Richard Lee's Ultra low Noise MC Head Amp

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#61
I am away until Monday, but will ‘re-craft’ the JFET phono design mentioned above to use an LSK489 or LSK389 (which are available) in place of the BF862.

I’m lucky in that I have a small stash of the latter, but of course not everyone is in that position.
 
#68
Just like to post a caveat about designs relying on EVIL 1000uF electrolytics.

Spend some time LISTENING to the noise of these AND with signals approaching the noise level. There are surprising differences beyond the obvious ESR.

In particular, Tantalums are TRULY EVIL and have a crackly noise rather like quantisation distortion. No. I don't have an explanation ... 😱 ... and it isn't a wannabe Golden Pinnae effect either. It's more obvious in the very low noise designs.

I say a bit more in my MicBuilders document.

.. and watch your PSU noise too. PSU electrolytics are often "in the signal path" too.
 
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#70
scott wurcer said:
I wish there were more discussions of gain structure and overload margin, folks still complain when they use a miniDSP in front of their 106dB sensitivity speakers and find it noisy. They are running everything throwing away bits to get the output level low.


Another thing about Mr. THX he talks about intermod with warp and LP eccentricity but this is not a pre-amp issue except in the extreme. The damage is done by the tracking angles and effective velocities changing, no amount of low pass filtering can fix that.
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I wish folks would go back to thinking in dB VU, and consider the system as having a working 0dB VU of some SPL (mastering SPL of +85 dB SPL is really way too loud for me; maybe something in the early 70's). Then amplifier peak output can be spec'ed as some chosen professional margin higher, maybe +14 or +20 or whatever, given speaker sensitivity, plus 3dB for two non-corellated sources, and estimated room dispersal.


When I hear someone talking about amplifier "watts", I just feel the need to slap the child. It's my upbringing.


And, yes, (almost! Pace, Bill) everything interesting in phonographs happens where the diamond meets the PVC, and two fundamental resonances.


All good fortune,
Chris
 
#72
.. if I may make a few corrections to my designs in Bonsai's compendium ..

On page 4, the powered version actually runs slightly less than 1mA

  • Q2 & 4 should be ZTX851 & 951 which are the lowest rbb' transistors available today (2019)
  • 1mA through each of these give Rnv 26R/2 = 13Rso with 2 in parallel, effective Rnv for the amp is 13R/2 = 6.5R giving Env 329pV/rt(Hz)
  • Of course this isn't the complete noise picture as we have to take the resistance of the MC cartridge into consideration. At 1mA, the head amp is optimised for a 13R cartridge for which the noise will be 3dB worse than the above.
  • The 329pV/rt(Hz) performance will hold for cartridges with less DC
  • C5, 6, 7, 8 can all be usefully the same supa dupa 470uF Aluminium electrolytics as at C2, 3.
 
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#73
The recommended version is the battery powered one on page 18. For this

  • Q1-4 should be replaced with 1 pair of ZTX851/951
  • R1, 2 are adjusted to match the cartridge resistance. As shown, the current is about 1mA which matches a 13R cartridge
  • With R1 & 2 = 13k, current is 2.9mA which matches a 4R3 cartridge
  • This higher current version is the one with measured Env 280pV/rt(Hz)
More on matching in the original document.
 
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#76
The reason I don't recommend the powered version is cos getting rid of PSU noise is a REAL PITA. It's MUCH easier for a DIYer (or indeed a true guru) to get SOTA noise with the battery version. The power version probably needs evil 1000uF caps. & other yucky stuff and the extra bits make the layout much larger & yuckier.

If I was doing a powered version again, I would look at doing it like this.
L C Audio Technology / RIAA/MC Amplifier

1000uF caps etc are really trying to float the PSU and the opto sources looks like a better way to do this. This implementation isn't the very best but better than most. If you want something off-the-shelf and single supply based, its definitely worth trying.

If you do buy one, please post on this thread. Us gurus & pseudo gurus will bug you for DC voltage measurements 😎
 

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#78
scott wurcer said:
Not actually playing an LP. We need to adjust our expectations for reality.
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Yes I know all that Guru Wurcer. But the pursuit of 280pV/rt(Hz) is only on par with Mark's pursuit of 33Vrms preamp output 😀

My listening test comments on supa dupa low noise are in the original document where I admit I may be pontificating from the wrong orifice.

But there's more to noise than the numbers. More recently, I've been looking at capacitor microphone noise.

The redder-than-pink '1G & capsule capacitance' noise dominates most condensor mikes. But its the 'white' FET/resistor/acoustic resistance noise which is most irritating and obviously not natural. Ambient noise, even in quiet studios, is almost always like the '1G & capsule capacitance' noise so you can often get away with higher levels of this ...

But the effect of the '1G & capsule capacitance' noise is to make ambient stuff sound 'thick' so its still worth using 5G resistors & other stuff from Ye Olde Unobtainium Shoppe in a very low noise mike. 🙂
 
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#79
billshurv said:
I'm still trying to work out if large electrolytics are more or less evil than FIRs 🙂 I feel Richard has an 'evil list' on his wall...
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Both of these, in particular circumstances, have been identified by my DBLT panel as EVIL.

That is my definition of EVIL. In my previous life, if I was a guru, it would be on using DBLTs to design stuff.

I'll claim my panel can identify my SupaDupa LN MCHA as 'better' in a DBLT while Mark will be hard put to find a situation where his 33V preamp shows audible benefit. 😀
 
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