api312 sowter input transformer question

Hi!

Im building some api 312 clones using fabio baumans pcb:s. My first attempt at diy, so far im having loads of fun!
Im using sowter in transformer and 4804 out, and melcor opamps.

I finished one channel today and tried it. Everything seems to work fine except for some groundnoise (i havent mounted them in an enclosure yet, so im hoping that will clear up once i rack them). The noise seemed to dissapear almost completely if i touched the in transformer or my star grounding point (i connected all the separate grounds to a screw).

But!
The sound seemed to lack alot of bass and lower mids. Kinda thin, but with good treble and uppermids. Another guy i know of had the same problem with the sowter transformer and solved it by connecting the yellow and gray wires ( he says he probably connected the windings in series, just like the oep alternative for baumans apis are wired).

I hope all of this makes sense to you, as english is not my native language.

Anyway,

is this a good idea, or is there any other way i could solve my problem? Are the colorcodings on the pcb wrong for the sowter, or have i missed something maybe?

thanks!
/Jonas

[quote author="Jonkan"]...Another guy i know of had the same problem with the sowter transformer and solved it by connecting the yellow and gray wires ( he says he probably connected the windings in series, just like the oep alternative for baumans apis are wired)[/quote]Yes, this would connect the primary in series. Here's the data sheet:
http://www.groupdiy.com/specs/pdfs/Sowter-9820-Data.pdf
Not sure why it'd give it more low-end. What kind of mic are you using? If you are using a dynamic and don't have the transformer properly terminated, it'll sound like that.

What's the deal with the 1+1:3.54 ratio? Should be able to get over 1:8. Typo maybe?

Conecting yel and gry will give you roughly four times the inductance, and therfore four times the bass response at the very low end.

Connecting pri's in paralell gives 2 times 3.5 = about 1:7 voltage gain.

pardon my ignorance (im just starting to learn all of this),
but what would be the "right" way to connect the sowter in this case?

In series or paralell? There almost seems to be too much bass if i connect the primary in series (connecting yellow and gray), and too little if i connect it like the pcb is made (paralell?)

/Jonas

[quote author="CJ"]Connecting pri's in paralell gives 2 times 3.5 = about 1:7 voltage gain.[/quote]Oh... 1 || 1 = .5.

[quote author="Jonkan"]...but what would be the "right" way to connect the sowter in this case?[/quote]Parallel (1:7). Check the API 312 schematic (in the META, I hope...) Do you have it configured like that circuit?

right now i have the yellow and gray wires connected, so that would be in series. Ill try and change it to paralell and see what happens.

/Jonas

So...I tried connecting the yellow and blue wires, and the gray and white. And it works fine, still alot of bass content, but unless my ears are fooling me, the sound is more "balanced" now. Maybe thats the way the api is supposed to sound (i have never heard one before).
The transformer seems to be connected that way in the original scematics if im not mistaken. So, would that be the proper way to connect the sowter by any chance? :green:

Also, another question:

which voltage would be the best to run the melcor opamp at? Right now im at +/-20V. Would 15v be worse (would i loose alot of headroom?). I have this crazy idea to use the same psu for 2 channels of green pre also if it can be done. If not, ill just build another one. Im thinking of putting 2ch of api, and two ch greenpre in the same box if it will fit (im too poor to afford two rack enclosures, and two power transformers atm, so i figured, why not?).

If i decide to build another psu, i can still use the same power transformer for it, right? (its a quite big torodoid, ill see if i can find the datas on it )

another question (sorry too many questions maybe? :green: ):

Looking at the original schematic I notice that theres two caps that are called C6,C7 that i cant find in the schematics for Baumans pcb, why is that?
Is it because modern transformers dont need it, or do i need to solder some 1000pf caps connected to the primaries this way on my api:s?
/Jonas

[quote author="Jonkan"]So...I tried connecting the yellow and blue wires, and the gray and white. And it works fine, still alot of bass content, but unless my ears are fooling me, the sound is more "balanced" now. [/quote]It's wired correctly - enjoy!

which voltage would be the best to run the melcor opamp at? Right now im at +/-20V.

Click to expand...

If it works at +/-20, it'll work at +/-15. You will loose headroom but if your output transformer is wired for 1:2 (like it should be) you don't need it. Anyway, the 2520s run at +/-15V, IIRC.

