Hi, You've seen my KRK Ewropeaidd or Rocket Six powered studio monitor speakers that I use for my video editing in before. I've done a repair video on these which was really fascinating which I'll link in down at the end or down below. There's one interesting thing not, we're not just specific to these: KRK Mondas but most studio monitors on the market regardless of the brand doesn't matter whether they're KRK the Yamaha their JBL there are leases. you know, amake any of the top brands, they will have a noise floor to them.

These powered monitors that have the amplifiers are built-in if you don't apply any signal to them whatsoever and you actually put your ear up to the tweeter like this and the woofer R to some respect as well, but it's more are pronounced on the tweeter. You can actually hear a low-level hiss on there, and it's got nothing to do with system interconnections or anything else. It's got nothing to do with the fact that I've repaired these all the black existing black gunk or anything like that. It's basically inherent in most studio monitors.

There are reported to be some that don't have any audible background noise floor hiss on them, but I think they're you know, reasonably rare. So it's as common as mud for these things to have them and I thought I just investigate why that's the case. First of all though, I'll get my mic and I'll actually put it art right up to it and I'll get see if I can actually record the noise floor for you. Okay, let's see if we can Actually do some simple analysis of or at least look at the FFT our response of our audio captures here that we got from the woofer and the tweeter.

Now this is the woofer file here. This point in here is just the background noise when I had it switched off. So from here to here is the powerup mute function. It lasts for about a half a second second or something like that.

And then this is the woofer noise in here and we can see the spectrum down the bottom here now. I'm using them NCH Wavepad here Australian Software I Don't know why they don't annotate these axes on here. So anyway, it's frequency and we can see the frequency on the cursor. You can see it here.

Note this peak at around hundred. Hertz This is what we're interested in with the woof. Eh, see how? it's right there there? in mute mode. there was a hundred, a significant, quite louder 100 Hertz peak there.

Now we're listening to the woofer. You can see that the spectrum here is actually quite broadband compared to the bass room noise. So this is just picking up from the microphone. and then that is the inherent noise floor of the woofer and the low frequency amplifier in there.

so you can see. you know there's not a huge amount difference in there. Once again, you have to put your ear right up to almost touch in the cone to hear this, but you'll notice that there is that hundred Hertz peak there. So of course hundred Hertz We have 50 Hertz mains here in Australia.

So the hundred Hertz is the full wave bridge rectified frequency of this thing, so that's obviously coming through the hundred Hertz ripple on the power supply. so that's coming through, so that's not great. System design is that they could have, you know, put some extra measures in place to filter that out, but they didn't But as I said, it's very low level, so it's not really a problem. but it's just you know it's there and you can technically hear it.

Everything else is like fairly broadband noise as you'd expect. So the tweto. This is a switch on point here. This is the mute period and this is the noise.

As you can see, it's quite high and you'll see that it is fairly broadband right across the spectrum there, as you'd expect. So this is the background. This is when it switched off. So this is the background room noise and then that is the signal lever.

You see it's much higher than we saw before on the woofer, so it is quite significant. You can see some roll-off there. it's what. 15 odd kilohertz there.

So I Don't know whether or not that's the measurement mic or you know, the speaker. rolling off. you get fairly consistent broadband noise across the band there, and that's what you expect from typical component thermal noise and resistor thermal noise and all the rest. You expect a broadband response and that's what we get.

We get no hundred Hertz there at all because it's a high-pass filter so it filters out. You know anything below. I Think crossovers like 2.8 kilohertz or something like that, so you wouldn't have expected that to come through. and it doesn't.

So there you go. Just an interesting look at the spectrum response of this room. Noise Tweeter: Hmm and this noise has absolutely nothing to do with the the volume. or you know, high frequency adjust or anything like that.

Makes no difference whether you turn that volume all the way down. It's an inherent system noise floor, and that's just manifests itself with the a wide dynamic range of speakers and the gain of the amplifier. But where is that noise coming from? That's what I'm interested in. And the other thing you've got to remember is that this noise.

