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

[Gold]

Thanks for the detailed response Wayne. I think I didn't quite understand what was going on. Correct me if I'm wrong. The top 1240 is acting as the third opamp in an instrumentation amp. This third opamp provides the CMRR in an instrumentation amp. The second 1240 is a clever way to provide a balanced out.

I'm not worried about the output of the 1240's. With enough gain at the final output you won't clip the input.

[mediatechnology]

That's exactly right Paul.
The top 1240 is the third instrumentation op amp.
The bottom 1240 just has its inputs connected in reverse polarity to provide an inverted output.
This is exactly what Cohen did in his "double-balanced" preamp.
The whole thing is essentially a fully-balanced mic preamp but with lower current noise inputs.

[ricardo]

In terms of quietness the front-end the choices (using bipolar input op amps) wound up being:

JRC NJM2114
NE5532
LME49720

Though I have not determined why, the LME49720 had popcorn noise.
I tried a handful of samples all of which were from the same lot.
I want to think that this is related to a specific lot of LME49720s but I don't know.

The NJM2114 has more bias current which provides a large amount of DC offset when the input is unterminated and it sees a 47K Rbias.
The same is true, to a lesser extent, for the 5532.
Both however are significantly quieter than the bias-current compensated LME49720.

Thanks for this Wayne.

I take it this is with your AT120. What's its Resistance & Inductance?

I've always found 5532 practically ideal for MM cartridges. It's possible to do better with (now Unobtainium) FETs like JR but IIRC, the advantage was about 1dB.

NJM2068 might be worth trying. Nwavguy reports less voltage noise than LM4562

For $$$, there's FET i/p AD745/3 with 3.2 nV/rtHz @ 1kHz
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I first encountered the EVILs of bias compensation in OP27/37. These are essentially souped up 5534s with bias compensation and better specs. This NEVER translated to better performance .. in fact always worse noise in real life applications.
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I don't think the Mike preamp ICs are quiet enough for MC cartridges. The 1nV/rtHz of the best eg THAT 1510 etc is Rnv = 62R5. You want Rnv nearer to 5R

[mediatechnology]

I take it this is with your AT120. What's its Resistance & Inductance?

AudioTechnica AT-120E: DCR 768 Ohms, L (1kHz) 548 mH

I also have several brand new Stanton 681s and styli I want to mount and try.

[ricardo]

I take it this is with your AT120. What's its Resistance & Inductance?

AudioTechnica AT-120E: DCR 768 Ohms, L (1kHz) 548 mH

I also have several brand new Stanton 681s and styli I want to mount and try.

Thanks for this Wayne. Got a recommendation for Capacitive loading?

This is what I've unearthed to look at MM noise.

Super XLM: R = 375 ohms, L = 300 mH, and C = 100pF
XLM & VLM Mk I R = 560 L = 260 200p 1.088mV/(cm/s) Audio jul72
XLM and VLM Mk II: R = 635 L = 350 450p 1.05
Shure V-15III: R = 1350 L = 500 450p 0.7 Kemble
Technics EPC205C III R = 500 L = 240 0.5mV/(cm/s)
Stanton 500AL-II R = 535 L = 400 0.92mV/(cm/s)
Ortofon VMS20E & II R = 800 L = 600
680/1 R = 1330 L = 910 275p 1.0 Miller
880/1 lower impedance 275p
Shure M97, M35 200-300p
AT 95E 100p 0.7
AT 120E R = 768 L = 548 1.0 Wayne

... just to prove there are even nerdier geeks than you Capacitances are total recommended for flat response

Sorry I haven't worked out how to do a table

[mediatechnology]

Got a recommendation for Capacitive loading?

Not for the AT-120 yet - Until I split the pre and EQ sections I was using long cables to the work bench and couldn't experiment with the C-load.
I do have a squarewave/tracking disc I inherited from a radio station I can use later.

[mediatechnology]

NJM2068 might be worth trying.

I'm going to try those ricardo.
The "double D" version is graded for voltage noise with a 2K2 Rsource and RIAA weighting.

One thing I found interesting about this is that later datasheets (Rev E) provide the specifications for RIAA weighted noise.
In the 2003 datasheet for the NJM2068 they have a note saying that they are "preparing" for a special D rank product.
By 2011 they've added the following table:


NJM2068DD Phono Preamp Noise Specification From Datasheet Rev E

From 2003 to 2011 JRC must have decided there was enough growth in phono preamps to provide a graded product.

NJM2068D, NJM2068DD Datasheet http://www.njr.com/semicon/PDF/NJM2068_E.pdf

[JR.]

Maybe I am wary of manufacturer specs, but what is RIAA noise? Is that shaped with a -40dB HF roll-off across the audio band? Like a noise weighting curve?

Might be good, might not I can't tell....

JR

[mediatechnology]

Maybe I am wary of manufacturer specs, but what is RIAA noise? Is that shaped with a -40dB HF roll-off across the audio band? Like a noise weighting curve?

NJM2068DD Phono Preamp Noise Test Circuit From Datasheet Rev E

Looks like a phono preamp.

[JR.]

OK so fair for comparing other similar op amps, using the same circuit, but perhaps not apples to apples vs other op amp input noise specs.

JR