EEVblog #1118 - Why Are Studio Monitors Noisy?

[Richard Crowley]

I posted the YouTube video over on the audio forum Gearslutz.com
https://www.gearslutz.com/board/geekslutz-forum/1229581-why-studio-monitors-noisy.html#post13503068

There was just a response from widely-respected design guru, Jim Williams (not the one from Linear Technology)
This Jim Williams is famous for modifying commercial audio gear to significantly improve performance.
He also consults for many companies that design and make commercial audio gear.
And he makes a boutique line of audio gear (at http://www.audioupgrades.com/)

Anyway, he said:

This is common on powered monitors as the amps are not well designed. Even the Meyer HD1's hiss. That is an issue for me. It masks low level details. My monitors are silent here, zero hiss 1/2" away from the center of the tweeters. The noise is the same whether the amp is on or off. It can be done.

[David Hess]

The datasheet says that closed loop gain needs to be at least 30dB for stability reasons.

So it does.  I looked for that in the datasheet but missed it and assumed it would be the case.

There are ways to lower the closed loop gain on a decompensated amplifier however they involve raising the noise gain so that is no solution.

It's somewhere in the side notes.
But basically all these power opamps have a minimum gain to keep them stable.

I am intimately familiar with the how and why leading to my surprise at not to find a minimum stable gain listed in the datasheet.  I just did not look closely enough as I am used to bold letters on the first page saying "Minimum Stable Gain of 32" or similar.

There are ways to add extra compensation and do other tricks.
From manufacture point of view those are mostly not very interesting because you will find yourself in unknown territory.
Which means spending a lot of time (money) on research.

The real restriction is not on minimum closed loop gain but on minimum *noise* gain.  It only takes a series RC circuit across the inputs to solve but raising the noise gain while lowering the closed loop gain produces the same output noise so nothing is improved.  Altering the internal compensation if possible would lower the full power bandwidth which I suspect is why they configured it this way in the first place; the full power bandwidth was insufficient without decompensation and a minimum stable gain is hardly an issue for an audio power amplifier.  It is too bad it suffers from such a high input noise but that is likely the result of transconductance reduction in the input stage being required as well.  Discrete audio amplifiers commonly employ both of these techniques.

Choosing such a noisy part seems dumb considering how much effort they put into the preamplifier circuits but it sure is economical for undiscriminating users.  Are all of the remaining integrated audio amplifiers noisy as well?  I have never heard of a quiet one.  Maybe there were no better choices short of a discrete implementation.

To solve the noise problem when using an integrated power amplifier likes this I might place it within the control loop of a lower noise amplifier but this is not something which could be easily retrofitted.

Also keep in mind that this noise isn't noticeable or only barely noticeable at the listeners position.  So it's a bit like solving a non-existing problem.

It drives me crazy when I come across it.  It used to be a more common problem when high efficiency speakers intended for vacuum tube amplifiers were used with solid state amplifiers.  Electronic headphones of various types all seem to have the same problem with excess noise.

[b_force]

The datasheet says that closed loop gain needs to be at least 30dB for stability reasons.

So it does.  I looked for that in the datasheet but missed it and assumed it would be the case.

There are ways to lower the closed loop gain on a decompensated amplifier however they involve raising the noise gain so that is no solution.

It's somewhere in the side notes.
But basically all these power opamps have a minimum gain to keep them stable.

I am intimately familiar with the how and why leading to my surprise at not to find a minimum stable gain listed in the datasheet.  I just did not look closely enough as I am used to bold letters on the first page saying "Minimum Stable Gain of 32" or similar.

There are ways to add extra compensation and do other tricks.
From manufacture point of view those are mostly not very interesting because you will find yourself in unknown territory.
Which means spending a lot of time (money) on research.

