CAPI/Hairball T-Pad Attenuator at the output of a Neve EZ1290

[corgan4321]

Hello,

I've got a pair of EZ1290s that I stuck CAPI's T-pads onto (+/- of the T-pad in between the +/- or the XLR output and the secondary of the Carnhill). I'm reracking my EZ1290 cards and figured I'd make sure the bias was still good. I discovered that when I turn the T-pad, the current draw of the card shot up from ~7mA to ~13mA. That didn't seem right... Am I misusing this T-pad? I thought it would remain constant impedance across it's full range? Also, if that's not true, why is the preamp drawing more current as I turn the attenuator left? I know the primary of the output transformer has DC across it, but I'm sure how that woudl effect the output?

Thank you!

 

[dmp]

Sounds pretty strange to me. Are you sure the T pad is hooked up on the output side?
I think if you measure the ohms of the tpad it does change as you turn it. I remember doing this and being a bit surprised. Manufacturing error.
But if it's on the output side it shouldn't change the bias of the output transistor.

 

[corgan4321]

Thanks for your reply!
Yes, it’s hooked up to pins 5 & 6 (with the little Zobel circuit still across those pins too of course) of the Carnhill output.
I have the trim and the gain all the way up and I’m feeding a 1kHz tone.
At full turn of the trim (volume all the way up) I’m at 22.6v on the case of the 2n3055.
With the trim knob at 12 o’ clock it drops to 21.2v and with the trim all the way down it does up to 21.7v.
So, the bias is changing I just don’t understand why. If I turn the gain down to where I’m not seeing any distortion on the oscilloscope, the bias doesn’t deviate: it stays at 22.6v