Ming Da amplifier improvements.

[Patrick Turner]

I have had the displeasure to fix a pair of 100W Ming Da power amps
with PP 845 and to fix a Ming Da preamp recently.

Should anyone want a schematic set of 4 sheets in .gif form for the
power amp schematics they can have a copy of them.

But the preamp was also a bother because the Left channel hummed
badly.

The tube line up has a pair of 2A3 which are used as rectifier diodes
for B+. There are two chokes and plenty of capacinace; in fact the PSU
is ridiculously over heavy and over engineered. Then there is an EL34
and 6SN7 for B+ regulation.

The gain is nearly 10x maximum from a SRPP pair of 6SN7 triodes, so 1
x 6SN7 per channel.
The anode output of the SRPP is directly coupled to a 1/2 6DJ8 as a
cathode follower.
The CF has a bootstrapped anode supply with a series 1/2 6DJ8 sitting
betwen CF anode and B+. The CF cathode has a third 1/2 6DJ8 operating
as a current source so that there is 19mA from in the 3 seriesed
triodes.

Now the MIng Da power amps have a voltage gain of about 27x, or nearly
30dB.

The preamp has its volume control pot right after the source select
switch and before the SRPP 6SN7 gain stage. This means that with gain
down to minimum, any noise in the gain stage and CF stage is amplified
27 times in the power amp.
Guess what, I found one 6SN7 to be bloody old and sputtery noisy, so
you hear all that because its amplified 27 times. It also was hummy,
but that took some trouble to sort out. One 6SN7 was quite
microphonic. Lemme tellya folks, when ppl buy 6SN7 NOS on the Net,
they rarely ever get NOS and oftenthey have thousands of hours and
have become noisy and or microphonic. So ppl sell the NOS on Ebay as
NOS and other stoopid ppl buy them......

Anyway, I lift the 0V rail from the chassis and connect it using 10
ohms x 10W plus a 2uF cap which helped keep HF hash in the output to a
minimum. But the L channel still hummed. Not much, but just visible on
the CRO when set to max sensitivity. Using a power amp it was clearly
audible. The ground lift did nothing to reduce hum and when I re-
routed output signal cables from where they were lashed tight to wires
carrying heater currents, hum reduction was minimal.
I tried screening the rear of the RCA inputs, hum stayed put. But then
when I brought hand around the SRPP input for the L channel I was able
to modulate the hum level. Now the L channel input 6SN7 is only 60mm
away from a 2A3 rectifier which has about 200Vrms or more of mains
signal on it so it them dawned on me that the SRPP input tube was
copping some hum from the 2A3 via the stray capacitance. So I cut up
two strips of olive oil can tin plate and made some tight fitting
sleeves around the 6SN7 and took a ground wire down throgh a hole I
drilled in the chassis top. The hum vanished almost completely. I
soldered shield up and painted the tinplate matt black.

NOW the Ming Da worked just fine except for the tube noises So I put
in a resistance divider after the CF output using 18k + 4k7 to reduce
gain from 10 to 2, and also reduce the tube noise.
Having gain = 2.0 is just fine because the power amps have so much
gain; 1 volt input gives 80W output. I then had a good search amoung
the box full of old 6SN7 and I found a pair better than the originals,
ie, less noisy and microphonic, and I know the customer will enjoy
find the inky black silence when no signal is present. The Rout is now
about 4k0, but that is quite low enough to get a signal across 3
metres of cable without HF losses to power amps.

The alternative to such alterations would have been to allow the
selected source signals to go straight to the SRPP stage and then to a
volume control and then to the CF output. Its a lotta trouble to do
it. But then when a CD player is putting out say 1.4vrms, the the
input stage produces 14Vrms, and THD and IMD is getting high, so one
has to add some shunt NFB to reduce the gain. And then if one does
that, well, you have to convert the SRPP to a µ-follower, and then the
thing works best with low THD. My own preamps at my website favour
this set up, and have a deletable gain stage.

But providing the input stage is quiet, the use of volume control
before the input stage is fine especially if you have an insensitve
power amp, such as a KT66 PP Williamson which needs 2V input for
clipping when making only 16W.

The trouble is finding quiet 6SN7s.

I don't have a clue how come all these ancient tubes were in these
amps to begin with. I guess the previous owner had bought them on Ebay
to replace the Chinese 6SN7.

I usually find that if the input tubes are very quiet and non
microphonic then they probably don't have many hours on them and they
usually sound very well.

