Question regarding Output Transformers and connectivity issues

[figuringstuffout]

Hi, I'm new here and hope i can get a little help with an issue I am having.

So i have recently built my first tube preamp, complete with jensen input and output trafos and while the thing sounds freaking amazing, I am having a slight issue with connectivity.

You see, when I connect it to any mixer (just plug the xlr into the input for a channel), firestudio, or any converter with a built in preamp, my tube preamp has tons of volume (more than my UA 610 has), and sounds amazing. However, when I connect it to my Apogee Rosetta 800, my tube preamp sounds thin and has very little gain before it starts to distort like crazy. The 610 has no problems here interestingly enough so i don;t think it is a problem with the rosetta......
My first thought was that is is an impedance mismatch but I don't see how that is possible....if anything it should be a mismatch going into a mixer since my Jensen Trafo runs an output of 600 ohms.
JT-10K61-1M

Note - i have a cathode follower 12au7 with a 25uf cap right before the output trafo

Potential Solution?
just thought of something, let me know if I am simply fooling myself.
The Jensen Trafo is 10k:600 rating in ohms. Thus, when taking the signal from before the plate resistor (somewhere in the 30K+ ohm rating) I would be properly seeing the 10k ohms in the Jensen and the output would be properly in the 600 ohm rating. However, since I am using a cathode follower right before the trafo, then I'm actually driving the Jensen at way under 10k ohms and thus throwing off the 600 ohm output, probably making it closer to the 150 ohm ish rating of a mic input. Thus when I run the preamp into a mixer preamp, or whatever, it sounds perfect with tons of gain because it is matching impedance, however the Rosetta gets pissy with it because it wants to see 600 ohms and is instead seeing way less.

Thanks for any help that might be offered.....

 

[lassoharp]

Try terminating the secondary of the OT with a 620-680r resistor and see if changes anything.  To get a 10Kish load on the primary the sec usually needs to see it's rated impedance.

I wouldn't guess the input Z on the Rosetta to be drastically different than other typical bridging inputs.

Double check cable or wiring connection issue.  Try all Rosetta inputs to check for consistency of results.

 

[figuringstuffout]

hey, thanks for the suggestion....i am in the process of trying it.

I did notice though that with (or without) the resistor, the impedance of the OT secondary is reading 47ohms.  Is this normal?

 

[figuringstuffout]

yeah, didn't fix the problem.

 

[PRR]

> with (or without) the resistor, the impedance of the OT secondary is reading 47ohms.  Is this normal?

You are measuring DC resistance.

Not the same as audio impedance.

 

[PRR]

> sounds thin and has very little gain

Usually means one side of the transformer is not connected to the input. You only get capacitive leakage.

With the wide variety of inputs used today, it is very possible this happens on some gear and not others.

 

[figuringstuffout]

> with (or without) the resistor, the impedance of the OT secondary is reading 47ohms.  Is this normal?

You are measuring DC resistance.

Not the same as audio impedance.

How do I then measure audio impedence?  I realize this is probably a noob question....

About the capacitive leakage suggestion....would that then explain why the preamp would sound full with some mixers and not others?  Currently the preamp sounds great with an Allen and Heath zed16 but terrible with a mixwizard....also great with a small peavey mixer but awful with the Rosetta 800 (all channels).  I'll check the connections again...

 

[figuringstuffout]

all connections are secure and correct.  I'm losing my freaking mind...

 

[pucho812]

can you post the schematic you used?

 

[figuringstuffout]

well as it turns out I think i fixed the issue.  I ended up removing the 12au7 cathode follower from the circuit (it was right before the output transformer) and just like that everything was fixed!  No more volume issues and no more distortion, etc.  The only guess I have about the whole thing is that the Jensen wants to see high impedance on the primary and the cathode follower was giving it very low impedance.

anyway, thanks for the help everyone.