Although you heard it like it sounded really bad there, it is barely audible. You've got to put your ear practically right up to the tweeter or the woofer in order to actually hear it. So you know, maybe in a dead, silent room you can hear it at a little bit of a distance. And it some monitors are worse than the studio monitors are worse than others in this aspect.

You can sort you know, hear the background hiss from further away. But it's really not an issue because when you play your music or your audio whatever it is you're mixing on your studio monitors, then it's You know so far in excess of this noise floor that it's not a problem you just here and it's literally down in the noise floor. Now if we have a look inside, we've got our main power amplifier board down here. which means-- has our main power amplifier chip in it for both.

There's a separate one for both the high frequency and the low frequency driver. It separates them and it does that over on this board here, which is the input board which has Japan radio Corp 4580 Op amps on that classic audio amplifier chip. You know they're quite low noise. they're low distortion.

Everything else, but we don't know is the noise coming from these? Is it coming from the power amplifier chips? Is it a combination of both? It is inherent system noise floor. No, it's got nothing to do with that. These boards aren't shielded or anything else you know, like in terms of can shielding. It's got the metal plate on the back.

But you know it's got nothing to do with that, its inherent electrical noise floor of the circuitry. But where does it come from? So what I'm going to do is I'm going to wire separate this preamp and filter board from the audio amplifier. Now the audio amp and power amplifier has its own pulldown resistor on the input, so if we disconnect the cable from the input, it should be fine. The signal will be pulled down to signal ground and not a problem.

So we the first thing to do is just isolate whether or not noise is coming from here or whether it's inherent in the power amplifier on the back board. Okay, so I've got it disconnected here. It's powered on, can't hear a thing. Aha! What I'm doing now is I've taken the front panel off I've connected my voltmeter across AC Voltmeter across the four.

Ohm! tweeter here and let's have a look. We get it about. This has like a normal, like a hundred kilohertz plus bandwidth. so it's more than capable of getting the average AC noise level.

Now if we switch it off just to show you that we actually get nothing there, just a little bit of residual book There we go. So we switch it on and it goes through. It's got an auto mute thing for a couple of seconds before it turns on. There you go.

you know, 245 micro volts, AC or there about as well. But let's try the high-frequency adjust here. it's droppin. look at that it is going down.

So that does show that this is of course on the input filter side of it. So here's the schematic for that. You can see where that is so it looks like the noise is coming from that front end with the Japan radio caught by 4580 Op-amps and if we actually disconnect the front end like that, hardly anything at all. There are so yeah, but it's certainly not at the same level.

We'll get in before. That's interesting and we can see that the volume control here does have a little bit of effect. So we're talking like you know, five or six micro volts here of extra noise, but so if we knock the high frequency adjust right down and the volume right down, we can go down to 220 Odd, but yeah, like it's we can still hear it anyway. as you can see on the schematic, we've got you one here, which is the input balanced and unbalanced amplifier and then it goes into the control part VR one here, which is the the back panel volume control and then that branches off into the low-pass amplified low frequency amplifier and the high frequency amplifier part of which half of which is in you two here.

So we know the volume pot has some factor into the noise. We know that the high frequency gain adjust here has some impact on the noise and things like that. So yeah, we need to start isolating the amplifier. But what's interesting here is the type of Op amp - you notice that it's the Jrc 4580 D and that D on the end actually matters because you can buy this chip in two different classes.

If you have a look at the data sheet, you can see that the not the standard Nandi part is actually only specifies a typical noise floor figure of art not 0.8 microvolts. and we won't worry about the conditions under which that's under. but you will see below that that the D part doesn't specify a typical, but it actually specifies a maximum of 1.4 microp microvolts are equivalent system noise floor. So there you go.

So they are actually using the higher quality part in here which has a guaranteed noise floor. Nice! Now this is where you might have to start getting into resistor noise as well. You know you've got to take this into account. So let's take the RH 10 there, which is a 20k resistor.

A 20k resistor at on over a bandwidth of 20 kilohertz has an inherent thermal noise of 2 micro volts. You know, which is enough all. but when you're around with this sort of stuff, you might have to take that into account. But let's go me.

