Load Testing Audio PA Amplifiers

[dicky96]

Hi, one of my customers who rents out equipment for events and carnivals asked me to make a passive dummy load so the can test amplifiers under full power without causing noise problems to other nearby businesses.

He wants 8 ohm and 4 ohm loads, originally he asked for 8 ohm 500w resistors, and I said we could use two resistors in parallel for 4 ohm 1000W

Then I found these cheap power resistors on Alibaba. It says they are 'Braking' resistors, but they seem to be suitably rated for my purpose

https://www.alibaba.com/product-deta...147691533.html

I figured with 8 of these (costs only 33 euros in total) connected in various series/parallel combinations he could have 2x 8R 2000W, 2 x 4R 1000W or 1x 4R 4000W load

He also wants to know if he can have a lower powered monitor speaker attached so he can hear the sound quality, but at a less than deafening volume, when load testing Amps.

See the attached dickyCAD image.

Q1. Can I use these resistors for Audio load testing? If yes, Maybe I should place cooling fan(s) near the resistors or bolt them to a large piece of metal?

Q2. How best to implement the Monitor speaker? Can it be done like shown in my diagram?


Rich

[stj]

Re: Load Testing Audio PA Amplifiers

you probably need to blow some air over those.
not a huge amount, but enough to stop them being able to burn anything - like wire insulation!

[redwire]

Re: Load Testing Audio PA Amplifiers

You can use mains heaters - ovens, stove elements as low cost load resistors. You must have a fan of course. 500W or 1000W convection cooled is no good.

You can blow amplifiers doing full power tests, a 15kHz sine-wave is very stressful for output transistors as an example. If it's just music, how does he know it's clipping? He'll need an ACV meter to show watts.

Since wirewound resistors have some inductance, it's a good idea to include an RC snubber as some cheap amplifiers have no (internal) protection diodes and the voltage spike kills transistors.

You can use mains heaters - ovens, stove elements as low cost load resistors. You must have a fan of course. 500W or 1000W convection cooled is no good.

You can blow amplifiers doing full power tests, a 15kHz sine-wave is very stressful for output transistors as an example. If it's just music, how does he know it's clipping? He'll need an ACV meter to show watts.

Since wirewound resistors have some inductance, it's a good idea to include an RC snubber as some cheap amplifiers have no (internal) protection diodes and the voltage spike kills transistors.

The Alibaba resistors are cheap and doubtfully "non-inductive" on a spiral wrap, and also rated power is forced-air cooling.

[dicky96]

Re: Load Testing Audio PA Amplifiers

Correct me if I am wrong though, but don't speaker coils also have some inductance?

To be honest those resistors are so cheap it doesn't seem worth the bother messing with stove elements

[barry wilkins]

Re: Load Testing Audio PA Amplifiers

When I was an engineer we had 1000W RF generators ,these actually American radio transmitters.
We had oil filled dummy loads for testing ,but they were 50 ohms.
You can get devices which are actually electronic resistors , I made a small load fo testing the exciters .

[stj]

Re: Load Testing Audio PA Amplifiers

i remember a 50ohm load project in a magazine years ago,
it was 2 sheets of tin with a huge number of parallel metal-film resistors soldered between them,
and then it was suspended in a paint-can filled with oil.

very cheap to build - totally stable.

[budm]

Re: Load Testing Audio PA Amplifiers

We use non-inductive loads in our lab to test amplifier output power, reactive loads for testing stability, we use lots of AP in our lab.
https://www.ap.com/technical-library...eactive-loads/
https://www.aikenamps.com/index.php/dummy-loads

[redwire]

Re: Load Testing Audio PA Amplifiers

My reply seems to have vanished

Correct me if I am wrong though, but don't speaker coils also have some inductance?

Yes loudspeakers are inductive, a large woofer has several mH.
But a crossover network has the Boucherot cell or Zobel whatever you like to call it which also acts like a snubber. Cheap amplifiers missing the output protection diodes and Zobel so they get nuked pretty easy.
So I would put a RC snubber like 0.1uF X-cap+10R-33R few W series resistor just to tame the inductive kick. You can easily check with a scope, inject a low power square-wave and see what the waveform looks like.

I'm not sure how "non-inductive" the cheap resistors are, they have a steel rod and brackets and who knows how they are wound, NiCr I think but they mention stainless steel

Mount the resistors on a metal, non-flammable enclosure, using high-temp silicone wire. Cooling fan pointing right at the resistor(s). There is a shock hazard 1kW 8R is 90VAC. I'd have LED or light to show some idea of the power output, you have to know if you're clipping or not. Before the guy blames the amp.
Feeding the monitor speaker at say 1/100 power should be OK.

[dicky96]

Re: Load Testing Audio PA Amplifiers

OK, so it seems not too difficult to get some suitable power resistors and fans.

How about a solution so he can hear the quality of music on a monitor speaker (at variable volume) while running the amp at high power into the dummy load?

This is something the customer is asking for, so we don't really need to discuss whether it is a good idea or not

[Agent24]

Re: Load Testing Audio PA Amplifiers

Add the speaker in parallel to the load but include a series resistor for it to lower the signal level so it doesn't get blown?

[momaka]

Re: Load Testing Audio PA Amplifiers

Add the speaker in parallel to the load but include a series resistor for it to lower the signal level so it doesn't get blown?

That may not work too well.

Because speakers are inductive and also generate back-EMF from the moving mass of the cone, a series resistor decreases the ability of the amplifier to shunt this back-EMF. This increases sound distortion, especially with bass. On that note, if you ever wondered about the Damping Factor (DF) parameter listed for on some amplifiers, it's the ratio of speaker impedance over amplifier's output stage impedance to ground. So for DF = 50 and rated speaker load @ 8 Ohms, the amp's output stage impedance is 8 / 50 = 0.16 Ohms. In that regard, higher DF means lower output stage impedance, which means the amplifier is able to drive the speaker cones more accurately and shunt more of their back EMF... which means less speaker distortion.

@ dicky96: how about just hook up an oscilloscope to the output of the amp and show the output with various synthetic waves at various power levels. Square and triangle waves should easily show any misbehavior, if there is any. Obviously don't push high frequencies at full power, as redwire suggested, as that may stress the output Q's harder than normal and blow something. Besides, NO ONE will ever dump 500 or even 1000 Watts @ 1 KHz, even in a carnival setup - that will blow most tweeters and midrange drivers right away and literally be a painful experience for anyone listening nearby. The bulk of power in music/audio is in the bass. Above 200-400 Hz, the power drops considerably.

As for the idea of using heating element as a load - I've done it before while also having a speaker hooked in series with it. Works great for pushing an amplifier to maximum volume levels without worrying about overloading the speakers.