Whoops, is that so ? Mk-6 ( dynacos 120w used a A-541 transformer or simular)
is'nt that available ?
If indeed the transformers are single-sourced , that is a big nono in my book ...
Hey I built a modified version of the VTA with their MK-3 boards.
I have another unit with the M125 like driver and input.
I seriously do not think that one can improve the sound after this...
I had one issue with the feedback wire picking up AC filament and mains noise.
Except that it works flawlessly. I use only 2 x kt120 per side instead of 4x
I use 100 watts output transformers.
The input stage is very adequate, the bias points great.
The driver stage is a very hifi tube able to drive well a quad of kt120. and the LTP is very well designed.
If you improve that design the result will be marginal...
From the design the things that improved the circuit and the sound I would class in order of most important to least important in percentage:
1. Get the best 12BH7 tube possible with matched sections(70%)
One would think the constant current sink null any imperfections, but a well matched low distortion selected tube will clean up the sound with a dramatic effect on sound clarity.
2. Adjust feedback for your speakers, i suggest a little higher or lower feedback resistor and hear test it, nothing major. (15%)
3. Use film capacitors ( around 100uf ) in the power supply input/driver tubes. (10%) , makes the top end a little more detailed.
4. Get good input tube (5%) , hard to see any improvement
I could not hear any difference much with coupling capacitors as long as they are decent quality film.
IMHO, the best value in audio today are the M-125 monoblocs. For not much effort you can build the kits yourself;or for another few hundred bucks VTA will build them for you. They are "plain Jane," all business so you're not paying 10 large for blue meters or somesuch. They will drive any speaker known to man, from Maggie IIs to Vandersteens to Tylers. Plus they have a great chat room (Basket). Bob Latino, who in conjunction with Roy Mottram at Tubdes4HiFi designed these amps and are a phone call away. The out-of-the-box components are very good and you can always add a few teaks for that extra 5 percent. I have had my M-125s about three years now and cannot imagine owning anything else. Wouldn't trade 'em straight across for the best Mac or ARC.
Another advantage I neglected to mention is that the M-125s is that you have a choice of noval (12BH7A) driver boards, which come as standard equipment, or octal (6SN7) driver boards. Bob Latino favours the novals but many M-125 owners, including me, like the wider selection of 6SN7s.
Again, you can't go wrong with this amp.
Again, you can't go wrong with this amp.
Thanks for the tip about the feedback wire. Got mine up and running this evening and there is some hum (20mV). I will try to solve that tomorrow
Edit: Other than that it sounds promising.
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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
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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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.
Compared to what's in the manual.
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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