wierd api mic pre output response

[pablobolche]

I am finishing a 6 preamp rack with self-etched pcbs except the opamps.. I asked a local tansformer guy to copy a output transformer I had some time ago
I am testing with a pink noise sound interface output with 150 ohm resistor across...
the unbalanced output from the opamp is pretty much flat and after these output transformers I get what looks like a hpf staring around 250hz...
Please look at the attached picture (gain difference is on purpose for the picture)...
I have no real apiish trafos and/or api micpres to compare to but it seems wrong to me.
should I get new trafos? or is this normal??
please advise...Thank you

 

[Bo Deadly]

Where is the 150 resistor? The input of the pre or the output? I would feed the output of the audio interface into a 3k-10k / 150 ohm resistor divider so that the pre sees 150 ohms. The attenuation also allows you to use a high level out but feed the pre with a low signal which is then amplified back to line level for feeding back into the interface input. So the source impedance seen by the mic pre should be 150 ohms and load on the output should be no less than 600 ohms. Unless you have the secondaries wired in parallel for a 2:1 step down. Then you can load it with 150R.

Post the schematic. Is there anything between the output of the amp and the transformer? Like a cap? Is there any DC across the transformer primary? Post the schematic.

 

[pablobolche]

hi Bo thanks for your feedback...

--150ohm resistor is between the interface and the mic pre..
--no DC at the transformer primary..
--between opamp and xfo there is a 47R resistor and then a parallel 470uf25v panasonic FC Electrolytic cap and .1uf ceramic cap..

when I disconnect the transformer and measure the frecuency response after the resistor and caps it is flat.
connecting connecting the trafo and measuring the trafo output I get a HPF starting around 250hz

I have attached the schematic

 

[Bo Deadly]

Well then it does look like the transformer is loosing some lows. But measuring this stuff is not trivial. It's easy to make a mistake in the test setup. If you think the transformer is really not good, then I would remove it and test it in isolation to be sure. Drive it with a low impedance (a function generator with 50 ohm output is low enough) and then load it with 10K. Adjust the level to 0dBu and look at a level in your DAW. Use a lower level if your interface cannot take a signal that high. Look at the level at 1 KHz, 500 Hz, 250, 125 ... etc and then zero in on the -3dB point exactly.

If the local shop is good at their job, they would test the unit and provide data and preferably plots of spectum and THD at different levels. If it's just some guy with a winding machine, then I would say that's on you.

 

[pablobolche]

great info Bo... I will test these xfos as you say and I will start to check all trafos before I put them in a build.
thanks very much