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VTA M-125 mono block kit.
VTA M-125 mono block kit.
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Old 10th January 2017, 03:41 PM   #31
dahlberg is offline dahlberg  Sweden
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So now the feedback wire is relocated as described in "post 25" and I
measured 3,5mV ac of the speaker connections. The lowest value measured
so far The remaining audible hum is almost non existant and I'm not going
to loose any sleep over it. In due time I will try some other tubes for input.
I'm using 5814A tubes for now, no idea if they are noisy or not.

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Old 6th January 2020, 05:26 PM   #32
IEales is offline IEales
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Default teaching VTA M-125 to sing

As built by VTA, my M-125 mono-blocks were very noisy, ≈55db unweighted


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After fixing build and design errors, the amps quietened almost 40db to ≈93db unweighted.

I suspect many DIY and retro designs may suffer similar problems.
In an effort to help others with similar problems, complete instructions:
ieLogical VTA M-125
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Old 6th January 2020, 05:59 PM   #33
Bob01605 is offline Bob01605  United States
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Ian's amps have the "autobias boards" that are sold by an independent distributor. Ian requested that they be added to his wired amp. The -95 dB signal to noise measurement for the M-125 was done 10 years before the autobias boards were ever created. When you remove the old bias pots and "piggyback" a second board on top the driver board and start adding wires here and there to connect the autobias board, I am sure that the amp's residual noise level will increase somewhat.

In short, any increase in the residual noise level of the amps is because of the addition of the autobias boards to each amp. I am now not recommending the addition of the autobias boards to any of the VTA amp kits or wired amps.

Bob Latino
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Old 7th January 2020, 07:27 PM   #34
IEales is offline IEales
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Per Mr Latino :"I can tell you that on the M-125's R39 can be either a 1K resistor or a jumper wire. I use a 1K resistor but a jumper wire will work OK also. What R39 does is that it sets the "range" over which the bias may be adjusted. Since the M-125's use either 6550, KT88 or KT120 output tubes, these tubes need more negative DC bias voltage to control them than the EL34 tubes that the VTA ST-70 uses. In the ST-70 we use a 7.5K resistor which drops the voltage into an area that is better for EL34 output tubes."

The noise problem is caused because the Audio GND is tied to Earth safety and in some implementations, causes a ground loop. Once the loop is eliminated the amps are fairly quiet, ≈78db.

The autobias boards only increase noise because the pick off point and R39 were incorrect. If anyone at Tubes4HiFi / VTA had tested an amp pre&post AB-Q install they would have seen the problem immediately. Just as I did.

Click the image to open in full size.

The 1k resistor vs link is going to make a ≈1v difference in the range: 1/(1 + 50/2 +27). It will however change the LP filter from ≈1.6Hz to none in a stock amp. 100Ω raises the filter to 16Hz, but that's still ≈15db down @ 60Hz.

78 + 15 = 93

IIRC, the Auto-Bias boards were shown on the Tubes4HiFi site when I bought the amps. There was no attempt to dissuade installation.

DTAF thread Auto-Bias board on Roy's website? goes on for 7 pages about the auto-bias on the VTA amps. There are many other threads relating to auto-bias. I read many of them prior to purchase and no negative posts by tubes4hifi or Mr. Latino

Today, 7-Jan-2020. the Tubes4HiFi site no longer has any auto bias products and the M-125 page is updated.

The as built noise was not caused by the auto bias per se. Just design and poor build.
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Old 7th January 2020, 08:59 PM   #35
MontanaWay is offline MontanaWay  United States
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VTA M-125 mono block kit. VTA M-125 mono block kit.
Quote:
Originally Posted by Bob01605 View Post
Ian's amps have the "autobias boards" that are sold by an independent distributor. Ian requested that they be added to his wired amp. The -95 dB signal to noise measurement for the M-125 was done 10 years before the autobias boards were ever created. When you remove the old bias pots and "piggyback" a second board on top the driver board and start adding wires here and there to connect the autobias board, I am sure that the amp's residual noise level will increase somewhat.

In short, any increase in the residual noise level of the amps is because of the addition of the autobias boards to each amp. I am now not recommending the addition of the autobias boards to any of the VTA amp kits or wired amps.

Bob Latino
That is an utterly incorrect statement! Just because one does not understand how to correctly implement the auto bias module into an amp does not mean the module causes issues!
The auto bias board DOES NOT add any noise to an amplifier, no matter what type of amp.
Only incorrect and bad wiring will create issues, which of course is not just limited to the auto bias module.
And I would also like to remind members here that VTA was an avid supporter of the very same auto bias module in question here, until things went south due to misunderstanding of how the auto bias module is to be correctly implemented and NOT due to any inherent fault/issues with the auto bias module!
And yes, I am that independent distributor of these auto bias modules.
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Old 8th January 2020, 06:11 PM   #36
IEales is offline IEales
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Pre-purchase Mr. Latino stated: "The sonics of the amp will be the same with the autobias board. You won't really hear any difference in sound quality with the autobias board installed. I personally have never compared the two (manual bias and autobias) MYSELF but others tell me that the sound is the same. The autobias board will give you greater convenience."
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