jdbakker
Well-known member
Hi all,
So I'm working on a simple transformer tester for the big bunch o' Beyers. The plan is to have a driver/receiver to connect to a sound card so I can get a rough frequency/phase plot for each transformer (and possibly distortion tests, too).
This is what I have come up with so far. All opamps are 5534s; I have omitted all supply decoupling capacitors and the transformer termination resistor/zobel for clarity. U3 and its entourage serve as a current sensor, so I can do impedance measurements. Most resistors are larger than I'd use for audio work because I want to keep the upper limit to the transformer loading high (and the sound card I plan to use doesn't exactly have a stellar noise floor, either). Yes, I'm aware that the system as drawn doesn't have an end-to-end gain of exactly 0dB.
Thing is, I would really like to have the entire signal path DC-coupled, to avoid coupling caps colouring the measurement results. I plan to null all 5534s once, before measuring, so even with worst-case drift I wouldn't expect more than 1..2mV of DC across the transformer. The Beyers have a DCR of ~3k, so that translates to less than a microamp of current. Is that sufficiently low to not have to worry about magnetizing the transformers' cores? Is there a rule of thumb about max DC, or is any DC completely verboten ?
Thanks,
JDB.
[tester source code will be published as and when I get it working]
EDIT: /me needs to stop writing 5532 when he means 5534, and vice versa
So I'm working on a simple transformer tester for the big bunch o' Beyers. The plan is to have a driver/receiver to connect to a sound card so I can get a rough frequency/phase plot for each transformer (and possibly distortion tests, too).
This is what I have come up with so far. All opamps are 5534s; I have omitted all supply decoupling capacitors and the transformer termination resistor/zobel for clarity. U3 and its entourage serve as a current sensor, so I can do impedance measurements. Most resistors are larger than I'd use for audio work because I want to keep the upper limit to the transformer loading high (and the sound card I plan to use doesn't exactly have a stellar noise floor, either). Yes, I'm aware that the system as drawn doesn't have an end-to-end gain of exactly 0dB.
Thing is, I would really like to have the entire signal path DC-coupled, to avoid coupling caps colouring the measurement results. I plan to null all 5534s once, before measuring, so even with worst-case drift I wouldn't expect more than 1..2mV of DC across the transformer. The Beyers have a DCR of ~3k, so that translates to less than a microamp of current. Is that sufficiently low to not have to worry about magnetizing the transformers' cores? Is there a rule of thumb about max DC, or is any DC completely verboten ?
Thanks,
JDB.
[tester source code will be published as and when I get it working]
EDIT: /me needs to stop writing 5532 when he means 5534, and vice versa