Phono Preamps

A resource for technical documentation. Datasheets, application notes, instruction manuals, books and links to resources are found in the Document Library.
User avatar
mediatechnology
Posts: 4134
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: Phono Preamps

Post by mediatechnology » Mon Dec 22, 2014 4:45 pm

One of the things I really like about the P-10 is the differential input and common mode rejection in the following stage.
There seem to be benefits to the balanced input that are realized even without re-wiring the head shell and tone arm leads to be fully-balanced.
https://ka-electronics.com

"5G Launches In Wuhan Weeks Before Coronavirus Outbreak" https://banned.video/watch?id=5e335e772f3c020019a423f8
"Biggest Medical Scandal In History Breaking! UN Comes Clean, Admits Vaccine Death/Damage Coverup" https://banned.video/watch?id=5e1b9ed434b802001ed99b14
Mirror/Download: https://proaudiodesignforum.com/content ... overup.mp4
More: https://proaudiodesignforum.com/content ... People.mp4

User avatar
mediatechnology
Posts: 4134
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

P10 RIAA EQ Network

Post by mediatechnology » Mon Jan 12, 2015 12:34 pm

While looking at your P10 - and for that matter most of your designs JR - I always notice something a little different and not immediately obvious.
Take the RIAA EQ network.
I've never seen another example like it though there may be some.


Image
Roberts P10 Phono Preamp RIAA Network

Most of the inverting and non-inverting "all-in-one" RIAA EQ networks typically have the feedback taken from the junction of R15 and R16.

Lets' see if I got this right:

The P10 RIAA EQ network is different: Only the 3180 us breakpoint is within the output to inverting input feedback loop.
The 3180 us breakpoint appears to be formed by R17(C7||C9) = 3178.9 us
The 318 us breakpoint is in the input leg formed by R16(C6||C8) = 317.89 us
The 75 us isn't so obvious: It's formed by (R15||R16)(C6||C8) = 9.02K * 8300 pF = 74.87 us

I don't know what your motivation actually was so let me take a guess:

By moving the feedback point to the right one node (the junction of R16.R17 vs R15.R16) the interaction between the 3180 and 318 us breakpoints is eliminated.
With the conventional location used for feedback, the 75 us and 3180 interact with each other to reduce the accuracy of the 318 us point.
With the modified location, the 75 us and 318 interact, the only major consequence of which is that R15's value get's "fudged" (raised) due to R16 lowering it's Thevenin equivalent.
This approach also requires only a single precision capacitance value. (The 100 pF tolerance doesn't have to be that tight.)

It also appears that the network to the left of the feedback point - the 75 us and 318 us - are actually preemphasis which "bump out" the falling 3180 us response to form the proper 318 us to 75 us shelf and eventual 75 us roll off.

I have some nice 8200 pF 2.5% polystyrene from the old days and building Jung's preamp.
I wanted to find something a little more modern in polypropylene.
Mouser doesn't stock the Wima films I'd like to use in 8200 pF and in looking for alternate values I was had-pressed to find an RC combination that hit, 75, 318 and 3180 with the same precision as 8300 pF.
You must have spent a lot of time finding the sweet spot of E96 combinations.
Fortunately Mouser do stock 6800 and 1500 pF so 8300 pF is easily accomplished.

How close am I and are there other benefits to the topology?
What is the 1 kHz gain?
https://ka-electronics.com

"5G Launches In Wuhan Weeks Before Coronavirus Outbreak" https://banned.video/watch?id=5e335e772f3c020019a423f8
"Biggest Medical Scandal In History Breaking! UN Comes Clean, Admits Vaccine Death/Damage Coverup" https://banned.video/watch?id=5e1b9ed434b802001ed99b14
Mirror/Download: https://proaudiodesignforum.com/content ... overup.mp4
More: https://proaudiodesignforum.com/content ... People.mp4

User avatar
JR.
Posts: 3152
Joined: Sat May 24, 2008 7:21 pm
Contact:

Re: P10 RIAA EQ Network

Post by JR. » Mon Jan 12, 2015 1:40 pm

mediatechnology wrote:While looking at your P10 - and for that matter most of your designs JR - I always notice something a little different and not immediately obvious.

Thanks i generally give these designs a lot of thought.

Take the RIAA EQ network.
I've never seen another example like it though there may be some.
AFAIK I was the first and probably only one. I never did like the conventional one stage NI preamp topology.
Image
Roberts P10 Phono Preamp RIAA Network

Most of the inverting and non-inverting "all-in-one" RIAA EQ networks typically have the feedback taken from the junction of R15 and R16.

Lets' see if I got this right:

The P10 RIAA EQ network is different: Only the 3180 us breakpoint is within the output to inverting input feedback loop.
The 3180 us breakpoint appears to be formed by R17(C7||C9) = 3178.9 us
The 318 us breakpoint is in the input leg formed by R16(C6||C8) = 317.89 us
The 75 us isn't so obvious: It's formed by (R15||R16)(C6||C8) = 9.02K * 8300 pF = 74.87 us
Yes the 3180 is the feedback pole.

