This is how I would go about making the attenuator, see the diagram below. Including a single pole, 5 position rotary switch, this would give you about +/- 6 dB of level difference for each change of the position of the switch. The volume control of the amp could be used to get an output sound level between what the circuit can provide, without substantially reducing distortion, I would think.

-Pete

The 6db switching method would work for most people, and it beats the problem with trying to buy a High Watt audio taper pot. I'm just following instructions to put a pot in the circuit. The guy I'm working for is a true blue nut case about sounds, but I give him what he asks for. If I could put a ten turn pot in, he would probably be so overjoyed that he would wet his pants. LOL!

View attachment 199833

 

If I could put a ten turn pot in, he would probably be so overjoyed that he would wet his pants.

So put one in. It doesn't have to connect to anything. If he's got golden ears he's bound to think it makes things sound better when he twiddles it.
And couldn't you bung a bit of inductance in series with the dummy load to add a bit of resonance for good measure?

 

So put one in. It doesn't have to connect to anything. If he's got golden ears he's bound to think it makes things sound better when he twiddles it.
And couldn't you bung a bit of inductance in series with the dummy load to add a bit of resonance for good measure?

I could add inductance. A little research would suggest the right value...or I could measure a guitar amplifier speaker. I have 5 of them and an inductance measuring jig I built. (Right now, I can't find my own blog pages or I would point to it.) The way this works in the real world is that the resistance and inductance of the voice coil, plus the mass of the cone, springiness of the surround, and the shape of the speaker cabinet combine and conspire to create the resonance and impedance changes. Just adding an inductor is a poor attempt at simulating a system with that many components.

Then there is, "where to put the inductor?" If I put it in series with the dummy load, the amplifier will interact with the inductance but the 82 ohm resistor effectively isolates the damping factor of the amplifier from the electro-mechanical world of a speaker. Maybe Audioguru will help me connect the damping factor to the speaker the customer will actually use. I did ask him about his design.

 

So put one in. It doesn't have to connect to anything. If he's got golden ears he's bound to think it makes things sound better when he twiddles it.
And couldn't you bung a bit of inductance in series with the dummy load to add a bit of resonance for good measure?

I found my blog
https://forum.allaboutcircuits.com/ubs/inductance-measuring-jig.478/

 

-18db (measured) and -25db (measured). First post
The math is: db=20log e2/e1

I was talking about the attenuator component value calculations.

ak

 

I looked in Google at "Celestion vintage guitar speaker" and found many of them that would resonate when an ordinary guitar plays. Of course the resonant frequency of a speaker increases when in an enclosure so just about any guitar speaker will sound like a bongo drum without having the amplifier output connected directly to the speaker.

 

I looked in Google at "Celestion vintage guitar speaker" and found many of them that would resonate when an ordinary guitar plays. Of course the resonant frequency of a speaker increases when in an enclosure so just about any guitar speaker will sound like a bongo drum without having the amplifier output connected directly to the speaker.

If I wanted a wonderful high fidelity system, I would worship at your altar. What I am doing here is working with people who have NEVER used an amplifier for the purposes you studied.

 

I was talking about the attenuator component value calculations.

ak

The worst limitation is the potentiometer. From PR = E^2 and a 100 watt amp designed for an 8 ohm load.
100W X8ohms =800 E^2 = 800
for a 5 watt pot where Pmax happens when the pot is at the same resistance as the limiting resistor I do:
5W R = 800
R=160
That means 80 ohms in the pot and 80 ohms external.
Next limit: When the pot is at it's high resistance limit, the external resistor feeds the speaker directly.
P (80ohms +8ohms) = 800
P=9.09W
Declare resistor as 18 watts.

Same calculations for a 1.5 Watt audio taper pot.

 

As AudioGuru again realized, if there is a great deal of resistance between the output of the amp and the speaker driver input terminals, then in the bass range the driver becomes completely unhinged. One website that I looked at claims that the average electric guitar has a low end of frequency range of about 80 Hz, and the initial transient of the lower notes of the guitar produce sub-harmonics of 80 Hz.

I think that even guitarists that like to hear a lot of distortion would not like the sound of the severely under-damped speaker.

But that lack of damping is not a (tremendous) problem- we can solve that!

 

Pete, your damped speaker will sound much better.

 

If I could put a ten turn pot in, he would probably be so overjoyed that he would wet his pants. LOL!

Surely you mean an ELEVEN turn pot!

 

As AudioGuru again realized, if there is a great deal of resistance between the output of the amp and the speaker driver input terminals, then in the bass range the driver becomes completely unhinged. One website that I looked at claims that the average electric guitar has a low end of frequency range of about 80 Hz, and the initial transient of the lower notes of the guitar produce sub-harmonics of 80 Hz.

I think that even guitarists that like to hear a lot of distortion would not like the sound of the severely under-damped speaker.

But that lack of damping is not a (tremendous) problem- we can solve that!
I'll do an ear test on that as soon as I get the new power transformer into my tube amp.

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Surely you mean an ELEVEN turn pot!

Spinal Tap

 

As AudioGuru again realized, if there is a great deal of resistance between the output of the amp and the speaker driver input terminals, then in the bass range the driver becomes completely unhinged. One website that I looked at claims that the average electric guitar has a low end of frequency range of about 80 Hz, and the initial transient of the lower notes of the guitar produce sub-harmonics of 80 Hz.

I think that even guitarists that like to hear a lot of distortion would not like the sound of the severely under-damped speaker.

But that lack of damping is not a (tremendous) problem- we can solve that!
I did the ear test. I couldn't get the bongo sound. Then I tried the damping resistor. 8 ohms in parallel with the 470 ohm resistor. No change in the tone, it just wrecks the way the knob works. The schematic in post one stands.

View attachment 200058

 

The customer loves it. He is using it at -53.5db on a 30 watt amplifier. That's about 2.75 on a knob labeled from zero to ten. That result makes me wonder if it will attenuate enough for a 100 watt amplifier.
I will wait to see how he adjusts the knob for a 5 watt amplifier before I consider changing the range of adjustment.

ps, no mention of bongo sounds or a monotone result.

 

If your customer loves it you did a great job.

 

ps, no mention of bongo sounds or a monotone result.

That maybe to the use of the correct speakers in the amp. It used to be, and suppose it still is, that instrument speakers were dampened more stiffly than a regular every day speaker. Back in the day you had to watch when buying replacement speakers for home use and instrument use, don't know about now.

 

Back in the day you had to watch when buying replacement speakers for home use and instrument use,

It's still true AFAIK. Instrument speakers in the 10" to 12" range usually stop responding significantly around 4KHz or 5KHz and they expect to perform with no support from an enclosed air volume. Their electro-mechanical resonance is unmodified by the box they are not in. (I think of them as more in survival mode than hi-fi mode.) Without a box full of air to shift, broaden, or flatten the resonance frequency, the tendency to monotone is real unless the Fr is below the instrument range. This is entirely possible because I have seen a lot of large speakers with Fr below 80 Hz but I haven't designed a speaker cabinet for 30 years and my memory is almost non-existant for the specs I read so very long ago.

In the case of the undamped musical instrument speaker, I will watch to see if the customer experiences any resonance effects as he tries the Power Soak with amplifiers from 5 watts to 100 watts.

 

but I haven't designed a speaker cabinet for 30 years and my memory is almost non-existant for the specs I read so very long ago.

That's why I wrote that I didn't know if they still did it. Many things that were mechanical in nature back in the day are done electronically today. I haven't kept up on that stuff.

 

The latest schematic from Pete has an 8 ohms resistor parallel with the speaker to help damp its resonance.