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Just wa=
nt to
share with fellow audiophiles a schematic of the Ming =
Da
MC-2A3 preamp and my experience as a curious owner.
bsp;
I have always been a believer of preamps being respons=
ible
for setting the tonal quality and power amps just delivering power with tra=
nsparency
and stability.
I have been a fan of 6SN7-based t=
ubed
preamps such as the MFA Luminescence, Melody 1688 Signature, and the Canary
CA906. Violin music just soun=
ds
magical with those 3. preamps.
which
I was not too impressed with due to the blurry bass and somewhat AM radio-l=
ike
mid-highs.
The description of the preamp @ the manufacturer's web=
site
sucks.
There has not been a review of this unit on the several
major audio-rags or sites like 6moons or enjoythemusic=
.
monstrosity.
The Ming Da MC-2A3 struck =
me as an
interesting design (tube rectification and regulation, dual stage chokes, a=
nd
of course… 6SN7) and I had the opportunity to pick up one at a bargain
price. The preamp arrived wit=
h the
cage detached from shipping and created a ding on one of the front support
“pillars”. No big=
deal,
just cosmetics.
Stock tubes included Shuguangan> 2A3
rectifiers, and the rest of the tubes are EH.
Initial impre=
ssions
Out of the box and as expected, there is a sweetness t= Off comes the= OK let’s take a look at what’s inside the The B+ power supply sure looks impressive. There seems to be 2?!= Looking at the circuitry, the 6SN7s are in SRPP Shotgun Appro= I decided to start with a shotgun approach by taking o= Schematics Thanks to a fellow audiophile who graciously shared wi= I looked at the 2 High Voltage bypass caps closest to = Colored sound= “So why didn’t you wait a few weeks to let While I believe in running in I refuse to believe in t= Why so finick= The same audiophile who provided me with the schematic= For the 6922 in the input/gain stage I tube-rolled amo= Each set of 6SN7s gave it a different sound as expecte= Oscillation!<= We looked at the output of the 6SN7 to the 6922 middle= Is that the o= So back to the pronounced mid-high= Faulty Jensen= At around the same time the preamp started to emit a Looking at page 168 of> So I decided to leave that cap OUT. Experiments w= It’s common assumption that = I tried resistor values ranging from 25 to 80kOhms at = Scrutinize the Running out of ideas, I decided to spend some time loo= http://www.pacificvalve.us/images/MC_2A3_Inside.JPG and here: http://www.audiocostruzioni.com/r_s/pre/ming-da-mc-2a3= do not show the 5.1kOhm res= According to “Beginner's Guide to Tube Audio Des= My unit has the green 300Ohm resistor but the unit dep= http://www.audiocostruzioni.com/r_s/pre/ming-da-mc-2a3= does not have that resistor= The 300Ohm resistor in my unit (and possibly all units The photo of the European version looks different. With this change to make the circuit a big step closer= Film caps I’m sure everyone has come across postings bashi= Fix the obvio= While trying to figure out how to squeeze 4 33uF film = Film caps for= After I figured out how to physically squeeze in 3 Even without any warm up I notice that there is an 1 More RC fil= Time for another fancy idea.> The result?&nbs= I want to keep 100uF bypassing the 6922s anyway so I e= Ripple-phobia= I was also surprised at how the preamp reacted to diff= I was suspicious with the quality of the choke-filtere= So I decided to squeeze a small film cap =3DGramE>( Drum RollR= Voila! <= I guess what I really should have done is replace the = This was indeed an interesting journey in audio. Beginner's Guide to Tube Au= Valve Amplifiers, Third Edi=
o the
violins that makes this preamp very desirable.
the
same weaknesses as the SLP98P I owned previously.
eiled
mid-highs, no top end extension.
The somewhat narrow soundstage destabilizes when playing complex
passages. Still the musicality is there that makes it an enjoyable preamp.<=
/p>
bottom
cover
unit. Wait… where are t=
he
Sprague vitamin Q gumdrops and the Mundorf M-ca=
ps in
the advertisements? All I see=
is
run of the mill SCR-MKP caps. >Oh
well no problem, I was gonna swap them out with=
my
collection of Mundorf Silver Oil, =3DSpellE>Dynamicaps,
Jensen Copper-oil, or Hovland llE>Musicaps
anyway.
pass.
17;s
the filament power supply hooked up?
Nice bridge-C-R-C to the 6SN7s but wait… it supplies it with an
unregulated 13V to be share serially among 2 6SN7s!
filament voltage of 6.5 on a good day at 0-6.5V and the other tube has it
raised to 6.5-13V! That’=
;s not
a good design imes New Roman";
mso-hansi-font-family:"Times New Roman";mso-char-type:symbol;mso-symbol-fon=
t-family:
Wingdings'>ings'>L
lts
shared between 3 6922s, that means one channel would have its filament at
absolute potential starting as high as 13.5V.
lower,
sounds logical. The 2 6922s o=
n the
side are a bit idiosyncratic.
