|
Amp/Preamp Asylum Looking for a new Amp or Preamp? If you're after tubes, post over here. |
|
|
|
Amp/Preamp Asylum Looking for a new Amp or Preamp? If you're after tubes, post over here. |
|
In Reply to: RE: Ming Da MC-2A3 preamp schematic posted by jwu - snoopy on July 26, 2008 at 11:54:54:
Eversince I wrote this I have been getting one email every few weeks asking me for details on how to do the mods. And recently I am seeing that Ming Da has decided to introduce blue-tinted EL34s and balloon CV181a as stock tubes in replacement of 6SN7EH. Gimmicky I think, but I dont think they ever addressed the problems, some more serious than others, in the design of the MC-2A3. I also see some units using black colored lytics instead of the blue "B-C" lytics after the choke... all I can say is "sigh"...
As I mentioned, you will need to do all these to get the sound fixed:
1. get 2 of the largest value Solen FastCap you can fit into the Chassis. 33uF 450V+ would be my choice
I was able to fit 4 33uF into the box, 2 in parallel for each section. That size is governed by the diameter of the cap whether you can fit it into the chassis. Those 150uF small blue caps sound bad. With this mod you essentially fix the muddy bass and somewhat veiled highs which is an obvious sonic signature.
2. get 2 x 3.3uF or 4.7uF Mundorf MSilverOil or maybe dynamicap or VCap for the output cap. (too expensive..... MSUpreme or Jensen copper is more reasonable)
3. get a pair of 1MOhm resistors and replace the 0.22uF + 1MOhm resistor combo next to each side-6922 tube. To rephrase: Keep the 1MOhm resistor but get rid of the 0.22uF cap.
4. take away (bypass) the 333Ohm green resistor and put 5.1k 3W resistor where the diagram shows 6922 B+ rail goes to 6SN7 side. The B+ going to the 6922 rail should be 220V.
Another way to put it: bypass the 300 Ohm resistor so that the regulator output
goes direct to the 6922 section. But disconnect the 6922 sections rail to the 6SN7 section and put a 5kOhm resistor as the connection. This will lower B+ of the 6SN7 section to approx. 200V.
5. Check filament voltage... the Ming Da is known to KILL tubes due to over voltage on the filament.
It pumps 7.3Vs into each filament instead of 6.3. Use an RC stage 0.25Ohm + 10000uF R-C filter in addition to what is there. Filament chokes need not apply due to size and filament regulators need not apply due to relatively small drop needed (1-2Volts).
6. Increase grid stop resistor to middle 6922 (the follower) from 200Ohm to 2k Ohm. This is necessary to avoid oscillation when you match NOS 6SN7 tubes with premium 6922s. Based on my experience anything other than the stock 6922EH will result in oscillation.
7. apply voltage balancing resistors to the pre regulation lytics (the 400uF and 330uF so called B-C electrolytics). This is needed to balance the voltage as you will find that each pair is wired in SERIES
and one of each pair will have a higher potential than the other, that's not good. Furthermore these resistors act as bleeders for safety in the event the regulator series pass tube (the EL34 goes bad).
So in order to accomplish the above these are the parts.
5k, 5-10watt resistor to fix B+ rail
2k, 1/4 to 1/2 watt resistor x 2 to fix oscillation
2 x 2.5Ohm 5W - 10W resistor x to fix filament
4 x 470KOhm 1/2 watt for bleeder (CAREFUL when working on those 2 pairs of lytics!!! Dont get electrocuted!)
4 Solen 33uF 400V or 4 x VALAB 47uF 500V from thetubestore or ebay to fix bypass (2 for 6922 section 2 for 6SN7 section, throw away those small blue lytics)
2 x 25V 10000uF electrolytic caps to fix filament supply.
2 x 3.3 - 4.7 uF Dynamicap or Mundorf M Supreme or something like that.
Hot glue to hold the output caps (larger than the stock fake Solen SCRs)
hot glue to hold the new 10000 uF lytics for the filament supplies.
Alternatively use tie wrap fastener blocks form hardware store or single terminal solder strips and use the screw / nut from the transformer to fasten the strip to chassis.
That's all I was able to do on my Ming Da before I sold it. My intention is to share my experience with this somewhat interesting preamp with fellow audiophiles. I assume no responsibility whatsoever for damage/injury that a DIY-er may inflict upon his equipment or himself as a result of applying these mods or any variation thereof.
Happy listening.
This post is made possible by the generous support of people like you and our sponsors:
Follow Ups
- RE: Ming Da MC-2A3 preamp schematic - phi70 13:46:44 05/18/09 (0)
- RE: Ming Da MC-2A3 preamp schematic - Frihed89 23:50:40 08/18/09 (0)
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2024, Audio Asylum, all rights reserved ]
