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

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mediatechnology
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Inverse RIAA/RIAA Encode-Decode Accuracy

Post by mediatechnology » Wed Jul 22, 2015 3:59 pm

I altered the schematic of the inverse RIAA to move the ultrasonic time constant to 3.3 us.
It's effect at 20 kHz turns out to be about 1/2 dB.
If the 100 pF Cfb is added the out-of-band ultrasonic HF rise is tamed further.
With both in circuit the 20 kHz drop is about 0.75 dB.

I originally checked the playback EQ accuracy using the Jung/Lipshitz network through the preamp.
The playback RIAA network I built for the protoboard used "8300 pF" 2.5% caps. (1500||6800 pF, 383K, 38K3, 11K8).
The overall accuracy was ruler flat using the Jung/Lipshitz Inverse RIAA with the RIAA playback network.
The Inverse RIAA network used 10 nF 1% caps and series resistors to get the R exact. (316K +2K etc.)

As a reality check I decided to check the Inverse RIAA feeding the RIAA playback EQ end-to-end.
I used a sine input.
Again they were ruler flat with the exception of the minor 20 kHz droop.

A picture is often worth a thousand words and time domain accuracy - square wave response - can speak volumes about frequency response.
Though we don't record square waves they can tell us a lot about the frequency domain in one quick glance.
If the Inverse RIAA and RIAA curves are precisely matched, then a square wave in should produce a square wave out.

RIAA pre-emphasis is a brutal thing.
The top trace is the input, the bottom trace the output from the Inverse RIAA filter.
To give you an idea of the dynamic range a pre-emphasized square wave generates, the input trace is 200 mV/div, the output trace is 5V/div.
Headroom is precious and to say that Inverse RIAA is a differentiator is an understatement.

Image
Output of an Inverse RIAA network driven by a 1 kHz square wave.

So what happens to waveform fidelity after RIAA de-emphasis?
Can this be put back together accurately?
Yes it can.

Image
Encode-Decode Response of Inverse/Playback RIAA networks passing a 1 kHz square wave.
Top trace is input, bottom trace is after encode-decode.


To provide absolute purist Inverse RIAA requires an op amp with almost infinite bandwidth.
Since we don't have those I'll accept the single pole rolloff and the 3/4 dB 20 kHz penalty.
Others might want a 2 pole pre-filter at 50 kHz.

The hits of the 60's and 70 used LM709s (Neumann SE-66) and LF356s (Neumann SAB-74) in these stages.
We should be able to do just fine today with a 5532, NJM2114 or LME49860.
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by JR. » Wed Jul 22, 2015 4:14 pm

sweet...

While I didn't do this a lot you can run full range audio through the inverse RIAA and listen to it through an experimental RIAA playback path for quick reality check.

JR
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 » Fri Jul 24, 2015 3:17 pm

Thank you John.
While I didn't do this a lot you can run full range audio through the inverse RIAA and listen to it through an experimental RIAA playback path for quick reality check.
I did eventually listen to the RIAA encoder-decoder end-to-end and really couldn't hear much difference A-B'ing.
I measured just under 40 degrees phase shift at 20 kHz end-to-end which is from the ultrasonic roll-off in the Inverse RIAA.
I got schematics of the Neumann SAB-74B and will publish them a little later on.
What they did was a 2 pole SK at 50 kHz ahead of the pre-emphasis.
They also had a linkable approx 20 kHz resonant peaking filter to bump the 20 kHz droop and perhaps provide some cutterhead correction.
The Neuamnn inverse RIAA isn't as accurate as this one. :geek:
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Flat Phono Preamp Jumpers, Jumpers, Jumpers

Post by mediatechnology » Fri Jul 24, 2015 3:41 pm

I'm now putting the finishing touches on the flat front end preamp board's layout.
It's coming in at 3 x 4".

One of the things I wanted were jumpers for the input loading and configuration.
Some of the preamps I see with selectable loading start out at 47 pF, then 94 pf etc. etc.
I decided to use 3 100 pF caps in a series parallel arrangement to get 50, 100, 150 and 200 pF additional load.

The preamp jumpers are:

J1, J2, J3 input capacitance.
All open - No added load.
J1 is 50 pF. J1+J2 is 100 pF. J1+J3 is 150 pF. J1+J2+J3 is 200 pF.

J4 is input resistive load. Open is 99.8K, closed is anything less than 99.8K.
With an added load of 88.7K the input resistance is 46.96K with J4 closed.

J5 is the input configuration: Balanced or RCA coaxial.
When balanced, the input load to ground (common mode) is 49.9K/leg.
When J5 is jumpered unbalanced the "-" input is grounded and the input resistance is 99.8K. (J4 open.)
FWIW I did try an InGenius-style bootstrapped configuration and it wasn't as quiet as a conventional low common mode impedance input.

