EEVblog #1118 - Why Are Studio Monitors Noisy?

[EEVblog]

Why do most powered studio monitor speakers have a low level audible background noise? Time for some investigation on the KRK Rokit 6 studio monitor speakers to track down where the noise is coming from.

[johnlsenchak]

It's because the output transistors (inside the chip package ) are slightly biased on, causing the noise/ripple in the  sound floor  to increase

[EEVblog]

It's because the output transistors (inside the chip package ) are slightly biased on, causing the noise/ripple in the  sound floor  to increase

Probably. Bottom line, it's the amplifier chip.

[thmjpr]

This is much worse on Class D powered monitors, especially the cheap ones. Same sort of inherent noise floor, regardless of gain (if lower), supply source, voltage, etc. I returned a pair of Kanto monitors because of the unbearable hiss. And yet a smaller chinese amp brand has managed to work around the issue very well: auto shutdown is incredibly fast when no sound is playing, so of course if its shutdown there is zero hiss, same as Daves A/B amp in mute. It also helps that they used a bit better output amp (STA328). Unless you are playing something with incredibly low volume, you would never notice the hiss.

Its one thing that is not really covered well in reviews or specs, especially for powered monitors.

[laneboysrc]

Interesting video as always
Would it help to decrease the gain of the output amplifier, for example by changing the 12k feedback resistor to a 5k6 one?

[Raj]

They can digitally add anti-noise into 'audio signal in-line' to cancel it, can't they?
Someone, get on it and make it happen

[EEVblog]

This is much worse on Class D powered monitors, especially the cheap ones. Same sort of inherent noise floor, regardless of gain (if lower), supply source, voltage, etc. I returned a pair of Kanto monitors because of the unbearable hiss. And yet a smaller chinese amp brand has managed to work around the issue very well: auto shutdown is incredibly fast when no sound is playing, so of course if its shutdown there is zero hiss, same as Daves A/B amp in mute. It also helps that they used a bit better output amp (STA328). Unless you are playing something with incredibly low volume, you would never notice the hiss.

Its one thing that is not really covered well in reviews or specs, especially for powered monitors.

That's why many of the monitors have auto-mute, there is no other reason for it than to mask inherent background noise.
Yes, I'm surprised by the lack of mention of any of this in reviews, when the go into insane depth on all the other stuff.

[b_force]

Most of it all has to do with PCB design.
Very often power ground and signal ground are just directly connected together.
Sometimes even in the amplifier chip itself!
In other cases these chips have an input side (signal ground) and output side (with power ground).
Make sure you divide the two properly and DON'T put them over the same wire back to the PSU.
In fact it's better to make a good start ground with enough capacitive buffering as close to the power amps if possible.

In the old days when we developed our own Class-AB (B) amplifiers, we could see an huge different in noise when connecting the gain stage to actually the signal ground.
Sometimes even as big as 6dB or more!

This was a huge factor with high power and high sensitive compression drivers (especially in line-arrays.
Nowadays there are more than enough Class-D amplifiers that are as good or even WAY better in terms of noise (and all the rest as well).

In application were there isn't much power needed for tweeters you can just simply add a resistor attenuator.
You only do have to compensate for the gain and keep an eye on the frequency response.
(I saw this was also mentioned in the comments by someone else)

It the end you just want a gain as small as possible, which is a bit tricky because of stability of the circuit.
It surprised me (find it hard to believe) that the pre-amp section wasn't contributing that much.
The rail regulation is pretty poor with a bunch of 15V zeners (which are noisy) plus these opamps aren't the best at all (against what Dave said)

@Raj
Yes in theory you could add pre-noise (and pre-distortion), but you will need a DSP for these things.
Noise is going to be pretty difficult since it's difficult to make the random characteristics.

Its one thing that is not really covered well in reviews or specs, especially for powered monitors.

There are so many things not covered at all in reviews.
One of the most important things is called directivity, were even expensive well known brands fail horribly.
Next to that is non-lineair distortion (especially in the lower frequencies).

[David Hess]

That was a great job Dave.  I like that you found that the mute function was powering down the amplifier confusing the noise issue.

I did the rough math and concluded that the power amplifier itself was contributing more noise than anything before it; I was expecting the input stage to be the problem.  While not being a particularly low noise circuit, whoever applied the RC4580 operational amplifiers used about the right impedance levels so they knew what they were doing.  The gain distribution could have been better, but with the TDA2052 contributing so much noise it did not matter.

Besides relatively poor input noise, the TDA2052 also only has power supply rejection of 40dB minimum and 50dB typical.  No detailed schematic is available but I suspect its high input noise comes from emitter degeneration of its differential input pair which is commonly done to reduce transconductance allowing for better frequency response and higher full power bandwidth which is required in an audio power amplifier.

I might consider adding a low noise voltage gain stage before the TDA2052 so it can operate at a lower voltage gain reducing output noise however the TDA2052 might not be stable at lower closed loop gains and the datasheet does not say.  It would be odd for it not to have a minimum stable gain because lower gains require more frequency compensation which would lower performance in its intended application.

[b_force]

BTW, could you share the schematic/service manual of the thing?

[Karel]

The power amp chip is poorly designed.
Power amps with discrete transitors usually don't suffer from this noise problem.

[David Hess]

The power amp chip is poorly designed.
Power amps with discrete transitors usually don't suffer from this noise problem.

I do not know about usually since everybody seems to use the same basic topology with emitter degeneration of an input differential pair but integrated amplifiers definitely have some disadvantages because the capacitors needed for complex frequency compensation are very expensive on an IC and economics means they use the least expensive power IC process available so they cannot take advantage of JFETs or high performance PNPs.  For instance no consumer device is going to use an expensive part from Apex Microtechnology.

