Speaker - amp attenuator topologies.

[diydidi]

I want to design and build an amp speaker attenuator. It will go between an amp and its speaker. I have been looking at L pads that parts express sells.
But before i delve into building it I decided to play and test two different topologies.
What are the pros and cons of each. Maybe one will have to add some form of treble bleed.
Please feel free to elaborate.
Regards

[doctor]

Apparently, such a device is intended for tube amplifiers to try to keep a certain “tube” sound at low levels. Such a sound, caused by specific distortions, is most characteristic at a power close to the maximum.
Here it is necessary to consider an important factor. The output power of a tube amplifier depends on the load nonlinearly, therefore the resulting load impedance (attenuator + speaker) must not exceed the range of 0.8R ÷ 1.5R, where R is the nominal impedance of the amplifier load. At this rate, the output power of the amplifier at the edges of the adjustment will decrease by no more than 20%.
Ohm's law will help.

[Chuck H]

Are you wanting to design something for the sake of designing it, or more simply build and have something. I ask because having something of my own design is often my own goal. If you would just like to build and have something I can share what I think is an excellent design.

EDIT: Oh, as to pros, cons on the above ideas... Both would have an amp running into a predominantly resistive load, so they're about equal in that regard. Not as bad as most people think either. They both have a very limited range of available speaker output. The first more so than the second I think. If you want to be able to just trim a little volume off for lower volume gigs or band practice I don't think either will suffice because there is so much attenuation just having them in the circuit. If just getting quiet for home practice is the goal then either should work fine.

[doctor]

I have absolutely no time for this. As well as for the step by step explanation of repairs.
That is why I confine myself to providing schemes or general advice.

[nickb]

I suggest the way to start is not coming up with typologies but rather defining your requirements. Here are some suggestions.

1) Impedance: Fixed (say 8 ohm) or selectable ( say 4/8/16)
2) Power handling capability
3) Attenuation range
4) Does it have to mute? Would a switch be OK for that?
5) Does it have to be continuously variable?
6) Does it have to go to full volume?
7) Do you care about trying to match the frequency response effects of a real speaker?
8) Budget
9) Headphone output?
10) DI output?

[diydidi]

Are you wanting to design something for the sake of designing it, or more simply build and have something. I ask because having something of my own design is often my own goal. If you would just like to build and have something I can share what I think is an excellent design.

EDIT: Oh, as to pros, cons on the above ideas... Both would have an amp running into a predominantly resistive load, so they're about equal in that regard. Not as bad as most people think either. They both have a very limited range of available speaker output. The first more so than the second I think. If you want to be able to just trim a little volume off for lower volume gigs or band practice I don't think either will suffice because there is so much attenuation just having them in the circuit. If just getting quiet for home practice is the goal then either should work fine.

Thank you Chuck H. Please do share.

[Chuck H]

Well drat! I'm trying, but I haven't been able to upload documents since the site change.?. I'll need to ask tboy about it. I think my problem may be different from what some others have dealt with because my documents file is at 399. Suspiciously close to 400, so maybe I'm tapped. And I can't find the procedure for deleting files to make room. I'll dig through the pile and see if I can find the resistive version. I know I've posted it here before so it should be in there. There's an active load version too but the upload in my archives has inaccuracies so I want to upload the updated image.

EDIT: Found it. This is for 8 ohms with a 20W amp. If you want to use it with a 50W amp you'll need to bump the 10r resistor to 50W. If you want to use it with a 100W amp you'll need to change the rating on all the components.

It's adjustable from -2dB down to, well I can't remember, but you could talk loudly over it The impedance varies from about 7 ohms to 10 ohms within the adjustment range.

I'll upload the active load version once I sort out problems with the forum.

[John_H]

The lowest cost, and simplest DIY solution is a non adjustable L-pad. There's a L-Pad calculator on the ted weber website http://www.tedweber.com/webervst/lpad.htm The closer you can get to the required resistor value, the better they sound. It pays to try different values of attenuation in the calculator to see what works out best for the lowest common denominator of total number of resistors needed. If you look at the values needed for 6db attenuation at 8 Ohm on the calculator, the results are 3.99r for the series, and 8.04 for the parallel resistors. It doesn't get any easier than that. The one in the picture is my midnight muffler. 12db of attenuation that turns 5w into 1/3w. It's still plenty loud, but I can get good tone without waking the neighbors.


I couldn't get my picture to upload. You're not missing much.

[Chuck H]

I can't upload anything either. I was only able to attach the above is because it was already in my forum files.

[John_H]

Here's a picture. I tried a new image hosting site.

[olddawg]

Interesting.. I have been using a fancy Weber attenuator with a “speaker motor” and all that for years and would normally swear by it. BUT for my last 20 watt EL84 amp build, I simply installed a 100 watt/8ohm, $12 Lpad built into it. Got it from “Parts Express” I think. Works great to my ears. But I don’t use an attenuator to squash the output. I usually just use it to take the edge off and hit that sweet spot without taking people’s faces off. Lol!

[Leo_Gnardo]

Here's a picture. I tried a new image hosting site.

The Mighty Midnight Muffler - I like that!

One fine day I was astonished when a customer asked me to make a fixed attenuator for his Champ. And then called me back with a rare "it works GREAT!" call. Couple resistors, a switch, bob's yer uncle. The way things oughta be.

[John_H]

Interesting.. I have been using a fancy Weber attenuator with a “speaker motor” and all that for years and would normally swear by it. BUT for my last 20 watt EL84 amp build, I simply installed a 100 watt/8ohm, $12 Lpad built into it. Got it from “Parts Express” I think. Works great to my ears. But I don’t use an attenuator to squash the output. I usually just use it to take the edge off and hit that sweet spot without taking people’s faces off. Lol!

You recommended the Parts express unit in another thread. Surprised at the price, I bought one. I've been using it with my 1482 Silvertone. It works just fine. I'll probably build one into the head cabinet. That's a great idea!

The Mighty Midnight Muffler - I like that!

One fine day I was astonished when a customer asked me to make a fixed attenuator for his Champ. And then called me back with a rare "it works GREAT!" call. Couple resistors, a switch, bob's yer uncle. The way things oughta be.

I love simple solutions. Especially when they're inexpensive.

[Chuck H]

I didn't like the simple L-pad solution. It looked funny on the scope and sounded sort of like a fixed Wha pedal at higher attenuation levels. I can't upload attachments from my poot right now for some reason or I would sketch an image of what I saw. It was pretty much a symmetrical, round protuberance sticking out of the center of the clipped ends of the wave form. The more I would attenuate, the more distended the protuberance. But I can't say if this was the same L-pad being sold by PE. I don't imagine the cheap L-pads are that dissimilar between brands though.

[Chuck H]

As promised

Here's the active load attenuator I said I would post when attachments were working again. I love this thing for it's transparency. I've had no need or desire for any tone tweak switches. Just turn it up or down. It uses something similar to the Aiken speaker load simulator that's tweaked for impedance and use of standard values and available parts. Notes on the smaller value inductor are important because you can't get within eight or nine feet from the thing with single coil pickups otherwise. Not cheap to build, but not terribly expensive either. Still cheaper than market offerings and much better as far as my own experience.