Electronics > Repair

Rigging up a load for audio amp repairs?

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tszaboo:

--- Quote from: rfengg on December 30, 2024, 08:55:48 pm ---Sorry guys, maybe I wasn't specific enough.....what I wanted to put together is a load that can be connected to the outputs of audio amps / receivers, so that I don't blow any expensive speakers when doing repairs.
Initially I thought that I would buy a couple of cheapo speakers, but didn't want the neighbors to start complaining thru my repair process.

--- End quote ---
That's what I'm talking about. Larger 2-3 channel speakers have crossovers in them, and you have no idea what will be connected to your amp. So any attempt at simulating inductance might be wrong. 8 Ohm resistive is OK.

floobydust:
In the shop we used wirewound dummy load resistors, with a divider take-off feeding loudspeakers at I think it was 1/100 power.
So you can heavily load down a power amp while listening to it for distortion etc. and not be really loud. I've even used headphones if it's some wierd distortion to track down. You can also connect a scope to the attenuated output.
At high power you need a cooling fan for the resistors. It was 4/6/8/16 ohm switchable plus fuse.

Loudspeaker inductance is huge plus the generator action of a moving cone. I say no need to include it because it's quite complex.
With wirewound resistor parasitic inductance I do put a snubber across them as some cheap amplifiers blow up if there are voltage spikes beyond their rails coming back from the load.

Swake:
Yes, good enough for repairs. It is not a super delicate precision job

Got 8 of those cheap 8 Ohm resistors 'with fins on them' screwed on a big metal plate for cooling (don't forget the thermal paste). Did not use a switch but 4 mm banana sockets on a patch panel so I can wire them in any configuration I need, down from mono 1 Ohm to quad channel 16 Ohm.

- These resistors exist in 'non-inductive' version. I guess that is going to be better but somewhat more expensive.
- If you use a switch, beware that heavy amplifiers can pump out many amperes, you'll need a big switch.
- You might want to add a pair of some higher Ohm resistors (32 Ohm; 80 Ohm; 250 Ohm; 600 Ohm ) to test headphone amps.

rfengg:
Great ideas @Swake....thanks a lot!!!

Jeff eelcr:
Most repair work does not require a load on the amp while troubleshooting (home audio equipment).
FUSE everything bench test speakers and unit under test. Most good speakers will not be damaged using a 1A fuse.
Tweeters are best protected by a 12V dome lamp.
Current monitoring variac is very helpfull also.
Testing is different, unit should be loaded at correct impedance noninductive is best and gets expensive.
However a cheep, soundless replacement could be a home AC heater coil, high power, adjustable with clips.
Jeff 

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