A good way to test for microphony is to connect the preamp up to a
power amp with a gain of at least 20 and clip a CRO on the power amp
output. Set the CRO to max sensivity.
Set the preamp volume to max.
Clap your hands loud as you can 600mm away from the preamp. Badly
microphonic tubes will create a splosh of signal on the CRO screen at
each hand clap, or maybe 30mV of signal.
Tapping the tube makes a ring tone which sometimes fades down slowly
like a tuning fork
It means that music in the room can modulate the signal and create a
tone in the tube and this will all measure more than the THD and IMD.
Hence this is one reason why some tubes sound different. Its the damn
microphony. Cupping a hand around an input tube and tapping it with a
fingernail will make the ring tone dissappear faster; ie, the mass
around the tube as you hold it damps the vibrations. But you need much
more mass around the tubes than those silly tube dampers ppl buy can
give.

Anyway, with nice low civised levels of music, microphony in preamps
is minimal.
With power amps like the Ming Da, I found the original preamp was set
at 9 o'clock or -30dB for most listening. It meant that only 30mV of
signal was going to the gain stage then on to the power amps. If the
noise in the tubes was 0.5mV, the SNR would be crap.

Si I never use my gain stage in my own preamp; the signal just goes to
a gain pot then to a CF which tends never to be microphonic or noisy
even with a tube that has seen better days because all of the output
of the CF is fed back as NFB in the follower connection.


There we are folks, this week's 2 bobs worth of how to fix crap gear.

Patrick Turner.

[Watt? Me worry?]

Hi RATs!

There is a difference between dirt ignorant and stupid. You had to
spend a bit of time to learn your skills :) It did not make you
smarter, it gave you more tools to use. Not the same thing.

I started out analyzing my then "new" old Fisher X-202-B and X-202-C
amps. From near zero, my Army training was just diagnose and fix, not
modify nor create :(

But even that small experience lead me to think their design was
mostly BS. A long line of 12AX7s to a P-P pair of 7591s. Lots of neat
monkey knob pots and switches, but, not much in terms of power supply
noise rejection, nor any noise rejection. They worked well enough to
sell. What else is there? ;) Plus, I learned that during the hay day
of tube Hi-Fi some companies put in tubes that lit up, but were not in
any circuit. Lots of tubes meant "Good" to gullible consumers...not
big companies, "boutique" shops, on the East coast.

So, I started reading and searching the WWW and asking lots of pig
ignorant questions and not getting many useful answers. Until I went
to the first VTV one day "Tube School" and met Eric and John. I drove
them crazy with emails and phone calls for a few years 8*D. We are
still friends - thirteen+ years later...

I started building amps from scratch, initially hoping to get enough
knowledge to "fix" the Fishers. Never happened. But, I did learn
enough to build some amps that worked better than the Fishers. OK, not
as many knobs, but... commercial amplifiers are like commercial
anything, some better than others, but, nothing really awesome, just
fancy. Not my idea of wonderful.

As my health collapsed, thinking about amps was my only link to
reality. It was interesting, and sometimes I could sit up and solder a
little. I had a turntable, amp and speakers in my bedroom. Also, tube
testers, signal generators and distortion analyzers... even an
O'scope! After a couple of years, I put the analysis tools aside and
just listened to the circuits I built. It was fun. I built some
speakers, too, but, with a bit less positive reinforcement :(

I did manage to learn what a wretched pair of ears I have, and what a
great imagination.

Yes, there is a lot of mediocre junk in the maketplace, but, even I
have learned enough to get many of the problems out of the way of the
music. Never made any money. Oh, well.

When I saw the article on the tubecad.com site about the Blumlein
garters, I got very excited, but, with my health thing, it took a
couple years to try it. It really is pretty decent, despite your
hostile analysis ;) Nothing else in my experience gave such a big
reward for such a small investment. Whatever turns me on, turns me on.

Happy Ears!

