Buy two? series or parallel to get what you need.
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Can I use a 12-inch woofer as a resistor to set amplifier gain?
- Thread starter abdo123
- Start date
They are more than adequate for quick power test. Could do more continuously if you cool them.
Two 8 ohm in parallel is best for simple testing.
You might find this QuantAsylum app note useful…
Using the QA451 with External Loads
Post by Matt One drawback of the QA450 was the fact that when the loads were disconnected, the output was muted. That changed on the QA451, and the measurement path was decoupled from the load path. This means that on the QA451 you can make measurements of an unloaded amplifier. But it also...
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JayGilb
Major Contributor
I have mine mounted on an old class A amp heatsink with cpu heat transfer paste.
Even a smallish fan helps a lot.
Need a dummy load? Use an old toaster 
Get a pair of alligator clips and your DDM, and just find a piece of toaster wire that will give you about 4 Ohm. Put your clips between that section, and power it with your amp
Get a pair of alligator clips and your DDM, and just find a piece of toaster wire that will give you about 4 Ohm. Put your clips between that section, and power it with your amp 
kiwifi
Active Member
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H
Have you considered using an incandescent light bulb as a dummy load?
Don Hills
Addicted to Fun and Learning
Light bulb resistance varies tremendously with temperature. Speaker resistance / impedance varies tremendously with frequency.
One or two high power resistors looks like the most cost effective way to get 4 ohms. Remember they don't have to be rated for the full 250 watts, for what you're testing you only need to apply signal for a few seconds at a time. You can get fancy and mount them to a heatsink or just drop them into a glass of water on the end of their leads. The bubbling of the water will tell you if you overdo it.
One or two high power resistors looks like the most cost effective way to get 4 ohms. Remember they don't have to be rated for the full 250 watts, for what you're testing you only need to apply signal for a few seconds at a time. You can get fancy and mount them to a heatsink or just drop them into a glass of water on the end of their leads. The bubbling of the water will tell you if you overdo it.
JayGilb
Major Contributor
Maybe design a dummy load that also works as a coffee mug warmer ?
I apologise if I sound thick but why don’t you measure the amplifier output without a speaker connected? A power amplifier is a voltage source and you want to measure the voltage amplification = gain.
Also why not use simple sine wave instead of burst tones?
restorer-john
Grand Contributor
Why? You'll be turning way past that nominal position with typical music recordings. You'll just rob yourself of a whole lot of available volume.
restorer-john
Grand Contributor
Because they are not voltage sources in the true sense. Not even close, especially with those little Aiyma things. The rails will sit way higher with no load and the results will be totally inconclusive.
I really don't know why he doesn't just grab a few 50/100W non-inductive resistors and use them in conjunction with the 'scope. They are so cheap.
Do you mean the amplifier gain change with load?
restorer-john
Grand Contributor
Not the gain itself, just the available output swing.
No load= highest swing due to no current being drawn (rail voltage highest)
8R load= rails will droop due to current draw and losses (I^2R) etc over OPT devices and the power supply leads/PSU itself.
4R load= rails droop even more due to double the current (i^2R) etc.
From what I interpret the OP wants, is he wants to find the spot where his 'rig' clips and set that as his 'reference' point. However, the concept is flawed. Plenty of ASR members do all their gain calculations and set their D/As to give what they mistakenly think is maximum unclipped power. Then they forget all about the fact that musical files are all totally different in levels. Some will need way less attenuation* than others.
If I did that with my pre/power and digital front end using a 0dBFS test track and scope, then put on say Dire Straits Love over Gold CD, I'd be robbing myself of about 90% of the power of my amplifier. I'd be rotating the volume control way past my 'reference' point, just to get a medium-high playback level. Every disc/track is different.
Hope that makes sense.
*note, I'm calling it attenuation not gain, because the amplifiers themselves are fixed gain, they just have an attenuator up front.
I estimated/calc'd those Aiyma amps to be like 25% to 30% reduction in supply voltage from loaded to unloaded. Sometimes maybe more. very loose.
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