I have the often discussed humming/buzzing amp problem. Yes, I’ve read many of the threads here and elsewhere addressing hum, but none fit my situation squarely enough to solve the problem.
I’ve got 2 Museatex MTR-101 solid state Class A amplifiers fed from a Museatex PA6i preamp. For those who don’t know these, the MTR-101s were a Class “A” amp built in the late 80s. I think they’re fantastic, but they both have a loud hum/buzz that is loud enough to be just audible from OUTSIDE the room with the door closed. This buzz issues from both of the speakers, as well as directly from the power supply areas of both amps.
Here are the details in point form:
Both amps buzz when they are plugged in, but not turned on.
Both amps will buzz, even if they are the only thing plugged in to AC and nothing else (no inputs or outputs are connected).
When I turn them on, the physical hum/buzz from the power supply areas increase minimally to moderately in volume. Then, of course, the speakers also start issuing the same, mostly through the midrange and tweeter.
The house I use them in has a proper and modern grounding scheme. The plugs have been checked for correct ground and polarity. I’m in Canada so 120V 60Hz.
The amps behave like this, no matter where they are plugged in in the house, or even in a neighbouring house
I have replaced ALL the electrolytic caps in the entire system (pre-amp and both mono blocks)
I have replaced the rectifier bridges in both mono-blocks
I have reflowed any suspicious looking solder
DC offset at the amp outlets is only three or four mVs
The amps actually sound very good except for the buzz, so yes, they function as intended, with of course the one exception.
The amps have single-ended inputs
The amps are fitted with 3-prong AC plugs and the AC cable is twisted 3-wire on one of the amps and not twisted on the other amp.
Ground loop may be part of the problem but not the majority. Remember, each amp hums even when plugged in to AC only.
It’s difficult to localize the physical source of the hum in the power supply area but it SEEMS that the toroidal transformer, two covered inductors and a very small transformer on the housekeeping board might all be sources.
I came to suspect DC offset on the AC lines but the problem persists (though admittedly lessened) even now that I have just built and employed three of Sjostrom Audio’s DCT-03 DC traps. (1 DCT-03 for each mono-block and 1 for everything else) All 16 diodes are installed in each.
I’m at my wits end, as the hum/buzz is very loud and I’ve done everything I can think of to address it.
I should note that unfortunately I cannot access or supply schematics for the Museatex equipment.
Any thoughts will be appreciated!
Now an additional but related question. Since I installed the DCT-03 traps the sound has improved in ways other than the slight reduction in hum/buzz. I’m not quite sure what I’m hearing but either the detail and dynamics have improved, or the treble has simply increased. I think I like it over all, but it might also be too analytical and “showy”. Does anyone have an informed opinion as to why these units should have this effect?
I’ve got 2 Museatex MTR-101 solid state Class A amplifiers fed from a Museatex PA6i preamp. For those who don’t know these, the MTR-101s were a Class “A” amp built in the late 80s. I think they’re fantastic, but they both have a loud hum/buzz that is loud enough to be just audible from OUTSIDE the room with the door closed. This buzz issues from both of the speakers, as well as directly from the power supply areas of both amps.
Here are the details in point form:
Both amps buzz when they are plugged in, but not turned on.
Both amps will buzz, even if they are the only thing plugged in to AC and nothing else (no inputs or outputs are connected).
When I turn them on, the physical hum/buzz from the power supply areas increase minimally to moderately in volume. Then, of course, the speakers also start issuing the same, mostly through the midrange and tweeter.
The house I use them in has a proper and modern grounding scheme. The plugs have been checked for correct ground and polarity. I’m in Canada so 120V 60Hz.
The amps behave like this, no matter where they are plugged in in the house, or even in a neighbouring house
I have replaced ALL the electrolytic caps in the entire system (pre-amp and both mono blocks)
I have replaced the rectifier bridges in both mono-blocks
I have reflowed any suspicious looking solder
DC offset at the amp outlets is only three or four mVs
The amps actually sound very good except for the buzz, so yes, they function as intended, with of course the one exception.
The amps have single-ended inputs
The amps are fitted with 3-prong AC plugs and the AC cable is twisted 3-wire on one of the amps and not twisted on the other amp.
Ground loop may be part of the problem but not the majority. Remember, each amp hums even when plugged in to AC only.
It’s difficult to localize the physical source of the hum in the power supply area but it SEEMS that the toroidal transformer, two covered inductors and a very small transformer on the housekeeping board might all be sources.
I came to suspect DC offset on the AC lines but the problem persists (though admittedly lessened) even now that I have just built and employed three of Sjostrom Audio’s DCT-03 DC traps. (1 DCT-03 for each mono-block and 1 for everything else) All 16 diodes are installed in each.
I’m at my wits end, as the hum/buzz is very loud and I’ve done everything I can think of to address it.
I should note that unfortunately I cannot access or supply schematics for the Museatex equipment.
Any thoughts will be appreciated!
Now an additional but related question. Since I installed the DCT-03 traps the sound has improved in ways other than the slight reduction in hum/buzz. I’m not quite sure what I’m hearing but either the detail and dynamics have improved, or the treble has simply increased. I think I like it over all, but it might also be too analytical and “showy”. Does anyone have an informed opinion as to why these units should have this effect?
Possible earth or ground loop may be a likely cause. Lift the ground on one amplifier with no preamps connected. Physical transformer buzz due to loose lamminations or eddy currents set up due to an earth fault.
There is no DC on AC mains. What you are hearing is a reduced current flow from the mains supply, a product of the series capacitor.
There is no DC on AC mains. What you are hearing is a reduced current flow from the mains supply, a product of the series capacitor.
