API 325->312

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thedug

Well-known member
Joined
Jun 13, 2004
Messages
235
I wired up some api 325 line cards as mic preamps.

Anyway, I racked these up awhile ago and was just checking it out on my scope. I noticed that pin 3 on the output is always 1/2 the voltage on pin 2. Is this right?

I basically wired the 2 top winding in parallel. Since this is an inverting amplifier, pin 2 is the botton of the winding and pin 3 is the top of the winding. So.. yellow & green tied (11&12) and pin 2 = 13 (blu) and pin 3 = 10 (orange).

This may just be indicative of my lack of knowledge of how to use a scope, but if I probe pin 2 then pin3 I see about a 1/2x drop in voltage.

I used these diagrams when I originally wired them up.

API 325
http://www.danalexanderaudio.com/ApiInfo/api325schem.jpg

API 312 (I used this as a reference for turning the 325 to a 312)
http://www.danalexanderaudio.com/ApiInfo/api312schem.jpg

Doug
 
sounds to me like your shorting when you probe between the 2 connections. which would be a drop of about 6 DB or a halfing of the voltage. It sounds like you wired what should be the ground to pin 3 by mistake. check your pinouts.
 
There is no way I am shorting with the probe because I used clips on the xlr connector.

I just looked at those diagrams again and I can't see how I used the wrong pin out on the transformer.

BTW: If I did short to ground, wouldn't the scope read 0 volts?

Doug
 
Doug,

I´m confused when you talk about pin 2 and 3, this is 0v/com. and -V on the 312 and 325 card ???

The transformer outputs on 312 are pin 4 to 8 and for 325 pin 10 to 15.

Normal the 325 card is not use to be a inverted input amp, (it can be configurated as a inverting/virtual earth amp) but there are a different between the internal transformer connection comparing the 312 and 325 card, because they have reversed all "HI" transformer connections brown, orange, green, violet and use them for "LO" on the 312 card.

--Bo
 
I don't think I ever mentioned "pin 1".

I am not sure what you are confused about but I will try to explain what I did....

------------


I took a 325 and converted it to a 312 using the to diagrams I linked.

I wired the output transformer with two of the secondary winding in series. I left the other secondary winding unconnected. To do this, I tied yellow & green together (11 & 12 on the card). Then used blue wire (card 13) for hi (xlr pin2 ) and orange (card 10) for lo.

When I place my scope probe on xlr pin3 I get x volts. When I place the probe on xlr pin2 I got 2x volts.

Doug
 
Your output transformer wiring is correct. This yeilds a 1:2 ratio output and 6db gain.

Leaving the extra windings unhooked is right.

I might desolder the transformers secondary leads from the card and hook them up directly to the XLR. Eliminate the PCB and edge connector from the equation, at least as a test if not permanently.

You also might try to check the AC voltage coming out of pin 2 and 3 with a DMM instead of using a scope.

Helsing

AKA Jon Erickson
 
I got the same results with a VOM. So I decided to investigate further.

All measurements are referenced to ground.

Transfomer primary (red wire) : 3.069V

Card pin 10 : 1.616V
Card pin 11 : 1.435V
Card pin 12 : 1.435V
Card pin 13 : 4.51V
Card pin 14 : 16mV
Card pin 15 : 3.068V

It appears that the transformer wiring is foo bar.

Doug
 
Ok there is something really wacky going on.

I removed the transformer wires from the board and measure the transformer wires relative to themselves, and everything is fine.

3V across each winding.
Then when I put the 2 windings in series I get 6V.

But.. Then if I measure the top of the series winding to ground I get 4 V
and if I measure the bottom of teh series winding to ground I get 2V.

Is this a dc offset or something?

Doug
 
Do you have a cap between the output pin and the transformer? Positive on the opamp side. Try it and see what happens if not.



Helsing
 
No the only cap actually on the output is paralleling the feedback resister.

I basically used this circuit..
http://www.danalexanderaudio.com/ApiInfo/api312schem.jpg
api312schem.jpg
 
Since this is an inverting amplifier

As a preamp this amp is not an inverting amp. Pin 10 should be XLR pin 2 and pin 13 should be XLR pin 3. Should also be noted that pin 11 & 12 do not go to pin 1 or ground. The output voltage on each pin when the output transformer windings are floating will be different due to the internal capacitance between the windings being uneven which gives the floating output an uneven reference to the ground on the primary side of the transformer. This is normal.

But if you want to check that there is no real fault. First check with an ohm meter that the output winding are not connected directly to anything with an ohm meter to prove they are floating. If they measure open circuit to ground that is right. Now add a 1k resistor between pin 2 & pin 1 and another 1k resistor between pin 3 and pin 1. Or connect pin 12& 13 to pin 1 of XLR just for this test. Then measure the voltage at pin 2 & 3 which should know be even now with a even resistive load to ground.


Joe

www.jlmaudio.com
 
Please excuse my inverting amplifier comment. This is not my problem.

It was late and I got confused. I rewired this like the 312, where the output of the ompamp feeds the negative side of the primary so that the negative side of the secondary then feeds pin 2.

So therefore it is correct that card pin 10 feeds xlr pin 2.

This is not a standard 325 anymore. I have rewired it. It should be identical to the 312 schem. I posted.
 
The inverting bit was only a minor point did you read the rest of the reply as what you are measuring is fine. You can just not measure a floating output with a meter to ground without some resistive elements holding the floating output symetrically around ground.

[/quote]The output voltage on each pin when the output transformer windings are floating will be different due to the internal capacitance between the windings being uneven which gives the floating output an uneven reference to the ground on the primary side of the transformer. This is normal.[/quote]

Joe

www.jlmaudio.com
 
Totally..

Thanks!

Sorry I didn't respond that.

So, I am guessing that that corret way to measure a blanced output with a scope would be across xlr 2 & 3?

Doug
 
Doug,

I don't think I ever mentioned "pin 1".

I am not sure what you are confused about but I will try to explain what I did....

Sorry, I mean pin 3, when you wrote pin 2 and 3 in your first post.

Now when you have EDIT your post and add the card connector numbers, I understand you mean the XLR connector when you only wrote pin 2 and 3.

As Joe already say, you can not measure a floating transformer output from ground to one of the winding sides, only between the windings.
But on a electronic/active balanced output you can measure from ground to the +hi side or -lo side, with a loss of -6dB compare to measure between the hi and lo.

Nice to here that you find the problem.

--Bo
 

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