You know the resistors aren't the problem here. If you're curious to know what I'm get in between the two ground points RH 10 over here and 29 over here. Yeah, 50 micro volts and that does actually go away if I switch it off but relatively speaking across Rh10 there which is the one including the ground reference here that goes over the cable to the power amplifier, Then that's our noise floor going into the power amp. And yeah, that'd almost explain it because the power amp has a gain as well.

Okay, what I've done is actually lifted Rh9 in there so that the signal is actually decoupled from all the input amplifier and everything else. And look at this. it's much higher. so that's referred back over the cable to the basically the input of the power amplifier so we've disabled.

Wealth is if there's a resistor on here, we're measuring across the resistor. But that's about it. It's it's disconnected from the rest of the circuit. And yeah, it look like an feeding noise.

There you go feeding crap just by touching that. And if you're curious to know, do we get still get the noise? Well, I can hear it from? Yeah. But here's the problem with audio system design: weathers. but we don't have a wire flapping around in the breeze kinda not the input wire.

This, we're still got that 20 K resistor that we're measuring across on that board. I Can hear it all from here. The hum is really actually quite loud, but if I actually disconnect this, the hum is gone. You'll have to.

trust me, it's now. not now. But the hiss is still there. so it's nothing to do with the input amplifiers there and all.

nothing deals. the balance input. There's nothing to do with that high-pass stage and all that sort of stuff. It's still there.

It's coming from that cable or the parent finally isolated it. So hopefully you can see the almost futility in trying to track down system stage noise. With a multimeter like this and these big antenna leads, we we've got some useful information back when over on Like, ah, 29 over here was it where? But it's like a low impedance. so when you got probing those low impedance stuff, all that crap you're picking up from your test leads and everything else isn't going to matter a rat's butt.

When we have got like just a 20k input resistor, which is the one over here that we're measuring and we disconnected that. We've got a relatively high impedance now, and when you start having these big antenna cables flapping around in the breeze just picking up all sorts of whatnot crap, it just introduces more noise than you're actually trying to measure. But of course you know we were just mucking around trying to met trace some system noise voltages there. We didn't have to do that.

We could have easily come to this conclusion without a multimeter at all. We could have just gone to the schematic and gone. well. let's just disconnect our H9 there, which disconnects basically all of the internal circuitry, but leaves the table in place going over.

Bingo. We've still got effectively the same our noise. We can hear it in the tweeter, so therefore it's none of the input circuitry here. So if we went and you know if we were an audio fool and went in and started changing or our Op amps to some super whiz-bang thing wouldn't have made a rat's arse difference, let's go over to the main power amplifier board.

and instead of disconnecting the cable over here which affects the mutant everything, what I've done is D soldered one end of Ch 100, which is the main input AC coupling cap for the things. So really, now, this power amplifier is not connected to that input cable at all. It's only got its own internal resistors, caps and traces, and everything else. Switch it on.

It still might even say it's a little bit lower. tongue at the right angle. Yeah, it's still, and you kind of expect that in theory, of course, cuz those all those Op amps on the input, preamp and filter stage as the Njm 45 ATS There really no low noise and they've even specified the particular part for that. But if you have a look at the power amplifier used in the tweeter and high-pass high frequency amplifier here, you'll notice its input referred noise is only a couple of micro volts as well.

But if we have a look at the schematic here and look at the feedback resistor 12 K and the input resistor 460 or Ohms or whatever. it's got a gain of about 26, so any input referred noise is going to get multiplied by that gain. But once again, if you look at the data sheet and that input referred noise figure, then it's only a couple of micro volts and you multiply that by the 26 gain. That doesn't get the output noise that we actually measured across the 4 Ohm speaker.

but if you look at the maximum figure, it could be as high as 10 microvolt. And in that particular case, ya, multiply that by 26. That's almost the exact figure that were seen. So are we seeing just the worst case amplifier noise here? I Don't think so, because and that'd be like this is inherent across all the speakers.

There's not just this oddball one that I've got here, so it's yeah. we're going to get our typical figures there. so that noise. There must be some extra noise being introduced somewhere in the power amp.

