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Advice for Building a Dummy Load for Amplifiers

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Bud_J

Member
Hi All,

I've recently renewed my interest in electronics by attempting to repair an old, broken guitar amplifier I have.

To avoid damaging the speaker, I'd like to build a dummy load for testing purposes. It's a 100W amp, though I have no intention of pushing it at full power while under test until I'm 100% sure it's fixed.

The amp takes a 4-ohm load. I found these on eBay:
http://www.ebay.com/itm/2-x-50W-Loa...ash=item2cb45f7fd7:g:g2sAAOSwcLxYC28o&vxp=mtr

My thought is to put them in parallel for a 4-ohm 100W dummy load. I also like the idea of having a 50W 8-ohm load for if/when I ever work on an amp that requires an 8-ohm load.

In looking around some of the other electronics forums, I see some people don't like the idea of using wire-wound resistors for dummy loads because they have an inductance. Others say the inductance is negligible.

I have two questions:

1.) Generally, what are your thoughts on using wire-wounds for dummy loads?
2.) I'm wondering why a (somewhat) inductive load wouldn't more accurately mimic an actual speaker? After all, the speaker has a coil. So I'm wondering why some people are against using wire-wounds for dummy loads?

All help/info appreciated. Am I over-thinking it?

Thanks all!
Bud
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Wirewound resistors are fine for dummy speaker loads - however, be aware that the 50W rating of those resistors requires them to be bolted to a huge and efficient heatsink, and will be hot enough to burn you.

I made my dummy loads simply by using 1 ohm 17W resistors, wired in series parallel across tag strips - with plenty of air spacing. Then suitably downrate the wattage, as their 17W rating is also with them hot enough to seriously burn you, but at least you don't need heatsinks - you can also blow a fan across them to keep them cooler :D
 

MikeMl

Well-Known Member
Most Helpful Member
Another trick I sometimes use when testing DC power supplies. Stick a 4Ω 5W or 10W wire-wound resistor in a glass jar, filled with Distilled Water. As long as the resistor hasn't yet boiled away the water, it is good for ~100W.

I have used non-inductive resistors immersed in (non-PCB) transformer oil (clean motor oil will work in a pinch) to test 1kW RF Power Amplifiers.
 

Bud_J

Member
Thanks, guys, all good ideas. I certainly have plenty of computer fans lying around ;-) "Large and efficient" heat sinks? Well, that's another story. I have some small and lame ones that I suppose are better than nothing for now. Will give the fan something extra to blow across.
 

JimB

Super Moderator
Most Helpful Member
I see some people don't like the idea of using wire-wound resistors for dummy loads because they have an inductance. Others say the inductance is negligible.
It has not been explicitly stated here, but wire wound resistors are no-no for radio frequency applications, ie testing radio transmitters.
In those cases the parasitic inductance of the wire wound resistor is a real killer.

For DC applications the wire wound resistor is fine.
For audio frequency applications, I have no direct experience, but as others have said it should not be a problem.

JimB
 

audioguru

Well-Known Member
Most Helpful Member
Those resistors have a power rating of only 25W on every Western distributor's website. ebay lies about everything.
 

tomizett

Active Member
1.) Generally, what are your thoughts on using wire-wounds for dummy loads?
Yea, wirewounds are no problem for testing at audio frequencies. As you say, loudspeakers are significantly inductive.
Despite the fact that a speaker is nowhere near a resistive load, resistors seem to be used nearly universally in testing, even by manufacturers.
 

dr pepper

Well-Known Member
Most Helpful Member
Speaker drivers also have inductance, reactance will be 4 or 8 ohms at dc, but nothing like that at ac, theres usually one or 2 bumps around resonance then the reactance gradually rises.
Wirewound resistors have a very small amount of inducatnce, they will not present a load like a speaker driver but ought to be good enough for most purposes, after all you just want to dump power to see if the amp is capable of doing so.
At radio frequencies the inducance could dominate over the resistance, so for radio applications wirewound isnt good.
 
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