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

Where we discuss new analog design ideas for Pro Audio and modern spins on vintage ones.
Gold
Posts: 677
Joined: Sat Dec 01, 2007 5:20 pm

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

Post by Gold »

The noise spectrum of buzz and hum is very different from rumble, pops and ticks. I'd expect to easily hear improvements. A good record should have a noise floor of about -65dBu.
brianroth
Posts: 91
Joined: Mon Jun 17, 2013 2:11 pm
Location: Salina, Kansas
Contact:

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

Post by brianroth »

Gold wrote:The noise spectrum of buzz and hum is very different from rumble, pops and ticks. I'd expect to easily hear improvements. A good record should have a noise floor of about -65dBu.
I'm seeing that -65 dBu number and wondering how far above 0 dBu is the "overload" point. Or, IOW, what is the total dynamic range of a good record?

Bri
Professional audio and video systems design/installation/maintenance.
www.BrianRoth.com
Gold
Posts: 677
Joined: Sat Dec 01, 2007 5:20 pm

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

Post by Gold »

brianroth wrote:
Gold wrote:The noise spectrum of buzz and hum is very different from rumble, pops and ticks. I'd expect to easily hear improvements. A good record should have a noise floor of about -65dBu.
I'm seeing that -65 dBu number and wondering how far above 0 dBu is the "overload" point. Or, IOW, what is the total dynamic range of a good record?

Bri
Overload is dependent on playback. A Fischer-Price turntable will clip before a high end setup. The EBU broadcast spec was +12dBu as the clip point. The equipment was built to that spec but was of course way over built.

Edit: A more correct way to say it is the EBU standard is 12dB headroom above reference level.
ricardo
Posts: 262
Joined: Sun Apr 10, 2011 7:24 am

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

Post by ricardo »

brianroth wrote:
Gold wrote:I'm seeing that -65 dBu number and wondering how far above 0 dBu is the "overload" point. Or, IOW, what is the total dynamic range of a good record?
Tomlinson Holman (da THX man) did an article for Audio magazine which looked at maximum theoretical & practical levels on vinyl using the SHURE and other data. It looked at the maximum levels cut on various records, what could be tracked and warps & other stuff too over the audio & sub/supersonic spectrums.

Wayne, that would be a good article to track down & archive.
User avatar
mediatechnology
Posts: 5442
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

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

Post by mediatechnology »

Thanks everyone for your comments.
Nice... I vaguely recall some carts that had a metal grounding tab bonding one channel ground to the tone arm shell.
Been meaning to mention that every cart I have with one of these has it so that the shell grounding clip is removeable and though I haven't tried it re-installable. Other than wiring up a balanced DIN female to XLRs there's not much work making the cartridge interconnect fully balanced.

The miniscule hum I'm hearing now is coming from the cartridge body "hearing" magnetic fields. The null point changes very slightly from the outer to inner diameter. Both channels have different null points. The hum is either in or below the noise floor to my ears.

I can't speak to vinyl but I do remember being able to hear with tape things below the noise floor.

Getting the high-order harmonics out makes it worthwhile to do IMHO.
More so than milling the box out of solid copper, oxy-free conductors, lots of discrete transistors etc. etc.

I saw the Holman data cited recently but don't remember where. I may have it. Will look.

Looking at the FFT of a 1 kHz tone played back from the STR-100 disc I can understand the "warmth" of vinyl.
User avatar
mediatechnology
Posts: 5442
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

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

Post by mediatechnology »

Tomlinson Holman (da THX man) did an article for Audio magazine which looked at maximum theoretical & practical levels on vinyl using the SHURE and other data. It looked at the maximum levels cut on various records, what could be tracked and warps & other stuff too over the audio & sub/supersonic spectrums.

Wayne, that would be a good article to track down & archive.
Tom Holman, "Dynamic Range Requirements of Phonographic Preamplifiers." Audio, July 1977.
I may have this...

