~QUESTIONS ABOUT "LOCAL"-NEGATIVE-FEEDBACK OUTPUT SECTIONS THAT ARE BASED ON A TERTIARY-WINDING OUTPUT TRANSFORMER~

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~PSYGN~
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~QUESTIONS ABOUT "LOCAL"-NEGATIVE-FEEDBACK OUTPUT SECTIONS THAT ARE BASED ON A TERTIARY-WINDING OUTPUT TRANSFORMER~

Post by ~PSYGN~ » Tue Jul 23, 2019 12:36 am

~hello all( especially Wayne )...
I've tried to do my due-diligence-web research on this subject, to make the muddy waters of this topology a little more clear & have not had much luck unfortunately. What little i could scrounge( & it's really not much ), aside from being vague & devoid of systematic absolutes on the process...most, if not all, was addressed to the context of guitar & bass amps/power amps for driving live, large-scale installations. I'm interested in getting a handle on the concept, as it applies to high-end professional preamps/eq's/comp-lim's etc. In other words, the most refined, elegant & broad-audio-spectrum accommodating iterations of such, that i can learn to implement. At this point, i've zeroed-in on one specific version of local-negative-feedback that, i want to understand & incorporate into some current builds.. which, is the kind derived by utilising custom output transformers, with a purpose-built tertiary(feedback) winding designed for that function. I know that those are normally associated with tube-output stages..but, the only info i could *somewhat* derive aside from that is, that it may be in a cathode-follower type configuration? If that is correct, is there a *best-of-the-best* version, which lends itself particularly well to high-end audio production gear? Is it ONLY something that can be implemented through a tube-output-stage configuration or, can it be utilised in solid-state circuit designs as well? I have a specific transformer set that was designed for this purpose, in it's original application...i have all of the transformer's parameter-data & specifications, as well as, a step-by-step method of how to utilise it in a *normal*, non-feedback based orientation..which, i'm sure will work quite well in it's own right, to be sure but...because of the build quality & top-shelf nature of the transformer itself & corresponding high-end mastering processor that, it was produced in relatively small-batch-orders to complete..i can't therefore help but, to wonder if this is a situation that warrants using it in it's intended configuration( i.e., will it make a significant difference over using it without the negative-feedback tertiary winding? )
I will post the relevant transformer data below & it bears mentioning that, these transformers were designed by R. Neve, for use in the output section of the mastering system he designed in collaboration with Legendary Audio called, the "Masterpiece". They were unused shelf-stock that, never found their way into a build, due to production ceasing on account of parts unavailability. They apparently are modern renditions in design & spec, of the output transformers wound for the "Air MontSerrat" studio console...beefy as hell toroids. Figured that it'd be worth looking into, especially considering that i will be using these in a mastering processor capacity as well. So, on that note, any information that anyone can provide on the subject would be IMMENSELY appreciated! Even if it's just pointing me in the proper research direction, ANYTHING WOULD BE GREAT!!! Thanks either way for enduring my rant 😜~
{{{ SPECS AS FOLLOW: }}}
Transformer Specs:
RED = Signal In
BROWN = Input Winding (Grounded in circuit)
Input Impedance Measurement = 7.4 Ohms
ORANGE = Signal Out +
YELLOW = Signal Out -
Output Impedance Measurement = 11.2 Ohms
BLUE = Tertiary (Feedback) Winding Input
GREEN = Tertiary Winding (Grounded in circuit)
Tertiary Impedance Measurement = 9.0 Ohms
BLACK (x2) = Screen/Case most likely. These should always be connected
to analog ground on the circuit board. (if there is a circuit board)

Configuration for passive use:

Connect the wires as below for +2.5 dB of gain and tone:

Blue remains open with no connection.

Red + Green = tie together, use for INPUT
Brown + Blue + Black (x2) = tie all to GROUND
Yellow = OUTPUT -
Orange = OUTPUT +

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Re: ~QUESTIONS ABOUT "LOCAL"-NEGATIVE-FEEDBACK OUTPUT SECTIONS THAT ARE BASED ON A TERTIARY-WINDING OUTPUT TRANSFORMER~

Post by mediatechnology » Tue Jul 23, 2019 4:42 am

Thanks for posting and joining us here!

Are the impedance values you gave the DC resistance?