Al Marcy

[[email protected]]

On Feb 26, 9:35*am, "Watt? Me worry?" > wrote:
> Hi RATs!
>
> There is a difference between dirt ignorant and stupid. You had to
> spend a bit of time to learn your skills :) It did not make you
> smarter, it gave you more tools to use. Not the same thing.
>
> I started out analyzing my then "new" old Fisher X-202-B and X-202-C
> amps. From near zero, my Army training was just diagnose and fix, not
> modify nor create :(
>
> But even that small experience lead me to think their design was
> mostly BS. A long line of 12AX7s to a P-P pair of 7591s. Lots of neat
> monkey knob pots and switches, but, not much in terms of power supply
> noise rejection, nor any noise rejection. They worked well enough to
> sell. What else is there? *;) Plus, I learned that during the hay day
> of tube Hi-Fi some companies put in tubes that lit up, but were not in
> any circuit. Lots of tubes meant "Good" to gullible consumers...not
> big companies, "boutique" shops, on the East coast.
>
> So, I started reading and searching the WWW and asking lots of pig
> ignorant questions and not getting many useful answers. Until I went
> to the first VTV one day "Tube School" and met Eric and John. I drove
> them crazy with emails and phone calls for a few years 8*D. We are
> still friends - thirteen+ years later...
>
> I started building amps from scratch, initially hoping to get enough
> knowledge to "fix" the Fishers. Never happened. But, I did learn
> enough to build some amps that worked better than the Fishers. OK, not
> as many knobs, but... commercial amplifiers are like commercial
> anything, some better than others, but, nothing really awesome, just
> fancy. Not my idea of wonderful.
>
> As my health collapsed, thinking about amps was my only link to
> reality. It was interesting, and sometimes I could sit up and solder a
> little. I had a turntable, amp and speakers in my bedroom. Also, tube
> testers, signal generators and distortion analyzers... even an
> O'scope! After a couple of years, I put the analysis tools aside and
> just listened to the circuits I built. It was fun. I built some
> speakers, too, but, with a bit less positive reinforcement :(
>
> I did manage to learn what a wretched pair of ears I have, and what a
> great imagination.
>
> Yes, there is a lot of mediocre junk in the maketplace, but, even I
> have learned enough to get many of the problems out of the way of the
> music. Never made any money. Oh, well.
>
> When I saw the article on the tubecad.com site about the Blumlein
> garters, I got very excited, but, with my health thing, it took a
> couple years to try it. It really is pretty decent, despite your
> hostile analysis ;) Nothing else in my experience gave such a big
> reward for such a small investment. Whatever turns me on, turns me on.
>
> Happy Ears!
>
> Al Marcy

Avery Fisher was a pioneer in the audio industry.
Sadly, the companies which owned the brand name since 1969 did not
continue his ideas and kept putting substandard products on the
market. Especially Sanyo. Most of pre -'69 of tubed Fishers were not
the terribly overpriced high end models but something decent that
could be bought by an average person. Made for entertainment
purposes it was worth the money. Avery Fisher was a very generous
person. It is not his fault that products bearing his name became a
laughing stock.

[Andre Jute[_2_]]

On Feb 26, 2:35*pm, "Watt? Me worry?" > wrote:
> Hi RATs!
>
> There is a difference between dirt ignorant and stupid. You had to
> spend a bit of time to learn your skills :) It did not make you
> smarter, it gave you more tools to use. Not the same thing.
>
> I started out analyzing my then "new" old Fisher X-202-B and X-202-C
> amps. From near zero, my Army training was just diagnose and fix, not
> modify nor create :(
>
> But even that small experience lead me to think their design was
> mostly BS. A long line of 12AX7s to a P-P pair of 7591s. Lots of neat
> monkey knob pots and switches, but, not much in terms of power supply
> noise rejection, nor any noise rejection. They worked well enough to
> sell. What else is there? *;) Plus, I learned that during the hay day
> of tube Hi-Fi some companies put in tubes that lit up, but were not in
> any circuit. Lots of tubes meant "Good" to gullible consumers...not
> big companies, "boutique" shops, on the East coast.
>
> So, I started reading and searching the WWW and asking lots of pig
> ignorant questions and not getting many useful answers. Until I went
> to the first VTV one day "Tube School" and met Eric and John. I drove
> them crazy with emails and phone calls for a few years 8*D. We are
> still friends - thirteen+ years later...
>
> I started building amps from scratch, initially hoping to get enough
> knowledge to "fix" the Fishers. Never happened. But, I did learn
> enough to build some amps that worked better than the Fishers. OK, not
> as many knobs, but... commercial amplifiers are like commercial
> anything, some better than others, but, nothing really awesome, just
> fancy. Not my idea of wonderful.
>
> As my health collapsed, thinking about amps was my only link to
> reality. It was interesting, and sometimes I could sit up and solder a
> little. I had a turntable, amp and speakers in my bedroom. Also, tube
> testers, signal generators and distortion analyzers... even an
> O'scope! After a couple of years, I put the analysis tools aside and
> just listened to the circuits I built. It was fun. I built some
> speakers, too, but, with a bit less positive reinforcement :(
>
> I did manage to learn what a wretched pair of ears I have, and what a
> great imagination.
>
> Yes, there is a lot of mediocre junk in the maketplace, but, even I
> have learned enough to get many of the problems out of the way of the
> music. Never made any money. Oh, well.
>
> When I saw the article on the tubecad.com site about the Blumlein
> garters, I got very excited, but, with my health thing, it took a
> couple years to try it. It really is pretty decent, despite your
> hostile analysis ;) Nothing else in my experience gave such a big
> reward for such a small investment. Whatever turns me on, turns me on.
>
> Happy Ears!
>
> Al Marcy

