power attenuators

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power attenuators

Postby TragicTravisty » 14 Sep 2008, 03:08

amps are very loud... so id like to build an attenuator. all of the resistor-based attenuators seem simple enough, but how is the heat dissipated?
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Re: power attenuators

Postby soulsonic » 14 Sep 2008, 03:34

It depends on the type of resistor you use. Some of them require heatsinks and others just require adequate ventilation of the enclosure.
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Re: power attenuators

Postby TragicTravisty » 15 Sep 2008, 21:06

idea: pot which works with speaker for attenuation. for an 8 ohm output, 16ohm pot, with speaker + and minus across on the first two lugs, and a 16 ohm resistor across the second two lugs, with the third lug to ground. would it work? do pots come in high wattage?
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Re: power attenuators

Postby soulsonic » 16 Sep 2008, 04:17

Well, lots of simple attenuators use high-wattage rheostats like that. You can see how it is in the Weber MiniMASS here:
members/soulsonic/schematic/WeberMiniMASS.gif
That circuit works well enough I suppose, but it doesn't keep a constant impedance to the amp.

You can also get high-wattage L-Pads that you can use the same way and it will give the amp a better constant impedance, but the only problem is that most of them can't handle the current of really high powered amps. I was trying a 100 watt L-Pad once and my 100 watt amp (turned up only about halfway - no where near full power) made it start smoking, so it would probably only be a good idea with lower powered amps. Or you could put a fixed L-Pad resistor network to bump down the power before the variable L-Pad, so it doesn't get the full force of the cranked amp. I've come up with a couple designs like that, where you have a fixed L-Pad that's switchable between a couple different levels, and then there's the variable one after it for fine-tuning the volume.
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Re: power attenuators

Postby TragicTravisty » 16 Sep 2008, 18:36

why wouldnt it give constant impedance to the amp? im not saying your wrong, i just dont get it.
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Re: power attenuators

Postby TragicTravisty » 16 Sep 2008, 19:42

oh nevermind i thought you were talking about my attenuator, not the weber one. what do you think of mine?
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Re: power attenuators

Postby .Mike » 16 Sep 2008, 21:28

First post at this forum...

If you're comfortable modifying tube amps and you know how to do it safely, you should check out "power scaling" or "variable voltage regulation."

The idea is that instead of attenuating the output of the amp, you scale the B+ voltage for the entire amp using a mosfet, four resistors, and a potentiometer. This apparently allows the amp to maintain natural interaction between the stages while significantly dropping the speaker output. According to what I've read and heard, it sounds better than straight resistors and L-pads. And also, as far as I understand, it causes less wear and tear on your tubes when scaled than running the amp full out and attenuating.

I first learned of this at the SEwatt and 18watt forums, which both require accounts to read topics, but I found a simple write-up here: http://yeomansinstruments.blogspot.com/ ... -ever.html

:)

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Re: power attenuators

Postby soulsonic » 19 Sep 2008, 08:55

The power scaling idea does seem interesting, but it seems to be something exclusive to distorted sounds. I mean, it's hard to imagine an amp putting out a decent clean tone at drastically lowered voltages. Like anything, I won't be able to really form an opinion about it until I experiment with it myself. Thanks for the link to the article... I'm not willing to pay $80+ for the TUT book just to learn about how he wires up a voltage regulator! :roll:
I suppose I could just start from scratch with a regulator design. The Art of Electronics certainly has all I'd ever need to learn about regulators... and it's a book I WAS willing to pay $80+, because it's REAL knowledge, not some amp mojo silliness. Seriously, I might consider purchasing TUT if I could actually read through it first and see if it's actually worth the money. It took me only five minutes of browsing in Barnes & Noble to be sold on The Art. Anyway, I know the main mojo behind the Power Scaling design is that it scales the biasing to match up with the plate voltage level.
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Re: power attenuators

Postby Fuzzer » 20 Sep 2008, 18:50

Yes, magnificaent book, totally recommended for everyone. By Paul Horowitz & Winfield Hill
The Freestompboxes Forum search function is soo great, use the search function..., the S E A R C H function.
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Re: power attenuators

Postby JHS » 20 Sep 2008, 20:27

Some Sundown Amps have a primitive powerscaling named RMS and Govenor. Work good if the limiting is small, with max. attenuation it sounds a bit like an OD-pedal.

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Re: power attenuators

Postby tunghaichuan » 20 Sep 2008, 23:02

JHS wrote:Some Sundown Amps have a primitive powerscaling named RMS and Govenor. Work good if the limiting is small, with max. attenuation it sounds a bit like an OD-pedal.


Here is a schematic that shows both:

http://www.schematicheaven.com/newamps/ ... dr100h.pdf

The Govenor is a post phase inverter master volume.

The RMS control is kind of like an adjustable/switchable cathode bias control. I remember the designer, Dennis Kager, teling me that he should have used a 10w pot for the Govenor control as the 5w units burned out quite frequently.

I've owned several of those Sundown amps, and the RMS and Govenor controls work as advertised.

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Re: power attenuators

Postby IvIark » 10 Oct 2008, 00:42

Here's a resistor based attenuator in stompbox format that I got a couple of years ago. I think you'll see how the heat is dissipated! :D

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