Ming Da MC34-AB

[zenelectro]

Incidentally, I was also about to order a Jensen electrolytic cap to replace the one remaining blue cap (as you pointed out in the other MingDa thread), but I notice that cap is under a bridge rectifier and is right near the small PCB that controls the volume control motor and associated logic. I speculate that the cap is just a power supply smoothing cap for a supply that only runs that logic board, and therefore there may be no point in replacing it if it is only to run that small digital circuit...?

Incidentally, I would be grateful if you sent me a schematic of the amp.

- r.

Anyone know what the bridge rectifier is for?

Z

[rab]

Oh, one more question: although the coupling caps for the front 4 valves have been replaced, i have not seen any mention or suggestion on this forum of replacing the 8 MingDa-branded coupling caps for the EL34s. Why is that? Presumably those caps are rubbish and I would have expected that those would be replaced before the resistors...?

[rab]

Anyone know what the bridge rectifier is for?

Am i missing something? Presumably the amp has different AC lines coming from (presumably different windings) of the external transformers... the bridge is just to rectify the AC to DC for the digital board... or so i speculate!

[mcb]

Incidentally, I was also about to order a Jensen electrolytic cap to replace the one remaining blue cap (as you pointed out in the other MingDa thread), but I notice that cap is under a bridge rectifier and is right near the small PCB that controls the volume control motor and associated logic. I speculate that the cap is just a power supply smoothing cap for a supply that only runs that logic board, and therefore there may be no point in replacing it if it is only to run that small digital circuit...?

Incidentally, I would be grateful if you sent me a schematic of the amp.

- r.

Hi Rab,

I don't think that cap is for the vol motor control.  Flemo replaced this cap in my (the ex flemo) amp a while ago and as you know, it does not have the remote volume control. 

With regard to you question about the coupling caps for the EL34's. I am pretty sure that they where on Mario's hit list.  I took one look at where they where located and decided that replacing them looked too hard for me.

I get the impression your brain is working overtime at the moment, scheming up possible mods .

Murray

[data]

Oh, one more question: although the coupling caps for the front 4 valves have been replaced, i have not seen any mention or suggestion on this forum of replacing the 8 MingDa-branded coupling caps for the EL34s. Why is that? Presumably those caps are rubbish and I would have expected that those would be replaced before the resistors...?

What value are they? 1uf aren't they?

I guess if replacing with something of high quality like Duelunds there is cost and also space could be an issue, for a Duelund Alexandra of 1uf ya' looking at around $100 each and they are 45mm x 62mm. There are 8 of them and space seems non existent for caps of the Duelund's size, would need to suspend them above (that being below when the amp is right way up)

[kajak12]

Oh, one more question: although the coupling caps for the front 4 valves have been replaced, i have not seen any mention or suggestion on this forum of replacing the 8 MingDa-branded coupling caps for the EL34s. Why is that? Presumably those caps are rubbish and I would have expected that those would be replaced before the resistors...?

Hi rab regarding el34 decoupling caps flemo didn't whant to spend the cash to replace them with something and yes the caps used will be sh!t.
flemo's amp was modded on the cheap heaps more can be done if funds are available.

[kajak12]

Incidentally, I was also about to order a Jensen electrolytic cap to replace the one remaining blue cap (as you pointed out in the other MingDa thread), but I notice that cap is under a bridge rectifier and is right near the small PCB that controls the volume control motor and associated logic. I speculate that the cap is just a power supply smoothing cap for a supply that only runs that logic board, and therefore there may be no point in replacing it if it is only to run that small digital circuit...?

Incidentally, I would be grateful if you sent me a schematic of the amp.

- r.

Anyone know what the bridge rectifier is for?

Z

negative bias for el34's

[crazikid]

i did the 8 x 1uf caps with jensen copper. stock was better. its a sh!te job to do as well.
In retrospect i wouldn't bother

[rab]

i did the 8 x 1uf caps with jensen copper. stock was better. its a sh!te job to do as well.
In retrospect i wouldn't bother

Hi crazikid: and interesting - and unexpected - result! And then (judging by flemo's pics) you changed the caps back again?

