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

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

Post by mediatechnology » Sat Aug 08, 2015 4:11 pm

I had to expand the scale to read the differences and it looks like low single digit dB (I think?). While better is always better. Do your analyzers have any bandwidth/weighting capability? I had a fairly crude IHF "A" filter (with gain) that I used the hell out of back in the day because those results correlated well with my ears.
Yep I think it's about a dB quieter.
And the NJM2068 is the least-expensive too...

My analyzer has filters but not good ones.
The documentation isn't that good either.

I needed an analog 20 kHz noise filter yesterday: viewtopic.php?f=6&t=749
had to settle for a 100 kHz 6dB/octave and a 157 kHz noise bandwidth.
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by JR. » Sat Aug 08, 2015 4:30 pm

mediatechnology wrote:
I needed an analog 20 kHz noise filter yesterday: viewtopic.php?f=6&t=749
had to settle for a 100 kHz 6dB/octave and a 157 kHz noise bandwidth.
I don't recall the exact cut-off but IIRC Paul Buff wrote a pretty good article on the subject of noise measurements back in the '80s (in RE/P). He suggested you could approximate a 20kHz measurement bandwidth with a one pole LPF at around 12kHz. For measurement purposes the HF cut off shape/slope was a major source of error. Starting the one-pole rolloff enough below 20kHz, balanced out the below 20kHz noise dropped, with the above 20khz noise still left in the measurement. Using a passive LPF eliminated filter/amplifier noise as a complication.


JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

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

Post by mediatechnology » Sat Aug 08, 2015 4:40 pm

That's a good idea.

The factor for a single-order is filter is 1.5708 so an fc of 12.73 kHz would have an equivalent BW of 20 kHz.

Equivalent Noise Bandwidth Chart. viewtopic.php?f=12&t=475&p=5331
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Noise Measurements of the MM Preamp

Post by mediatechnology » Tue Aug 11, 2015 5:15 pm

I finally had a chance to wire the preamp umbilical up properly.
I trimmed the shielded twisted pair balanced DIN tonearm leads to 3 feet.
The line output leads from the flat preamp are 48 feet feeding the RIAA EQ/Monitor protoboard.
I had NJM2068s already installed in the flat preamp so I used them.
It "sounds" quiet. Scary quiet.

I wanted to see what the noise level was when gain was set to produce +4dBu output.
The final output has 6 dB gain and the RIAA decoder was set to unity gain.
The remainder of the gain, all but 6 dB, was dialed into the preamp front end.

The ATR-96 MM cartridge is rated 5mv at 5 cm/s 1 kHz. They don't specify whether modulation is mono or single-channel.
The CBS STR-100 test record I'm using for level set is 3.54 cm/s with one channel modulated.
If I'm not mistaken this should work out to be 5 cm/s with both channels driven.
Based on that premise, I set the single channel gain to a reference level of +1 dBu.
Both channels driven should produce +4 dBu at 5 cm/s (lateral).
I may stand corrected on that...

The RIAA EQ'd (but not A-weighted) output noise with a real cart, sitting on a real turntable (not a resistor) produced an output noise floor of about -72 dBu.
This measurement bandwidth is 157 kHz. (fc=100 kHz.)
The signal to noise, unweighted, referenced to +4 dBu is about 76 dB.
Some of that is hum at Line and 2X, 3X and 5X line.

Its quiet, really quiet.
I suspect most of its quietness is from the balanced interface and common mode rejection.

BTW for someone wanting a FET input, the OPA2134 worked pretty good.
It was about 3 dB noisier (with a resistive input termination) than the NJM2068.
The OPA2134 may actually be better with the cart, compared to a resistor, due to its lower current noise.
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RIAA EQ/Monitor Switcher Schematic

Post by mediatechnology » Thu Aug 13, 2015 12:01 pm

After using and living with the flat phono preamp and RIAA EQ monitor switcher for awhile I think I know where to put all the jumpers.
Pauls' suggestions, particularly the inverse RIAA EQ for dub plate cutting, really helped.


I tried to put in a lot of flexibility and make it a "Swiss Army knife."
Someone wanting a simple RIAA EQ'd phono preamp needs only to build that part of the board.
My application, for LP archiving using RAW flat transfer, requires the full circuitry.

This is the schematic for the RIAA EQ and Monitor switcher:

Image
RIAA EQ, Inverse RIAA EQ and Monitor Switcher for the Flat Phono Preamp.

Signal Flow

The design has two signal paths. One is for recording the second for confidence monitoring or simultaneous recording and playback.
An RIAA EQ section can be placed in either the record path or playback path.

Flat transfers can be made through the record path while non-destructively switching monitor modes from monitoring flat record, record with EQ or playback from the DAW output.
Having analog RIAA EQ for monitoring saves ears and permits later de-clicking, done in the RAW un-EQ'd domain, to be heard in context.
Final EQ of the transfered files is done in DSP.