The real restriction is not on minimum closed loop gain but on minimum *noise* gain.  It only takes a series RC circuit across the inputs to solve but raising the noise gain while lowering the closed loop gain produces the same output noise so nothing is improved.  Altering the internal compensation if possible would lower the full power bandwidth which I suspect is why they configured it this way in the first place; the full power bandwidth was insufficient without decompensation and a minimum stable gain is hardly an issue for an audio power amplifier.  It is too bad it suffers from such a high input noise but that is likely the result of transconductance reduction in the input stage being required as well.  Discrete audio amplifiers commonly employ both of these techniques.

Choosing such a noisy part seems dumb considering how much effort they put into the preamplifier circuits but it sure is economical for undiscriminating users.  Are all of the remaining integrated audio amplifiers noisy as well?  I have never heard of a quiet one.  Maybe there were no better choices short of a discrete implementation.

To solve the noise problem when using an integrated power amplifier likes this I might place it within the control loop of a lower noise amplifier but this is not something which could be easily retrofitted.

Also keep in mind that this noise isn't noticeable or only barely noticeable at the listeners position.  So it's a bit like solving a non-existing problem.

It drives me crazy when I come across it.  It used to be a more common problem when high efficiency speakers intended for vacuum tube amplifiers were used with solid state amplifiers.  Electronic headphones of various types all seem to have the same problem with excess noise.

You're talking about a composite or nested amplifier I think?
That is actually a perfect way to get not only rid of the noise, but also improve the total performance significantly.
Totally forgot about these lol.

Yes, with high efficient compression drivers it can be an issue.
I have worked on huge line-array systems myself with 12 compression drivers in it, even the best amplifiers struggle with that.
Luckily these are being used pretty noisy environments.
In these situations you definitely can hear what a proper PCB layout and good parts (using NP0 instead of X7R etc) can do.
For home audio it's basically like Dave showed. You have to put your ear almost into the tweeter to hear anything at all.
It does depend what kind of speakers and how the filter is made.
Like I said, just adding a passive attenuation circuit can reduce the noise by 6dB minimum (because of the bafflestep compensation)
Since most tweeters are also much more sensitive than woofers, that number can easily go to about 12dB.
Of course that will cost a bit more power, but home hifi tweeters need around 10-15W anyway

The latest Class-D amplifier IC's are in terms of noise pretty much the same or better.
Professional modules are a lot better actually.

[boB]

My Mackie HR-824s (not made anymore) are virtually dead quiet.  I can hear slight hiss if I put my ear right up to the tweeter but it is very quiet.  These used discrete amplifiers and not a module like most of the monitors today use.

I hear that Genelec monitors are also very quiet.   Anything made to be cheap and usually Chinese just aren't going to be everything you could ask for.

That's why I buy used on Ebay.

boB

[Bassman59]

I hear that Genelec monitors are also very quiet.

I have a pair of Genelec 1031A which I bought new in 1996. They are dead quiet. The output amplifiers are discrete. Input stage is nothing fancy, just a differential amplifier made with a 5532.

[Kleinstein]

The residual noise problem is no specific to monitor speaker. It also sometimes happen with normal Hifi equipment if high efficiency speakers are used. I had that with an otherwise good amplifier and self build speakers too. Many of the old chip amplifiers are made with low efficiency speakers in mind. Speakers can be quite different in how much sound they make from a given electrical power.

Those monitor speakers tend to prefer high efficiency speakers as they could save on the amplifiers power this way.

[b_force]

The residual noise problem is no specific to monitor speaker. It also sometimes happen with normal Hifi equipment if high efficiency speakers are used. I had that with an otherwise good amplifier and self build speakers too. Many of the old chip amplifiers are made with low efficiency speakers in mind. Speakers can be quite different in how much sound they make from a given electrical power.

Those monitor speakers tend to prefer high efficiency speakers as they could save on the amplifiers power this way.

To my professional experience and knowledge I have never seen or come across significant differences in the sensitivity between home hifi and studio monitors.
The biggest difference is that studio monitors are mostly active speakers while home hifi speakers use a passive filter.
This passive filter already reduces the noise by at least 6dB because of the baffle step correction.
Since most tweeters have an higher sensitivity this attenuation can be easily around 9-12dB.