No instrument can substitute for the experience of say 25 years in
concert halls.

Andre Jute
I'm not a know-all. I don't need to be. I know who to ask.

[Andre Jute[_2_]]

What this comes down to, Patrick, is that,

-- If the Ming Da is cheap enough
--And if you are capable of redesigning it
--Buy it
--You will then have a decent power supply, a punched case, a good
deal of wound iron, tubes in sockets, lots of other components
--Which you can rebuild into a simpler, quieter, better sounding amp
--For a less than it costs to order a brandnew custom amp from someone
competent

But is it still so economical when you have to give the Ming Da to the
"someone competent" to rebuild?

Note that I'm assuming that in the first instance the Ming Da isn't
very expensive for what it is. If it is expensive to start with, you
may as well start from scratch.

Andre Jute
Visit Jute on Amps at
http://www.audio-talk.co.uk/fiultra/
"wonderfully well written and reasoned information for the tube audio
constructor"
John Broskie TubeCAD & GlassWare
"an unbelievably comprehensive web site containing vital gems of
wisdom"
Stuart Perry Hi-Fi News & Record Review


On Feb 26, 11:46*am, Patrick Turner > wrote:
> I have had the displeasure to fix a pair of 100W Ming Da power amps
> with PP 845 and to fix a Ming Da preamp recently.
>
> Should anyone want a schematic set of 4 sheets in .gif form for the
> power amp schematics they can have a copy of them.
>
> But the preamp was also a bother because the Left channel hummed
> badly.
>
> The tube line up has a pair of 2A3 which are used as rectifier diodes
> for B+. There are two chokes and plenty of capacinace; in fact the PSU
> is ridiculously over heavy and over engineered. Then there is an EL34
> and 6SN7 for B+ regulation.
>
> The gain is nearly 10x maximum from a SRPP pair of 6SN7 triodes, so 1
> x 6SN7 per channel.
> The anode output of the SRPP is directly coupled to a 1/2 6DJ8 as a
> cathode follower.
> The CF has a bootstrapped *anode supply with a series 1/2 6DJ8 sitting
> betwen CF anode and B+. The CF cathode has a third 1/2 6DJ8 operating
> as a current source so that there is 19mA from in the 3 seriesed
> triodes.
>
> Now the MIng Da power amps have a voltage gain of about 27x, or nearly
> 30dB.
>
> The preamp has its volume control pot right after the source select
> switch and before the SRPP 6SN7 gain stage. This means that with gain
> down to minimum, any noise in the gain stage and CF stage is amplified
> 27 times in the power amp.
> Guess what, I found one 6SN7 to be bloody old and sputtery noisy, so
> you hear all that because its amplified 27 times. It also was hummy,
> but that took some trouble to sort out. One 6SN7 was quite
> microphonic. *Lemme tellya folks, when ppl buy 6SN7 NOS on the Net,
> they rarely ever get NOS and oftenthey have thousands of hours and
> have become noisy and or microphonic. So ppl sell the NOS on Ebay as
> NOS and other stoopid ppl buy them......
>
> Anyway, I lift the 0V rail from the chassis and connect it using 10
> ohms x 10W plus a 2uF cap which helped keep HF hash in the output to a
> minimum. But the L channel still hummed. Not much, but just visible on
> the CRO when set to max sensitivity. Using a power amp it was clearly
> audible. The ground lift did nothing to reduce hum and when I re-
> routed output signal cables from where they were lashed tight to wires
> carrying heater currents, hum reduction was minimal.
> I tried screening the rear of the RCA inputs, hum stayed put. But then
> when I brought hand around the SRPP input for the L channel I was able
> to modulate the hum level. Now the L channel input 6SN7 is only 60mm
> away from a 2A3 rectifier which has about 200Vrms or more of mains
> signal on it so it them dawned on me that the SRPP input tube was
> copping some hum from the 2A3 via the stray capacitance. So I cut up