I'm thinking you might find some design examples of feedback windings in test equipment outputs.
Maybe the AP output patent?
https://ka-electronics.com

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Re: ~QUESTIONS ABOUT "LOCAL"-NEGATIVE-FEEDBACK OUTPUT SECTIONS THAT ARE BASED ON A TERTIARY-WINDING OUTPUT TRANSFORMER~

Post by KMN » Tue Jul 23, 2019 11:58 pm

Did you measure the signal coming off the tertiary when you energized the primary? The 9 ohms could mean there is a step up there too but might also just mean smaller wire diameter.

Curious if the term you've used "tertiary winding" is that yours or is it in the spec sheet? The reason I ask is that may be a clue. I think there may be very specific meanings for it used in certain contexts.

Sorry I'm not of more use. By the way. for more useless knowledge There are a ton of switching power supplies with using extra "feedback" windings, usually in a sort of flyback mode for self powering off high voltage.

Now I'm curious about the schematics too. Had to play with transformer amp feedback in spice for a few minutes. oo transformers are magic.

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Re: ~QUESTIONS ABOUT "LOCAL"-NEGATIVE-FEEDBACK OUTPUT SECTIONS THAT ARE BASED ON A TERTIARY-WINDING OUTPUT TRANSFORMER~

Post by mediatechnology » Wed Jul 24, 2019 4:46 am

I wished I had more experience with this to comment too.

The real expert on this IMHO is CJ at GroupDIY.
Joe Neil was working on the Pye comp about the time he died and I recall it used transformer NFB.
https://ka-electronics.com

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Re: ~QUESTIONS ABOUT "LOCAL"-NEGATIVE-FEEDBACK OUTPUT SECTIONS THAT ARE BASED ON A TERTIARY-WINDING OUTPUT TRANSFORMER~

Post by JR. » Wed Jul 24, 2019 2:45 pm

I have tried to remain mum since this is another one of many areas I lack hands on experience. Back in the 70s I wasted some bench time trying to glom together a cheap transformer with opamps to make something better than each alone... my effort was not promising so summarily abandoned.
~PSYGN~ wrote:
Tue Jul 23, 2019 12:36 am
~hello all( especially Wayne )...
I've tried to do my due-diligence-web research on this subject, to make the muddy waters of this topology a little more clear & have not had much luck unfortunately. What little i could scrounge( & it's really not much ), aside from being vague & devoid of systematic absolutes on the process...most, if not all, was addressed to the context of guitar & bass amps/power amps for driving live, large-scale installations. I'm interested in getting a handle on the concept, as it applies to high-end professional preamps/eq's/comp-lim's etc. In other words, the most refined, elegant & broad-audio-spectrum accommodating iterations of such, that i can learn to implement.
I am not aware of much work in this specific application area. I have seen trick transformer applications in summing amps, and my first technician job in the 60s involve magnetic amplifiers used for current limiting inside an early switching PS.
At this point, i've zeroed-in on one specific version of local-negative-feedback that, i want to understand & incorporate into some current builds.. which, is the kind derived by utilising custom output transformers, with a purpose-built tertiary(feedback) winding designed for that function. I know that those are normally associated with tube-output stages..but, the only info i could *somewhat* derive aside from that is, that it may be in a cathode-follower type configuration? If that is correct, is there a *best-of-the-best* version, which lends itself particularly well to high-end audio production gear? Is it ONLY something that can be implemented through a tube-output-stage configuration or, can it be utilised in solid-state circuit designs as well? I have a specific transformer set that was designed for this purpose, in it's original application...i have all of the transformer's parameter-data & specifications, as well as, a step-by-step method of how to utilise it in a *normal*, non-feedback based orientation..which, i'm sure will work quite well in it's own right, to be sure but...because of the build quality & top-shelf nature of the transformer itself & corresponding high-end mastering processor that, it was produced in relatively small-batch-orders to complete..i can't therefore help but, to wonder if this is a situation that warrants using it in it's intended configuration( i.e., will it make a significant difference over using it without the negative-feedback tertiary winding? )
In theory, and I am not a transformer guy, a floating sense winding could reveal data about the state of the magnetic flux inside the transformer.

