API 325 & Cinemag CMMI-7C...

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soundguy

Well-known member
Joined
Jun 4, 2004
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Location
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I am doing something massively wrong and cant figuure heads to tails.

Ive got an api 325 line amp which is working fine.

Ive got a cinemag cmmi-7c which appears to be working fine. I can put a 1K, .1v signal between the brown/orange winding and get about .7v out from the yellow/green winding. That seems about right.

When I hook the transformer up the the card, however, I get a really loud hum, and no signal. This is with the case and screen floating. Connecting the screen to pin 1 on the mic input doesnt do anything. Connecting it to power ground makes the hum go away. I still can get no signal into the amplifier through the transformer. Ive got the secondary connected directly to pins 4/5 on a stock 325 amp. with a 600 ohm line level feeding the amp transformerless, it works fine.

This one seems fairly free of variables, I cant see what I could be doing wrong here. Any ideas?

thanks

dave
 
Are you sure about the color coding? You might want to ohm it out. I have a C-mag right here that uses org-yel as a winding and brown-red as a winding.

Sometimes a xfmr will pass signal to a scope even when it is mis wired.
 
Im just going by the color coding that is on the PDF, maybe its wrong. It would be cool if its that simple. Check it out-

http://www.cinemag.biz/mic_input/CMMI-7C.pdf

I can measure the following:

brown/orange 23 ohm
red/brown 12 ohm
red/orange 12 ohm
yellow/green 1.2K
white/black 2.6M

no readings between orange/yellow or green/red, so I guess the pdf is correct.

anyone ever run into this type of thing? Ive tried it on two or three different 325's and get the same thing. What could I be doing wrong here?

dave
 
Try the generator straight into a card with no xfmr.

If your using a gain pot, it has to be in the circuit as it is in series with the signal.
 
the card works fine with a line level plugged straight in stock, no input transformer.

the transformer appears to be working fine with a signal gen on the primary and the scope looking at the secondary.

mic-transformer-325, I get no signal. With the sheilds floating, there is really bad hum, ground the shields and the hum goes away but there is no signal. Ive been plugging a 421 into the transformer.

Is there something about that transformer that the 325 doesnt like? I figured that one would work pretty well.

dave
 
Doh, I just read you tried it straight in on your first post..

Hmmmm, try ohming out pins 4 and 5 of the card.

Try wiring up the transformer with the line source into the primary, and the scope on the pri.

If you are loading it with the card, you will see a drop in signal when you connect the sec to the card, unless you have a pretty stiff audio source.

.
 
Looking at the datasheet it appears that this trafo wants to see closer to 100k of load impedance. Second paragraph of the spec mentions "moderately high impedance secondary..." I would assume that the 12k of the 325 is too low for it to drive. Or am I missing something? This whole audio transformer thing is kinda new to me.

Analog Packrat
 
Possibly. But even with 12K, you should hear something.

I do not have a schematic right here, but there is a cap in the input signal chain, correct?

I am driving a pair of Melcors with C-mag iron, and am not having any problems.
 
Your circuit is different from this, right?

api312schem.jpg
 
How about just trying to drive a variable dummy load and see if it can deal with it. Lower the load and see where it craps out. You could even use the test circuit on the datasheet. Maybe also try the input on just the red/brown, though it's hard to see how you could have good DCR readings and still have a winding issue on the primary side. :?:
 
CJ, cant see that schem, get prompted for a pw. My 325 is identical to this

http://www.waltzingbear.com/Schematics/API/API_325.htm

On one of the cards I tried, I heard a really worbly distorted sound and then the sound died, I freaked out, thought I had killed the 2520, but its cool. On the other I tried, I got nothing at all.

Can I add some resistors between the transfo and the 325 to create a higher load? Would say a 100K resistor in series on the +/- input do the trick?

Im away from the studio for a few days (thank god) but will get to this when I get back next week.

dave
 
cool. its defintely a 325, not a 312.

I'll try the cap. what in theory is that supposed to do?

thanks for the help!

dave
 
Dave,

Don't know much about this stuff, but is it possible that the negative side of the input tranny secondary goes to ground - like the 312?

In other words, secondary + to 5 and secondary - to common.

That would mean that in hi (5) and in lo (4) represent input gain, rather than balanced input.
 
Ron,

I'm pretty sure those are the high and low sides of the balanced inputs, but PRR once posted a long time ago that if you're working on these cards on the bench and not in a chassis then the low side has no return to ground. I wonder if that could be the issue? Dave, try a 10k resistor from the low side of the input to ground.

:thumb:
 
Dave,

It looks like you've already done this with Jensen trannies - your website page refers to the 325 with pins 3 and 4 bridged - which directly grounds the input secondary low:

http://www.glideonfade.com/recording/api325.html
 
I figured grounding the low side would make it work, but Ive always felt that doing that was my original kludge to make it work, not necessarily the right way to wire it. Maybe Im wrong about that?

dave
 
ok, I grounded the low side of the transformer and am getting signal through now ok. Shoulda just went with what works instead of trying to reinvent the wheel with my logic.

The transformer is passing this sorta high end phasey sound, is this transformer "ringing" that people talk about? I know I dont have the secondary properly terminated, so Im assuming so but while Im waiting for the parts to come in, figured I would ask. The high end is kinda shimmery in a not at all good way, just sounds sorta phasey, I could see this being described as "ringy". Is that just the high frequency taking off?

dave
 

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