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Yaqin MC-5881A amplifier improvements
Yaqin MC-5881A amplifier improvements
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Old 12th September 2007, 04:05 PM   #41
DanDini is offline DanDini  United Kingdom
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Hi Salas,

I've sorted out the gnd loop that goes via the volume and switch pcb by breaking the track on the sub-pcb in a couple of places. What hum is left is still there with that disconnected (and the inputs suitably loaded).

I will proceed with my track-breaking and get back to you...

Dan
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Old 12th September 2007, 04:22 PM   #42
kevinkr is offline kevinkr  United States
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Yaqin MC-5881A amplifier improvements
One other thing you might consider since this thing has a 220V power transformer is to use a small external transformer to buck the mains voltage. You have a 50% chance of phasing it correctly to buck the mains voltage the first time around, if it is boosted just reverse either (not both) the primary or secondary - whichever is easiest.

Mount the transformer and mains socket in a small metal box.
Ground the box through the mains cordset, and also connect mains socket ground connection to safety ground on your cordset. Install a 3A (or whatever the transformer is rated for) fuse in the fuse holder in the mains plug.

This or a variac will greatly reduce the losses in the amplifier's power transformer core which at ~10% high line probably have the transformer approaching saturation. Should you be in an area of the British Isles where mains voltages approach 250V this could be a real problem. (Harmonization indicates 230V, but I have heard that it can still be much higher in parts of the UK.)

Note that you can also use a transformer with taps and switch between them to get a closer match. (24VCT gives you both 12V and 24V bucking.)

If you choose a mechanically quiet transformer for the "buck box" you won't even know it is there, far quieter in a lot of instances than the typical variac, and if acquired ex stock, far cheaper too.

One other thought if the power transformer in this amp is approaching core saturation due to load and a high mains voltage it may be spewing harmonics in its magnetic field all over the amplifier. It may quiet down significantly..

See the schematic below:
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File Type: jpg buck_transformer.jpg (23.6 KB, 1019 views)
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Old 16th September 2007, 04:26 PM   #43
DanDini is offline DanDini  United Kingdom
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Thanks for the suggestion Kevin. I have just picked up a variac in need of some repair and once done I can test on a lower voltage.

I've sorted the grounding as best I can now, and have certainly calmed it down a lot!

The attached pictures show the channels at startup (quite a loud hum) and then after the valves are operational. The relatively smooth wave you see is at idle is 50Hz.

Startup (noisy):

Click the image to open in full size.

Warmed and at idle (still some audible hum):

Click the image to open in full size.

I've also messed about with CRC and CLC filtering, but the choke I have is not up to the job full time (current handling too low) and is already too big to fit in the chassis!

Thanks,

Dan
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Old 16th September 2007, 08:22 PM   #44
Salas is offline Salas  Greece
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Yaqin MC-5881A amplifier improvements
Very well done, ehmm Dan.

What exactly did you do for the GND? How does the sorted out amp sound?
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Old 18th September 2007, 09:14 AM   #45
DanDini is offline DanDini  United Kingdom
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All I really did was ground all the shields and the pcb in the same place. The main problem was the input selector and volume earthing. Once the chassis of those switches was disconnected and grounded separately from the input itself, things got a lot better.

There is still some noise, but I'm running out of patience (and ideas) with the thing to do much more.

As for sound, my garage is not quiet enough for a good test (freezer running in there etc.) but I will feedback when I have put it all back together and had a chance to plug it in somewhere quiet.

Thanks,

Dan
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Old 18th September 2007, 03:11 PM   #46
Salas is offline Salas  Greece
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Yaqin MC-5881A amplifier improvements
Default A successful thread.

I think that you have done much, and this thread is a good guide for anyone with this amp. You sorted the heater voltage, the output stage bias, the bulk of the ground noise, and you upgraded the supply. Plus quality upgrade hints were given, and the circuit got known. With your soon to come performance description, I think that the original questions of the thread starter are covered, and more. A short, to the point, hands on thread. Very nice.
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Old 24th September 2007, 01:53 PM   #47
DanDini is offline DanDini  United Kingdom
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Check out this guy selling these in the UK! He is claiming the the MC-5881A is point to point wired. I've emailed him asking how he is dealing with the various safety issues they have on 240v.

Should be interesting:

http://yaqin.co.uk/mc5881a.htm

Dan
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Old 24th September 2007, 02:06 PM   #48
Salas is offline Salas  Greece
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Yaqin MC-5881A amplifier improvements
Ha - ha! Right! Point to point, a super regulated PSU, and a hefty mark up to match! Ok Dan, you kick him first for lying through his teeth to the UK audio public and me next for they use Greek alphabet for their logo!
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Old 24th September 2007, 02:22 PM   #49
DanDini is offline DanDini  United Kingdom
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Sounds like a plan!

One other thing a friend of mine mentioned is that the transformers in this amp are all core-aligned. This could be the source of the 50Hz ripple on the output?

See their EL84 effort which has the mains tranny rotated:

Click the image to open in full size.

Dan
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Old 24th September 2007, 02:38 PM   #50
Salas is offline Salas  Greece
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Yaqin MC-5881A amplifier improvements
Yes it can give some hum when not in right angles. Do you still have audible hum with your speakers at listening position? Did you manage to listen critically at all?
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