How do you stress test an amplifier?

[Axtman]

I bought an Ampeg PF500 bass amp head. I know that early versions of this amp were prone to failure. There are lots of stories on the internet of these amps shutting down on a gig. Before I take this amp out on a gig or if I decide to sell it I want to besure that it is not faulty. What is a safe way of stress testing the amp? Thanks.

[Jon Snell]

It's an early class D amplifier relying on the IRS20957 driver chip and a very unstable muting circuit.
The switching transistors are rated at 130Amps so unlikely to fail before your speakers do.
You can supply it with a 1kHZ tone and drive it into a dummy load and watch your load glow to test it.
In short, Chinesium stuff made on a budget by Loud Technologies. (Not too harsh I trust, just telling it, how it is).

[Axtman]

Jon, I know the designer of the amp, Rene' Jaeger. Maybe I will ask him. Rene' told me the hated that Ampeg would get broken amps in and just board swap them not even bothering to find/fix the fault. Yes he also told me that early PF500 amps had QA problems with their Chinese manufacturers.

[Enzo]

Stress test? Do you practice or do sound checks?

[Axtman]

Yes we practice, but in a small living room. We also do a sound check at the gig. Then people start gathering (when they used to) and making noise and we start cranking the amps. Or sometimes we play outside where we need more volume.

I would like to stress the amp and have it fail on my work bench then at a gig mid song. Nobody likes the bass player to drop out on 'Mustand Sally'! ;-)

[mozz]

I'd use lower than 1khz. Square wave even. How about wide open and some F chords?

[Enzo]

I would play music into it at 3/4 output and let it cook. A steady sine wave is not easy on an amp, and music doesn't look like a sine wave. It is much more peaky and spacey.

[drewl]

Crank it up and play it before the gig.

Also make sure you have a good heavy duty speaker cable (not an instrument cable !)

Make sure your cabinet is good, no loose speaker jacks, etc.

[Jon Snell]

Jon, I know the designer of the amp, Rene' Jaeger. Maybe I will ask him. Rene' told me the hated that Ampeg would get broken amps in and just board swap them not even bothering to find/fix the fault. Yes he also told me that early PF500 amps had QA problems with their Chinese manufacturers.

The 'amplifier' was designed by International Rectifier back in 2008. The preamps etc are possibly Ampeg.

The power supply is the main issue and it is a replacement board only, due to the glue and RTV used to stop coils from whistling when being used.

[J M Fahey]

Realistic testing?
Just carry it to the gig, but have a backup head handy. As near as already plugged into mains, and sitting above or below it (depending on size/weight) or on a chair besides your setup.

Absolute worst case, pulling/replugging input and speaker cables takes less than 30 seconds.

Stage volume and real speaker load are more stressful (hence realistic) than a sinewave on a bench and a *resistive* load.

FWIW on my own bench I always have around a "2/3" load, meaning my 4 ohm one is made out of three 8 ohm loads in parallel.

Normal diagnostic setup is 2 of them in parallel for resistive 4 ohm like anybody else, but for overheating/possible short protection/nuisance fuse blowing/etc. (hint: "stress"), I switch the third one in parallel.

Quite realistic by the way, since at some frequencies speaker Z drops a lot, 8 ohm nominal often having about 6 ohm DCR (sometimes even less), scale that to other impedances.

A| *perfect* load should have a strong inductive components, resonance at low frequencies, etc. ... like any plain vanilla speaker has, but again, my 2/3b load has always helped me.

My rule of thumb is that, say, a 4 ohm amp must flawlessly drive .2.6 ohm (3 x 8 ohm in parallel) with any input signal, even hard clipping, but protection should trigger or a fuse blow with a 2 ohm load.

[mgalloway]

Maybe a little off topic since it has not been resolved but it might help determine the weak points for a gigging musician. I'm looking at a PF500 for a friend at the moment (definitely not an experienced amp tech but have a pretty decent knowledge of electronics). His is the newest board revision and was playing through it with active pickups and he lost sound and it went into fault. I visually looked it over and no components had any signs of damage but checked the 2 output IRFB4227s (Q10 and Q11) and both were shorted. Removed them to confirm and they were still shorted out of circuit. I ordered 10 and replaced both. I do not have a scope so I couldn't trace sound but I ran a 4 ohm dummy load in the cab output and powered up the amp and it seemed stable, no fault light after 5-10 mins. He wanted to try it for practice so I gave it back to him before trying it out through one of my cabs or really checking voltages and when he powered it up he immediately got the fault light. I'll get it back from him this week and look into it a little further to make sure the driver is getting the +/- 82V and see if the power mosfets have shorted again and go from there. He said that he ran the same cab through another head and the cab was fine but I am wondering if it is a cab/cable issue or an issue with the driver circuit or driver.