Amp Dyno

[Neener]

I always wondered about trusting the sticker on the back of an amp and how one really knows exactly how much power an amp actually puts out.

Interesting YouTube video --> Clicky

[JimmyM]

Unless you have an accurate way to measure it, you really don't know.

[fdeck]

Standards for bass amp power ratings range from loose to nonexistent. I have assiduously measured the output power of all my amps, but have honestly not learned much.

[phatbass]

Don't try this at home unless you know what you are doing. Although it's easy, it's NOT a home handyman job. You could seriously **** up your amp or yourself through innocent ignorance. Although if you have an oscilloscope you probably know how to use it....

You need a signal generator and an oscilloscope. Many oscilloscopes have a built in signal generator.

Put a resistive load (like a resistor, but will take a lot of power - use one rated at least at your amp's rating) across the speaker output. You can actually use a speaker cab, but this test involves turning the amp up to maximum clean volume, so you may want to take pity on your ears...

Input a 1000hz sine wave, and put the 'scope's leads across the resistive load. Turn the master (if your amp has one) to maximum, then bring the gain/preamp/whatever up until the sine wave on the oscilloscope's screen starts to visibly flatten. Record the voltage shown on the oscilloscope.

Now using the formula Power = Voltage squared, divided by Resistance (impedance), work out your power. e.g. an amp putting out 100 watts will measure a little bit under 30 volts across an 8 ohm load. So the sine wave visibly begins to flatten when it's measuring just under 30 volts on the oscilloscope.

[fdeck]

Originally Posted by phatbass

Don't try this at home unless you know what you are doing. Although it's easy, it's NOT a home handyman job. You could seriously **** up your amp or yourself through innocent ignorance. Although if you have an oscilloscope you probably know how to use it....

You need a signal generator and an oscilloscope. Many oscilloscopes have a built in signal generator.

Put a resistive load (like a resistor, but will take a lot of power - use one rated at least at your amp's rating) across the speaker output. You can actually use a speaker cab, but this test involves turning the amp up to maximum clean volume, so you may want to take pity on your ears...

Input a 1000hz sine wave, and put the 'scope's leads across the resistive load. Turn the master (if your amp has one) to maximum, then bring the gain/preamp/whatever up until the sine wave on the oscilloscope's screen starts to visibly flatten. Record the voltage shown on the oscilloscope.

Now using the formula Power = Voltage squared, divided by Resistance (impedance), work out your power. e.g. an amp putting out 100 watts will measure a little bit under 30 volts across an 8 ohm load. So the sine wave visibly begins to flatten when it's measuring just under 30 volts on the oscilloscope.

RMS volts?

[T-Bird]

Hi.



Kits like that have been one of the most favourite DIY audio gadgets for decades.

Useless if You ask me.

Without an actual distortion figure, it's even more useless than the old oscilloscope method that determines the power before clipping.

I guess Pt Barnum said it best.

Regards
Sam

[B-string]

Originally Posted by T-Bird

Hi.



Kits like that have been one of the most favourite DIY audio gadgets for decades.

Useless if You ask me.

Without an actual distortion figure, it's even more useless than the old oscilloscope method that determines the power before clipping.

I guess Pt Barnum said it best.

Regards
Sam

This way to the great egress?

[grendle]

Originally Posted by B-string

This way to the great egress?

+1

Does anyone actually care as long as they don't have the volume knob cranked and can't hear themselves? I have a hartke 3500, and a 1200 watt power amp with an svt 2, and an ampeg b4r (1000 w). With my 8x10 cab I never need to turn the volume on any of then over 4. That's with 2 loud guitarists and a loud drummer. Oddly enough I usually play with all of them at about 3 reguardless of the wattage. Lol

[T-Bird]

Hi.

Originally Posted by B-string

This way to the great egress?

Applies as well I guess (had to Google that phrase to be honest).

I was thinking more along the lines of :" There's a sucker born every minute" though.

As for caring about what certain amp is able to do, then I'd have to say, I'm one of those people who do care.
Benching an amp does tell quite a bit about its suitability for the application it's intended to, and reveal any shortcomings that may spell disaster if not handled properly.

Then again, a quote from the racing world: "We don't tend to race on dynamometers".
The power alone is pretty much meaningless.

Regards
Sam

[B-string]

Originally Posted by T-Bird

Hi.





Applies as well I guess (had to Google that phrase to be honest).

I was thinking more along the lines of :" There's a sucker born every minute" though.

As for caring about what certain amp is able to do, then I'd have to say, I'm one of those people who do care.
Benching an amp does tell quite a bit about its suitability for the application it's intended to, and reveal any shortcomings that may spell disaster if not handled properly.

Then again, a quote from the racing world: "We don't tend to race on dynamometers".
The power alone is pretty much meaningless.

Regards
Sam

I knew that actually, I was having a little fun. Although I think the one I quoted was pure genius by him.

[chucko58]

It's not so hard, and I bet many of us have most of the gear to do it.

You need an accurate voltmeter, a signal source of known quality (e.g. an MP3 player loaded with test tones), a computer with a sound card, a recording program with a spectrum analysis plugin, a resistive pad to knock the amp's output voltage down to something the sound card can handle, and (the hardest part) a high-power load resistor of the appropriate resistance.

The load resistor is the hardest part because it has to withstand and dissipate all the power coming from the amp without changing its value. Oh, and it has to be non-inductive, because that will make the load appear to be higher resistance at higher frequencies.

So, once you have an appropriate load resistor:

(1) Take the spectrum of the signal source for comparison purposes
(2) drive the amp until the spectrum of the output no longer resembles that of the input, then turn it down just to where they match again
(3) measure the RMS voltage on the load
(4) turn off the amp and measure the load resistor ASAP, before it cools down
(5) P = V^2/R

Simple, yes?