Mitsos API 312 / 990 Build Thread

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ok, so I did some rewiring on two channels:

1. took the spare 3rd secondary winding and put it in series with the primary winding (unsoldered red, joined it with violet and put gray back to 0V). Result: no level change

2. used the spare 3rd secondary winding instead of the first and second winding in series. Result: no level change

so it seems that the circuit doesn't really care whether it is a 1:2 or 1:1 output? I will never fully understand iron
 
Rewiring the output transformer as you did should normally give you 6dB less gain.  Not sure what's going on there. Maybe double check?

 
Hi Mitsos,

thanks for this amazing project...

I have one question... Can I use MOD1 opamp with this preamp?

Some info about MOD1 opamp:
- http://sureshotstudio.blogspot.com.tr/2011/09/urei-mod-1-opamp.html
- cca 27 dB gain (27.634551dB)
- "Also there's probably some DC offset on the output so using a DC blocking capacitor is very much needed" - has your preamp PCB this bypass capacitator on the opamp´s output rails? I see  0,1 uF C4 and C3... But I´m not sure...

Best

Milan
 
Vac11 said:
Hi Mitsos,

thanks for this amazing project...

I have one question... Can I use MOD1 opamp with this preamp?

Some info about MOD1 opamp:
- http://sureshotstudio.blogspot.com.tr/2011/09/urei-mod-1-opamp.html
- cca 27 dB gain (27.634551dB)
- "Also there's probably some DC offset on the output so using a DC blocking capacitor is very much needed" - has your preamp PCB this bypass capacitator on the opamp´s output rails? I see  0,1 uF C4 and C3... But I´m not sure...

Best

Milan
Most any opamp should work just fine. The capacitor you are looking for is not PSU decoupling cap (C1-C4), but the AC coupling cap at the output of the opamp, C8 and C7 if you want to bypass it with a film cap. 
C8 is generally about 470uf.
Put a jumper across R21, and if you use a 2503 type output transformer, put a 47R in R19, it eliminates the slight low end bump with that transformer. Not needed if using a different output transformer, and definitely dont stuff R19 if using a nickel output transformer.

Download the BOM if you havent already, it has description of most parts and their functions.

Good luck
 
Thanks for your quick answer...

I just collect parts... I have 4 your PCB´s, 2503-L (Litz wire) output transformer, EA2622 input transformer and APP PSU2 on the way... I want to try MOD1 opamp, because it is simple and inexpensive.

I will let you know.

Milan
 
Man just when one starts to think they got a handle on s***.
Well here goes;
A good while ago I needed to borrow a DOA so opened up my four channel 3124 using mitsos boards and I saw the wiring looked a little loose in places and decided to make my harness tighter.  I spent a little bit redoing the molex and such, finished and turned it on to make sure it passed audio.
To my frustration it didn't.  None of the four channels. I grabbed some DOA's from an eq and still all 4 channels were passing nothing. Getting frustrared even more I was about to bust out my scope when I saw the  sl-2520 red dots I had in 3 channels beginning to smoke.  After another evaluation I realized to my horror that I had accidentally switched the 48v and 15v wires coming from the psu.
6 DOA's dead. 3 of which were not cheap.
So ive redone the wiring correctly but now im reading -29v at the -v of the doa pin. The +v side is okay at 15v. What can I check?  What other damage have I done?  What caps should I replace?
Yeah that was dumb.
Sigh.
 
Its putting out +15 and -15 and +48 just fine so im pretty certain the lm337 would be fine then. .  Its the five fish 2448 btw.  Throughout the harness at all points its fine. 
 
Yeah i figured theyd be suspect so I'm gonna replace those. I'll let you know.
Any chance I damaged the xfmrs?
Oh and thanks cyanide
 
So an update;
There was a dude on the api thread who pretty much did the same thing which helped a little. I changed all the caps except the bipolar and the film not in the - path and d2 which seems to have worked. Ive tested each card seperately and it looks good.
On one card i didnt change d2 (1n4004) and when connecting the harness it effected the others (reverse voltage???)
I am surprised that the 1n4004 would be effected along with the film caps which were rated at 63 and 100 which just shows i got more to learn.
Its been humbling nevertheless.
 
Hello,
I've read this complete thread although I still have some doubts and need your help.

If I want to use a 990 opamp I see that I need to use a lower turns ration input transformer and  a Bifilar and Nickel core output transformer.

INPUT
Does the input transformer needs to have a nickel core also?
Would the  CM-75101APC be a choice for the 990 if setup for 1:4 instead of the normal 1:10 for the 2520? Or it needs to be a 1:2 ratio transformer?
couldn't find info on the core material for this transformer

OUTPUT
from Cinemag theres 4 different Bifilar output transformers with a Nickel core,
CMOB-1H
CMOB-2H
CMOB-3H
CMOB-4H
Which one should be the best to match the 990?


 
Im in the same boat as Whoops:

Input: 75101 wired for 600 ohm

opamp: 990c

Output Iron: CAPI 2503-L wired to 1:1

What are some of the challenges that can occur and how can these be used together?




Whoops said:
Hello,
I've read this complete thread although I still have some doubts and need your help.

If I want to use a 990 opamp I see that I need to use a lower turns ration input transformer and  a Bifilar and Nickel core output transformer.

INPUT
Does the input transformer needs to have a nickel core also?
Would the  CM-75101APC be a choice for the 990 if setup for 1:4 instead of the normal 1:10 for the 2520? Or it needs to be a 1:2 ratio transformer?
couldn't find info on the core material for this transformer

OUTPUT
from Cinemag theres 4 different Bifilar output transformers with a Nickel core,
CMOB-1H
CMOB-2H
CMOB-3H
CMOB-4H
Which one should be the best to match the 990?
 
Hi everyone!

I know I'm resurrecting  an old (but awesome) thread here - would anyone be willing to hook me up with the Mitsos 312/M1 PCB etch files, please? I'm hoping to build 4 channels based closely on the M-1, or rather M-2, in my case. I'd be deeply grateful. Thanks to everyone who has already contributed such a wealth of knowledge!
 
Coul be really hard to get a bunch of them.
But you can also try Whistlerock ML12, these PCBs you get from Peterson and it’s also a great 312 build.
 
Hi all

Back from the dead.......

Does anyone have the etch files for these PCBs? Our quarantine  has been extended by another 2 weeks and I need a few projects to keep me busy.

Many thanks
Peter
 
I don't think there ever were any files for etching yourself. The pcb is a two-sided affair, you can find the documents on the first page of the thread.
 
Wouldn't anyone happen to have the original schematics and component placement files for the Mitsos boards? I have requested google drive access a few over the past month, but it never seems to get approved.
 
I have built the 990 / Hardy inspired version. Not sure if you have enough info to build the 312 but should be enough to get you started
 

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Wouldn't anyone happen to have the original schematics and component placement files for the Mitsos boards? I have requested google drive access a few over the past month, but it never seems to get approved.

I just tried and I can access all the files
 

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  • Mitsos API 312 docs.zip
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