> two strips of olive oil can tin plate and made some tight fitting
> sleeves around the 6SN7 and took a ground wire down throgh a hole I
> drilled in the chassis top. The hum vanished almost completely. I
> soldered shield up and painted the tinplate matt black.
>
> NOW the Ming Da worked just fine except for the tube noises So I put
> in a resistance divider after the CF output using 18k + 4k7 to reduce
> gain from 10 to 2, and also reduce the tube noise.
> Having gain = 2.0 is just fine because the power amps have so much
> gain; 1 volt input gives 80W output. I then had a good search amoung
> the box full of old 6SN7 and I found a pair better than the originals,
> ie, less noisy and microphonic, and I know the customer will enjoy
> find the inky black silence when no signal is present. The Rout is now
> about 4k0, but that is quite low enough to get a signal across 3
> metres of cable without HF losses to power amps.
>
> The alternative to such alterations would have been to allow the
> selected source signals to go straight to the SRPP stage and then to a
> volume control and then to the CF output. Its a lotta trouble to do
> it. But then when a CD player is putting out say 1.4vrms, the the
> input stage produces 14Vrms, and THD and IMD is getting high, so one
> has to add some shunt NFB to reduce the gain. And then if one does
> that, well, you have to convert the SRPP to a µ-follower, and then the
> thing works best with low THD. My own preamps at my website favour
> this set up, and have a deletable gain stage.
>
> But providing the input stage is quiet, the use of volume control
> before the input stage is fine especially if you have an insensitve
> power amp, such as a KT66 PP Williamson which needs 2V input for
> clipping when making only 16W.
>
> The trouble is finding quiet 6SN7s.
>
> I don't have a clue how come all these ancient tubes were in these
> amps to begin with. I guess the previous owner had bought them on Ebay
> to replace the Chinese 6SN7.
>
> I usually find that if the input tubes are very quiet and non
> microphonic then they probably don't have many hours on them and they
> usually sound very well.
>
> A good way to test for microphony is to connect the preamp up to a
> power amp with a gain of at least 20 and clip a CRO on the power amp
> output. Set the CRO to max sensivity.
> Set the preamp volume to max.
> Clap your hands loud as you can 600mm away from the preamp. Badly
> microphonic tubes will create a splosh of signal on the CRO screen at
> each hand clap, or maybe 30mV of signal.
> Tapping the tube makes a ring tone which sometimes fades down slowly
> like a tuning fork
> It means that music in the room can modulate the signal and create a
> tone in the tube and this will all measure more than the THD and IMD.
> Hence this is one reason why some tubes sound different. Its the damn
> microphony. Cupping a hand around an input tube and tapping it with a
> fingernail will make the ring tone dissappear faster; ie, the mass
> around the tube as you hold it damps the vibrations. But you need much
> more mass around the tubes than those silly tube dampers ppl buy can
> give.
>
> Anyway, with nice low civised levels of music, microphony in preamps
> is minimal.
> With power amps like the Ming Da, I found the original preamp was set
> at 9 o'clock or -30dB for most listening. It meant that only 30mV of
> signal was going to the gain stage then on to the power amps. If the
> noise in the tubes was 0.5mV, the SNR would be crap.
>
> Si I never use my gain stage in my own preamp; the signal just goes to
> a gain pot then to a CF which tends never to be microphonic or noisy
> even with a tube that has seen better days because all of the output
> of the CF is fed back as NFB in the follower connection.
>
> There we are folks, this week's 2 bobs worth of how to fix crap gear.
>
> Patrick Turner.

