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power attenuator

Started by Joe, June 26, 2015, 06:17:41 PM

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Joe

Looking to make a power attenuator for my Dual Rectifier amp. After looking over some schematics, some things are not clear to me. I would like to use silicone wirewound resistors for cost reasons, if that's ok. Also, some circuits are purely resistive and others have bipolar capacitors in addition to the resistors. Do the capacitors matter enough to bother with? Thanks.

Enzo

What impedance and what power handling rating do you want your load to be?

Yu can use any type resistor you like.  I like the aluminum kind with the mounting holes, so you can mount them to a chassis.  Regular wire lead resistors are cheaper, but only come up to a certain wattage.  So for say a 50 watt load, I can use 5w wire lead resistors, but I'd need ten of them.  Those seem to start at about 70 cents and up.  Or I can use a single 50 watt resistor, which is like $5.51.  Saves me a dollar and a half.
http://www.mouser.com/ProductDetail/Vishay-Dale/RH0508R000FC02/?qs=sGAEpiMZZMtbXrIkmrvidAxPoX60y5sEfHy7goO%252b%252b5M%3d

Sometimes in larger wattages, four 50 watt resistors is cheaper than one 200 watt resistor.

A straight resistive load works.  Adding caps and inductors is done in some units to emulate the impedance of a real speaker.  It affects the tone somewhat, but it is up to you whether the difference is enough to matter.

Joe

The amp is 100W/16 ohm cab, and is mounted in a rack so I'm thinking about putting the resistors on a faceplate and bolt it on the back above the amp. Will probably shoot for about 25W. Thanks for the tips, looks like those aluminum housed things would work really well.


J M Fahey

QuoteWill probably shoot for about 25W.
25W`total resistor dissipation?
Won't do.

Least you need is a 16 ohms 25W`resistor in parallel with the speaker and an 8 ohms 50W`resistor inseries with both, so you keep total ompedance 16 ohms, and the speaker gets 25W , which is perfect for rehearsal or small club use.

And with such little attenuation, sound will still be fine.

It gets complicated when people attenuate amps to bedroom levels (say, 1W) , not because of amp or speaker or attenuator itself, but because human ear loses a ton of treble and bass sensitivity at lower levels, and everything sounds thin and undefined/brittle.

Not your case.

LateDev

I am surprised you want to do this, other than you want to put the head on a smaller cab.

I do agree that 25W is no where near good enough, given that you need to have something which will protect your amp.

Probably the easiest way would be to use a switched resistor in line with the volume control, but this takes a little bit of a calculation and use of a signal generator and a scope and a dvm. A scope will show you if the signal is clean or clipping.

Output impedance should always be matched as J M Fahey has stated and the combination of series and parallel resistors is of course a must, don't forget that the speaker forms part of this.

Resistor Wattages should really be all the same not only for the ease of purchasing but because you are strapping a resistor across a speaker whose impedance does change with frequency.  If a speakers impedance increases, you will find that the load resistor across the speaker has to dissipate more Wattage. You can use wire wound Aluminium Can or Ceramic resistors, whichever has the values you need.

Speakers are reactive in nature so are Not, strictly speaking, resistive elements. They are really just motors.
As far as the capacitors you have seen are concerned, they could form part of a reactive filter circuit in the output.

Joe

I misspoke and meant to say to cut the power down to that of a 25W amp, not that the resistors would be 25W. Still something to be careful about.

My motivation for doing this is simply to make the volume control a little more usable. Right now it goes from "zero" to "loud" with a 1MM turn of the knob. It's loud or nothing basically.

Thanks for the tips. I guess using oversized resistors and maybe a fan wouldn't hurt. Will post my circuit for review when I get it planned out.

Enzo

A 25w amp will be 6db less loud than a 100w amp, if all else is equal.  What is 6db?  That is the difference between the high and low input jacks on a zillion Fender amps.

LateDev

Quote from: Joe on June 29, 2015, 03:13:07 PM
My motivation for doing this is simply to make the volume control a little more usable. Right now it goes from "zero" to "loud" with a 1MM turn of the knob. It's loud or nothing basically.
That is rather strange and makes me think there is a problem with the volume control, as in the track is broken on one end.
A volume control is a logarithmic pot which allows a gradual change of the volume level. If the track is broken at one end either by way of a dry joint or the carbon track is broken, you will get a leap in volume for a small movement of the volume control. Volume pots usually form a voltage divider with one end to the out of the preamp, the wiper out to the power amp, and the other end of the pot going to ground. If the ground side has lifted you will get this effect.

Joe

Fortunately I figured out a way around my problems by simply putting a patch cord between effects send/return. This allows the loop master to control the overall volume of the amp.

Sounds better this way too, presumably since the individual channel masters aren't sitting so close to zero now.  8)