How to measure an amplifier

[Razorhelm]

What equipment do you need to measure the performance of an amp?

Also what are the pertinent measurements to make?

Many Thanks

 

[solderdude]

The fact that you have to ask suggests making meaningful and accurate measurements is not something for you to do.
Those making measurements already know what to measure, how to measure it and how to interpret the results.

 

[Razorhelm]

Sorry I find asking people who know more than me an effective way to learn, fine if that isn't how you do things but would you mind not replying if its just to be rude.

I knew nothing about using REW to do room correction, however I asked on this forum and some very helpful people stepped in and helped me get started.

I am in the beginner section am I not?

 

[sergeauckland]

Firstly, you need to decide on what it is that you want to measure and why.

I measure amplifiers both for a bit of fun (yes I know, I should get out more, but that's currently not possible.....) and when I've repaired something to make sure it's working correctly.

Let's assume for the sake of this post that your interest is in working out if the amplifier is working correctly, not because you're designing amplifiers or verifying production.

In order to test an amplifier you'll need the following:-

A source of signal with very low distortion, from, say 1mV up to 10v and from 10Hz to 100kHz. Square wave option is also useful.

A millivolt meter that can measure audio levels from -100dBu to +30dBu ( 15uV to 50v) unless you're going to be testing some ultra-high power amps. It should have a bandwidth of 100kHz and it would be useful if that could be switched to 20kHz and have some standard weighting filters.

A distortion meter that can measure down to 0.01% is sufficient to make sure the amplifier will be completely transparent. It might not be enough to verify if the amplifier fully meets its specification, but my view is if it's so low I can't measure it, I don't need to worry about it.

An oscilloscope so you can see what's happening. A 20MHz bandwidth is sufficient. Dual trace is very useful indeed, almost obligatory.

A dummy load that can be set to 2, 4 & 8 ohms, of sufficient power handling for the largest amplifier you'll ever want to test. Having two will allow you to do 'both channels driven' testing. It's also useful to have a loudspeaker simulator load that provides some reactive loading, but there's no standard for that, so comparisons with other tests will be pretty meaningless. I use 8 ohms in parallel with 2uF as my standard, especially for square-wave tests.

Some of those instruments can be combined, and a lot can be done using a high quality PC sound card and some free software like REW. The one limitation of PC software I've found is that none I've come across measures distortion by nulling out the fundamental and displaying the residual. All do it by FFT, which is a much more powerful technique, but doesn't show things like crossover distortion as obviously as looking at a distortion residual on a 'scope.

You'll also need a set of interface cables for ins and outs, a load of adapters for the different sockets used and plenty of patience.

Good luck. Measurements can be a fun hobby in its own right.

S.

 

[Koeitje]

There is one thing that I would like to know how to measure: actual watts my amplifier is putting out at the loudspeaker terminals. I know that if I play loud I pull about 90 watts from the wall, but that doesn't say a lot because of efficiency (class ab) and because of the peaks the amplifier might actually be outputting that are taken care of by its reserves.

 

[sergeauckland]

There is one thing that I would like to know how to measure: actual watts my amplifier is putting out at the loudspeaker terminals. I know that if I play loud I pull about 90 watts from the wall, but that doesn't say a lot because of efficiency (class ab) and because of the peaks the amplifier might actually be outputting that are taken care of by its reserves.

It depends if you want to know what the amplifier is capable of, i.e. under continuous tone conditions, or what it's putting out at any one time whilst you're listening. For the former, you need the equipment mentioned in my post above. To put it simplistically, you inject a signal and crank up the level until you can just see clipping on the 'scope, then measure the volts. Power is V^2/R, so 28.3volts into 8 ohm is 100 watts.

If you want to see how much instantaneous power you're putting out at any one time whilst listening, you'll need a calibrated peak-meter that's measuring across the loudspeaker terminals. Those are rather rare, and you'll probably have to build one yourself from a true-peak PPM or similar.

S.

 

[Razorhelm]

Thanks for the help Serge!

Are you actually based in Suffolk? I grew up in Ipswich

 

[Koeitje]

It depends if you want to know what the amplifier is capable of, i.e. under continuous tone conditions, or what it's putting out at any one time whilst you're listening. For the former, you need the equipment mentioned in my post above. To put it simplistically, you inject a signal and crank up the level until you can just see clipping on the 'scope, then measure the volts. Power is V^2/R, so 28.3volts into 8 ohm is 100 watts.

If you want to see how much instantaneous power you're putting out at any one time whilst listening, you'll need a calibrated peak-meter that's measuring across the loudspeaker terminals. Those are rather rare, and you'll probably have to build one yourself from a true-peak PPM or similar.

S.

I'm mainly interested in actual listening conditions, because I already know how much power it can produce for a continuous tone. I just want to see how much I'm putting in my speakers

 

[solderdude]

Sorry I find asking people who know more than me an effective way to learn, fine if that isn't how you do things but would you mind not replying if its just to be rude.

I knew nothing about using REW to do room correction, however I asked on this forum and some very helpful people stepped in and helped me get started.

I am in the beginner section am I not?

Serge already went into more detail about what you need.
You really need some specific gear such as an oscilloscope, dummy load(s), meters, frequency generator (preferably sine and square-wave) and then some analysis and plot software.
Then you need to know how and what to test and what the setup must be.
It is handy to have experience so not to fall into 'traps'. It is very easy to get incorrect measurement (results) and if you don't know what to expect you may not get meaningful results.