If i decide to build another psu, i can still use the same power transformer for it, right?

Click to expand...

Right - same transformer, but different bridge rectifiers. But, like I said above - you don't need another supply.

Looking at the original schematic...

Click to expand...

What schematic?

Thanks for answering my questions!

I found the schematic over at Bo hansen's site:
http://web.telia.com/~u31617586/

Another quick question:
Comparing the data for the sowter 9820 http://www.groupdiy.com/specs/pdfs/Sowter-9820-Data.pdf
vs the api 2622 data I found that the "pinout" isnt the same. The primaries are labeled different, mirrored really (2 on 2622 is 1 on the sowter, 6 is 5, 1 is 2, and 5 is 6) but the secondaries have the same order 3 on "top", 4 at the "bottom".

Does this mean that i might screw up the polarity of the signal or doesnt it matter? Also, which one of the secondary leads should go to ground on the sowter, 3(pink) or 4(brown)?

the output transformer (4804) ought to be correctly wired if baumans colorcodings on the pcb is correct. But to be sure, what is the easiest way to check this?

/Jonas

I still don't see C6 and C7, but if it isn't on the board, don't worry about it.

The polarity would only matter if you were using 2 or more at a time to record the same instrument. Usually transformer diagrams have a "dot" indicating the polarity, but this one doesn't. In this case I'd just go by pin numbers

For output transformer wiring, go to the Bo hansen link you posted and look at the 312. Should be like that.

Thanks!
I'll just look at the schematic for baumans pcb then and try to figure it out.
/Jonas

I'm just building my four API channels right now (Using Baumans API PCB's and so forth)!
I'm using Sowter input trafos nad I just can't figure out how to fasten it and how to "draw" the cables from it?
How is the Sowter input trafo fastend to the PCB?
Is the cables drawned thrugh the PCB to the back of the PCB board and soldered on the back of the PCB or how is it done?

Many thanx for fast answeres :razz: !!!!

alo
to fasten sowter to the board,use the empty hole near the i/t,with a L shaped metal piece.
hope i explained myself right.
best regards
pedro

[quote author="louder"]alo
to fasten sowter to the board,use the empty hole near the i/t,with a L shaped metal piece.
hope i explained myself right.
best regards
pedro[/quote]

Thanks!

I glued the Sowter to the PCB och drew the cords below the PCB instead....seems to work good..maybe eaven better... :wink: :razz: !

from Jonkan:

which voltage would be the best to run the melcor opamp at? Right now im at +/-20V. Would 15v be worse (would i loose alot of headroom?). I have this crazy idea to use the same psu for 2 channels of green pre also if it can be done.

Click to expand...

As already suggested by I-forgot-who in one of the other threads you
could also choose a supply that is somwhere in between: the Green will
also run fine at a higher supply voltage than +/-15V.

I'm also combining 'Green' & 'API' on the same PSU and will go for
something like +/-17.5 V to keep the TL07x & LM339 in the Green happy.

Bye,

Peter

thanks clintrubber!

I suppose then i dont have to build a separate psu for the green, great!

Now i only have to fit everything in one tiny box..lol

Is it ok to stack two greens on each other, to get them a bit farther away from the psu transformer?

atm, im running the api at +-20v, so i guess +-17.5 wouldnt be much different.

/Jonas

I suppose then i dont have to build a separate psu for the green, great!

Now i only have to fit everything in one tiny box..lol

Click to expand...

You're going for four channels in total, right ? Since I'm putting the gain-switches for the Greens next to the PCBs it'll only be a single API added here (1HE box).

Is it ok to stack two greens on each other, to get them a bit farther away from the psu transformer?

Click to expand...

If it doesn't make shorts of course then all will be fine. I think using a toroid will help for sure, you're using this kind of power-TX ?

atm, im running the api at +-20v, so i guess +-17.5 wouldnt be much different.

Click to expand...

I don't know anything about an eventual sweet spot w.r.t. the supply voltage, maybe others.

Im using a big fat toroid, so i should be ok i guess...

I just have to order those damn parts for the green soon... atm my api preamps are lying in the box "naked" with wires everywhere. Not a pretty sight, but they work good.

Then im off to neve 1272 land, if i can just get smarter and figure out which parts i need to order :green:

/Jonas