[Patrick Turner]

On Feb 27, 1:35*am, "Watt? Me worry?" > wrote:
> Hi RATs!
>
> There is a difference between dirt ignorant and stupid. You had to
> spend a bit of time to learn your skills :) It did not make you
> smarter, it gave you more tools to use. Not the same thing.
>
> I started out analyzing my then "new" old Fisher X-202-B and X-202-C
> amps. From near zero, my Army training was just diagnose and fix, not
> modify nor create :(
>
> But even that small experience lead me to think their design was
> mostly BS. A long line of 12AX7s to a P-P pair of 7591s. Lots of neat
> monkey knob pots and switches, but, not much in terms of power supply
> noise rejection, nor any noise rejection. They worked well enough to
> sell. What else is there? *;) Plus, I learned that during the hay day
> of tube Hi-Fi some companies put in tubes that lit up, but were not in
> any circuit. Lots of tubes meant "Good" to gullible consumers...not
> big companies, "boutique" shops, on the East coast.
>
> So, I started reading and searching the WWW and asking lots of pig
> ignorant questions and not getting many useful answers. Until I went
> to the first VTV one day "Tube School" and met Eric and John. I drove
> them crazy with emails and phone calls for a few years 8*D. We are
> still friends - thirteen+ years later...
>
> I started building amps from scratch, initially hoping to get enough
> knowledge to "fix" the Fishers. Never happened. But, I did learn
> enough to build some amps that worked better than the Fishers. OK, not
> as many knobs, but... commercial amplifiers are like commercial
> anything, some better than others, but, nothing really awesome, just
> fancy. Not my idea of wonderful.
>
> As my health collapsed, thinking about amps was my only link to
> reality. It was interesting, and sometimes I could sit up and solder a
> little. I had a turntable, amp and speakers in my bedroom. Also, tube
> testers, signal generators and distortion analyzers... even an
> O'scope! After a couple of years, I put the analysis tools aside and
> just listened to the circuits I built. It was fun. I built some
> speakers, too, but, with a bit less positive reinforcement :(
>
> I did manage to learn what a wretched pair of ears I have, and what a
> great imagination.
>
> Yes, there is a lot of mediocre junk in the maketplace, but, even I
> have learned enough to get many of the problems out of the way of the
> music. Never made any money. Oh, well.
>
> When I saw the article on the tubecad.com site about the Blumlein
> garters, I got very excited, but, with my health thing, it took a
> couple years to try it. It really is pretty decent, despite your
> hostile analysis ;) Nothing else in my experience gave such a big
> reward for such a small investment. Whatever turns me on, turns me on.


You have the knack of telling a life story in one page!

Just about all commercially made gear can be improved.

But what ****es me off is that so much commercially made gear has NOT
been improved in a second or third approval of prototype process, so
hence problems as they exist in gear like MingDa don't get picked up
and fixed by the makers before it is splashed all over the Internet
and promoted as God's Gift to Hi-Fi, available for a huge price.

Patrick Turner.


>
> Happy Ears!
>
> Al Marcy

[Patrick Turner]

On Feb 27, 10:10*am, Andre Jute > wrote:
> What this comes down to, Patrick, is that,
>
> -- If the Ming Da is cheap enough
> --And if you are capable of redesigning it
> --Buy it

Well, the whole Ming Da amp line up isn't dirt cheap at all.

The gear is top of the line Chinese Hi-Fi gear.

A customer here spent about USD $2,400 on the set second hand which
was far below the cost it would have been new.

It is quite apparent Ming Da set out to make a visually stunning amp
set. In the power amps, there are huge 845 driven with a pair of 300B,
then driven with a pair of 6SN7.
The preamp has two 2A3 for HT diodes, then 3 x 6SN7, 1 x EL34, and 3 x
6DJ8 all just for a line stage with gain of 20dB.

I have nothing against the use of "circuituria spectaculari."

I just don't want to be able to improve on the distortion figures,
noise performance, stablity, ease of bias setting, relibility, and
bias failure prorection.

Huge improvements to the performance are possible, and in good
expensive gear, this just should not be possible.


> --You will then have a decent power supply, a punched case, a good
> deal of wound iron, tubes in sockets, lots of other components
> --Which you can rebuild into a simpler, quieter, better sounding amp
> --For a less than it costs to order a brandnew custom amp from someone
> competent

The Chinese power trannies are average, and often made to work on 220V
or 110V and do not have the added tappings for 240V. But on many days
we measure 250V mains. So HT and heater voltages are all a bit high.

The case quality is marginal. The chassis in many Chinese amps is made
so the top plate has two layers of steel sheet about 6mm apart so that
the bottom one has all the bolted bits and peices and the top one is
free of a single screw head. But the result is that if you want to
replace a tube socket or remove a transformer the whole ****in amp
must be dismantled and its a major ****in job. Ming Da have use some
very nice 4 pin ceramic sockets for the 2A3 rectifiers in the preamp.
These have pin grippers with C shaped steel springs to ensure good
contact no matter how many times you replace tubes. But the ceramic
sockets for the 300B in the power amps are utterly attrocious and are
the very cheapest crappy sockets ever ****in made by anyone. Despite
the gold plating, the pin grippers expand and stay bent open and
sloppy after changing tubes a few times or wobbling the tubes in their
sockets; its inconsistency in production.