Learning by asking is a great way. It is relatively easy to learn how to use REW. Afterall there are plenty of tutorials and only requires a PC, REW, and a mic.
You don't need specialized equipment at all for this other than the mic.
Yes, you can do some basic measurements using REW but you really need to know how to make attenuators and how to calibrate so you get meaningful results.

When you have to ask what to measure and how then I can only conclude that you need to learn some basics first.

I never reply just to be rude.

 

[sergeauckland]

Thanks for the help Serge!

Are you actually based in Suffolk? I grew up in Ipswich

Yes, just outside Bury St Edmunds. Been here since the mid 1980s.

S.

 

[solderdude]

I'm mainly interested in actual listening conditions, because I already know how much power it can produce for a continuous tone. I just want to see how much I'm putting in my speakers

What amp and speakers are you talking about ?
Amps, for instance that double the output power when halving the impedance will have the same 'peak power' as nominal power (sort-of) but if your amp doesn't do that there is likely a bit more (very short moment) peak power available.

I have a peak-power meter you could loan (assume you are Dutch) I built specially for this type of measurements.
Within NL I can send it over.

 

[Koeitje]

What amp and speakers are you talking about ?
Amps, for instance that double the output power when halving the impedance will have the same 'peak power' as nominal power (sort-of) but if your amp doesn't do that there is likely a bit more (very short moment) peak power available.

I have a peak-power meter you could loan (assume you are Dutch) I built specially for this type of measurements.
Within NL I can send it over.

Marantz PM-16 + Revel M106:

I believe a German magazine measured it and its actual output is 10-20% higher.

And thanks for the offer, but its mostly out of interest to see how much is true about the "you only need x watts" story you hear all the time .

 

[Blumlein 88]

To the OP, the Quant Asylum might be a good start for someone new to measuring amplifiers.
https://quantasylum.com/collections/frontpage/products/qa401-audio-analyzer

You will need more, like a load for power amps. And the QA isn't state of the art like an Audio Physics unit, but it can be used to good effect on nearly all audio gear. It would let you do all the normal measures on an amp. Also can measure other audio devices. It does appear to have a problem with its noise floor being modulated by the signal, but for basic stuff it would do nicely while reducing how many bits of gear you need for some power amp measuring.

As for what to measure, I'd suggest reading thru Amir's amp reviews and asking questions about them. His measures aren't standard, but are usually sensible. Other measures can be made he doesn't do depending upon one's needs. You'll probably want to study some basic electronics as well if you aren't up to speed on it already.

 

[wwenze]

It is easy to measure stuff.
It is hard to measure stuff in a way that generates data in a way that can be directly compared to data measured by others. $$$ is usually required.

Anyway...

If all you want to know is how loud you are setting the amplifier, all you need is a multimeter. Go as loud as you want. Then wear hearing protection and play a 50Hz tone and use the multimeter to measure the AC voltage.

 

[DonH56]

Note most speakers' impedance varies greatly over frequency and somewhat with power so just measuring the voltage at the terminals will not in general provide the correct power.

REW has the ability to do impedance sweeps, I think, so that plus test tones plus a meter (make sure it has >20 kHz bandwidth; many of the very cheapest do not) will get you there.

HTH - Don

 

[solderdude]

Marantz PM-16 + Revel M106:
View attachment 58986

I believe a German magazine measured it and its actual output is 10-20% higher.

And thanks for the offer, but its mostly out of interest to see how much is true about the "you only need x watts" story you hear all the time .

The meter shows actual peak levels using music. What is interesting to see real time is not when you reach the maximum but how much one usually uses under normal and louder than normal usage.

The 10 to 20% higher peak readings are plausible, of course 20% more power is just 0.8dB so inconsequential.

 

[Razorhelm]

Yes, just outside Bury St Edmunds. Been here since the mid 1980s.

S.

Haha, I started my existence on this earth in Ipswich in the 80's!

Nice to meet you, small world!

 

[Razorhelm]

Thanks for the link to Quant Asylum @Blumlein 88 .
They have some great resources on their site too.

@solderdude ok cool that you're not trying to be rude but I still think you're wrong.

To your point:
"Those making measurements already know what to measure, how to measure it and how to interpret the results."
At one stage they didn't know how to measure either, they had to learn! This is the first stage of my learning! Hope you wish me well

 

[bribur]

This thread on diyaudio tells how to do distortion measurements with a soundcard and REW:

https://www.diyaudio.com/forums/software-tools/338511-howto-distortion-measurements-rew.html

 

[Blumlein 88]

Thanks for the link to Quant Asylum @Blumlein 88 .
They have some great resources on their site too.

@solderdude ok cool that you're not trying to be rude but I still think you're wrong.

To your point:
"Those making measurements already know what to measure, how to measure it and how to interpret the results."
At one stage they didn't know how to measure either, they had to learn! This is the first stage of my learning! Hope you wish me well

BTW, thought I'd comment on the tone of the replies. People aren't being as unhelpful as you might think. If you know some basic electronics you can figure out on your own or easily enough from someone else how to measure various factors. OTOH, if you don't know much of that, we probably have several times given someone what we thought were simple instructions only to find out how many ways that can go wrong. With power amps you have the substantial chance of harming speakers, harming the amp or some other piece of measuring gear. With DACs not so much can go wrong, but with power amps a little extra care is needed.

So while we could give you some step by step instructions it can be incredibly tedious to do that in a way the person reading the instructions cannot misunderstand and damage some gear as a result. Even with the best instructions a costly misunderstanding is too easy until you learn some of the basics of how things work. So replies aren't so much that we won't help you, as much as saying jumping straight to full amplifier measurements can be dangerous to the gear involved. So go easy and go slow until you get your feet on the ground.