As Wayne already mentioned AP has at least one patent for the work they did in that area. (United States Patent 4614914 Hofer 1986) That was in the context of test equipment but low distortion should always be a good thing.
I will post the relevant transformer data below & it bears mentioning that, these transformers were designed by R. Neve, for use in the output section of the mastering system he designed in collaboration with Legendary Audio called, the "Masterpiece". They were unused shelf-stock that, never found their way into a build, due to production ceasing on account of parts unavailability. They apparently are modern renditions in design & spec, of the output transformers wound for the "Air MontSerrat" studio console...beefy as hell toroids. Figured that it'd be worth looking into, especially considering that i will be using these in a mastering processor capacity as well. So, on that note, any information that anyone can provide on the subject would be IMMENSELY appreciated! Even if it's just pointing me in the proper research direction, ANYTHING WOULD BE GREAT!!! Thanks either way for enduring my rant 😜~
{{{ SPECS AS FOLLOW: }}}
Transformer Specs:
RED = Signal In
BROWN = Input Winding (Grounded in circuit)
Input Impedance Measurement = 7.4 Ohms
ORANGE = Signal Out +
YELLOW = Signal Out -
Output Impedance Measurement = 11.2 Ohms
BLUE = Tertiary (Feedback) Winding Input
GREEN = Tertiary Winding (Grounded in circuit)
Tertiary Impedance Measurement = 9.0 Ohms
BLACK (x2) = Screen/Case most likely. These should always be connected
to analog ground on the circuit board. (if there is a circuit board)

Configuration for passive use:

Connect the wires as below for +2.5 dB of gain and tone:

Blue remains open with no connection.

Red + Green = tie together, use for INPUT
Brown + Blue + Black (x2) = tie all to GROUND
Yellow = OUTPUT -
Orange = OUTPUT +
I can not make any useful sense of that... but may be my own personal problem.

Deane Jensen RIP was the smartest transformer guy I ever knew. "Beans" RIP the old transformer design engineer at the vendor who made truckloads of power transformers for us at Peavey last century knew stuff, but mostly about power transformers. Reg Williamson also RIP was famous for making transformer coupled tube amplifiers high fidelity (IIRC the bandwidth matters when transformers get included in NF loops as you would expect).

For people still alive, CJ and several others over at GroupDIY are more capable than I to provide transformer wisdom. Bill Whitlock just joined up over there, he knows stuff about transformers.

JR
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Re: ~QUESTIONS ABOUT "LOCAL"-NEGATIVE-FEEDBACK OUTPUT SECTIONS THAT ARE BASED ON A TERTIARY-WINDING OUTPUT TRANSFORMER~

Post by KMN » Wed Jul 24, 2019 3:02 pm

I found this. I beg your pardon, Wayne, if linking it turns out to be some copyright violation. Some of the schematics use an extra transformer winding for feedback.http://lcweb2.loc.gov/master/mbrs/recor ... Misc.).pdf

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Re: ~QUESTIONS ABOUT "LOCAL"-NEGATIVE-FEEDBACK OUTPUT SECTIONS THAT ARE BASED ON A TERTIARY-WINDING OUTPUT TRANSFORMER~

Post by emrr » Wed Jul 24, 2019 8:22 pm

offhand:

Altec 428B
Collins 356A
Langevin AM-5116-B
RCA BA-21A
Several of the original UA preamps, I think also one or more of their SS types. LA-3A?
There are several Lundahl designed for SS with tertiary feedback I think.
There are others.
Best,

Doug Williams
Electromagnetic Radiation Recorders

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Re: ~QUESTIONS ABOUT "LOCAL"-NEGATIVE-FEEDBACK OUTPUT SECTIONS THAT ARE BASED ON A TERTIARY-WINDING OUTPUT TRANSFORMER~

Post by mediatechnology » Wed Jul 24, 2019 8:57 pm

KMN wrote:
Wed Jul 24, 2019 3:02 pm
I found this. I beg your pardon, Wayne, if linking it turns out to be some copyright violation. Some of the schematics use an extra transformer winding for feedback.http://lcweb2.loc.gov/master/mbrs/recor ... Misc.).pdf
Kevin - Thanks for posting that. It looks like there are a lot of examples in those docs of the Neve OP transformer providing NFB to the BA440.
https://ka-electronics.com

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Re: ~QUESTIONS ABOUT "LOCAL"-NEGATIVE-FEEDBACK OUTPUT SECTIONS THAT ARE BASED ON A TERTIARY-WINDING OUTPUT TRANSFORMER~

Post by Gold » Thu Jul 25, 2019 12:14 pm

Lundahl suggests the Zero Field approach with negative source impedance at least for solid state Outputs.I don't think they make a transformer with a tertiary winding for solid state. I've never looked at their offerings for tube output.

Take a look at the Neumann PV46 schematic I posted. I believe that has two dedicated windings for negative feedback on the Output transformer primary for each polarity.
http://www.proaudiodesignforum.com/foru ... &start=240

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