The flat balanced input/balanced output moving magnet preamp can be remotely located and powered from the monitor switcher permitting long cables between the turntable and workstation.

The flat/wideband preamp output is received by a THAT1240 line receiver.
It is then AC-coupled into a variable gain inverting buffer.
Relay RY2 selects either flat or RIAA EQ recording mode. RY1 must also be in Record.
The operation of RY1 and RY2 are destructive.
The record out is available as an RCA connection or fully-balanced by a THAT1646.

The RIAA EQ section can be switched between the Record and Playback Monitor paths by RY1.
A post-EQ gain stage can be used to provide stepped gain trim and additional post-EQ gain if needed.

The DAW playback output is fed into the THAT1246-based line receiver.
RY3 selects either Flat or EQ'd monitoring from either the Play line input, Preamp Output or RIAA EQ output.
The operation of RY3 is non-destructive to recording.

Jumper Description

J1 bypasses the balanced input stage. This is used when the flat preamp is in the same chassis as the EQ/Monitor Switcher.
When J1 is connected directly to the input, the THAT1240 on the EQ board and one THAT1240 on the flat preamp may be removed.

J2 (optional) permits the preamp gain to be raised or lowered in 3dB steps. It can also be a front panel switch.

J3 is a rumble/warp filter insert.

J4 is for alternate EQ. It can be used to bypass the RIAA stage and for inverse RIAA dub cutting allow the insert of a 50 kHz ultrasonic filter. (e.g. Neumann 2 pole SK)

J5A/J5B switch the RIAA filter to inverse RIAA to permit the board to be used for cutting discs.

J6 is an optional cartridge correction filter insert point. J6 may also be used to provide a return for alternate EQ.

J7 permits the post-EQ gain to be switched in 3 dB steps. J7 can also be used to sum cutterhead feedback with modulation.

J8 is a second playback cartridge correction filter insert. J8 can also be used for additional cutterhead EQ.

J9 selects the flat monitoring source. J9 can either be connected to the Playback line input or it can follow the RIAA EQ. J9 can also be made a switch. For transfers which are going to primarily be made without RIAA EQ, "RAW," then J9 should be jumpered to follow the RIAA EQ. If RIAA EQ'd transfers are the norm, placing the RIAA EQ in the record path, J9 should be jumpered to Play line in to permit confidence monitoring or simultaneous editing of files already recorded with EQ.

Board Stuffing Options

There are lots of them but these are the highlights.

The preamp gain can have an optional trim pot to adjust flat gain.
Gain is typically trimmed in the flat moving magnet preamp.
For moving coil preamps, fine adjustment of gain is not always possible.
A trim in this location allows minor amounts of "helper" preamp gain and adjustment of channel balance for moving coil carts.

If the flat preamp is located with the EQ/Monitor switcher the THAT1240 balanced input is not required.
The flat preamp can have one THAT1240 removed.
Due to signal inversion, the inverting output of the flat preamp and inverting input of the EQ board are used to maintain correct polarity.
This is also the reason the THAT1240 appears "upside down" with the + input connected to the - THAT1240 input.

The RIAA EQ shows optional capacitors bridging the 10 nF.
The 10 nF do not require them.
If precise 10 nF are not available, a combination of 6800 and 1500 pF can be stuffed.
The resistor values then become 11K8, 31K8 and 318K with links in the unused resistor locations.

For applications which do not require post-EQ gain trim it may be eliminated.

Workflow Enhancement

Two simultaneous signal paths allow flat transfers to take place while other files are de-clicked, edited and rendered with RIAA EQ in DSP.
Having two paths is a huge time saver for archival transfer because it allows to things to be done at once.
There will be no cigarette breaks for interns doing transfers with this box - if you got time to lean you got time to edit.
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by mediatechnology » Wed Aug 19, 2015 1:27 pm

I did a drawing of the proposed front panel to visualize the switching options.

Image
RIAA EQ and Monitor Switcher Front Panel Switches for the Flat Phono Preamp

The switches from left to right operate RY1, RY2 and RY3.
The fourth switch, which is optional, is at J9.

Image
RIAA EQ, Inverse RIAA EQ and Monitor Switcher for the Flat Phono Preamp.
https://ka-electronics.com

"States Ordered To Fraudulently Inflate COVID-19 Cases 15 Times Actual Rate"
https://banned.video/watch?id=5efab695672706002f367a0a
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by JR. » Wed Aug 19, 2015 9:06 pm

Back when I was doing phono preamps I had switches for RIAA vs IEC (pole at 7950 uSec) and even a polarity reverse (decades ago).

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

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

Post by mediatechnology » Sun Aug 23, 2015 6:12 pm

I'm very close to hitting the "submit" button for a small ProtoPro order on the EQ/Monitor switcher.

