Amp power and SNR measurements

[EngineerNate]

Had a thought about amplifier measurements reading some of the PA amp reviews.

If you have an amp rated at 50W and one at 500W, measuring SNR at 5W will almost always favor the 50W amp, as the voltage/current gain applied to the noise signal coming out of the preamp stage will almost always be fixed, and the 500W amp is applying 10X more amplification than the 50W amp.

Attenuation happens before/at the input stage, so the self noise of the input stage is fixed. That self noise will get amplified along with the signal, so when we attenuate the signal to match the outputs of two drastically different power level amps, unless something is broken the less powerful amp should always win.

That's not taking into account the fact that to get high power levels often means paralleled output devices, which inherently increases noise as well.

It would be interesting to start testing SNR at some fixed percentage of rated output capability rather than at fixed outout power levels. 500W full range would be instant deaf for most of us in practice, but academically it'd be interesting to see how quiet big amps can be on a "level" playing field.

Cheers,
Nate

 

[antcollinet]

Had a thought about amplifier measurements reading some of the PA amp reviews.

If you have an amp rated at 50W and one at 500W, measuring SNR at 5W will almost always favor the 50W amp, as the voltage/current gain applied to the noise signal coming out of the preamp stage will almost always be fixed, and the 500W amp is applying 10X more amplification than the 50W amp.

Attenuation happens before/at the input stage, so the self noise of the input stage is fixed. That self noise will get amplified along with the signal, so when we attenuate the signal to match the outputs of two drastically different power level amps, unless something is broken the less powerful amp should always win.

That's not taking into account the fact that to get high power levels often means paralleled output devices, which inherently increases noise as well.

It would be interesting to start testing SNR at some fixed percentage of rated output capability rather than at fixed outout power levels. 500W full range would be instant deaf for most of us in practice, but academically it'd be interesting to see how quiet big amps can be on a "level" playing field.

Cheers,
Nate

Except that is not how amps are used.

5W is chosen since this will be a typical "loud" level in domestic listening. Even if it not for you- then regardless of what amp yo are using - your normal listening won't be at a higher level just because you are using a more powerful amp.

So the test/measurements show you a comparable basis for the noise difference you will experience in your listening.

 

[EngineerNate]

I don't think you read the last paragraph of my post.

 

[EngineerNate]

To me it's about measuring a device's capability in the regime it was designed to operate under. An 800W PA amp isn't "fit for purpose" to be used at 5W on a pair of bookshelves. Judging it based off of those criteria is, on some level, very silly.

It's also kind of... An uninteresting? And predictable result. Something applying 10x more gain to a signal is going to be noisier from first principles.

 

[Oddball]

To me it's about measuring a device's capability in the regime it was designed to operate under. An 800W PA amp isn't "fit for purpose" to be used at 5W on a pair of bookshelves. Judging it based off of those criteria is, on some level, very silly.

It's also kind of... An uninteresting? And predictable result. Something applying 10x more gain to a signal is going to be noisier from first principles.

Try Kippel test. Your ears are unfortunately much less capable than you would think.

Listening Test

www.klippel.de

And there is nothing wrong with "noisy" amps as long as they are "right".

Rotel RB-1070 Amplifier Review

This is a review and detailed measurements of the Rotel RB-1070 stereo power amplifier. It was released in early 2000. Can't find the cost then but I see it used for around $400 to $500. I have always found Rotels to be most attractively designed power amplifiers. One look at that front panel...

www.audiosciencereview.com

Academically, you are right - this whole story does not seem to add up but we happen to live with it happily ever after - every day. Don't invent a problem and there will be none, or at least less of them.

 

[antcollinet]

I don't think you read the last paragraph of my post.

No, I read it all. Perhaps you are not understanding my point.

 

[EngineerNate]

I guess what I'm trying to say is that criticising a 1000W PA amplifier because it isn't dead quiet at 5W output (as many many of the comments on the PA amplifiers here do, some going as far as to call the designers completely incompetent) is akin to saying an F1 car is a bad car because it's not good at your daily commute.

I don't think there is a problem, other than things being criticised for not performing under operating conditions they were never designed to operate under.

I agree with you in that most people won't hear even noisier amps because their speakers self noise will dominate, and few of us actually have golden ears.

 

[EngineerNate]

No, I read it all. Perhaps you are not understanding my point.

I'm rephrase.

It would be interesting to have both sets of measurements.

 

[antcollinet]

I'm rephrase.

It would be interesting to have both sets of measurements.

Not to me.

 

[EngineerNate]

Not to me.

Okay.

 

[EngineerNate]

If what I'm proposing isn't of interest why comment in the thread? If 5W is the intended use case the PA amps make no sense to even care about reviewing. There are cheaper/less power hungry/quieter (no fans) products that do the job. My 40 year old 45W receiver in the living room system does a great job at those power levels. Job done.

This is a forum where were are supposed to care about the technical excellence or lack thereof of products, and if something is being measured in a regime outside it's intended operating conditions that should at least make us dig a little deeper to see what the engineering is capable of achieving within the operating range it was intended to function. Otherwise why bother testing at all?

It's like doing a 0-60 on a Peterbilt and not doing any more testing because it's slower than a sports car.

 

[NTK]

Why can't you get the information you want from plots like this one which are in (almost) all amplifier reviews?

 

[kyuu]

Edit: NTK beat me by a hair, just refer to his post above.

 

[EngineerNate]

Why can't you get the information you want from plots like this one which are in (almost) all amplifier reviews?

Perfect! I have to admit on my phone these charts don't pop as clearly as the big SINAD number in the initial 5W measurement. It'd be nice to see that point measurement done at both 5W and at max rated just for clarity.

 

[DVDdoug]

There are lots of ways to measure noise and SNR (including A weighted), making it very difficult to compare manufacturer's specs.

The 5W reference (along with the spectrum that Amir shows) means the measurements are comparable. If you have a 10W amp and a 1000W with the same measurements you'll hear (or hopefully won't hear ) the SAME noise from your speakers at your listening position.

...And you can make a calculation if you want to reference maximum power.

 

[EngineerNate]

That's fair! I guess it just seems like there's some expectation that big amps will perform the same as smaller ones, when that's generally not possible from a physics standpoint, if you're starting from the same input sensitivity. Front loading the 5w measurement (and using in the ranking charts) and burying commentary on the full power performance would seem to reinforce this and give an unnecessarily negative impression of the amp under test.

It may very well be the amp sucks for doing 5W, but it might be a great amp for doing something else.

 

[Matias]

As for big amps, there are kilowatt amps with the biggest sinad on the market today, having best of both worlds. A kilowatt noisy amp is just not competitive anymore, and these test expose this.

See the spreadsheet on my signature.

 

[EngineerNate]

As for big amps, there are kilowatt amps with the biggest sinad on the market today, having best of both worlds. A kilowatt noisy amp is just not competitive anymore, and these test expose this.

See the spreadsheet on my signature.

Those Purifi and Hypex based amps sure do seem to change the game. Are any of the kilowatt low noise amps traditional A/B amps? I did some scrolling and the first five or six I checked were all of those two varieties under the hood.

 

[DVDdoug]

Actually if you take an amplifier, beef-up the MOSFETs & heatsinks, and increase the power supply voltage (without otherwise changing the gain or anything in the circuit) and the noise will be about the same if not exactly the same.

 

[Matias]

There are some Accuphase, Halcro and Boulder amps that are class AB, low noise and tons of power. But cost dozens of not hundreds of thousands of dollars.

Hypex and Purifi are class D and therefore smaller and sane prices.