There's something interesting here. We didn't hear it when we disconnected the input cable which I puts it into mute mode. And sure enough, when I power this thing up, there is no noise at all. No really, no audible noise, baby.

Chinese Little half of these bigger noise. But if you have a look at the schematic which shows how the mute system actually works, it's just switching between a different input which either goes to ground via a 22 K resistor or goes via the input, that capacitor that we disconnected up here and a parallel 10k resistor. So the thermal noise of a 10k resistor around about you know just under two micro volts or there abouts over the bandwidth that we're talking about. So really it can't be that.

plus the fact that the in mute mode you're going across a 22 K resistor which is more than doubles. So yeah, it's not that. I still think maybe it's a you know, a small contribution, but I think the noise is coming from somewhere else. It's got to be like system related, layout, ground, everything else.

So what I've done is actually just for kicks. Shorted out our H 100 here which is that 10k input resistor and note just tested that noise is still there and like that and even if you disconnect that input trace which is what then takes the trace all the way up to the top to the cap which we've taken out up here, then it's still there. So it's all happening down around that power amp. If you have a look at the power amp here, this is the trace that comes down from the input that AC coupling cap jumps over.

This goes over this jumper here, goes across. There's our 10k down to ground and there's the parallel cap as well. and that just goes into pin 7 if I were up there. So it doesn't get any simpler than that.

So our power amplifier is now simply terminated with a 10k resistor and a cup there. That's it Yet it still generates the hum. So yeah, we've isolated down to just the amp. and if you might think it's you know something to do with the mute path.

While the pin 5 is the muting put there and there's the resistor, you're jumping straight over to the same ground there. so no problems whatsoever. It's not sneaking in there. And even if I disconnect our H 106 here which goes off to the mute function via the diode off to the input and I ground that so I'm actually disabling the mute function altogether and always having it in play mode.

I Still get the noise Where and when this amplifier goes into mute mode, it actually mutes the output properly even though it's just switching the input according to this schematic. Uh-huh. Check this out. It's different to the schematic.

The schematic just shows that the mute function just switches between the inputs here right? and doesn't disable the output pair amps. So that's why I was getting. you know noise when I selected the pin seven. Here, when you select the non mute function the just the regular play mode and you disconnect the capacity, Here you disconnect all the traces, you've only got a 10k going to ground and we get the noise.

We get the hiss, but then you select mute mode pin five here and you don't get it which has the exact same well. it's got a double the amount of resistance going to Ground Weed. But if you actually go look at the actual internal schematic. aha, there it is.

You can see that the mute standby switch here goes through into these two comparators here and these are the voltage threshold limits. So anything below 1.8 volts, it actually switches into shutdown. Here's the output amplifier block and it shows a it going into there and disabling the output amplifier. So that's why there is no noise that's completely silent when you put it in standby mode.

So that explains that. white silent in mute mode. And but anyway, it looks like all the noise is coming from the power amp itself because there as I said if I disconnect CH 100 there and all the traces go into it. Even if I disconnect the mute, you know I Thought: maybe there's some, you know crosstalk between the mute pen and noise coupling over.

or you know, some weird thing like that's removed IH 106 Here, we still get the noise with just this. 10 K input resistor here. What do you do? I mean it's only got a couple of micro volts. Noise.

A couple of micro volts for the chip. It doesn't really add up to what we're measuring. Its inherent in that I Don't know. Is there any pin compatible one you could whack in that's inherently not low noise, but it's already pretty low noise.

But with the thing, they're a massive high dynamic range of this tweeter, even a convert or micro volts a noise is audible. You can hear it. you know it's down in the microwatts region or whatever, but you know it's like 80 DB down on one watt. but you can still hear it.

And they actually designed the pre amplifier input with spec to those low noise Op amps so it didn't really contribute much if anything to that, a system noise level which is quite nice. Couldn't really hear the difference when I like disconnected the input and so those 45 ad Op amps really aren't contributing. It's pretty much anything at all to that. So there you go.

Fascinating. We've narrowed it down inherent in the amplifier anyway. I Hope you found the interesting if you did. give it a thumbs up and as always you can assess us down below or on the Eevblog forum.

Catch you next time.