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Old 05-25-2022, 12:42 PM   #1
Nevillet
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Default Dummy loads for amplifier fault finding

Hi All,

I'm looking at upgrading my load resistors for power amplifier diagnostics and have a few questions:

1. is it possible to use e.g. a kettle element as a load?

and at the other end of the financial spectrum..

2. Electronic loads - would they work?

3. What's the gold standard method?

Many thanks.

Last edited by Nevillet; 05-25-2022 at 12:49 PM..
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Old 05-25-2022, 03:04 PM   #2
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Default Re: Dummy loads for amplifier fault finding

How many gigawatts? How many ohms? How long to reach 88MPH?
Appliance heating elements can be used as long as the heat can be dissipated. An empty kettle, or space heater with no fan, is no good for longer periods.
Most heating elements are quite non-linear for resistance. Cold will be a fair bit lower in resistance than hot.
Also they have some inductance, especially the coil on coil wound ones, just like wire-wound resistors so I add RC snubber top prevent amplifier damage.

Many people are buying the cheap power resistors out of china.
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Old 05-26-2022, 02:14 AM   #3
Nevillet
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Default Re: Dummy loads for amplifier fault finding

Lets say a 1000W and 8ohms. Not sure about the speed...
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Old 05-26-2022, 07:30 AM   #4
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Default Re: Dummy loads for amplifier fault finding

As frequencies go up, the harder it is to make a dummy load. DC is easiest, audio slightly harder, uhf is tough, microwave is really tough...
inductance in power resistors are lower than actual audio speakers...
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Old 05-26-2022, 09:12 AM   #5
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Default Re: Dummy loads for amplifier fault finding

Many thanks all,

Regarding finding something suitable & cheapish what search term should I use?

Also are there any particular far east vendors?
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Old 05-26-2022, 11:41 AM   #6
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Default Re: Dummy loads for amplifier fault finding

Quick note.. I've found many suitable resistors on Aliexpress.

Like this: https://tinyurl.com/2p8tau9t
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Old 05-26-2022, 12:17 PM   #7
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Default 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...
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Old 05-26-2022, 12:41 PM   #8
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Default Re: Dummy loads for amplifier fault finding

and regarding source material:
spot frequencies, sweep tones, pink or white noise or all over the above?
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Old 05-26-2022, 02:22 PM   #9
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Default 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://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.
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Old 05-26-2022, 03:48 PM   #10
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Default 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!!
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Old 05-26-2022, 11:00 PM   #11
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Default 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.
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Old 05-27-2022, 01:36 PM   #12
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Default 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...

Last edited by Nevillet; 05-27-2022 at 02:39 PM..
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Old 05-27-2022, 05:22 PM   #13
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Default 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.
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Old 05-27-2022, 05:33 PM   #14
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Default Re: Dummy loads for amplifier fault finding

Quote:
Originally Posted by Topcat View Post
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.
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Old 05-27-2022, 06:22 PM   #15
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Default Re: Dummy loads for amplifier fault finding

Quote:
Originally Posted by CapLeaker View Post
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.
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Old 05-27-2022, 07:39 PM   #16
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Default Re: Dummy loads for amplifier fault finding

Quote:
Originally Posted by Topcat View Post
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.
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Old 05-27-2022, 07:44 PM   #17
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Default Re: Dummy loads for amplifier fault finding

Quote:
Originally Posted by CapLeaker View Post
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!
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Old 05-27-2022, 07:53 PM   #18
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Default 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.
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Old 05-27-2022, 08:27 PM   #19
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Default Re: Dummy loads for amplifier fault finding

Quote:
Originally Posted by CapLeaker View Post
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!
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Old 05-29-2022, 10:03 AM   #20
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Default 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 ....
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