Gyrator loaded tube for simulating OT and speaker

[earthtonesaudio]

I was thinking of using a gyrator to simulate the inductance of a tube amp's output transformer and speaker, in order to get the same response from the tube at lower volumes and with lighter weight components.

For a high voltage tube stage one could use a high voltage MOSFET to implement the gyrator; for starved-plate things, an op-amp would be nicer.

I imagine this is something that's been done already, but my searches have been unsuccessful.

[PRR]

A gyrator will be a nearly perfect inductance. The main result will be a bass-cut. And there are much cheaper ways to get a bass-cut (undersize the next coupling cap).

The effects of an under-sized over-stuffed low-price cheap-iron OT are much more complicated than an inductance, and not easily emulated.

[earthtonesaudio]

A gyrator will be a nearly perfect inductance. The main result will be a bass-cut. And there are much cheaper ways to get a bass-cut (undersize the next coupling cap).

The effects of an under-sized over-stuffed low-price cheap-iron OT are much more complicated than an inductance, and not easily emulated.

Define "easily emulated." 

I can guess that a tube output transformer will have some inductance in series with winding resistance, plus self-capacitance.  Also, it will exhibit some magnetic hysteresis (the amount of which depending on a class A or AB output stage, and whether/how much it is clipping).

The self capacitance can be modeled with a capacitor, the winding resistance can be modeled with a resistor in series with the gyrator, but the hysteresis losses will require a separate solution.  Perhaps instead of a resistor the tube could get power through a voltage-controlled current source, which could in turn be driven by an altered version of the input signal.

In theory, the result of all this will be a complex reactive load to get the tube behaving as if it was powering a speaker through an OT.  A simple voltage signal is easier to deal with than using a dummy load, an isolation box, re-amping, or the various other techniques that have been used to get "tube sound" at lower volumes.

[dschwartz]

Marshall uses a gyrator circuit in some o their amps, to reproduce the output impedance of a tube amp (that means, OT)
It gives a resonant peak at around 100Hz..Wich is commonly recognized as "tube punch"

search google "marshall gyrator impedance"... there´s a patent for it available.

[R.G.]

Define "easily emulated." 

OK.    Folks have been trying to do this for decades in a convincing fashion.  If it was easy, they'd already have done it. Like tube emulation by JFETs, MOSFETs, bipolars, opamp circuits, DSPs, etc. there are many patents, but precious few successful products. There's money to be made if you're good at this. The money hasn't been made yet, in spite of the patents belonging to big names in the music industry. So it's at least harder than that. 

I can guess that a tube output transformer will have some inductance in series with winding resistance, plus self-capacitance.  Also, it will exhibit some magnetic hysteresis (the amount of which depending on a class A or AB output stage, and whether/how much it is clipping).

Lemme make it easier. Go find a copy of Nathan Grossner's "Transformers for Electronic Circuits" and you'll find a simple, more complex, and yet more complex model for the transformer. You don't need to guess.

There is a nonlinear primary inductance in parallel with the reflected load, and in parallel with that a nonlinear core eddy current loss, and in parallel with that a nonlinear hysteresis loss. There is a distributed capacitance/resistance in parallel with that. There is a capacitance, again distributed and not lumped, from primary to secondary, but good first order results can be had by tweaking a lumped capacitance. There is a reflected distributed cap/resistance of the secondary windings, and then there's the reflected speaker load, which is itself part linear and part nonlinear, part of it being the electrical result of the mechanical and acoustic resonances of the speaker, and then the speaker in the box, and to some extent the room the speaker/box is in.

How hard can it be to just model that? 

There have been various degrees of success with all that. "Nonlinear" has typically meant "it could be anything" and "distributed" has historically meant "you can get as close as you like as long as you keep working at it, but you'll never get it all".

This is another one of those things where you get 80% of the benefit from the first 20% of the effort, as witness the simple progression of second order lopasses, fourth order lowpasses, additional resonators added, nonlinear diode shaping stuff, and so on as long as you'd like to keep adding parts. DSP does much the same, adding complexity as you add code. It's easy to get something where the initial results are very encouraging, but as you listen more critically, you have to refine it, and that's closer. Then you refine that. Then you refine that. Then... well, you get the idea.

The other 20% of the work takes the other 800% of the time.

But there's probably something very simple and effective that I'm overlooking.

Good luck. I'll buy one when you get it done if it's really good! 

[R.G.]

I forgot. One more complexity specific to gyrators as opposed to inductors is noise and bandwidth.

Real inductors, even tiny, high inductance ones like for instance a wah inductor, have small real resistances. A wah inductor may only have 50 ohms of resistance for 1/2 H inductance. Thermal noise is proportional to resistance, so the thermal noise in a real inductor tends to stay low. Gyrators though tend to have either resistors or active devices which generate thermal noise, and the noise is amplified by the active devices, and worse yet the impedance, by definition, rises monotonically until the active device runs out of gain to keep it going up. This is a situation made for creating noise.

I've been pretty uniformly disappointed any time I didn't use a gyrator as part of a tuned circuit where the cap associated with the circuit rolled off the composite impedance at the high end. Likewise, the gyrated inductance gets less ideal as the device runs out of gain. These are not killers, and some circuits may work just fine. But I have bad feelings about putting a gyrator inductor in parallel with the path for audio otherwise.

But your mileage may vary. 

[earthtonesaudio]

Good points, R.G.  I think "80% right" seems like a good goal.

Thanks for the tip, Daniel.  I found the Marshall patent.  I knew someone must have tried this idea before, I just couldn't find it with my searching.

[R.G.]

Good points, R.G.  I think "80% right" seems like a good goal.

Thanks for the tip, Daniel.  I found the Marshall patent.  I knew someone must have tried this idea before, I just couldn't find it with my searching.

It's certainly worth while to mess with. You can apply all the goodness and refinement you want. 80% of the goody for 20% of the effort is a great deal if you can get it! 

[petemoore]

Albert Einstein once said "The definition of insanity is doing the same thing over and over again and expecting different results".
  Can I repeat it?..oh..I just did, again.
  Here's a couple more of my absul-favorite Quotes:
  *Reality is merely an illusion, albiet a very persistant one.
  *The only thing that interfered with my learning was my education.

[PRR]

> Go find a copy of Nathan Grossner's "Transformers for Electronic Circuits"

VERY excellent book.

Focuses on "electronic" transformers, both power and "signal". You don't wade through a lot of stuff about big distribution iron.

Main "Con": you have to read it page by page. He builds like Ellery Queen. If you flip-around you won't know what's-what.

Also there is considerable RF, switcher, and pulse info, less interesting to us.

www.ABE.com has several copies around $40. If you honestly want callouses on your pencil finger, this is a bargain.

[PRR]

Grossner references Reuben Lee (sounds like a piano player) and his Electronic Transformers and Circuits for audio distortion thoughts. This is another good book and available free:

http://www.tubebooks.org/Books/Lee_1955_Electronic_Transformers_and_Circuits.pdf (23MB PDF)
http://www.vias.org/eltransformers/index.html (online HTML pages)
http://www.archive.org/details/ElectronicTransformersAndCircuits (Archive.org: PDF, Kindle, DjVu, etc)

Not transformers, but very excellent basic text:
Fundamentals of Electrical Engineering and Electronics T.R. Kuphaldt
http://www.vias.org/feee/

[earthtonesaudio]

Thanks Paul, I'm going to check those out.