½ of each “side” 6922 serve as the constant curre=
nt
source to each half of the the middle 6922.
er
½ of each of the “side” 6922s is the top tube of the
bootstrapped cathode follower for the middle 6922.
onvinced
me that a lot of thought has been put into the circuit (well… read on=
).
ach
ut the
SCR MKP output caps ( was 2.2uF paralleled with =
some
PIO-like 0.01uF) and replace them with Mundorf =
2.2uF
Silver Oils. The 0.22uF caps
between the cathode follower and the top 6922 was also replaced with Jensen
0.22uF Copper Oil. The sound =
opened
up quite a bit and every sounds more well
defined. However, it came at a
price of a pronounced mid-high and the bass is equally as flabby.
o with
the output cap than the cap between the cathode follower and the upper foll=
ower
tube’s grid.
th me
a copy of the schematics.
the
6SN7s and right at the regulator outputs and according to the schematic they
are 100uF 400V rated. So in w=
ent to
Blackgate 100uF caps in place of the blue 150uF=
caps
marked Philips and BC (they looked identical and based on the markings on t=
he
cap I have reasonable suspicion that they are some Kung E>Pao
Mushu counterfeit caps).
fuller but it still sounds slow and muddy.=
There is a slight incoherence in the mid-high that I associated with=
the
0.1uF bypass cap across strapped from B+ to ground.
span
class=3DSpellE>lytics do not like to be bypassed so I took it out.
style=3D'mso-spacerun:yes'> Marginal improvement I called it. =
and
lethargic bass
everything run in?” You=
might
ask.
he
400+ hours of break in time. =
If
something does not sound decisively better than the old component
games on you cos your ears are getting used to =
being
pierced. Anyway, the preamp s=
till
sounds lethargic and presents a somewhat colored midrange.
y about
tubes?
s was
very kind and generous to loan me some of his prized collection of tubes.
style=3D'mso-spacerun:yes'> So the MC-2A3 is now equipped with=
RCA
2A3s, an uber expensive Philips pellE>Miniwatt
EL34 as regulator series pass (yes!), my trusty RCA 6SN7GT smoke as compara=
tor,
and mega-expensive pinched waist 6922s as followers and constant current so=
urce
(yes!).
ng
aged Sovtek (don’t laugh, they ARE compet=
ent),
Ken Rad, RCA-smoke,
w:st=3D"on">Sylvania
nd CBS
6SN7GT and CBS 5692.
d, and
I won’t go into the details.
But what I found was that with certain 6SN7s the preamp sounds
WEIRD. Also with some known g=
ood,
known quiet 6SN7s the preamp started to hum as if grounding is a problem.
style=3D'mso-spacerun:yes'> The midrange gets additional harmo=
nic coloration
which I know from using the tubes in other applications is definitely NOT a
characteristic of that tube. =
So
onto the bench it goes and we checked for gremlins.
o:p>
tube
and found there is SERIOUS oscillation when RCA smoke-glass 6SN7 mates with=
my
(couldn’t resist so paid the big bucks) pinch-waist 6922.
eal
that Ming Da uses a 220Ohm grid stop resistor b=
etween
the 2 stages, that’s about 1/10th of what we’d expec=
t in
a circuit like this. Out goes=
the
stock 220Ohm and in comes a 2kOhm grid stop.
ere
but the strange noise is gone and the voltage measures normal.
ate
the tube combos without this tube-capacitance-matching problem.
utput
caps?
s.
for a few weeks the preamp still shows this tonal imbalance so I swapped in=
2
2uF Dynamicaps.>
Initially it sounded more buttoned down but the air and decay went
away. No my cup of tea.
, a
blessing in disguise
“puff puff putt putt=
”
sound and it was traced to a faulty Jensen cap in the boot-strap-follower u=
pper
tube. Out goes that cap and t=
he
preamp actually sounds cleaner. an>
then an
Obligato 0.1uF with no success.
cap (actually it’s 2, one per 6922 on each side) is in place the
coloration comes back.
“Valve Amplifiers, Third Edition by Morgan Jones”
cathode follower is for distortion cancellation but even Morgan Jones comme=
nted
that it “invites further problems” and “requires a separa=
te
heater supply” which the MC-2A3 certainly does not have.
ith NFB
preamps
with NFB gives a punchier sound.&nb=
sp;
So I proceeded to investigate whether a small amount of NFB, impleme=
nted
using a cap and a resistor looping either at the output cap or before the
output cap might serve to tighten up bass.
