Attenuators between a tube amp and the guitar speaker: some measurements and theory

[nickb]

Suppose we haver a perfect device that can sample the voltage across a guitar speaker without affecting it and reproduce it exactly into any load at any power level. Connect an identical guitar speaker to the output. Would this second speaker sound just like the one one that is sampled? I think it must. Add a volume control to this device and you can scale the output while maintaining the sound as close as possible to the original given that the speaker is somewhat power sensitive as is human hearing.

Now replace the speaker that is being sampled with a silent device that has the same impedance, and therefore, the same voltage across it. Now "the device" and the speaker connected to it are a very good attenuator or booster. Off course, a good ss amp approaches this ideal device very closely. Given that electric guitar is not exactly audiophile, the ss amp might not have to be the very best. A light, inexpensive switching amp might be good enough.

Unlike many ideas, this one is double blind testable. You can can have someone else switch between the two, or not, and see what you can hear without knowing which is which ahead of time.

This has similarities to the trans-impedance power amplifier used in some VOX's and the Fender Recording amp. The difference is the TI amp reflects the impedance of the actual speaker back the the driver amplifier's output terminals.

[Malcolm Irving]

...
According to Zollner ….
... in the literature (Zollner).

Sorry to go slightly off-thread, but the book by Zollner does seem to be very useful and interesting. I found a rough English translation on the web, but is there any plan to publish a full English version?

[Leo_Gnardo]

Suppose we haver a perfect device that can sample the voltage across a guitar speaker without affecting it and reproduce it exactly into any load at any power level. Connect an identical guitar speaker to the output. Would this second speaker sound just like the one one that is sampled? I think it must. Add a volume control to this device and you can scale the output while maintaining the sound as close as possible to the original given that the speaker is somewhat power sensitive as is human hearing.

Now replace the speaker that is being sampled with a silent device that has the same impedance, and therefore, the same voltage across it. Now "the device" and the speaker connected to it are a very good attenuator or booster. Off course, a good ss amp approaches this ideal device very closely. Given that electric guitar is not exactly audiophile, the ss amp might not have to be the very best. A light, inexpensive switching amp might be good enough.

Unlike many ideas, this one is double blind testable. You can can have someone else switch between the two, or not, and see what you can hear without knowing which is which ahead of time.

I think this idea was tested pretty well around 1973-74:



For the guitars & bass anyway, each column of speakers powered by pretty good hi fi amps for the times, McIntosh MC2300's. Driven from Twin Reverb Fenders. Yeah, I'd say "that works!" I'm sure the crowds agreed. Those who like it, like it a lot. Those who don't, well what can ya say...

[Mike Sulzer]

This has similarities to the trans-impedance power amplifier used in some VOX's and the Fender Recording amp. The difference is the TI amp reflects the impedance of the actual speaker back the the driver amplifier's output terminals.

What I am thinking of here is two simple pieces:

1. A perfect voltage follower, or rather a follower with gain.

2. A silent device that has the same impedance as a speaker.

The first is what a good ss amp strives to be, The second has not yet been discussed here, but I think a passive device should do the job. Chuck H has sone posts with some pretty nice components, including a hum canceling inductor, and so I was thinking of building on that. For example, it appears that the inductor needs some eddy current losses. Could one use a short stack of really large steel washers as a poor's man toroid? I think I will try that and see how it works.

[Gregg]

A very informative thread on Aiken's reactive dummy load:

https://www.thegearpage.net/board/in...-load.1072793/

Speaking of reactive attenuators one must also take into consideration that hooking up a speaker to a reactive load will boost the reactive effect (if not properly designed).

[Mike Sulzer]

I think this idea was tested pretty well around 1973-74:



For the guitars & bass anyway, each column of speakers powered by pretty good hi fi amps for the times, McIntosh MC2300's. Driven from Twin Reverb Fenders. Yeah, I'd say "that works!" I'm sure the crowds agreed. Those who like it, like it a lot. Those who don't, well what can ya say...

That takes care of the boost! Not so sure about the attenuate.

[Helmholtz]

Could one use a short stack of really large steel washers as a poor's man toroid? I think I will try that and see how it works.

Why not? Problem: Undefined resistance between the washers. Eddy currents will want to travel around the cross section.
Even the aluminum mounting/separating plate in Chuck's design helps somewhat.
But why not use a solid steel rod as core in a 1mH choke?

I use a modified Marshall PowerBrake PB 100 with 1mH//33 Ohm but plan to split the choke.
I measured my 60s VOX alnicos and found average series inductance values of 0.7mH@1kHz and 0.4mH@10kHz.

Zollner shows a design with 0.5mH//6R in series with 0.33mH//60R. The bass resonance is realized with 12mH//250µ.

