[martthie_08]

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

[mitsos]

Rewiring the output transformer as you did should normally give you 6dB less gain.  Not sure what's going on there. Maybe double check?

[Vac11]

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

[mitsos]

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

[Vac11]

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

[Rocinante]

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.

[MountCyanide]

Damn.
How about checking the LM337 on your psu?

[Rocinante]

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. 

[MountCyanide]

How about the diode and associated caps where the -v comes in? D2, C1, C3.

[Rocinante]

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

[Rocinante]

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.

[Whoops]

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?

[GasJunkie_Jabz]

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?

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?

[Markus R. Els]

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!

[TillM]

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.

[peterc]

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

[volker]

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.

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