MB200 hiss = 1 watt AM transmitter? scope photos included.

WingKL · #1 07-27-2011, 06:28 PM

So I just picked up a new MB200 from RMC. Hooked it up to my Acme Low B2 and noticed hissing with nothing connected. It's even noisier than my modded Thunderfunk with its many filter stages. So I decide to put it on the scope to try and measure the noise voltage for a more objective understanding of how noisy it actually is.

Looks like it's putting out a 3-4 volt peak to peak 500Khz signal. It's pretty hard to try to measure just the noise alone and tweaking the volume and treble does increase the hiss and shifts the waveforms but does not increase it's overall amplitude. The noise is hiding in the carrier.

Unloaded.

This one is loaded with the Acme cab. The amplitude is lowered slightly by the load.

At .5 uS/div timebase ~ 500KHz eyeballed.

Can anyone tell me if this is normal? I'm trying to decide if I should go through the trouble of sending it to GK only to have them tell me it's normal.

If I had one of those spectrum analyzer thingys, I might be able to come up with a noise measurement.

5port · #2 07-27-2011, 07:14 PM

You do realize this is a switching amplifier thus it will have some switching noise on the output. The only thing the designer/manufacturer can do is try to minimize it (switching noise). Having said that my MB200 is quite acceptable and am very happy with it. If I turn up my other Class D (BX1500) amplifier and put my ear against the speaker I can also hear the switching noise and I expect to.

BogeyBass · #3 07-27-2011, 07:35 PM

4 volts of switching noise is acceptable?
You could easily fix that with a BFH

(Big ####ing Hammer)

Sorry man aside from the others who will say " i have one and it sounds fine"

unless you feel like moding it yourself to sound better can you return it? whatever Chip GK used did they decide to skip out on the Ferrite bead on the output.

Maybe you could pop one in somewhere and see if it helps, or check the datasheet for whatever class D amp is in there and find out what GK decided wasn't necessary...and then add it

Jerrold Tiers · #4 07-27-2011, 08:11 PM

That should not pass the emissions testing..... no way, no how*. Presumably that model actually DOES pass, so most all of them won't do that.

Something is wrong with it, quite possibly.

* Now that I think of it, though, if that is all "differential" signal, and not "common mode", it could actually pass enissions.... so I guess that would be..... "maybe: way, how".

billfitzmaurice · #5 07-27-2011, 08:18 PM

Quote:

Originally Posted by WingKL

Looks like it's putting out a 3-4 volt peak to peak 500Khz signal.

500 kHz? Even those with canine teeth that really are canine teeth can't hear 500kHz.
I don't know if it's supposed to be there or not, I suspect not as the FCC might have a problem with it, but anything higher than 25kHz is moot where being audible is concerned.

WingKL · #6 07-27-2011, 09:02 PM

I don't really have a problem (yet) with the 500khz signal. It's presence however makes it very difficult to eyeball the noise voltage without the use of a spectrum analyzer or some sort of filter. All I really wanted was some numbers so I can contact GK to see if the amount of hiss I'm hearing should be there.

@Jerrold, out of curiousity, how would I measure a differential vs a common mode signal, if it is even possible in this case? I got the traces by connecting the ground line from the scope to the sleeve and the probe to the tip connection of an exposed 1/4 jack into the amp output.

fdeck · #7 07-27-2011, 09:53 PM

Are you using a load? Give me a few minutes, and I'll scope mine.

WingKL · #8 07-27-2011, 10:04 PM

Yup, the middle trace is the one loaded with my Acme cab measured at the other jack on the cab.

5port · #9 07-27-2011, 10:05 PM

Quote:

Originally Posted by fdeck

Are you using a load? Give me a few minutes, and I'll scope mine.

**************Unloaded.***************

fdeck · #10 07-27-2011, 10:12 PM

I just measured it, with and without 8-Ohm resistive dummy load. In both cases, I get the same pattern as you do, at a level of roughly 2.8 V p-p.

Now I know how I smoked the audio input on my computer, even though the volume was turned all the way down.

Do not hook up those output terminals directly to your PC audio input.

The two output terminals both have the same signal, albeit the second terminal is inverted. This would reduce the emissions.

