation, my feeling is the base decoupling caps can stay at 220uF, but the output coupling caps should be large - say 1000uF. This way you lose no headroom and you get no peaking about say 1 or 2 Hz where it is so small anyway that it is insignificant.

If the collector load (the output cap Xc + the total output load resistance including damper or no damper + the 100 Ohm to ground) increases, the gain will increase and you will get some kind of peaking. The solution therefore it seems is to keep Xc flat to << 1~2 Hz.

Just my thoughts.


Last edited by Bonsai; 21st July 2019 at 07:31 PM.
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Old 21st July 2019, 09:18 PM   #743
Hans Polak is online now Hans Polak  Netherlands
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Quote:
Originally Posted by Bonsai View Post
Are you not losing head room because the voltage on the collector before the output coupling cap is peaking at LF?

Looking at this situation, my feeling is the base decoupling caps can stay at 220uF, but the output coupling caps should be large - say 1000uF. This way you lose no headroom and you get no peaking about say 1 or 2 Hz where it is so small anyway that it is insignificant.

If the collector load (the output cap Xc + the total output load resistance including damper or no damper + the 100 Ohm to ground) increases, the gain will increase and you will get some kind of peaking. The solution therefore it seems is to keep Xc flat to << 1~2 Hz.

Just my thoughts.

Bonsai,
Good question about headroom, let's do a worst case calculation, which is for a 0.5mV 5R Cart, see image in #741.

1) Vmax for the collector voltages of the Duraglit is ca. 500mV rms.
2) Looking at the image in #741, collector voltage at 10Hz is 20dB above output voltage, so Vmax rms out @ 10Hz is 50mV.
3) With Riaa premphasis 0.5mV@5cm/sec@Khz drops to ca. 0.05mV@10Hz.
4) A 0.5mV Cart needs a Headamp gain of 20dB to be compatible with the MM input.
5) So Vin@0.05mV@10Hz gives a Vout of 0.5mV with this gain of 20dB

(Vout max)/(Vout@0dB@10Hz) = 50mV/0.5mV resulting in 40dB overload margin.
Where 30dB is regarded to be very good, 40dB is more than excellent and nothing to worry about.

Using 5mF + 10uF will give just one single Caps solution that fits all applications independent of Gain and Rs, whereas using 200uF or even larger output caps with a damping resistor will remove headroom over the complete FR and needs adjustment for each individual Cart / gain combination.
That's why to my opininion the 5mF +10uF solution seems to be the clear winner.


Hans
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Old 21st July 2019, 10:50 PM   #744
billshurv is offline billshurv  United Kingdom
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Richard Lee's Ultra low Noise MC Head Amp
What would one recommend for a 5mF cap?
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Old 21st July 2019, 11:06 PM   #745
Hans Polak is online now Hans Polak  Netherlands
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Originally Posted by billshurv View Post
What would one recommend for a 5mF cap?
What about 2x 2.7mF 2.5Volt Oscon Alu Polymer ?

Hans
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Old 21st July 2019, 11:11 PM   #746
syn08 is offline syn08  Canada
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Originally Posted by Hans Polak View Post
What about 2x 2.7mF 2.5Volt Oscon Alu Polymer ?
Or those mentioned here Richard Lee's Ultra low Noise MC Head Amp

Absolutely no noise contribution, to my measurements.
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Old 21st July 2019, 11:26 PM   #747
billshurv is offline billshurv  United Kingdom
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Richard Lee's Ultra low Noise MC Head Amp
Well yes I would love an excuse to use wet slug tants, but the budget won't stretch to that.
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Old 22nd July 2019, 05:15 AM   #748
kgrlee is offline kgrlee
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What about 2x 2.7mF 2.5Volt Oscon Alu Polymer ?
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Originally Posted by syn08 View Post
Or those mentioned here Richard Lee's Ultra low Noise MC Head Amp
Absolutely no noise contribution, to my measurements.
  • Does this mean the Nichicon 2V5 high value 'CAP ALUM POLY's vary from value to value or perhaps individually? syn08 appears to have had good results while Gerhad did not.
  • Are we also saying wet slug Tantalums (whatever they are) don't have this EVIL 1/f & greater noise?
  • My 2c is that in the 80s, cheapo Aluminium electrolytics worked well especially the Panasonic Lo Leakage ones.
  • Also Jurassic caps with slightly higher voltage rating had a paper ESR rather less than the very low voltage ones and this was borne out by simple listening tests in a test jig. My original document specified 25V caps because of this. I'm not sure if this holds in da 21st century.
  • I think my recommendation to actually listen to the noise of the base capacitors is still valid for those wishing the very best performance. But surely with all the expertise here, we can now point to something in the datasheets that tells us what to look for? aboos has suggested low leakage for Aluminium electrolytics. Is this enough to discard the EVIL organics & polymers? It obviously doesn't hold for Tantalums but we know to avoid these.
Excuse me if I belabour this point but I feel it is the last unknown obstacle to a DIYer making a battery powered Duraglit MC head amp with true SOTA noise.

Last edited by kgrlee; 22nd July 2019 at 05:42 AM.
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Old 22nd July 2019, 05:38 AM   #749
kgrlee is offline kgrlee
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Originally Posted by Bonsai View Post
Are you not losing head room because the voltage on the collector before the output coupling cap is peaking at LF?

Looking at this situation, my feeling is the base decoupling caps can stay at 220uF, but the output coupling caps should be large - say 1000uF. This way you lose no headroom and you get no peaking about say 1 or 2 Hz where it is so small anyway that it is insignificant.

If the collector load (the output cap Xc + the total output load resistance including damper or no damper + the 100 Ohm to ground) increases, the gain will increase and you will get some kind of peaking. The solution therefore it seems is to keep Xc flat to << 1~2 Hz.
Bonsai, I think Hans' solution is the correct one for Duraglit.

I note my original Wireless World 1981 article specified 22u output capacitors and I'm going to claim I investigated all this fully then then forgot all about it

Thank you Guru Hans for your fine work

A more formal version of Hans' 'worse case calculation' would be to look at Tomlinson's data which he originally presented in Audio magazine. I'm not sure which of his AES papers has the data which I think is still definitive for recorded levels and also what to expect from warps. Wayne found the original Audio article some years ago and I think its archived on his proaudiodesign.com website.

It will show that Hans is right and the use of 10u output capacitors will not be anywhere near overload at LF while damping resistors will be sailing close to the wind at mid frequencies & higher at times.

BTW, the Wireless World article also shows the onset of 'overload' starting above 100mV for a 3R cartridge and a 1v5 battery. It's quite 'gradual' and very different from the clipping of most other circuits. The 6R cartridge has overload starting above 150mV. Below these levels, THD is almost all due to the input stage.

Running higher currents gives less THD.
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Old 22nd July 2019, 08:12 AM   #750
Bonsai is offline Bonsai  Europe
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Here are the Holman articles

Technical Requirements of Phono Preamplifiers by Tomlinson Holman
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