There are two preamp gain trims for each channel.
J6 selects which gain adjustment pot is used.
This is for normal gain, typically 5 mV at 5 cm/s, and low gain for very hot, "+12 dB" dub plates.
Though it would likely not be quiet enough for MC inputs the jumper could also switch between MM and MC gains.
Someone using this mode often would likely make the jumpers a switch or relay.

I'll be sending boards out for a pre-production run next week and will start on the RIAA EQ/Monitor Switcher next.
https://ka-electronics.com

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

Post by JR. » Fri Jul 24, 2015 4:58 pm

Didn't I send you one of my gold 4 pos dip switches from my cap loading board kit?

JR
Visit https://circularscience.com to hear what properly "cleared" drums sound like.

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Flat Balanced Input/Balanced Output Moving Magnet Phono Prea

Post by mediatechnology » Tue Jul 28, 2015 7:20 pm

This is the latest schematic of the Flat Balanced Input/Balanced Output Moving Magnet Phono Preamp.
The preamp can be located remotely at the turntable and/or lathe.
Another potential application for this board is a cutter head feedback coil preamp.

I'll have boards to test in a couple of days.

Image
A Flat Balanced Input/Balanced Output Moving Magnet Phono Preamp

The next step is the RIAA EQ/Monitor Switcher...
Didn't I send you one of my gold 4 pos dip switches from my cap loading board kit?
I remember the boards and will look for the DIP switches.
I decided to use jumpers on this one.
Thought I might be able to pack it tighter.
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by mediatechnology » Thu Jul 30, 2015 7:33 pm

The board:

Image
A flat, non-EQ'd, balanced input, balanced output phono preamplifier for RAW transfers using DSP-based de-clicking and DSP RIAA EQ.

The initial run of four boards were virtually perfect.
The only thing I found to change was to slide the "Power" silkscreen over so the "P" doesn't land on a feethrough.

Image
A Flat Balanced Input/Balanced Output Moving Magnet Phono Preamp

I split the gain stop resistors in the INA to reduce the potential reduction of HF CMR from pin capacitance with off-board gain switching.

I'm very happy with the NJM2114 dual op amps.
I have some JRC2068DD op amps coming in tomorrow to also try.
The "DD" "double-D" is the selected for low noise RIAA-weighted parts.
Now that this is off the protoboard I'll try more of the LME49720/LME49860.

If this board is to be used to directly drive a soundcard or A/D I would install C4/C5/C24/C25.
I left mine completely DC-coupled since the RIAA/Monitor switcher will have a 1 uF PP downstream film cap.
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by JR. » Fri Jul 31, 2015 9:42 am

It seems a little odd that this late in the life cycle of vinyl we can buy op amps selected for RIAA noise, while back in the day about the only dedicated parts designed for vinyl playback was National Semi's LM381/387 parts.

JR
Visit https://circularscience.com to hear what properly "cleared" drums sound like.

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Noise Test Using The NJM2114, 2068, 5532, 33078 and LME49720

Post by mediatechnology » Thu Aug 06, 2015 5:14 pm

I just compared the noise performance of the NJM2114, NJM2068, NJM5532, MC33078, LME49720 and LME49860.

And the winner is: The NJM2068DD "double-D" "D-rank" grade.
(Measured with a 2K Rsource at 40 dB gain. I decided not to use a dummy cart for this test.)

The NJM2144 is a close second.
The 5532 and MC33078 are tied at third.

The LME49720 and LME49860 are unuseable due to burst noise. (See: https://e2e.ti.com/support/amplifiers/a ... 6/t/415907)

These are noise comparisons of using both channels.
This is the flat output of the preamp before RIAA EQ.

Image
Noise comparison of the Left and Right Channels using NJM2114s in both channels.

Image
Noise comparison of the Left and Right Channels. NJM2068 in Red, NJM2114 in Green

The NJM2068 had the lowest noise and was used as the basis for comparison.

Image
Noise comparison of the Left and Right Channels. NJM2068 in Red, NJM5532 in Green

Image
Noise comparison of the Left and Right Channels. NJM2068 in Red, MC33078 in Green

The LME49720 and LME49860 are a solid "fail."
I tried numerous samples from various date codes and could not find a sample suitable for test.
Both the LME49720 and LME49860 suffer from burst noise:

Image
LME49720 burst noise. Measured at INA output 40 dB gain. 5 mV/2ms per division.

In between bursts the LME49720 performs quite well - as good as the NJM2068.
TI needs to get a handle on this - I'm not the only one that has found it.
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by JR. » Fri Aug 07, 2015 3:40 pm

I am struck by how well the technology has evolved to make pretty noise floor plots. Could I get one of those systems delivered to my bench circa 1980? :lol:

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.

That shot noise looks unacceptable for an audio path, unless those are silly cheap and can only be used at low closed loop gain. A few tens of microvolts will not be a huge issue in a unity gain buffer, but again better is always better. I hope it's only a process issue and not the underlying technology, but luckily for me I can let you guys sort that out. 8-)

Nice stuff...

JR
Visit https://circularscience.com to hear what properly "cleared" drums sound like.

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