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Old 11th September 2007, 07:38 AM   #31
DanDini is offline DanDini  United Kingdom
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Quote:
Another idea is to use an extra 100uF 450V cap before the 470uF that is already there, and link them with a 10R 10W. CRC will cut somehow on your PSU ripple, and you will lose just 10V B+. A no space for 2 good chokes situation can benefit somehow from the CRC alternative suggestion. Appreciably better than the single cap you have there now. [/B]
I'm unsure of the layout you mean. You obviously want the caps in parallel but can you draw the circuit with the resistor in so it makes sense to me?

Thanks again...
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Old 11th September 2007, 08:37 AM   #32
DanDini is offline DanDini  United Kingdom
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...another thing I note is that the power cap is running over spec. I'm getting 485Vdc out of it and it's a 450v rated cap.

The transformed supply voltage is claimed to be 295Vac on the schematic but due to the 220/240 difference in supply voltage it's now out of spec. It's actually a little more out of spec than that difference would imply:

220/240=0.917
295/343=0.86 (343 is a guess at the transformed voltage based on 485/1.414.

The heaters were originally 7.38v average, which is
6.3/7.38=0.85

The last two at least correspond with what I'm seeing but it suggests that the thing would be in spec at about 205Vac supply voltage.

Dan
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Old 11th September 2007, 06:04 PM   #33
Salas is offline Salas  Greece
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Quote:
Originally posted by DanDini


I'm unsure of the layout you mean. You obviously want the caps in parallel but can you draw the circuit with the resistor in so it makes sense to me?

Thanks again...

In the attached scan you can see what I meant, graphically.

I spec the caps for 500VDC since you have a 'wild' secondary on British mains. Nippon Chemicon caps are obtainable, logically priced, reliable, and will sound better anyway than the soon to dry from over voltage stock one in your amp.
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Old 11th September 2007, 09:06 PM   #34
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Sorry to go a little bit off topic.

Hello, I have a Yaqin MC5881A also.
Wen I turn it on it makes a Hum like yours but in 4 or 5 seconds almost goes to silence (not an absolut silence).
I would like to ask you about the amp's sound, what do you think? Is it ok for you? Can you improve the sound with some other valves?
What valves sould I buy to replace the chinese ones?

Sorry about my english.
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Old 12th September 2007, 07:33 AM   #35
DanDini is offline DanDini  United Kingdom
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Hi Rui, personally I haven't used mine much just to 'listen' as it was obvious from the start that there were some issues that I wanted to deal with before integrating it into my system. I have the same situation as you where the hum is louder initially. It does quieten off, but it is still too loud in my opinion.

If I could resolve this hum I would be happy enough with the sound I think. I have bought some 1980's Russian tubes which are yet to arrive, so I will let you know how they sound.

Thanks for joining in - this could be become the MC-5881 owners' thread if we try hard enough!

Dan
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Old 12th September 2007, 07:36 AM   #36
DanDini is offline DanDini  United Kingdom
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A Chinese colleague of mine at work has kindly translated the little bits of text on the schematic I posted. There isn't much and you could have guessed it more or less, but it's nice to have...

http://seis.bris.ac.uk/~chdma/files/mc-5881_english.jpg

Dan
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Old 12th September 2007, 01:49 PM   #37
Salas is offline Salas  Greece
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Hello again. My suggestion for output valves would be =C= 6L6Gs. I would be skeptical about substituting the stock Sino 6N1s with just any 6922 because 6N1s are generally smoother. I would discard the blocky gray coupling caps and put Auricaps. I would tweak the PSU to the CRC suggestion I made earlier, discarding the stock cap. I would use Nippon or even better, RIFA. (The initial additional hum, is the sound of the only PSU cap roaring with ripple as it charges). If I could fit it somehow, I would use a choke! CLC instead of CRC. L in the position of the R element in the schematic I posted above. I would trace the GND layout and try to break the ground loops with resistors. Last but not least, I would smack the living daylights out of those luna park 6E2s!

P.S. I attach a picture of the best valves you can use for drivers. Amperex 6DJ8-ECC88. Since 4 of them NOS are going to cost more than the amp itself, just keep em in mind in case you bump on them in some used old circuit or get them for peanuts from some non audiophile ex TV service old man.
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Old 12th September 2007, 02:20 PM   #38
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another quick little fix might be to stick a CL-90 on the transformer primary, assuming there isn't some other slow-start device implemented. i'll calm down any current rushing into your heaters, and drop a few volts, which eases the strain on your PT. exceeding the voltage spec on the windings is not a good thing.
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Old 12th September 2007, 03:18 PM   #39
DanDini is offline DanDini  United Kingdom
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Thanks for all the suggestions. I did put in place a CRC filter, but the basic hum is still there. I think it's from the gnd trace on the PCB. My intention is to break it where I can and run gnds to a common point.

I'll feed back.

Dan
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Old 12th September 2007, 03:59 PM   #40
Salas is offline Salas  Greece
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The CRC is for sounding better. Nothing to do with the basic ground loop. Keep fiddling with the GND routes. I am optimistic, you will sort it out. You mentioned that one major part of ground noise was that volume and input switch sub PCB loop via chassis and you could not isolate the metal bodies. What about disconnecting the ground side of the coax cables that go to PCB from that sub PCB? Maybe the pot can still see good GND via chassis.
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