[Leo_Gnardo]

But why not use a solid steel rod as core for a 1mH choke?

I figure that's a good idea. If you're trying to emulate a real speaker voice coil, they have steel magnet poles both inside & outside the coil. Which is bound to have some inductance magnifying effect over a voice coil type inductor without any iron/steel nearby.

[Helmholtz]

I figure that's a good idea. If you're trying to emulate a real speaker voice coil, they have steel magnet poles both inside & outside the coil. Which is bound to have some inductance magnifying effect over a voice coil type inductor without any iron/steel nearby.

Of course I meant winding it to around 1mH with core or start with lower value air core coil.

[Helmholtz]

What I am thinking of here is two simple pieces:

1. A perfect voltage follower, or rather a follower with gain.

2. A silent device that has the same impedance as a speaker.

Exactly that^^ would have been one of my proposals too. BTW, when I use the term "reactive dummy load", I always mean "A silent device that has the same impedance as a speaker". The HF choke doesn't necessarily need to have Eddy current losses. The upper frequency response of a speaker can be modelled/fitted very well with 2 chokes and parallel damping resistors. The bass resonance requires an additional parallel resonant circuit wired in series with the DCR and the HF choke(s).

[Malcolm Irving]

Suppose we haver a perfect device that can sample the voltage across a guitar speaker without affecting it and reproduce it exactly into any load at any power level. Connect an identical guitar speaker to the output. Would this second speaker sound just like the one one that is sampled? I think it must. Add a volume control to this device and you can scale the output while maintaining the sound as close as possible to the original given that the speaker is somewhat power sensitive as is human hearing.

Now replace the speaker that is being sampled with a silent device that has the same impedance, and therefore, the same voltage across it.
…

In addition to having an impedance, the real speaker generates a back e.m.f.
Is it necessary for the 'silent device' to also generate this back e.m.f.?

[Helmholtz]

Sorry to go slightly off-thread, but the book by Zollner does seem to be very useful and interesting. I found a rough English translation on the web, but is there any plan to publish a full English version?

Yes, Prof. Zollner's book is highly valuable (to me) and the most comprehensive opus on electric guitars and equipment I know. The german version comprises more than 1200 pages in 2 volumes (I payed ca. 150€ for both some years ago). I think they call it POTEG= Physics of The Electric Guitar in English. Meanwhile he has published a third volume covering electroacoustics for stage and studio (about 600 pages).
Zollner is the mastermind of GITEC. AFAIK they started English translation activities about 2 years ago. You may find out about the progress contacting them via their webpage:
https://gitec-forum.de/wp/en/gitec-c...tars-and-gear/

[Helmholtz]

In addition to having an impedance, the real speaker generates a back e.m.f.
Is it necessary for the 'silent device' to also generate this back e.m.f.?

Yes, but this reflects in the complex impedance as this determines the interaction and relation between current and voltage. If the reactive dummy load shows the same impedance, it should be fine. The real speaker stores and releases motional energy and re-converts it into electrical energy while the reactive load directly stores and releases electrical energy by its reactive components.

[Mike Sulzer]

In addition to having an impedance, the real speaker generates a back e.m.f.
Is it necessary for the 'silent device' to also generate this back e.m.f.?

As Helmholtz said.... But let's also look at it from a bit different viewpoint. Do the following for a frequency, and then generalize it for a function of frequency in the audio range:

1. Apply a sinusoidal voltage. It has an amplitude and the locations of its zero crossing can serve a phase reference.

2. Measure the resulting current, both amplitude and phase relationship to the driving voltage. (We assume that a single sine wave is produced; this is a result of linearity.)

3. Then ask this question: is there any other property that the response could have?

The answer is no. The ratio of amplitudes and difference of the phases is all the information there is because a sinusoid is completely determined by its amplitude and phase.

[Mike Sulzer]

Why not? Problem: Undefined resistance between the washers. Eddy currents will want to travel around the cross section.
Even the aluminum mounting/separating plate in Chuck's design helps somewhat.
But why not use a solid steel rod as core in a 1mH choke?

I use a modified Marshall PowerBrake PB 100 with 1mH//33 Ohm but plan to split the choke.
I measured my 60s VOX alnicos and found average series inductance values of 0.7mH@1kHz and 0.4mH@10kHz.

Zollner shows a design with 0.5mH//6R in series with 0.33mH//60R. The bass resonance is realized with 12mH//250µ.

One can spray the washers with lacquer to insulate them, choosing washers with the right thickness. All I found in the garage was a nut that fits on a 3/8" rod. The inductance drops a factor of three between 120 and 1 000 Hz. That is too much.

I kind of like the "toroid" because the flux is confined and the complete magnetic path gives more H. But the rod could be more practical.