Have you fed a sine wave into it? Note that there's a lovely piece of software, Visual Analyzer, that is my fave for PC based audio analysis. It has both audio generator and analyzer.

cataract · #11 07-27-2011, 10:23 PM

I don't know anything about this particular subject..
.. but those photos are probably the coolest photos I've seen in a long while.

**B-string** · #12 07-27-2011, 10:39 PM

Quote:

Originally Posted by WingKL

I don't really have a problem (yet) with the 500khz signal. It's presence however makes it very difficult to eyeball the noise voltage without the use of a spectrum analyzer or some sort of filter. All I really wanted was some numbers so I can contact GK to see if the amount of hiss I'm hearing should be there.

@Jerrold, out of curiousity, how would I measure a differential vs a common mode signal, if it is even possible in this case? I got the traces by connecting the ground line from the scope to the sleeve and the probe to the tip connection of an exposed 1/4 jack into the amp output.

Warranty
christine@gallien.com
Ext.106
Tech
daniel@gallien.com

jason@gallien.com

Parts
lailany@gallien.com
or (209) 234-7300 extension 120. 9am to 4pm PST USA
Sorry don't have the ext.# for Daniel or Jason but I'd start with Christine or Brian Clark at Ext. 105

will33 · #13 07-27-2011, 10:50 PM

I'm more interested in what these electrical guys are going to come up with. This kind of stuff is why it's cool to keep track of this place.

WingKL · #14 07-27-2011, 11:32 PM

My MB200 only has one 1/4 inch/speakon combo connector. So the 1/4 inch and speakon connections(assuming a 2 pole speakon) have outputs inverted to each other? If thats the case, I would have to crack open the case to check it out but I'm not about to do that since this is brand new and under warranty.

I'm surprised that a 3V PP 500 khz signal could smoke the inputs to an audio card. Aren't inputs supposed to be high impedance or does the RF frequency do something bad?

I haven't fed it any sine waves since I was just checking for noise. I'll download Visual Analyzer and play with it one day. It's the one from Visual Analyser 2011 XE right?

Edit: I did some research: combo connectors are probably internally connected +1to tip and -1 to sleeve, so the inverted outputs refer to the tip and sleeve. It is a paradigm shift for me as I'm used to thinking single ended. With that in mind here is a dual trace of the tip and sleeve with the inverted outputs. I wonder if this passes FCC regulations. I still think the amp could be less noisy, hiss wise.

Mark Reccord · #15 07-28-2011, 05:40 AM

I'll throw mine on the scope lunch time and see what is to be seen!

KJung · #16 07-28-2011, 07:24 AM

I've found the entire GK MB line to be among the noisiest amps out there regarding upper treble his. I always attributed that to the rather goosed upper treble bump (and also the treble extension that seems to go way up over the 10K+ range) they seem to have baked into the voicing of the pre.

Wes Whitmore · #17 07-28-2011, 07:50 AM

Mine hissed like a box of snakes...just send it to GK for repair. Mine is now, like many others, dead silent. CALL THEM. Do not email them. It's a couple of email process that tends to get delayed when I need to send something back...
Wes

fretno · #18 07-28-2011, 07:59 AM

I have never noticed any noise from mine . The DI is dead quiet as well, op did you DI yours ?

Mark Reccord · #19 07-28-2011, 09:54 AM

Quote:

Originally Posted by Jerrold Tiers

That should not pass the emissions testing..... no way, no how*. Presumably that model actually DOES pass, so most all of them won't do that.

Something is wrong with it, quite possibly.

* Now that I think of it, though, if that is all "differential" signal, and not "common mode", it could actually pass enissions.... so I guess that would be..... "maybe: way, how".

I just scoped it. I'm getting about 1V p-p at 500kHz and 100kHz on each leg single-ended. I'm getting about 50mV p-p when I connect the differential probe.

WingKL · #20 07-28-2011, 02:23 PM

As suggested, I checked out the DI and it is not noisy. Using the line out with headphones is quiet but when I switch to headphone mode and have to lower the gain, it is as hissy as the main outs. Looking at the MB200 block diagram, it seems like the main amp is the noisy section. Also playing a bass through this particular amp unloaded but with the output hooked up to a scope and DI hooked up can make it unstable and go into protect. Looks like I'll be calling GK.

As a stupid MB200 trick, maybe someone with an AM radio could try tuning to around 500 khz and play bass through the MB200 to see if they can get their own personal pirate radio station.