I decided to take a break from staring at the layout to listen to some LPs using the flat preamp PC board and the RIAA/Monitor switcher on the Protoboard.
With any luck I should be able to re-claim the Protoboard next week and have a working PC board.
I'm seeing and learning some things I didn't expect to find.

Cartridge Crosstalk

While setting levels using test tones from the CBS STR-100 disc I noticed something very curious: MM cartridge crosstalk (1kHz AT AT-96) is out of polarity.
Left-only modulation is producing a right channel output about -20 dB down exactly out-of-polarity.
I think this may have to do with disc mechanics.
What occurred to me seeing out-of-polarity crosstalk is that a cart may measure 20 dB separation based on level, but it will sound far wider, and produce more apparent separation, than what it actually measures.
An in-polarity crosstalk component will pull left and right together, an out-of-polarity crosstalk component pushes them further apart.
This may have been a trick used by the designers of stereo phono playback.

I don't have other HF spot tones to test. I suspect the out-of-polarity relationship extends from the LF to mid-band.
Above mid-band azmith and electrical phase errors may grow.

Cartridge Loading

A lot has been written about cartridge load and I decided to experiment with the various jumpers on the flat phono preamp.
The balanced STP to DIN cable is about 90 pF conductor-to-conductor.
I don't know what the internal arm wiring is.

While playing around with the jumpers and monitoring the flat output on the oscilloscope I can see the ultrasonic HF material "bloom" on the scope if the cartridge does not have enough Cload.


Most people playing around with loading have to listen (or monitor with instruments) the RIAA-EQ'd output.
Monitoring un-EQ'd output the differences are profound.
The HF material on the 'scope literally explodes as the cart begins to resonate.

With no added Cload cymbal crashes screech like fingers on chalkboards and there is audible mistracking.
The scope has HF material way off-scale.
Add 50-100 pF and it sounds sweet again and HF energy returns to normal.
Ultimately I decided on 100 pF and 47K.

Monitoring HF Tracing Distortion using the Wideband Output

I didn't see this one coming.

While monitoring the wideband flat preamp output in X-Y on the scope I started noticing some strange HF vectors not visible when EQ was applied.

The first quadrant, Q1 looked normal. Q1 is when both outputs are positive.
The third quadrant, Q3, is diagonal to Q1 and is when both Left and Right are negative.

Q3 exhibited an unusual curvature suggesting high even-order harmonic distortion compressing both channels negative excursions.
I suspect this is tracing distortion.

From the Neumann website:
One interesting phenomenon in this connection is the tracing distortion that results from the difference in geometric shape between the tool used to cut the grooves and the playback stylus. The cutting stylus is shaped like a spade, while the playback stylus is spherical. During playback, this results in tracing distortion, which mainly contains the 2nd harmonic. In 1968, Neumann built the Tracing Simulator that solved this problem.
I rechecked my cart's alignment with the Mobile Fidelity Geodisc and adjusted the overhang slightly but the problem continued.

To make along story short the problem worsened.
Cleaning the stylus (and the record under water) ultimately solved the problem. Duh.

The interesting thing is that I saw the gunked-up stylus tracing error develop in the wideband vectors before I actually heard it.
Monitoring the wideband output can provide a lot of information masked by RIAA de-emphasis.
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"States Ordered To Fraudulently Inflate COVID-19 Cases 15 Times Actual Rate"
https://banned.video/watch?id=5efab695672706002f367a0a
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by JR. » Sun Aug 23, 2015 9:22 pm

we all remember having a clump of smutz on the stylus. Often you can hear distortion.

I just looked at my turntable that I haven't used in more than a few(?) years, and even inside the cabinet behind a glass door, there is a thick layer of dust on the record... still on the turntable... I'll need to hose that off before I can read the label. :oops: Judging by the dust probably decades. :lol:

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

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

Post by mediatechnology » Sun Aug 23, 2015 10:07 pm

I'm really curious - in fact dying to know - what the last LP was that you listened to John.
After you remove the dust please tell us.

I've been doing a few needle drops of stuff that was best forgotten.
There were a lot of bands then (the 70s) that sound like Spinal Tap.
I hear those records today and think "I hope I got this as a promo."

There's one LP's credits I looked at that had a quote from a label exec that said: "It takes $15,000 to make a hit album. That's why I'm giving you ten."

Then there's the Joe Sample Carmel album I rediscovered.
Made digging through the other ten worth it.
https://ka-electronics.com

"States Ordered To Fraudulently Inflate COVID-19 Cases 15 Times Actual Rate"
https://banned.video/watch?id=5efab695672706002f367a0a
"Plandemic/Doctors In Black" https://proaudiodesignforum.com/content/Plandemic.mp4

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