[BrianHG]

I'm surprised.  (Talking about the power amp stage, not the puny input preamp noise) The cheapest discrete design power amp, when I say cheap, it usually means simple, but a proper design, would have low enough noise floor to make any hiss inaudible, even with a high power 200 watt design, and the power supply rejection ratio of the supply rails should be high enough that even if you have lemon dying caps on your power supply, the 100hz should also be inaudible when there is no source audio connected.

It's like someone put together ok effort on speakers, ok effort case, ok effort power transformer, ok effort front end, then, any amp IC that just happen to be available which had enough power and it just had to be a 2$ by the spec, no engineering solution.

[Nominal Animal]

Am I the only one who noticed a wisp of magic smoke escaping from that thing opened on the right side of the speaker at 9:27?  I swear I could smell something..

[b_force]

Am I the only one who noticed a wisp of magic smoke escaping from that thing opened on the right side of the speaker at 9:27?  I swear I could smell something..

That's reflection

[EEVblog]

Most of it all has to do with PCB design.
Very often power ground and signal ground are just directly connected together.

Not in this case. I shorted the input to ground and the layout looks good, same noise was still there. Seems inherent in the amplifier chip.

[EEVblog]

BTW, could you share the schematic/service manual of the thing?

Service manual and schematic:
http://eevblog.com/files/RP6G2schA1.pdf
It's excellent actually.

[EEVblog]

I'm surprised.  (Talking about the power amp stage, not the puny input preamp noise) The cheapest discrete design power amp, when I say cheap, it usually means simple, but a proper design, would have low enough noise floor to make any hiss inaudible, even with a high power 200 watt design, and the power supply rejection ratio of the supply rails should be high enough that even if you have lemon dying caps on your power supply, the 100hz should also be inaudible when there is no source audio connected.

It's like someone put together ok effort on speakers, ok effort case, ok effort power transformer, ok effort front end, then, any amp IC that just happen to be available which had enough power and it just had to be a 2$ by the spec, no engineering solution.

But it's not just KRK. Yamaha, JBL, Alesis and others all have it. And even someone in the youtube comments mentioned Adam as well.

[EEVblog]

That was a great job Dave.  I like that you found that the mute function was powering down the amplifier confusing the noise issue.

Many speaker have an auto-mute function to mask the noise, that's it's only function. I believe the KRK Gen 3 has this.

[boB]

I was hoping you would go one step further and reduce or short the 12K power amp feedback resistor to see if reducing the gain would reduce the noise. 

If so, then you'd have to makeup the gain in front of it IF it could even be done quietly.

boB

[BrianHG]

I'm surprised.  (Talking about the power amp stage, not the puny input preamp noise) The cheapest discrete design power amp, when I say cheap, it usually means simple, but a proper design, would have low enough noise floor to make any hiss inaudible, even with a high power 200 watt design, and the power supply rejection ratio of the supply rails should be high enough that even if you have lemon dying caps on your power supply, the 100hz should also be inaudible when there is no source audio connected.

It's like someone put together ok effort on speakers, ok effort case, ok effort power transformer, ok effort front end, then, any amp IC that just happen to be available which had enough power and it just had to be a 2$ by the spec, no engineering solution.

But it's not just KRK. Yamaha, JBL, Alesis and others all have it. And even someone in the youtube comments mentioned Adam as well.

It is not possible to argue with selecting from what's available & the design choices.  I'm using a pair of Paradigm Monitors, but, originally fed with a homemade and amp which used OP27 opamps for voltage gain, feeding a NPN/PNP darlington emitter follower 75 watt amp, and today, a new external 200 watt amp both tied directly to the headphone output of my laptop and there is 0 hiss or hum.  So, technically, a proper amp can be designed, it's just that KRK, Yamaha, JBL, Alesis just picked a noisy IC audio amp off the shelf.

I personally do not blame them.  In a self powered monitor speaker system, they have a number of criteria and features to meet.  For the electronics, they went with published off the shelf reference designs and components as they do not have a specialist in each specific field and making an elaborate discrete audio amplifier (which takes many more components and engineering know how), or, choosing a quality premade 25$-200$ amp module was just out of the question.

If I were to re-equipt today with a cheap budget in mind, I would still choose a set of discrete monitor speakers similar to my Paradigms, but, maybe a 4 ohm version.  I would then either choose a higher quality Alibaba type Class D amp with a single 48v telecom switching supply, or, an Ebay used quality high end amp.  I do not think I can tolerate the performance bar of KRK, Yamaha, JBL, Alesis because at times, I need to crank up the volume and I cant tolerate any weakness or distortion, and at low volume, hiss and hum is something that should be obsolete in today's digital age.  Though, I still cannot faulter that you don't need an external amp with the KRK, Yamaha, JBL, Alesis.

[b_force]

Most of it all has to do with PCB design.
Very often power ground and signal ground are just directly connected together.

Not in this case. I shorted the input to ground and the layout looks good, same noise was still there. Seems inherent in the amplifier chip.

What makes you think that shorting the input bypasses the PCB layout?

I was hoping you would go one step further and reduce or short the 12K power amp feedback resistor to see if reducing the gain would reduce the noise. 

If so, then you'd have to makeup the gain in front of it IF it could even be done quietly.

boB

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

[SilverSolder]

It's interesting that the HF noise viewed in the FFT application seems to start to drop off at around the crossover frequency?

Perhaps the power amp noise is spat back into the input, and is actually attenuated by the preceding stage...  but only above the crossover frequency?

[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.

[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.

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.

Like I said before the quickest and easiest way is just add a resistor divider after the power amplifier.
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.