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

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mediatechnology
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Cartridge Upgrade and Cart Internal Grounding

Post by mediatechnology » Wed Sep 16, 2015 7:04 pm

My article in Linear Audio Vol. 10 is out. I cover IIR and min phase FIR using a trick that I have not seen elsewhere. The IIR coefficients are tablated there for <.005dB accurate response even at 48K sampling for RIAA and most of the historical equalizations. I only posted the FIR's for RIAA but supply the Python code to do any you might want. The min phase FIR at 96K is only ~5k taps but does .000002 dB and .000001 Degrees even down to 20Hz.
Scott - Just now saw your post. Thank you for joining us!
I guess I'll have to get a real job too and subscribe to Linear Audio.

I'm pretty happy with how DC8's RIAA sounded compared to CoolEdit's RIAA but I have a DC8 learning curve.
I had strange things happen with it truncating files but that might be because I was using 32 bit float as my test source.

Now that I have the preamp in a box I decided it was time to upgrade phono cartridges.
I did this on the cheap having two Stanton 681EE bodies and a 2/3 full three-pack of D6800EL styli given to me by a radio station many years ago.
I've basically been using the AT-96 for testing since it was also free.
The AT-120 from my youth needs a new stylus...

One thing I noticed about the AT-96 is that it has no grounding strap from either the left or right ground connection to the body.
All of my other carts do.
The ADC LME-2 head shell is plastic and galvanically isolated from any tonearm lead.
The AT-96 also has a plastic outer shell and plastic mounting ears.

The AT-96 tended to have - depending on arm position - electrostatic induced noise the left channel didn't have.
When the arm was hovering over the turntable all was well but on the tonearm rest it was about 3 dB noisier when the room's dimmer was on located about 3 feet from the cartridge.
Approaching the cartridge body or mounting screws the left channel remained silent but the right channel would have this slight noise increase.
I got the feeling that the AT-96 had it's internal shield or frame bonded to the right channel ground.
Playing a record both channels were quiet.

When I mounted the Stanton I purposely left the ground strap on.
Sure enough this cartridge had the same noise signature but in the opposite channel.
Touching the metal body produced bodacious amounts of hum in that channel which is what we should expect in a balanced and floating connection.
It unbalances the circuit.

The bonding clip to the cartridge metal outer body can be slid off the ground post to provide a true floating connection.
(The grounding clip can also be re-installed - the clip slips between the outer metal shell and the cart's plastic terminal block.)
When I removed the bonding strap both channels were equally quiet regardless of arm position.

For a true balanced connection the clip should be removed and the metal cartridge body allowed to float.
That way I get the best CMR in both channels.

The Stanton 681 sounds absolutely fantastic.
The AT-96 was OK but now that I'm done experimenting with the preamp it was worth the no-cost upgrade.

Image
Noise Signature of Balanced Input Phono Preamp Input Termination Stanton 681 on Turntable with RIAA EQ
0 dBFS is +8 dBu, Gain at 1 kHz approx 34 dB


Image
Noise Signature of Balanced Input Phono Preamp Input Termination Stanton 681 on Turntable Flat, No RIAA
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Comparing the NJM2068 to the OPA2134 in the Flat Preamp

Post by mediatechnology » Wed Sep 23, 2015 1:39 pm

I later found that the OPA2134 is actually better with a cart, compared to a resistor, due to its lower current noise.
In the midrange the NJM2068, NJM2114 or NE5532 out-perform the OPA2134 where it's higher voltage noise dominates.
Got some numbers or preferably some noise spectrum plots for this? Yes. I know it will vary with cartridge ;)
Got some numbers ricardo.

The following two captures are baseline measurements with a Stanton 681EE cart and the NJM2068DD installed in both channels.
The first is with RIAA EQ, the second flat.

Image
Phono Transfer System Balanced Input Preamp Stanton 681 RIAA EQ NJM2068DD In Both Channels

Image
Phono Transfer System Balanced Input Preamp Stanton 681 Flat NJM2068DD In Both Channels

These images are with the NJM2068 installed in the Left channel the OPA2134 installed in the right.

