Yaqin MS 12B phono amp- reviews - Page 11 - diyAudio
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Old 17th September 2013, 02:38 PM   #101
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Join Date: Jun 2012
Default bypass volume control

Hi,
Really hope someone here might be able to chime in. Im hoping to bypass the volume control on my ms12-b. Do i need to put a resistor from signal to ground at that point to simulate the pot being turned all the way up (full volume) or can I just go straight? If I need a resistor should I go with 50k (which is what the pot is) or something else. Circuit diagram is here

http://i1223.photobucket.com/albums/.../YaqinMod2.jpg

Many thanks
Nick
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Old 18th September 2013, 10:17 PM   #102
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ooops wrong link. here is the schenatic, as given on page two of this thread

http://www.diyaudio.com/forums/attac..._schematic.jpg
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Old 24th May 2014, 06:05 PM   #103
Hollyv is offline Hollyv  France
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Hi,
I back this topic hoping to find help. Sorry for my basic English.
I have the MS12-B yaqin past few weeks. On a French forum I also read that the 12AX7 phono stage was not polarized cathode resistor, so this is the gate leakage current, passing through the R 2 megohms, which polarizes the tube. Which is technically a design error. It would add a cathode resistor of 1 kb and reduce the R 2MO to 220 kB for the tube work in good conditions. I also read that the 12AU7 tube works with only 1 mA per triode, he is expected to spend between 5 and 10 mA. Can someone make me a plan to correct the circuit, I would be very grateful. thank you.
Hollyv
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Old 25th May 2014, 10:07 AM   #104
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Join Date: Mar 2012
Hi Folks
I bought one off EBay a while back whilst my other preamp was being modified. It could be described as 'ordinary' and 'nothing special.' I'm sure it would have benefited from an upgrade of components but sold it for a price similar to what I paid in the first place.
Chris
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Old 25th May 2014, 05:10 PM   #105
Hollyv is offline Hollyv  France
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This is a great device at value for money but could be improved at the technical level. Can someone explain how to polarize the 12AX7 phono stage correctly and amperage 12AU7 to a value between 5 and 10 mA? It seems to me unnecessary to change before components to correct a technical error...
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Old 7th July 2014, 10:45 AM   #106
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Quote:
Originally Posted by frapa02 View Post
I recently acquired one of these and after some minor work I can say that it does sound very good, especially for the cost (paid 100 plus cost of my mods.)
Basically, you have to check the 12AU7 heater voltage first - this was a serious issue and could ruin some expensive valves. This was alarmingly high at 15.4V here in the UK with 240V mains on a 220V unit. These heaters are fed via a resistor from the unreg supply and I think the resistor was incorrect, even for 220V mains. It was 15R and I replaced with a 22R and the voltage is now much better at 12.90V, with 240V mains. This also cured a small hum problem and some very hot valves! A better option would be to fit another regulator but it was a bit tight.
Next, I replaced the 1uF coupling caps with ICEL 1.0uF 10% 400VDC. These are very cheap but seem to have a very natural sound, although they won't appeal to those who swear by PIOs! Well at least they fit.
Finally, the output coupling caps, also 1uF, were replaced with ICEL 4.7uF 10% 400V. This improved the bass extension when driving solid state amplifiers.
The simple mods made the MS-12B sound like it would challenge a much more expensive unit. So far, the stock Chinese valves have been used and they do improve a lot with use. I will try some more refined ones next.
Another comment, I am beginning to love this as a pre-amp buffer from my home brew DAC. It improves the dynamics and everything sounds more solid. I am not sure why yet.
The only problem I can report is that it takes a while to strip and re-build the unit to work on it.
Paul.
I've got 11,73 on the 12ax7 and 13,8 on the 12au7... worth doing something?
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