The 75uSec pole and 318uSec zero is in the input network.
I don't know what your motivation actually was so let me take a guess:

By moving the feedback point to the right one node (the junction of R16.R17 vs R15.R16) the interaction between the 3180 and 318 us breakpoints is eliminated.
yes the 3180 pole is simpler, but 75 and 318 still interact.
With the conventional location used for feedback, the 75 us and 3180 interact with each other to reduce the accuracy of the 318 us point.
With the modified location, the 75 us and 318 interact, the only major consequence of which is that R15's value get's "fudged" (raised) due to R16 lowering it's Thevenin equivalent.
This approach also requires only a single precision capacitance value. (The 100 pF tolerance doesn't have to be that tight.)
Yes this was indeed part of it.. I had a couple common values of good sound quality polystyrene (I used 1000pF and 8200pF in my earlier parametric EQ)
It also appears that the network to the left of the feedback point - the 75 us and 318 us - are actually preemphasis which "bump out" the falling 3180 us response to form the proper 318 us to 75 us shelf and eventual 75 us roll off.
ding ding ding... and exactly the same result as any well executed approach that tracks RIAA accurately.

One not so subtle characteristic is that the 75uSec pole continues to roll off forever so no zero at HF like with NI topology.

A subtle consideration is that the pole in the RIAA at 3180 uSec is not hugely different than the pole in the TL07x open loop transfer function so for much of the audio range the loop gain margin (difference between open loop and closed loop gain) is in phase and constant. Sorry this was pretty tweaky and well over the head of most customers. In theory making the input error voltage more well behaved should help, while arguably over-engineered.

Today in hindsight I might claim that I used the inverting topology to eliminate the FET input distortion from mismatched impedance. :lol: (but that would be a lie)

For the rest of the story, No less than Stanley Lipshitz (of Waterloo University fame) pointed out an error in my 75uSec math due to reactance of the series coupling caps. The error was only 1 or 2% IIRC but to make it closer to perfect (on paper at least) I replaced the .33uF cap with a 20 ohm resistor. The .15uF cap (optional IEC 7950 uSec pole) got an added 510 ohm resistor in series to tweak the 75/318 uSec time constants to be precisely on RIAA centers.
I have some nice 8200 pF 2.5% polystyrene from the old days and building Jung's preamp.
I wanted to find something a little more modern in polypropylene.
Mouser doesn't stock the Wima films I'd like to use in 8200 pF and in looking for alternate values I was had-pressed to find an RC combination that hit, 75, 318 and 3180 with the same precision as 8300 pF.
You must have spent a lot of time finding the sweet spot of E96 combinations.
Fortunately Mouser do stock 6800 and 1500 pF so 8300 pF is easily accomplished.
Honestly I don't think I gave it a ton of thought. I had the 8200 pF in stock and used the 100pF in my input termination, so when I could nail the time constants using 1% resistors I stuck a fork in it.
How close am I and are there other benefits to the topology?
What is the 1 kHz gain?
I do not recall the gain of that particular stage but I generally dialed in my MM phono preamps for +40dB at 1kHz total. The front end is roughly flat +30 dB making that EQ stage +10dB at 1kHz (give or take).

This preamp was over engineered for the task, my later p-100 was just silly excessive design. I even had next generation ideas kicking around after that but finally grew up and realized the world didn't need any more new phono preamps. :oops:

JR

PS: Note the P-10 only used electrolytic caps inside global feedback loops so they were sonically well behaved. Audible frequency poles used film caps.
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

carlmart
Posts: 169
Joined: Mon May 19, 2014 6:11 am

Re: Phono Preamps

Post by carlmart » Mon Jan 12, 2015 2:26 pm

Code: Select all

PS: Note the P-10 only used electrolytic caps inside global feedback loops so they were sonically well behaved. Audible frequency poles used film caps.
Isn't the RIAA filter inside the global feedback too? So why do you need to use top quality parts there too?

User avatar
mediatechnology
Posts: 4134
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: Phono Preamps

Post by mediatechnology » Mon Jan 12, 2015 2:55 pm

One not so subtle characteristic is that the 75uSec pole continues to roll off forever so no zero at HF like with NI topology.
Yes, I see that now.
As long as the inverting input is held at virtual ground it rolls off forever because C6 and C8's right-hand side is "grounded."
Very cool.

I suspect that when designing RIAA networks designers have looked at the curve reading left to right as we do in English and seeing 3180, 318 and 75.
Then they design the feedback network the same way dealing with 3180 first, then 318, 75 etc.
Drawn as feedback, that network flows from right to left 3180, 318, 75.
In most "all-in-one EQ" stages we see the big R/mid-size C, flowing left to the smaller R/smaller C, then to the inverting input.
The schematic follows the linear thought process.

The P10 network follows 75, 318, 3180 in a reverse order and conforms to "JR's passive poles early" doctrine.