Incidentally, I trawled the web and came across an Audiogon advert for a modded one of these amps with all of those caps replaced with "Russian K-40s and K-75s", see here: http://new.audiogon.com/listings/30344.
Pic:


According to the seller, "The change made a dramatic difference in sound and detail and continues to improve as they are still breaking in." But then he changed the coupling caps for the small valves at the same time, so I suppose the change could have been dominated by those...

I now have a schematic of the amp: to my untrained eye, the circuit looks about as simply as you could imagine for a high power push-pull topology. I was interested to see that the only differences between the 'power amp' and 'integrated' modes seem to be that the input selector and volume controls (but interestingly nothing else) are bypassed and that a different (parallel RC) filter is used for the global feedback loop. Any ideas why the latter would be the case? I

[rab]

I don't think that cap is for the vol motor control.  Flemo replaced this cap in my (the ex flemo) amp a while ago and as you know, it does not have the remote volume control. 

Interesting. I will try to trace the rectifier output the next time i have the amp open. I note that the volume control logic circuit is not included in the MingDa schematic. The rectifier for the bias voltages is shown as being part of the power supply in a separate schematic, which I assume (possibly incorrectly) means that circuit is in the separate supply box thingy. I note there are two 3.15VAC taps coming off the same circuit... perhaps those are for the digital circuitry? If so, there would need to be a third bridge rectifier.

I get the impression your brain is working overtime at the moment, scheming up possible mods .

Well, not really... i lack the knowledge to come up with my own mods! I will be relying on Mario's expertise... I am just trying to understand the amplifier circuit on paper and in hardware!

[data]

I don't know if It's just the photo, but those four caps in the center look to be positioned badly, unless they are raised higher than what it looks.

I would be inclined to stand the caps well away from the valve pins, say about 1 inch away, maybe a bit more. Edit: defiantly more now that I think about the HV of the anode and screen.

[rab]

I don't know if It's just the photo, but those four caps in the center look to be positioned badly, unless they are raised higher than what it looks.

Agreed. I don't think it's the photo: those central caps will be cooked!

[data]

Possibly cause more serious problems too.

[tuyen]

i did the 8 x 1uf caps with jensen copper. stock was better. its a sh!te job to do as well.
In retrospect i wouldn't bother

i would have to agree with this one.

its fun to play with different caps, but personally, I wouldn't recommend spending too silly $ on them.  For example 8x1uf Jensen coppers ($60x8=$480+postage).   

[kajak12]

i did the 8 x 1uf caps with jensen copper. stock was better. its a sh!te job to do as well.
In retrospect i wouldn't bother

i would have to agree with this one.

its fun to play with different caps, but personally, I wouldn't recommend spending too silly $ on them.  For example 8x1uf Jensen coppers ($60x8=$480+postage).   

the jensen pio are very thin sounding cap 

[rab]

i will ring you on the weekend and explain what you will be doing to the amp.

OK Mario, I am now ready for your call when you are!

I have run the amp for over 100 hours now. I haven't yet sat down and listened carefully, but my impression is that the sound has not improved substantially, if at all.

It strikes me that these amps are pretty simple in topology, as presumably is your own amp (simplest is best?), which I think also runs 8 Winged C EL34s. Would it therefore be possible to effectively convert my MingDa into something close to your amp by first making it have the same topology (presumably not that different in any case?) and substituting parts over time? ... or am I dreaming?

[kajak12]

i will ring you on the weekend and explain what you will be doing to the amp.

OK Mario, I am now ready for your call when you are!

I have run the amp for over 100 hours now. I haven't yet sat down and listened carefully, but my impression is that the sound has not improved substantially, if at all.

It strikes me that these amps are pretty simple in topology, as presumably is your own amp (simplest is best?), which I think also runs 8 Winged C EL34s. Would it therefore be possible to effectively convert my MingDa into something close to your amp by first making it have the same topology (presumably not that different in any case?) and substituting parts over time? ... or am I dreaming?

rab post some internal photos of your amp or email them to me high rez
my amp uses valve rectification i am sure paul could do yours but he needs your amp