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-   -   Richard Lee's Ultra low Noise MC Head Amp (https://www.diyaudio.com/forums/analogue-source/339246-richard-lees-ultra-low-noise-mc-head-amp.html)

Bonsai 16th June 2019 07:52 PM

Richard Lee's Ultra low Noise MC Head Amp
 
For reference and future posterity, I've loaded Richard Lee's head amp schematic and his commentary about it up on hifisonix.com - its there if you need it.

Any questions, please post them here - maybe Richard can chime in.

Richard Lee’s Ultra-Low Noise MC Head Amp


And here is the updated (03 July 2019) MC Head Amp Compendium:-

MC Head Amp Circuit Compendium

Bonsai 17th June 2019 07:55 PM

I've collected a compendium of 15 or so MC head amp circuits with quick sim results. Here's the link to the pdf file (c. 1.3 MB)

MC Head Amp Circuit Compendium

syn08 17th June 2019 08:05 PM

Quote:

Originally Posted by Bonsai (https://www.diyaudio.com/forums/analogue-source/339246-richard-lees-ultra-low-noise-mc-head-amp-post5826493.html#post5826493)
I've collected a compendium of 15 or so MC head amp circuits with quick sim results. Here's the link to the pdf file (c. 1.3 MB)

MC Head Amp Circuit Compendium

At a quick look, you can parallel JFETs on the page 15 circuits until the cows come home; the 68ohm source resistor will limit the noise to some 1.1nV/rtHz.

Bonsai 17th June 2019 09:08 PM

Yes. You can lower it but the power consumption goes up. I will post some further designs up in the next few weeks that address this.

See page 18 for better implementation.

billshurv 17th June 2019 09:29 PM

Richard did post a couple of variants for lower R cartridges (such as ortofons). Next time I make contact with him will ask if they can be added, along with a discussion of any obtainium bipolars that would work

Bonsai 17th June 2019 09:34 PM

It’s quite easy to adjust the gain on his design by changing the load resistor.

Transistors are an issue because of the required complementary. There are some ultra-simple designs in the compendium that circumvent this, but noise is 3dB higher and distortion is higher as well. Something that needs a bit of dev work.

billshurv 17th June 2019 09:38 PM

This wasn't gain change, it was re-optimised for the impedance of the cartridge.

Bonsai 17th June 2019 09:42 PM

Ahh. Ok

syn08 17th June 2019 10:38 PM

Quote:

Originally Posted by Bonsai (https://www.diyaudio.com/forums/analogue-source/339246-richard-lees-ultra-low-noise-mc-head-amp-post5826596.html#post5826596)
It’s quite easy to adjust the gain on his design by changing the load resistor.

Transistors are an issue because of the required complementary. There are some ultra-simple designs in the compendium that circumvent this, but noise is 3dB higher and distortion is higher as well. Something that needs a bit of dev work.

Only differential stages come with a 3dB noise penalty, single ended complementary push-pull stages come with a 3dB gain in noise (assuming n/p transistors with the same noise).

kgrlee 17th June 2019 10:49 PM

Thanks for posting this Bonsai.

I've just uploaded another version to Yahoo MicBuilders. You have to join. I try to keep that version my most updated 'public' version.

It includes more optimisation for different cartridges and also my comments about Diodes Inc ZTX851/951 which are now the lowest rbb' BJTs I'm aware off.

Their advantage is for cartridges with DC less than 4R3 where you can run more than 3mA up to 5mA. (The older Unobtainium d786/b737 et al don't improve noise above 3mA cos their rbb' becomes dominant.)

Best of all, ZTX851/951 don't come from Ye Olde Unobtainium Shoppe :D

For best real life performance, it is important to match the amp to the cartridge. This holds for other designs too though they are more difficult to match.

The document also has my notes on MM noise .. all a bit moot now as in Mar this year, I got rid of my entire record collection and bits of record playing gear which I have not touched this century to the Op Shop (Oz speak for Charity Shop).

In the week that followed, I had to be physically restrained from going to the Op Shop to buy back some of my records :eek:

If you are doing a commercial design, I would look at Opto power source as in the TODO section at the end. If you do this, please let me know as I would love to have more info on these SFH206K devices.

At these noise levels, PSUs are a real pain and its easiest to measure noise on the battery versions.


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