Thanks for the insight.
https://ka-electronics.com

"5G Launches In Wuhan Weeks Before Coronavirus Outbreak" https://banned.video/watch?id=5e335e772f3c020019a423f8
"Biggest Medical Scandal In History Breaking! UN Comes Clean, Admits Vaccine Death/Damage Coverup" https://banned.video/watch?id=5e1b9ed434b802001ed99b14
Mirror/Download: https://proaudiodesignforum.com/content ... overup.mp4
More: https://proaudiodesignforum.com/content ... People.mp4

User avatar
JR.
Posts: 3152
Joined: Sat May 24, 2008 7:21 pm
Contact:

Re: Phono Preamps

Post by JR. » Mon Jan 12, 2015 3:49 pm

carlmart wrote:

Code: Select all

PS: Note the P-10 only used electrolytic caps inside global feedback loops so they were sonically well behaved. Audible frequency poles used film caps.
Isn't the RIAA filter inside the global feedback too? So why do you need to use top quality parts there too?
Sorry if I wasn't clear. In the P-10 front end, a variant "Cohen" topology (I really hate calling it that since I never heard it called Cohen until decades later), I used electrolytic caps in the open-loop gain path. Global closed loop gain effectively trades off high open loop gain to linearize and reduce the contribution of any non-ideal open loop behavior (it's how negative feedback works).

OTOH capacitors used for RIAA time constants are in the closed loop NFB path so any deviation from ideal performance will express into the actual transfer function. What''s even worse is that the RIAA filter poles and zeros are smack dab in the middle of our hearing sensitivity (50 Hz,500 Hz, and 2.1 kHz so any related artifacts "could" be audible. I am not suggesting that RIAA EQ caps are a huge problem but if any caps are audible these seem like obvious candidates (not counting passive speaker crossovers). In my P-10 and later P-100 I used polystyrene for the RIAA EQ time constants, and polyester for the LF HPF.

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

User avatar
mediatechnology
Posts: 4134
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: Phono Preamps

Post by mediatechnology » Tue Feb 04, 2020 2:48 pm

I've been meaning to scan these two 1987 examples of balanced input preamps from "He who shall not be named."

Image

Image

Going through old books recently I stumbled on these two examples.
It pays to go back and review what I may have overlooked at the time.
https://ka-electronics.com

"5G Launches In Wuhan Weeks Before Coronavirus Outbreak" https://banned.video/watch?id=5e335e772f3c020019a423f8
"Biggest Medical Scandal In History Breaking! UN Comes Clean, Admits Vaccine Death/Damage Coverup" https://banned.video/watch?id=5e1b9ed434b802001ed99b14
Mirror/Download: https://proaudiodesignforum.com/content ... overup.mp4
More: https://proaudiodesignforum.com/content ... People.mp4

User avatar
JR.
Posts: 3152
Joined: Sat May 24, 2008 7:21 pm
Contact:

Re: Phono Preamps

Post by JR. » Wed Feb 05, 2020 11:56 am

In case I haven't mentioned it recently, back in the early '80s after I published the P-10 kit article, some puke approached me for royalty payments because he managed to convince the patent examiners that a balanced input phono preamp was novel. :roll: :roll:

I ordered a copy of the patent wrapper (arguments with examiner) and apparently the wet behind the ears examiner was blown away by the new to him concept of balanced inputs.

I sent him a zerox of an old tube phono preamp with transformer (balanced) input. To avoid dueling lawyers I unbalanced my kit instructions. :lol:

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

User avatar
mediatechnology
Posts: 4134
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: Phono Preamps

Post by mediatechnology » Wed Feb 05, 2020 12:16 pm

That would have been Logitek: https://proaudiodesignforum.com/images/ ... lifier.pdf

BTW the "5W" Zener in the above-referenced MC preamp should have been 0.5W since it's only dissipating 56 mW.

Question for you JR...

If we connect an MC cart between the emitters instead of a gain resistor in a balanced BJT front end like the flat MC preamp (mine not the one above) it get's called a "current input."

But is it really a current input?
A differential voltage is developed across the cart with feedback brought to the ends.

Looking at it sideways it's a common base amp with current feedback, in the form of a U-pad, brought to the emitters and cart +/- nodes.
The emitter/cart node can't be a "virtual ground" because the opposite end of the cart isn't at AC ground - it's also floating.
A differential potential develops across the cart in this topology despite the feed back being "current feedback."

When a cart is connected into a balanced transimpedance input isn't it unterminated differentially?
What defines the cart's differential load impedance?
Without feedback wouldn't the differential input impedance be the sum of the (two) common mode input impedances of the "common base" amps?
With feedback applied and returning current that cancels that of the input current flowing through the E-B junctions what does the differential input impedance become?
https://ka-electronics.com

"5G Launches In Wuhan Weeks Before Coronavirus Outbreak" https://banned.video/watch?id=5e335e772f3c020019a423f8
"Biggest Medical Scandal In History Breaking! UN Comes Clean, Admits Vaccine Death/Damage Coverup" https://banned.video/watch?id=5e1b9ed434b802001ed99b14
Mirror/Download: https://proaudiodesignforum.com/content ... overup.mp4
More: https://proaudiodesignforum.com/content ... People.mp4

Post Reply