The OPT has very lirrle interleaving and saturates at way too high a
frequency. Sure the amps can be forced to work with stability, but
there is not one single Chinese amp maker who understands how to do it
properly by beginning with a well designed OPT. Ming Da and the
Quad40, both Hi-End Chinese product have awful OPTs.

>
> But is it still so economical when you have to give the Ming Da to the
> "someone competent" to rebuild?

I'd charge someone about USD $12,000 for an Australian made amp set
with PP 845 and a decent preamp.

Its a lot cheaper than if ARC or Conrad Johnson had made it.

My customer paid way below the above prices so if he spends $500 on me
to fix the junk up then sure he has a bargain, and it will sound
pretty well.

>
> Note that I'm assuming that in the first instance the Ming Da isn't
> very expensive for what it is. If it is expensive to start with, you
> may as well start from scratch.


The Ming Da preamp is the model shown at
http://www.ornec.com/Meixing-MingDa-MC-2A3_QQC100299B

It is a heavyweight preamp of about 15Kgs.

It has a very unecessarily elaborate circuit so that each channel uses
5 triodes for the audio signal to get a gain of 10.

I have achieved excellent sound and technical performance for a two
channel line stage with just two 12AU7 and a few MJE340 and MJE350 as
current source loadings and a small power supply. See what can
achieved with frugality at
http://www.turneraudio.com.au/Line-preamp-2003.htm

Nobody needs to make a preamp the way Ming Da does it.

Patrick Turner.




