Dummy loads for amplifier fault finding

[check12]

Re: Dummy loads for amplifier fault finding

5mins @ 6kW is plenty of time for a barrage of tests ...

[redwire]

Re: Dummy loads for amplifier fault finding

I think the water volume goes up around 4% from 20-100°C so 20L becomes 20.8L... time for an expansion tank lol. I wonder how those oil-filled electric radiant heaters handle it.
Otherwise it's cooling fans I guess, on larger resistors not immersion heating elements.

[check12]

Re: Dummy loads for amplifier fault finding

Yeah thought about distilled water when putting it together (which is pretty cheap) but got offered the oil free & it gives plenty time ... easy to build in thermometer or pressure releases too but I've never bothered ..

[stj]

Re: Dummy loads for amplifier fault finding

water expands and contracts - not good in a can!

[redwire]

Re: Dummy loads for amplifier fault finding

It's interesting to do some math.
With 1,000W (total) input you have around 35 minutes before the oil heats to 100°C, in a 20L barrel. So 2x500W as well. Water is much better at 105 minutes.
Very few, if any, audio amplifiers are designed to output full power as a sine-wave for long periods of time. It simply costs too much to size heatsinks, power transformers etc for that. Many are not rated for all channels driven, just a single channel.
My old (fanless) home-made gear can do it, but that is a DIY norm for audio power amps.
I hate parties where the crappy cheapy A/V receiver overheats and cuts out or blows up after 1/2 hour. Buzz kill!

[stj]

Re: Dummy loads for amplifier fault finding

nice

[check12]

Re: Dummy loads for amplifier fault finding

Here's a little something I put together for about £50 ... 2x 6kW 220v elements.

https://www.aliexpress.com/item/3284...21ef1802cXZQmO

A metal bin that I bought from local adverts and some suitable connectors and wire. Had to bend the elements a bit to fit in (pretty tough but they can be bent). I fitted them through opposing holes just below the bottom ridge on the metal bin (for the max amount of oil above them).

Elements are almost exactly 8ohms, very little inductance.

My son works parts time in a restaurant so we got 20L of used vegetable oil, filtered it and that is what cools the elements.

Switchable 2x 8ohms, 1x 4ohms ... Been using it for ages now and had no problems. I can now properly stress test BIG amps without worrying about how hot the load was getting after a few seconds ....

[Topcat]

Re: Dummy loads for amplifier fault finding

He wanted US$ 600 for it. Was a bit over CAD 800.

That was cheap, even if it was just the unit missing all the accessories! No wonder it's gone!

[CapLeaker]

Re: Dummy loads for amplifier fault finding

He wanted US$ 600 for it. Was a bit over CAD 800. I do not much over eBay. Only things I really need and can’t find it anywheres… like a replacement OLED for my Agilent U1253B.

[Topcat]

Re: Dummy loads for amplifier fault finding

Yes… found it on eBay.ca and was a Canadian seller. However, now I don't see it anymore.

They go fast! What was it priced at?

I almost had one about a year ago.... Complete 1-owner rig in perfect condition for $1000 shipped. Someone was literally seconds faster on the BIN button!

[CapLeaker]

Re: Dummy loads for amplifier fault finding

They pop up now & then. A complete one with all the leads goes for an easy $1500USD. I'm not seeing one on right now, maybe the one you're seeing is a CA seller that won't ship to the US? All I see for the PA81 is just the power brick.

Yes… found it on eBay.ca and was a Canadian seller. However, now I don't see it anymore.

[Topcat]

Re: Dummy loads for amplifier fault finding

There is one on eBay.

They pop up now & then. A complete one with all the leads goes for an easy $1500USD. I'm not seeing one on right now, maybe the one you're seeing is a CA seller that won't ship to the US? All I see for the PA81 is just the power brick.

[CapLeaker]

Re: Dummy loads for amplifier fault finding

Sencore PA81 is probably the best there is for this....but good luck finding one and bend over if you do.

There is one on eBay.

[Topcat]

Re: Dummy loads for amplifier fault finding

Sencore PA81 is probably the best there is for this....but good luck finding one and bend over if you do.

[Nevillet]

Re: Dummy loads for amplifier fault finding

So I now need to convince 'she who must be obeyed' that I can buy a Lab Grupen behemoth amplifier to heat some oil filled radiators as part of the house heating system….
Worth a try I guess...

[redwire]

Re: Dummy loads for amplifier fault finding

Oh yeah it's fine and dandy until the oil starts to boil lol. The Heathkit Cantenna was like that, a mineral-oil filled paint can with a power resistor inside.

[stj]

Re: Dummy loads for amplifier fault finding

so, let me describe a stable dummy load i remember from "back in the day"

you take a metal paint can,
put a connector through the lid - PL259, N or whatever
inside you put 2 metal discs with a shitload of holes drilled in them and a shitload of 1w or 2w metal oxide 1% resistors between them so they are all in parallel.
choose the resistance so it adds up to what you need
fill the can with oil and assemble it.

the result is a very stable non-inductive oil cooled load that is shielded.
we used to build these to load transmitters on CB radio's!!

[redwire]

Re: Dummy loads for amplifier fault finding

At 1kW power level, for any significant length of time you need a cooling fan or a water/oil bath.
The resistor you linked to has to be mounted on a very large heatsink of course.
You can use a few 4R parts to make 4,6,8R loads. I've seen people use stove heating elements. VFD braking resistors are a low cost load, use a few.
Also talked about here https://www.badcaps.net/forum/showthread.php?t=91984

Doing a 20kHz sine full power test will damage output transistors.
I check at low frequency say 1kHz to see if full power can be delivered into the load, with no clipping/distortion/power supply rail sag. You can test the SOA current-limiting/protection circuits as well but a bit dangerous.
I'll do a "speed test" so a sweep at high frequencies to see if something is switching too slow, like a damaged but not fully dead transistor, yet not at full power. Some issues show up at less than full power. Only output and driver stages are really stressed at full power, when those transistors are supplying high current and hFE droop is an issue.
You can also see if bias-current is tracking properly once an amp is heated up.

There's a thread somewhere where a guy puts a big dummy load on an amplifier and then resistors to drop that down- i.e. 1,000W to the load bank and 1W to the loudspeakers and he listens to music for an hour to test amplifiers.

[Nevillet]

Re: Dummy loads for amplifier fault finding

and regarding source material:
spot frequencies, sweep tones, pink or white noise or all over the above?

[eccerr0r]

Re: Dummy loads for amplifier fault finding

BTW typically "electronic loads" are meant for testing power supplies. It's nice they are adjustable, but unsuitable/incorrect for speaker outputs. Electronic loads are typically constant power or constant current, but constant resistance is what audio amplifiers need.

Speaker outputs a mostly resistive load is probably best - while a speaker's resistance is not constant, it can be modeled as such for a dummy load. Nothing better than a real speaker for testing, except if that speaker or your ears are valuable...