10k
steps and various capacitor sizes without success.
gives no effect, and once the value is small enough (50k) to tighten the ba=
ss
it makes the preamp sound nervous and constricted.
schematic
king
at the schematic. The unit th=
at I
got as well as the units depicted in THIS picture
/mc-2a3.htm
istor per
the schematic on the B+ rail separating the 6SN7 stage and the 6922 follower
stage. The way my MC-2A3 was =
wired
the resulting B+ to the 6SN7 be the same as the 6922s top triode.
ign by
Bruce Rozenblit”, this kind of an arrange=
ment
could even result in coupling between the 2 anodes.
icted
here
/dentro.jpg
.
coming into the
separates the regulated B+ from the rail going to the tubes.
ut to
about 202V or so. No problems
here. BUT if you take a closer
look, this means the 100uF filtering capacitor (now a =
blackgate)
at the output of the regulator has to go through the 300Ohm before getting =
to
the plates of 2 6922 and 2 6SN7.
With this arrangement, the only bypass cap directly serving t=
hose
tubes would be one single 100uF lytic
).
separating the output of the regulator and the 6922 anodes.
922
anode rail from the 6SN7 anode rail.
This means (and it agrees with the schematic) that the 6922 sees 220=
V B+
and the 6SN7 sees 190V. This =
prevents
anode coupling between the gain stage and the cathode follower.
ss to
100uF of bypass and the 6922s get their share of a full 100uF of bypass.
style=3D'mso-spacerun:yes'> This all makes a lot more sense.
style=3D'mso-spacerun:yes'> It’s time to follow the sche=
matic
and start making corrections.
to
the schematic, the midrange coloration (an occasional overtone noticeable in
complex passages) is completely gone.
ng the
lytic caps.&nb=
sp;
Sure I’d like to use film caps for bypass but how do I fit a t=
otal
of 200uF or more into that chassis?
Also since Blackgates are known for thei=
r deep
bass will I be going the wrong direction by attempting to replace the 2
class=3DSpellE>blackgates with film caps?
om the
MC-2A3 now but the problem is with the quality of bass I am getting.
lam,
and a clear after-decay, not the “farty&#=
8221;
bass I am getting.
us and
snub them Zeners!
caps
into the chassis, I notice that there are 2 (well technically 3) places whe=
re
Zeners are known for their diode noises =
but
they are a necessary evil for voltage reference to the constant current sou=
rce
and the main B+ voltage regulator. <=
/span>
oss
the zener.&nbs=
p;
The zener used in the 6922 constant curr=
ent
source for each channel has 2 stage snubbing.
are used. I duct-taped it wit=
h a
cascade of film caps so as to ensure no high frequency >zener
noise gets into the CCS. Also=
I
noticed the zener snubber<=
/span>
in my unit is wired across 1 diode only and not both as depicted in the
schematic. I ADDED another ca=
p just
to make very sure there is no zener noise getti=
ng
into the EL34 series pass. The
result was not dramatic but noticeable.&nb=
sp;
The noise floor was lowered and I can hear more details.
bypass
class=3DSpellE>Solen 33uF caps, I used them in parallel for
post-regulation B+ on the 6922 rail before hitting getting to the 6SN7s whi=
ch
are still bypassed by a Blackgate.
owered
so I did not expect any drop in bass response.
improvement that is not subtle. an>The
lethargic sound is gone. Dyna=
mic
contrast is outright scary with absolutely no sacrifice in smoothness or
coherence. This is a much big=
ger
improvement than swapping in Mundorfs and
ter
stage may not improve sound
or and
put back the Blackgate 100uF lE>lytic
to ADD another C-R filtering stage before the 6922 rail?
d end
up with only 66uF servicing the 6922 (2 Solens
instead of 3).
p;
A slight but noticeable incoherence and
of
what I’d expect from an additional C-R filter.
nded
up taking out the 300 Ohm resistor which is not in the schematic anyway.
erent
EL34s as regulators. A good
regulator should dissipate incoming power and provide smooth constant volta=
ge
to the B+. The EL34 is not
“playing music” so why is the preamp sensitive to that tube.
style=3D'mso-spacerun:yes'> Also why did a Groove Tube E34LS h=
igh
current EL34 which is supposed to have great heat dissipation sound bad as a
regulator in my preamp?
d high
voltage going into the regulator and I came across this posting
ttp://user.tninet.se/~vhw129w/mt_audio_design/tweakpage_dac.htm
0.47uF Auricap) right between the EL34&#=
8217;s
plate and ground. In theory t=
his
cap should serve as a fast reacting capacitor filtering out any high freque=
ncy
noise or small ripple going into the regulator.
30;.
span
class=3DGramE>Another MAJOR improvement.
l,
air, decay, limitless dynamic headroom and soundstage stability.
pre-regulation is THAT important.
(Are those “BC lytic caps”
fake). Th=
e preamp
us now uncolored and coherent.
2 blue
300uF 400V lytic caps with Nichicon.
least
for now.
point-to-point wiring gave me much confidence in trying certain things.
o the
authors of these 2 books
dio
Design by Bruce Rozenblit
tion by
Morgan Jones
