Flat Phono Preamp Based on John's P10 and 2SK389

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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by mediatechnology » Sat Sep 03, 2011 7:49 pm

Remember I'm presently a beach bum.
Did not know that. Isn't it winter there?
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by JR. » Sat Sep 03, 2011 10:22 pm

I posted a brief explanation of why a mic preamp was not optimal for noise with MM carts to a thread over on prodigy where Gold originally asked about using one, and I pointed him to my old design (and here).

The first actually comprehensive input noise tutorial I recall that laid out the sundry noise sources and even got into tweaking current density was an old national semi app note for the LM381 phono pre IC. I don't recall who wrote it but there were several strong guys at national back then ('70s).

It was probably in the pile of old stuff (one of the smaller brown covered app collections) I sent to Wayne a while back.

http://www.national.com/an/AN/AN-104.pdf

They mention an AN-64 but I couldn't find it...


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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by mediatechnology » Sun Sep 04, 2011 6:09 pm

I just went to the old house today to pickup some stuff and grabbed the National file.

Sure enough, right at the front, I had a copy of AN-64 for the LM381 by Byerly and Long of National dated 1972. AN-64 does go into RIAA preamp design but I think what we may be looking for is in the Audio Handbook. They don't have much of a noise tutorial in AN-64. I'll ckeck the copy of the Audio Handbook you sent John. I think yours may be the first edition.
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by JR. » Sun Sep 04, 2011 6:51 pm

Nah no worries, perhaps the AN-104 link was the noise toot, I was thinking off... This was only a few decades ago... so my memory is not going to be precise.

I thought i recalled National talking about tweaking the current density of the LM381 but you know how memory gets.

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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by mediatechnology » Sun Sep 04, 2011 7:26 pm

AN104 that you provided may be what ricardo was talking about. Thank you for posting that John.

From scanning National's AN104, it looks like it demonstrates how you can calculate noise "piecewise" and then by (power) "summing" the bands deal with all the frequency-dependent things the cartridge and EQ curve throw at you.

I hope Paul got a backgrounder in converting spectral noise voltage density to rms voltage noise from that ADI You Tube vid. The principles for spectral noise current density (e.g. pA per root-hertz) are the same - the result is simpy "x" uA or pA rms current for a given BW.

Things get really hairy though once you throw in the cartridge model and EQ. But, to make a long story short for our readers, the steeply rising impedance vs. frequency you see in a phono cartridge isn't present in a mic which is why a THAT1510 or INA217 make great MC preamps but not-so-good MM preamps. Also why FETs excell as MM preamps with their low current noise.

I'm not sure how much that last dB of noise or so is going to matter since de-noiseing will happen in the post-transfer process. I'm more concerned about aliasing on ticks with a broadband preamp.
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by ricardo » Mon Sep 05, 2011 3:39 am

AN104 that you provided may be what ricardo was talking about.
This is the sort of thing I remember but I think the AD note didn't apply the A wt directly and actually plotted the 8ve or 1/3 8ve noise. I could compare this with a 1/3 8ve noise plot which I've just managed to dig up. 30 yr old memories :o

How do I post a pic of this? I could put it on micbuilders but then you'd have to join to look at it.

Great Guru Baxandall pontificated at length about this including tweaking the i/p stage current for various loads. In da old days, MM preamps with old LN BJTs (B >400) ran from 20uA to 100uA.

GG B's 1968 Wireless World gospel is in my Files directory on the Micbuilders Yahoo group. Sorry!

His last words on LN design are a chapter in the Microphone Engineering Handbook - ed Gayford. This is Unobtainium if prices on Amazon are any indication. However via the magic internet http://www.leonaudio.com.au/cohen.htm has "MEH Chapter 8" (23 meg file) BTW, this is Graeme Cohen's website, a rather younger LN guru who designed probably the quietest mike preamp in the known universe.

Among the nuggets in MEH Ch 8 is GG B on designing Transformers. It is his clearest and simplest explanation of many topics but doesn't cover everything in WW68 so wannabe LN gurus probably need both .. to read daily on their knees facing his grave in Malvern.

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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by mediatechnology » Mon Sep 05, 2011 6:46 am

How do I post a pic of this?
Use an attachment? Send a link to the file and use the img tag? Send it to me and I'll post it?

Thank you for sharing the MEH handbook. I had forgotten about Ian's scan of it.

Speaking of Leon Audio, Ian is a member here. He shared what he had on Cohen here with us. https://www.proaudiodesignforum.com/for ... f=12&t=161 I really appreciate him sharing it with us.

Cohen's work in RF gave him a lot of insight into preamp design and it is indeed "representative of good contemporary practice." (See MEH
http://www.leonaudio.com.au/microphone. ... pdf#page=3)

While writing an update to Chapter 12 of Glenn Balou's Handbook for Sound Engineers I spent quite a bit of time thinking about and asking what made the Cohen circuit Cohen's. People give him credit for an overall topology - the op amps wrapped around the differential transistor pairs establishing DC bias and also CFB - that predates him. Cohen did a lot of subtle things that made it "representative of good contemporary practice." Not sure I want to reopen responses to that can of worms again here but anyone wanting to revisit can follow this link and post there: https://www.proaudiodesignforum.com/for ... p=887#p887

I found citations for the basic topology going as far back as 1968 from Demrow at ADI. Whoever wrote that sentence in MEH Pg. 305 (pdf 38) put it well. The basic circuit has been around a long, long time. This is not what makes Cohen's circuit his own. The confusion is ours, not his.

The following 1968 example is not a low-noise preamp but that wasn't Demrow's original point.

Image

Note the similarities of Demrow (above) to our phono preamp. (Yes, I know it's AC-coupled.)

Image
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by JR. » Mon Sep 05, 2011 10:38 am

Not to feed the veer, and my views on this are well explored in the other thread, but I was momentarily confused by the "unity CM gain" notation and different schematic symbol (from later opamp) used inside the two front end loops. Upon reflection I believe the "unity CM gain" notation is an observation of how those blocks act together in response to CM signals at the very input, not about how the internal gain blocks respond to local inputs, with only one input per shown.

Sorry if TMI... Yes the ancients have been busy with this topology...

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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by emrr » Mon Sep 05, 2011 10:52 am

ricardo wrote: Microphone Engineering Handbook - ed Gayford.
Wow, salivating after a quick glance. I must need to eat....
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by mediatechnology » Mon Sep 05, 2011 10:56 am

Upon reflection I believe the "unity CM gain" notation is an observation of how those blocks act together in response to CM signals at the very input, not about how the internal gain blocks respond to local inputs, with only one input per shown.
I think you're right about that. EDIT: Note also the common mode shield drive connection shown in Demrow.

Just posted National AN-64 in Document.

Also found this preamp over at AudioExpress using an SE 2SK389 (paralleled) married to an AD797: http://www.audioxpress.com/magsdirx/ax/ ... in2774.pdf

There's an interesting (though not surprising) statement in the article WRT MM vs MC and the effect of MM cartridge inductance at high frequencies.
Above 7kHz, the typical 0.5mV MC cartridge has slightly better SNR (lower noise with reference to signal) than the MM cartridge, even though the total (full band integrated) SNR is 5.6dB better with the MM. This is from the HF noise increase, due to the high MM inductance not effectively shunting the 47k5 load’s thermal noise, as I described.
A couple of other citations for our readers:

On phono cartridge loading from Hagerman Technology: http://www.hagtech.com/loading.html

And the RIAA Curve from Stefan Lidström: http://www.tanker.se/lidstrom/riaa.htm
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