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

[SoulFetish]

Mike (this may be a dumb question), but when analyzing the effect on high frequencies, were you measuring the phase shift to differentiate between the real/imaginary components of the impedance?

I suppose this answers my question -

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.

[SoulFetish]

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'm doing a little catch-up on this thread. This is a great project, so I've been wanting to take some proper time to read up on the conversation.
So this is an interesting solution. Send the full output into constant load using a silent device, rather that splitting the output ratio between the speaker and attenuator load. Then sample and amplify the voltage into the load... got it.
Interesting. Is there much added complexity in designing purely passive adjustable reactive attenuator, which led to this approach?

[catalin gramada]

That the best way found it aso with the mention I used a speaker simulator for the best. A Palmer in my case. The question is what kind of load have to be use. I blowed up two power stages using speaker equivalent reactive loads into dimmed Marshall amps. Pretty much enough for me.
The reactive devices are terrible loads.You can enjoy for a while, but is it safe enough ?

[Mike Sulzer]

I reactive load with the same impedance as the speaker should not blow up the output stage if the speaker does not. That said, there is more than one way to mess up the design or implementation.

That the best way found it aso with the mention I used a speaker simulator for the best. A Palmer in my case. The question is what kind of load have to be use. I blowed up two power stages using speaker equivalent reactive loads into dimmed Marshall amps. Pretty much enough for me.
The reactive devices are terrible loads.You can enjoy for a while, but is it safe enough ?

[SoulFetish]

I have built a prototype using stuff from the garage, simulating as necessary to get reasonable response before building.

Here is the simulated circuit also with a description of the components used in the prototype.
[ATTACH=CONFIG]51285[/ATTACH]I

Here is the Python program used for the simulation.

[ATTACH]51286[/ATTACH]

Next post, look at the computed and measured responses to look for reasonable agreement.

Could someone explain how to calculate the voltage and current distribution in the modeled circuit? I'm not sure how to specify power handling requirements where the network has reactive/storage devices and resistance/dissipation components. I'm eager to build this prototype.

[Roberto]

Interesting thread! Thanks.
If can be of any help, the best sounding attenuator I've ever tried is this one:

[Helmholtz]

That the best way found it aso with the mention I used a speaker simulator for the best. A Palmer in my case. The question is what kind of load have to be use. I blowed up two power stages using speaker equivalent reactive loads into dimmed Marshall amps. Pretty much enough for me.
The reactive devices are terrible loads.You can enjoy for a while, but is it safe enough ?

What exactly was damaged? Power tubes, screen resistors, OT?

[Helmholtz]

I think it might make sense to increase the DCR of the speaker emulation load by 20% or more. The reasoning is that the voice coil heats up considerably with power. A temperature increase of 100°C and more seems possible at the speaker's power limit. This will cause an increase of the copper resistance by up to 40% and more at real max. power operation.

[Helmholtz]

Could someone explain how to calculate the voltage and current distribution in the modeled circuit? I'm not sure how to specify power handling requirements where the network has reactive/storage devices and resistance/dissipation components. I'm eager to build this prototype.

This is best done with a circuit simulation at say 5 discrete frequency points: Below resonance, at resonance, around 400Hz and three or more frequencies above.
I don't intend to do that at the moment. Anyone volunteering?

[Helmholtz]

Is there much added complexity in designing purely passive adjustable reactive attenuator, which led to this approach?

The problem with passive attenuators is that in order to drive a speaker, the load network needs to be additionally loaded by relatively low resistances/impedances to extract some power. This in turns changes the impedance as seen by the amplifier. Also different attenuations often result in different loading.

With the active approach, a high resistance (>1k) voltage divider can be used across the load to produce a suitable input voltage for the slave amp.

[catalin gramada]

What exactly was damaged? Power tubes, screen resistors, OT?

Hello, it was the power tubes which collapses. I used a hefty reactive dummy load inspired by RA site. I enjoyed it picking up signal directly by it, send it through a Palmer simulator, then reamping into a ordinary SS but tubes collapsed in the end.
This was my attempt:

[Helmholtz]

Hello, it was the power tubes which collapses. I used a hefty reactive dummy load inspired by RA site. I enjoyed it picking up signal directly by it through a Palmer simulator, then reamping into a ordinary SS but tubes collapsed in the end.
This was my attempt:

[ATTACH=CONFIG]51657[/ATTACH]

Thanks!
So you did not use the Palmer power attenuator but something like a Palmer DI box with speaker simulation together with your self made load? Why did you need the Palmer simulator? It only makes sense with full range (not guitar) speakers.

Did you measure the impedance frequency response of your load?

[mhuss]

Very interesting thread.

I've often wondered if the bad rep attenuators get for blowing up output stages isn't due to the fact that most people wouldn't run their guitar amps on "11" if they had to deal with the full volume. Except for maybe arenas, it is sort of self-limiting. With an attenuator, one is much more likely to run the amp way hotter then normal. And particularly with the tubes being made today...

[Jazz P Bass]

Fully dimed through an attenuator, I have seen the screens actually sag & then melt/ short out.

It may help to bump up the screen resistor value.

[catalin gramada]

Thanks!
So you did not use the Palmer power attenuator but something like a Palmer DI box with speaker simulation together with your self made load? Why did you need the Palmer simulator? It only makes sense with full range (not guitar) speakers.

Did you measure the impedance frequency response of your load?

I used the Palmer cause the raw signal recovered from ouputs was not usable from a tone perspective to be reamped into a PA amp even guitar speaker was used. No, I didn.t measure impedance response of my device, the values of rc/rlc networks was commin from calculation, pretty close what a Celestion speaker does in respect of resonant freq and impedance. The simulation was published on mentioned source, I just build it as copy/paste

And to respond to mhuss: yes, it was used maxed or almost dimmed but for one hour period or so, the sound was huge but as any goodies was finished very fast