Rigging up a load for audio amp repairs?

[rfengg]

Hi all,

Am trying to rig up a "load" for audio repairs and was wondering if using a couple of high power (50W)  8ohm thick film resistors with a switch to change it between 8 ohms and 4 ohms (by switching another 8ohm in parallel to the 8ohms) is good enough or if we really need a inductance similar to that of  the voice coil as well to "simulate " the actual load?
Any advice is welcome.

[tszaboo]

Once you simulate the inductance, then you can take it one step further and realize that crossovers , which will make generic speaker loads very unpredictable. And it will mess with your measurements and calculations.

[coppercone2]

I had this problem too. I did not know if I should use a load @ that frequency or just a resistor, because I could not get that resistor in the manual for a amplifier. The frequency was on the low end of ultrasonic.

I ended up making a special load with a capacitor that is exactly right. To me it seems that if your gonna calibrate something, calibrate it on a impedance similar to the DUT, then I determine that is the calibration aritfact and calibrated all the amps of that type with it.

But if your going for a speaker, that is going to be different.


my take on it is a balance between realism and stability. For multi channel, you want them to be equal, and you want it equal over time. If you get too complicated it might drift or be unstable.

[rfengg]

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.

[coppercone2]

Yes, I know, what I did is use a compensation capacitor to null out inductance for higher frequencies because my resistor was wire wound.

I like it just being Ohmic. That is best case, I think, and then you can make it less ideal.

It might help you get to the bottom of a problem if there is a more ohmic load

[tszaboo]

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.

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!!!