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VTA M-125 mono block kit.
VTA M-125 mono block kit.
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Old 4th January 2017, 03:49 AM   #11
gabdx is offline gabdx  Canada
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Nice to see that you are happy with the m125

Got mine up and running this evening and there is some hum (20mV)

5 mV should be attainable , good luck
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Old 4th January 2017, 01:50 PM   #12
dahlberg is offline dahlberg  Sweden
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Replaced the feedback cable with a screened one and got it down to 15mV. What's really strange is that now I can hardly here any hum att all, maybe I have lost my low frequency hearing over night
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Old 4th January 2017, 10:32 PM   #13
Bob01605 is offline Bob01605  United States
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Quote:
Originally Posted by dahlberg View Post
Replaced the feedback cable with a screened one and got it down to 15mV. What's really strange is that now I can hardly here any hum att all, maybe I have lost my low frequency hearing over night
When you test for hum/noise on the VTA M-125, you should use shorting plugs on the input jacks OR plug in an interconnect to the VTA M-125 and then plug the other end of the interconnect into your preamp. What you hear through the speaker is the residual noise of the amp. If you leave the input jack open, the input jack will pick up some hum/noise from the amp's circuitry.

The most common cause of noise/hum in the M-125 is the FRONT driver tube. Sometimes just reversing the front and rear input tubes can drop any residual hum to an inaudible level.

Another thing .. The VTA M-125 has a 3 wire IEC connector and power cord. If another component in your music system ALSO HAS a 3 wire cord, you could have set up a "ground loop". Try floating the ground with a 3 wire to 2 wire "cheater plug" and see if the hum level drops.

Bob Latino
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Old 4th January 2017, 11:26 PM   #14
dahlberg is offline dahlberg  Sweden
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Quote:
Originally Posted by Bob01605 View Post
When you test for hum/noise on the VTA M-125, you should use shorting plugs on the input jacks OR plug in an interconnect to the VTA M-125 and then plug the other end of the interconnect into your preamp. What you hear through the speaker is the residual noise of the amp. If you leave the input jack open, the input jack will pick up some hum/noise from the amp's circuitry.

The most common cause of noise/hum in the M-125 is the FRONT driver tube. Sometimes just reversing the front and rear input tubes can drop any residual hum to an inaudible level.

Another thing .. The VTA M-125 has a 3 wire IEC connector and power cord. If another component in your music system ALSO HAS a 3 wire cord, you could have set up a "ground loop". Try floating the ground with a 3 wire to 2 wire "cheater plug" and see if the hum level drops.

Bob Latino
I have done all the connections you described, both shorted out and connected to the preamp/dsp. I have also cheated a bit by simply putting some electrical tejp over the ground connector of the wall plug (eu). I also tried to exhange the tube rectifier for a solid state one. Only thing that gave any measurable result was the feedback cable. It went from 22mV to 15, not that much difference but for some reason now it's very close to inaudible. I have to put my ear right up close to the drivers to hear any hum. Actually thought that the change in rectifier gave a bit less noise as well

I'm running JAN 5814A for driver tubes, supposed to be matched pairs but I belive I messed up wish ones was pairs I will try to rotate the tubes some day but it doesn't seem to be any real hurry for that now.

I have done the other channel as well and will be doing some more listening tomorrow That channel had less hum to start with so.... Ok I will rotate the tubes tomorrow :-)
One a clock in the morning, guess I should get some sleep now.
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Last edited by dahlberg; 4th January 2017 at 11:40 PM.
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Old 4th January 2017, 11:47 PM   #15
dahlberg is offline dahlberg  Sweden
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They turned out like this by the way.

Click the image to open in full size.
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Old 6th January 2017, 11:36 PM   #16
gabdx is offline gabdx  Canada
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awesome looking
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Old 7th January 2017, 02:24 PM   #17
boatheelmusic is offline boatheelmusic  United States
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Where are you measuring this hum - I see around 12 mv ac at the B+ point, 2 mv ac or less at the bias test points and 2 mv or less at speaker jacks.
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Old 9th January 2017, 07:58 PM   #18
dahlberg is offline dahlberg  Sweden
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Quote:
Originally Posted by boatheelmusic View Post
Where are you measuring this hum - I see around 12 mv ac at the B+ point, 2 mv ac or less at the bias test points and 2 mv or less at speaker jacks.
I'm measuring at the speaker connections. There is still some hum and I think
that I might have messed up the wiring a bit. It's not identical to the manual
and it's at least somewhere to do some work before stashing up on input tubes.
Better wiring is a win/win anyway so.....
These are mine.
Click the image to open in full size.

Compared to what's in the manual.
Click the image to open in full size.
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Old 9th January 2017, 08:34 PM   #19
dahlberg is offline dahlberg  Sweden
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Measured ac voltage at the speaker connections are now @ 6,5mV. I have added a buck transformer to take away 10v (from ~225-238v) and also a dc blocker. That helped a bit.
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Old 9th January 2017, 08:46 PM   #20
BinaryMike is offline BinaryMike  United States
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Wow. You don't often see new amplifiers using the chassis for the ground system these days. 2mV of hum at the output is very respectable for this method of construction. You might try cleaning and re-tightening all of the chassis ground connections if your layout is correct and nothing else helps.
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