I think no ground loop possible when amps will hum when each plugged in to mains and nothing else. No interconnect or speaker connections.
And poor quality AC power from utility can have a DC offset. Appliances and other loads, even from nearby homes, can also introduce a DC offset.
just pluging in probably powers up some auxillary circuitry such as a soft start
I found this picture of what Museatex call a floating charge power supply. Do your amps have these odd coils and if so is it the toroid or these coils buzzing? Never heard of floating charge can anyone elaborate?
I would say the "improvement" in sound is due to less harmonic load on the power supply and if you have reduce the hum you are less distracted so listening more to the detail.
I would say the "improvement" in sound is due to less harmonic load on the power supply and if you have reduce the hum you are less distracted so listening more to the detail.
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You’re probably right, but loud enough to hear from outside the room with door closed? I’d say that something is amiss, especially when these amps were known to be very quiet.
You’re probably right, but loud enough to hear from outside the room with door closed? I’d say that something is amiss, especially when these amps were known to be very quiet.
Yes, my MTR-101s have these 2 coils, or something like that. Mine are under non removable cans/(sheilds?).
The physical buzz seems to come from all of the big toroidal transformer, these 2 coils and a very small transformer on the housekeeping board. It’s really hard to tell, even with a stethoscope. All seem affected. The smaller the source, the smaller the contribution to the overall sound (hum/buzz) level.
I spoke with John at Museatex. Unfortunately, he’s currently unable to take a look at the equipment, but beyond my other interventions he suggested putting anti-parallel diodes in the mains. That suggestion was part of my inspiration to use the Sjostrom DCT-03 units, as, unless I’m mistaken, they do exactly that.
Why don’t you make a recording using your iPhone and post that up. Make sure to record any sound from the amp itself, and the speakers. Often you can pinpoint the general cause ie mains DC, loose laminations or ground/cross channel/ground loop or common impedance just by listening to the recording.
As both units show identical problems, there may be a fault been build in during the "restoration".
Presumably the grounding concept was not understood and was confused. Fixing such an amp without a schematic and some understanding/ experience with amps may be very problematic.
Why don't you post a number of clear pictures, made in bright light, so some here might get an idea about this unicorn?
PS not publishing a schematic of such an old concept may be a little bit strange. Maybe he still makes a living out of repairing these stone age MOSfet amps.
Presumably the grounding concept was not understood and was confused. Fixing such an amp without a schematic and some understanding/ experience with amps may be very problematic.
Why don't you post a number of clear pictures, made in bright light, so some here might get an idea about this unicorn?
PS not publishing a schematic of such an old concept may be a little bit strange. Maybe he still makes a living out of repairing these stone age MOSfet amps.
Sorry to say DIYAudio won't let me attach a video file. I've never posted to YouTube before but I'll look into it. Until then I'll say that walk in the door and it's immediately noticeable. More so from the amps themselves than from the speakers.
Her are some pics. I was going to replace the transformers next, but I read on Museatex's website that the transformers were custom designed for this amp.
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You have to determine if it’s electrical hum (earth loop or suchlike), electromechanical hum (DC on the mains causing core growling) or mechanical buzz/rattle (loose laminations - normally only applies to EI transformers). A recording will help you do that.
I'll say too that the hum is mostly unchanged with the replacement of all the caps and the rectifier. The amps hummed before and they hum after. The only change with the restoration is that the sound improved, separate from the persistent hum/buzz. The DC traps have reduced the perceived sound level by about 1/3.
Can there be a loop if the amp is running with no connections except the AC mains? As far as DC on the mains, each amp is on its own dedicated DCT-03 DC trap by Sjostrom Audio.
First thing the large toriod does not seem to be fixed down in any way? It needs to be on a rubber base and clamped. Secondly you say it/they have always hummed and the company you spoke with suggested DC blockers. That points to this being a known problem. The two coils are DC chokes between the brown and black capacitors and should help with regulating the supply but I'm guessing it is taking more current than the supply is happy with.
The toroid was attached to the chassis via a substantial bed of silicon adhesive, and remains so. This solution was successful years ago and I don't doubt the effectiveness of this strategy now. The caps were similarly mounted.
When I say they have always hummed, I mean that this is the case since I got my hands on these particular units. In their day, this model of amp was eerily quiet. A tech I know, who used to work with Museatex, says that these units that have been upgraded with a scritch soft-start and had some circuitry removed and modified to allow that new circuit. Both of these amps have apparently been outfitted with this upgrade, though I am unqualified to make that determination myself. My contact has said that, with this mod, some MTR-101s are more susceptible to, as he has put it, "growling at each other". So, yes, with the mod, this is a known problem. At his direction, the anti-parallel diodes of the DCT-03s were fully expected to resolve this, but they did not. Normally, it would make sense for me to simply go to the tech with this issue but unfortunately, recent developments for him might make that difficult. Thus my turning to this community.
When I say they have always hummed, I mean that this is the case since I got my hands on these particular units. In their day, this model of amp was eerily quiet. A tech I know, who used to work with Museatex, says that these units that have been upgraded with a scritch soft-start and had some circuitry removed and modified to allow that new circuit. Both of these amps have apparently been outfitted with this upgrade, though I am unqualified to make that determination myself. My contact has said that, with this mod, some MTR-101s are more susceptible to, as he has put it, "growling at each other". So, yes, with the mod, this is a known problem. At his direction, the anti-parallel diodes of the DCT-03s were fully expected to resolve this, but they did not. Normally, it would make sense for me to simply go to the tech with this issue but unfortunately, recent developments for him might make that difficult. Thus my turning to this community.