Image
Phono Transfer System Balanced Input Preamp Stanton 681 Flat EQ NJM2068DD Left Channel OPA2134 Right

Image
Phono Transfer System Balanced Input Preamp Stanton 681 RIAA EQ NJM2068DD Left Channel OPA2134 Right

The NJM2068 is slightly better in the midrange at 1/10th the cost of the OPA2134.
The OPA2134, considering that its a FET input with higher voltage noise nevertheless has outstanding performance.

The DC differential offset of both channels measured at the preamp output was <2mV so from a DC perspective they are also just about equal performers.

One more interesting factoid: The measurements above were made with the Phono Transfer System powered from a Meanwell P25A14E-R1B switcher.
The noise performance was identical to the linear bench supply.
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by emrr » Wed Sep 23, 2015 2:00 pm

Those Meanwell supplies have not failed me yet.
Best,

Doug Williams
Electromagnetic Radiation Recorders

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Re: Comparing the NJM2068 to the OPA2134 in the Flat Preamp

Post by ricardo » Tue Sep 29, 2015 2:42 pm

I later found that the OPA2134 is actually better with a cart, compared to a resistor, due to its lower current noise.
In the midrange the NJM2068, NJM2114 or NE5532 out-perform the OPA2134 where it's higher voltage noise dominates.
Got some numbers or preferably some noise spectrum plots for this? Yes. I know it will vary with cartridge ;)
Got some numbers ricardo.

The following two captures are baseline measurements with a Stanton 681EE cart and the NJM2068DD installed in both channels.
The first is with RIAA EQ, the second flat.
.........
These images are with the NJM2068 installed in the Left channel the OPA2134 installed in the right.
.........
The NJM2068 is slightly better in the midrange at 1/10th the cost of the OPA2134.
The OPA2134, considering that its a FET input with higher voltage noise nevertheless has outstanding performance.
.........
The DC differential offset of both channels measured at the preamp output was <2mV so from a DC perspective they are also just about equal performers.
.........
Great stuff Wayne :)

Looks like NJM2068 on paper looks slightly better on paper (test gear?) than OPA2134 including the important 4kHz region. I sorta expected OPA2134 to do better in that region.

Were you trying to say the OPA2134 sounded quieter though it measured slightly worse? This is entirely possible and I've used this phenomena to cheat ... I mean present figures which more truly reflect the excellent user experience with my designs :D .

Do you have Inductance and Resistance for your 681EE? I make Stanton 680/1 (their previous top model) 910mH & 1k3

The one item we haven't added in this investigation is the effect of the optimum load capacitance. In the 1975 Boston Audio Society article I sent you as part of my MCamp.doc, you can clearly see the effect of this on V15 III which demands a very high capacitance.

I'm tempted to resurrect my spreadsheet based on that NI application note on MM noise :ugeek:

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

Post by mediatechnology » Tue Sep 29, 2015 3:45 pm

Great stuff Wayne :)
Thank you!
Looks like NJM2068 on paper looks slightly better on paper (test gear?) than OPA2134 including the important 4kHz region. I sorta expected OPA2134 to do better in that region.
The NJM2068 was just a little bit better.
Were you trying to say the OPA2134 sounded quieter though it measured slightly worse? This is entirely possible and I've used this phenomena to cheat ... I mean present figures which more truly reflect the excellent user experience with my designs :D .
The OPA2134 originally measured worse than it actually was so it sounded almost as quiet as the NJM2068. But it turned out to measure really close so what I heard correlated to what I later measured. (Thanks to the benefit of shielding.)
I can't really A/B but I suspect the NJM2068 may still come in a tad quieter audibly based on its midrange advantage.
Do you have Inductance and Resistance for your 681EE? I make Stanton 680/1 (their previous top model) 910mH & 1k3
Stanton 681 927 mH 1.25K for another 681 found in the same box. (Am I lucky or what?)
The one item we haven't added in this investigation is the effect of the optimum load capacitance. In the 1975 Boston Audio Society article I sent you as part of my MCamp.doc, you can clearly see the effect of this on V15 III which demands a very high capacitance.
Well maybe we have....

I had a chance to check it with both 47 K and various capacitance.

