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

[SoulFetish]

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/

Thanks for the link! Looks like a really great site

[Helmholtz]

I kind of like the "toroid" because the flux is confined and the complete magnetic path gives more H.

And a toroidal core has minimal leakage and avoids the PU interference issue as reported by Chuck. But as the choke will have to handle several amp(ere)s, saturation may be an issue without an airgap.

On the other hand I think a steel box for the load should effectively shield against radiated mag. fields interfering with the PUs even with a rod core coil.

[Zozobra]

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.

The bad cat unleashed was sort of like that iirc.

In more recent news Boss have just announced something similar with a lot of bells and whistled to boot:
https://www.boss.info/us/products/wa..._amp_expander/

[Mike Sulzer]

The bad cat unleashed was sort of like that iirc.

In more recent news Boss have just announced something similar with a lot of bells and whistled to boot:
https://www.boss.info/us/products/wa..._amp_expander/

bad cat:

HERE'S HOW IT WORKS

The Unleash V2 converts the high voltage speaker output of any guitar amplifier up to 100 watts into a line level signal, sends it to our proprietary reactive load circuit that responds like a speaker and then into an ultra-transparent class D amplifier which allows any wattage amplifier to be adjusted between 1-100 watts without any change in dynamics or tone.


It looks as though they are attenuating the amp output, and then feeding that through a network that simulates a guitar speaker. IMO that misses the point completely; the idea is to load the amp properly so it responds as it would into a guitar speaker, and then attenuate, or reamplify, and then feed a guitar speaker. Perhaps it is just a very bad explanation.

Boss:

The boss might be loading the amp properly, but what it does after that is not clear to me, except there is an ss amp for reamplifying.

[Mike Sulzer]

And a toroidal core has minimal leakage and avoids the PU interference issue as reported by Chuck. But as the choke will have to handle several amp(ere)s, saturation may be an issue without an airgap.

On the other hand I think a steel box for the load should effectively shield against radiated mag. fields interfering with the PUs even with a rod core coil.

Yes, you would need enough steel so that it would not saturate. On the other hand, the rod would have an effective permeability of under 10, and so I think you would need a lot more turns. I am not sure which weighs less: the steel shielding box or the steel toroid.

I have some 1 mH (I think) coils intended for crossovers, air core. Maybe it would be easiest to make a "Chuck H" sandwich, and then experiment with adding more metal until the eddy current loss is about right.

[Chuck H]

I have some 1 mH (I think) coils intended for crossovers, air core. Maybe it would be easiest to make a "Chuck H" sandwich, and then experiment with adding more metal until the eddy current loss is about right.

Once you take that sandwich to such gourmet levels I think it's fair to rename it We'll call it the "Big Mike"

[nickb]

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.

And that is exactly what a properly configured transimpedance amp will do for you. The forward amp is a voltage amplifier with variable gain. That puts whatever voltage that appears on the tube amp terminals across the actual speaker. You have a second reverse amplifier that senses the speaker current and voltage which loads the tube amp so that it thinks it's loaded by a real speaker.

[Mike Sulzer]

And that is exactly what a properly configured transimpedance amp will do for you. The forward amp is a voltage amplifier with variable gain. That puts whatever voltage that appears on the tube amp terminals across the actual speaker. You have a second reverse amplifier that senses the speaker current and voltage which loads the tube amp so that it thinks it's loaded by a real speaker.

This is a deeper use of "transimpedance" than I am used to. I was thinking of a transimpedance amp as one that converts a current into a voltage, nothing more. I did spend some time last week thinking abut a method to transfer the impedance of the speaker that the forward amp drives back to the tube amp. I did not figure out anything that did not involve taking a ratio (too difficult and complicated) and so am probably missing something, and so I am going with the "load it, follow it, and forget it" method.

[Mike Sulzer]

Once you take that sandwich to such gourmet levels I think it's fair to rename it We'll call it the "Big Mike"

And that ain't no whopper.

[nevetslab]

The Boss Waza Tube Amp Expander box sounds like the sort of device everyone has been trying to achieve. Sort of takes all the fun out of the physics lab efforts, as the chase is so invigorating, as seen following your thread. Guess I'll have to make it to NAMM show this January, or stop by at the Roland Industry Office across the street to see what they know about it. We have Artist Relations offices here at our rehearsal studio complex...Fender, Zildjian, Paul Reed Smith, Roland, to name a few.

[nickb]

Yes, you would need enough steel so that it would not saturate. On the other hand, the rod would have an effective permeability of under 10, and so I think you would need a lot more turns. I am not sure which weighs less: the steel shielding box or the steel toroid.

I have some 1 mH (I think) coils intended for crossovers, air core. Maybe it would be easiest to make a "Chuck H" sandwich, and then experiment with adding more metal until the eddy current loss is about right.

Just a suggestion. Before putting humongous effort into reproducing every nuance that you can measure might it not be a good idea to figure out which ones actually have an audible effect? I bring this up as I was particularly struck by olddawg, a regularly gigging musician on the road and in the studio, who finds a simple resistive attenuator quite satisfactory.

[Mike Sulzer]

The Boss Waza Tube Amp Expander box sounds like the sort of device everyone has been trying to achieve. Sort of takes all the fun out of the physics lab efforts, as the chase is so invigorating, as seen following your thread. Guess I'll have to make it to NAMM show this January, or stop by at the Roland Industry Office across the street to see what they know about it. We have Artist Relations offices here at our rehearsal studio complex...Fender, Zildjian, Paul Reed Smith, Roland, to name a few.

Maybe we will find out more about that next year when it is available. Its vast array of effects will be appealing to some, perhaps many, but that all comes at a price. What we are discussing here has one specific simple purpose.

[Mike Sulzer]

Just a suggestion. Before putting humongous effort into reproducing every nuance that you can measure might it not be a good idea to figure out which ones actually have an audible effect? I bring this up as I was particularly struck by olddawg, a regularly gigging musician on the road and in the studio, who finds a simple resistive attenuator quite satisfactory.

That makes sense, and I think the way to do that is to build this thing we are discussing, which is really not so complicated, and then compare it to a simple attenuator. This might sound unnecessarily indirect, but in my experience nothing not so loud sounds exactly like something loud because there are effects related to hearing and speaker performance. Thus you want to compare the not so loud things and see what is best.

[Dave H]

The Boss Waza Tube Amp Expander box sounds like the sort of device everyone has been trying to achieve.

But is it waterproof?
Up here in the north of England to waz means to urinate. A waza would be someone who urinates.

[nickb]

But is it waterproof?
Up here in the north of England to waz means to urinate. A waza would be someone who urinates.

Another marketing triumph. It's like the Toyta MR2 was a disaster in France as "merde"sounds like MR2 and means "sh^t".

Sorry Mike - I promise not to divert your excellent thread again.