>
> Andre Jute
> Visit Jute on Amps at
> *http://www.audio-talk.co.uk/fiultra/
> "wonderfully well written and reasoned information for the tube audio
> constructor"
> John Broskie TubeCAD & GlassWare
> "an unbelievably comprehensive web site containing vital gems of
> wisdom"
> Stuart Perry Hi-Fi News & Record Review
>
> On Feb 26, 11:46*am, Patrick Turner > wrote:
>
>
>
> > I have had the displeasure to fix a pair of 100W Ming Da power amps
> > with PP 845 and to fix a Ming Da preamp recently.
>
> > Should anyone want a schematic set of 4 sheets in .gif form for the
> > power amp schematics they can have a copy of them.
>
> > But the preamp was also a bother because the Left channel hummed
> > badly.
>
> > The tube line up has a pair of 2A3 which are used as rectifier diodes
> > for B+. There are two chokes and plenty of capacinace; in fact the PSU
> > is ridiculously over heavy and over engineered. Then there is an EL34
> > and 6SN7 for B+ regulation.
>
> > The gain is nearly 10x maximum from a SRPP pair of 6SN7 triodes, so 1
> > x 6SN7 per channel.
> > The anode output of the SRPP is directly coupled to a 1/2 6DJ8 as a
> > cathode follower.
> > The CF has a bootstrapped *anode supply with a series 1/2 6DJ8 sitting
> > betwen CF anode and B+. The CF cathode has a third 1/2 6DJ8 operating
> > as a current source so that there is 19mA from in the 3 seriesed
> > triodes.
>
> > Now the MIng Da power amps have a voltage gain of about 27x, or nearly
> > 30dB.
>
> > The preamp has its volume control pot right after the source select
> > switch and before the SRPP 6SN7 gain stage. This means that with gain
> > down to minimum, any noise in the gain stage and CF stage is amplified
> > 27 times in the power amp.
> > Guess what, I found one 6SN7 to be bloody old and sputtery noisy, so
> > you hear all that because its amplified 27 times. It also was hummy,
> > but that took some trouble to sort out. One 6SN7 was quite
> > microphonic. *Lemme tellya folks, when ppl buy 6SN7 NOS on the Net,
> > they rarely ever get NOS and oftenthey have thousands of hours and
> > have become noisy and or microphonic. So ppl sell the NOS on Ebay as
> > NOS and other stoopid ppl buy them......
>
> > Anyway, I lift the 0V rail from the chassis and connect it using 10
> > ohms x 10W plus a 2uF cap which helped keep HF hash in the output to a
> > minimum. But the L channel still hummed. Not much, but just visible on
> > the CRO when set to max sensitivity. Using a power amp it was clearly
> > audible. The ground lift did nothing to reduce hum and when I re-
> > routed output signal cables from where they were lashed tight to wires
> > carrying heater currents, hum reduction was minimal.
> > I tried screening the rear of the RCA inputs, hum stayed put. But then
> > when I brought hand around the SRPP input for the L channel I was able
> > to modulate the hum level. Now the L channel input 6SN7 is only 60mm
> > away from a 2A3 rectifier which has about 200Vrms or more of mains
> > signal on it so it them dawned on me that the SRPP input tube was
> > copping some hum from the 2A3 via the stray capacitance. So I cut up
> > two strips of olive oil can tin plate and made some tight fitting
> > sleeves around the 6SN7 and took a ground wire down throgh a hole I
> > drilled in the chassis top. The hum vanished almost completely. I
> > soldered shield up and painted the tinplate matt black.
>
> > NOW the Ming Da worked just fine except for the tube noises So I put
> > in a resistance divider after the CF output using 18k + 4k7 to reduce
> > gain from 10 to 2, and also reduce the tube noise.
> > Having gain = 2.0 is just fine because the power amps have so much
> > gain; 1 volt input gives 80W output. I then had a good search amoung
> > the box full of old 6SN7 and I found a pair better than the originals,
> > ie, less noisy and microphonic, and I know the customer will enjoy
> > find the inky black silence when no signal is present. The Rout is now
> > about 4k0, but that is quite low enough to get a signal across 3
> > metres of cable without HF losses to power amps.
>
> > The alternative to such alterations would have been to allow the
> > selected source signals to go straight to the SRPP stage and then to a
> > volume control and then to the CF output. Its a lotta trouble to do
> > it. But then when a CD player is putting out say 1.4vrms, the the
> > input stage produces 14Vrms, and THD and IMD is getting high, so one
> > has to add some shunt NFB to reduce the gain. And then if one does
> > that, well, you have to convert the SRPP to a µ-follower, and then the
> > thing works best with low THD. My own preamps at my website favour
> > this set up, and have a deletable gain stage.
>
> > But providing the input stage is quiet, the use of volume control
> > before the input stage is fine especially if you have an insensitve
> > power amp, such as a KT66 PP Williamson which needs 2V input for
> > clipping when making only 16W.
>
> > The trouble is finding quiet 6SN7s.
>
> > I don't have a clue how come all these ancient tubes were in these
> > amps to begin with. I guess the previous owner had bought them on Ebay
> > to replace the Chinese 6SN7.
>
> > I usually find that if the input tubes are very quiet and non
> > microphonic then they probably don't have many hours on them and they
> > usually sound very well.
>
> > A good way to test for microphony is to connect the preamp up to a
> > power amp with a gain of at least 20 and clip a CRO on the power amp
> > output. Set the CRO to max sensivity.
> > Set the preamp volume to max.
> > Clap your hands loud as you can 600mm away from the preamp. Badly
> > microphonic tubes will create a splosh of signal on the CRO screen at
> > each hand clap, or maybe 30mV of signal.
> > Tapping the tube makes a ring tone which sometimes fades down slowly
> > like a tuning fork
> > It means that music in the room can modulate the signal and create a
> > tone in the tube and this will all measure more than the THD and IMD.
> > Hence this is one reason why some tubes sound different. Its the damn
> > microphony. Cupping a hand around an input tube and tapping it with a
> > fingernail will make the ring tone dissappear faster; ie, the mass
> > around the tube as you hold it damps the vibrations. But you need much
> > more mass around the tubes than those silly tube dampers ppl buy can
> > give.
>
> > Anyway, with nice low civised levels of music, microphony in preamps
> > is minimal.
> > With power amps like the Ming Da, I found the original preamp was set
> > at 9 o'clock or -30dB for most listening. It meant that only 30mV of
> > signal was going to the gain stage then on to the power amps. If the
> > noise in the tubes was 0.5mV, the SNR would be crap.
>
> > Si I never use my gain stage in my own preamp; the signal just goes to
> > a gain pot then to a CF which tends never to be microphonic or noisy
> > even with a tube that has seen better days because all of the output
> > of the CF is fed back as NFB in the follower connection.
>
> > There we are folks, this week's 2 bobs worth of how to fix crap gear.
>
> > Patrick Turner.- Hide quoted text -
>
> - Show quoted text -