This is 47K Ohm, 100pF added, approximately 100 pF cable, tonearm capacitance unknown. Total Cload approximately 200 pF.
Stanton 681 CBS STR-112 Group 1A.

Image
Stanton 681EE 200pF CLoad 47K Rload 1kHz 5cms Square Wave Lateral Modulation. Left channel is top trace.
I'm tempted to resurrect my spreadsheet based on that NI application note on MM noise :ugeek:
Let me scan what I have in the original Audio Handbook. It's completely different than the modern TI version and spread over two sections.
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Re: Comparing the NJM2068 to the OPA2134 in the Flat Preamp

Post by JR. » Tue Sep 29, 2015 3:46 pm

ricardo wrote:
Great stuff Wayne :)

Were you trying to say the OPA2134 sounded quieter though it measured slightly worse? This is entirely possible and I've used this phenomena to cheat ... I mean present figures which more truly reflect the excellent user experience with my designs :D .
:ugeek:
Back in the day I used a little battery powered gain stage for noise measurements. The 40 dB gain stage had IHF "A" (?) noise weighting curve built in. It ignored both low bass and very HF noise so made measurements look a bunch better than wide band, but it was also more representative of how human hearing sensitivity/response curves work. Back in the day I never used wide band measurements for tweaking noise.

JR
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Re: Comparing the NJM2068 to the OPA2134 in the Flat Preamp

Post by Gold » Tue Sep 29, 2015 4:00 pm

ricardo wrote: The one item we haven't added in this investigation is the effect of the optimum load capacitance.
I was looking at the Jensen MC transformers last night. It surprised me to see that their test circuit loaded the secondary with a C as well as an R. That's the first time I've seen that for an MC cart.

I ended up nowhere near the recommended load for the DL103 with the Lundahls. Off by a factor of 10x. The transformer is 1:16. R2 and R3 of the preamp are 1MΩ. This measured 100kΩ differentially. My load resistor is 221kΩ. That's 221kΩ in parallel with 100kΩ if I have that right. I come up with 68.85kΩ. Right?

Any lower value sounded darker and higher values didn't make much of a difference. It still sounds darker than the source. it sounds like there is not much above 12k or so. The sheet that comes with the DL103 shows it ruler flat to 20k. The roll off miraculously is like a tape hiss reducer so I think it might have been designed in on the sly. Or it could just be the limitations of the conical stylus. This is for QC and I like the way the DL103 breaks up. It doesn't sugar coat things.

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

Post by JR. » Tue Sep 29, 2015 4:12 pm

IIRC Deane Jensen would terminate transformers with RC to flatten out the response of the transformer...So may not be optimal for cartridge loading.

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

Post by dirkwright » Tue Nov 17, 2015 4:17 pm

I don't understand the desire to provide a phono preamp that does not have the RIAA de-emphasis EQ built into it. Since the vast majority of LP's were recorded using analog filters for the RIAA emphasis, then they have phase shifts that are built into the LP itself. The analog RIAA de-emphasis EQ has the opposite phase shifts so I think that the way to correct the response in both amplitude and time domains is to use an analog filter instead of a digital one.

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

Post by mediatechnology » Tue Nov 17, 2015 4:32 pm

Hi -

The purpose of flat recording is to make DSP-based de-clickers work better. I found that to be the case with Virtos, a DirectX8 plug-in running under Cool Edit.

The IASA writes: http://www.iasa-web.org/tc04/mechanical ... ualisation
5.2.6.10 There is some debate as to whether noise reduction tools for the removal of audible clicks, hiss etc are more effective when used before an equalisation curve is applied rather than afterwards. The answer very likely varies according to the specific choice of tool and the nature of the job to which it is applied, and in any event will be subject to change as tools continue to evolve. The most important point in this regard is that noise reduction equipment, even fully automated tools with no user-definable parameters, ultimately employs subjective and irreversible processes, and so should not be used in the creation of archival master files.
We have indeed discussed the problem with linear phase DSP RIAA decode.
It turns out that DC8 from Diamond Cut does follow analog phase characteristics; Cool Edit's FIR did not.
Scott Wurcer has posted here his link to the Linear Audio article he's written.
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