Found: https://www.proaudiodesignforum.com/for ... f=12&t=738
mediatechnology wrote:"Dynamic Range Requirements of Phonographic Preamplifiers," Tomlinson Holman, Audio, July 1977, page 72-79.
The curve in figure 1 then gives the sine-wave low-distortion power response of the cutterhead and cutting stylus, cartridge, preamplifier combination. That is, no single-frequency component of program material may exceed the limits for low reproduced distortion. But, because overload is a peak phenomenon for which all frequency components instantaneously add, the actual spectral output of the preamplifier must run substantially below the sine-wave limit line.
"Dynamic Range Requirements of Phonographic Preamplifiers," Tomlinson Holman, Audio, July 1977, page 72-79 article pdf: https://proaudiodesignforum.com/images/ ... y_1977.pdf

Image
Holman, Dynamic Range Requirements of Phonographic Preamplifiers Fig 1.
User avatar
mediatechnology
Posts: 5442
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Switchable RIAA/Inverse RIAA EQ

Post by mediatechnology »

Paul asked me to provide inserts in the Phono Transfer Preamp to accommodate Inverse RIAA EQ.
I decided to explore making the RIAA EQ section jumper-selectable to provide both playback and Inverse RIAA EQ for cutterhead modulation.
Most published Inverse RIAA networks are passive, low-level, devices for phono preamp testing.
It turns out to be simple to implement a line-level active network to provide playback and Inverse RIAA.


Image
A switchable RIAA/Inverse RIAA EQ Circuit for Playback or Cutterhead Modulation.

Jumpers reverse the 75/318 us and 3180 us networks' feedback position.
This changes the slope from de-emphasis to pre-emphasis.
When the jumper is in position A, RIAA EQ is applied; when in position B Inverse RIAA EQ is applied.

I used 10 nF 1% Wimas which are available from Mouser and series resistors to get the time constants nearly exact.
This is the circuit adapted from John's P10 and later used by SSM.
The 75 us pole isn't immediately obvious. It is [(9K76+53R6)||(31K6+200R)] * 10nF = 74.99 us

An Inverse RIAA has a response that rises indefinitely into the ultrasonic until the op amp runs out of steam.
An interesting article about RIAA inverse preemphasis and the so-called "Neumann pole" can be found here: http://www.pspatialaudio.com/neumann_pole.htm

When the jumpers are in the "B," Inverse RIAA position, a 332R resistor provides an ultrasonic pole.
Testing the Inverse RIAA cascaded into the RIAA playback network shows a very slight approximately 0.6 dB rolloff at 20 kHz.
The input impedance at HF is also low - the 332R allows a high current op amp output to drive it.
An optional local Cfb around the op amp to adds a second ultrasonic pole.
With the Cfb and the 332R ultrasonic pole the response at 20 kHz is down about 0.75 dB.
Without the Cfb it's about 0.5dB.

The RIAA EQ stage has AC coupling (not shown) which follows it due to its high DC gain.
User avatar
JR.
Posts: 3700
Joined: Sat May 24, 2008 7:21 pm
Contact:

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

Post by JR. »

Interesting... Back in the day I was a fan of using a real pole at 75uSec not because of accuracy but because I didn't expect much music in the octaves above 20kHz and preferred to keep the active stages well behaved.

Ironically perhaps the inexpensive one op amp RIAA stage had an inherent zero up around 200kHz that may be more accurate, while less well behaved.

JR
Cancel the "cancel culture", do not support mob hatred.
User avatar
mediatechnology
Posts: 5442
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

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

Post by mediatechnology »

Both Neumann designs had two pole Sallen-Key ultrasonic rolloff.
The "inversable" RIAA in the previous post only has one ultrasonic pole.
My thinking is that in a dub cutter application the input source (DAW, CD etc.) will already be band-limited by its reconstruction filter.

Walt Jung also has an interesting article, "A High Accuracy Inverse RIAA Network," The Audio Amatuer, 1/1980. http://waltjung.org/PDFs/A_High_Accurac ... etwork.pdf
I have Walt's passive Inverse RIAA that I purchased from Old Colony and it's quite accurate.
Like most RIAA networks (and unlike the "P10") the 75 us and 3180 us interact to produce the 3180 us pole.
Mine doesn't have the 12M||75K|5pF to tweak it the last 0.003 dB. :geek:
User avatar
JR.
Posts: 3700
Joined: Sat May 24, 2008 7:21 pm
Contact:

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

Post by JR. »

Yup, I have one of those inverse RIAA networks somewhere in my lab. Handy for design and testing.

JR
Cancel the "cancel culture", do not support mob hatred.
Post Reply