Transformers and DC: how much is too much ?

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jdbakker

Well-known member
Joined
Nov 24, 2005
Messages
1,431
Location
Amsterdam, The Netherlands
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
 
Ha JD,

Nice !

If this circuit is open for discussion then let's first get the nitpicking be done with: you'll be using 5534, right ? The 5532 doesn't have the... etc

Or you could null the 5532's just as easy, but in another way.


Nice this shows up, maybe I should build me some testjig as well for some other type of line-TXs I just got in. I'm also thinking of adding a few discrete BJTs to the driving opamp for checking line-output-TXs.

Contrary to what I thought at first, those discretes won't be required when wanting to measure a mic-TX; the load can indeed be higher (less Ohms) than the allowed 600 for the 553X, but signal levels will be small.



Bye,

Peter
 
[quote author="jdbakker"][tester source code will be published as and when I get it working][/quote]
Hmm, what will be going on there :?: :razz: No RMAA but something dedicated ?

Bye,

Peter
 
[quote author="clintrubber"]If this circuit is open for discussion then let's first get the nitpicking be done with: you'll be using 5534, right ? The 5532 doesn't have the... etc[/quote]
D'oh! Yes, of course. Original post updated.

JDB.
[I always mix these up. Good thing I have a few dozen of either...]
 
[quote author="clintrubber"][quote author="jdbakker"][tester source code will be published as and when I get it working][/quote]
Hmm, what will be going on there :?: :razz: No RMAA but something dedicated ?[/quote]
RMAA is nice, but it's a bit too closed for my liking. I want to do phase measurements, automated distortion tests and impedance calculations; I don't believe there's a way to add stuff to RMAA. Oh well, I get to do enough FFTs and PN-codes in my day job, might as well apply it to audio measurements.

[quote author="clintrubber"]Nice this shows up, maybe I should build me some testjig as well for some other type of line-TXs I just got in. I'm also thinking of adding a few discrete BJTs to the driving opamp for checking line-output-TXs.[/quote]
Don't those THAT line driver chips that Wayne sells have enough oomph to drive line-out transformers? That's what I was going to use until you talked me out of it.

JDB.
 
[quote author="RogerFoote"]I am interested in why you were talked out of using the THAT driver chips to drive output transformers. I did non scientific listening tests with the THAT1646 driving Edcor WSM600s and Jensen JT11 ELs and it seemed to work very well.[/quote]
I'm mostly being impatient/lazy. Peter suggested that I might as well use 5534s; I have plenty of these in my parts bin, whereas for THAT16xx chips I'd first need to order some from Wayne and then wait for the USPS to get around to delivering them to me. The transformers I'm working on ATM are line-in, so drive requirements are minimal. For a 'production' design I would prefer the THAT1646/1606 (or even a 990) over a 553x.

[The 1606 looks particularly good for DAC-to-line apps, as it would allow for a fully-differential path from the DAC through an OPA1632 to the '1606. Not that I'm one of those who believe that there's something magical about differential vs single ended, but it makes the design cleaner and simplifies ground current immunization]

[quote author="mediatechnology"]jdb You've stumbled upon something all of us need: A soundcard interface for lab work. I've almost thought about providing a sound card monitor port on my HP334B so I can use the input attenuator.[/quote]
Working on it. I've found I need one to properly test/tweak the mic in channels for my HDD recorder project ('it slices! it dices!'). What I have in mind is a small board with S/PDIF and AES/EBU I/O, a SRC, a good ADC/DAC combo and line drivers/receivers. Plenty cheap sound cards can be found with S/PDIF I/O, and for most tests 24/96 is good enough.

[The trouble with 24/192 for analyzer purposes is that most 192k ADCs have a very slow roll-off in their digital filter, which is great for audio but no good for out-of-band measurements (think aliasing). The only options that I know of are the flagship AKM converter (impossible to get in small qtys) or the PCM4222 with a custom filter in an FPGA (life is too short)]

The original question remains: how much DC is too much, specifically for a small ungapped 10k:10k line-in transformer ? When it comes to iron I'm a neophyte, and I'd hate to fry all these nice Beyers.

JDB.
 
[quote author="RogerFoote"]I am interested in why you were talked out of using the THAT driver chips to drive output transformers.[/quote]
Was based on a passing remark from me that those drivers (or the SSM-predec.) could have internal series-resistors. But I didn't look it up then to confirm this. So I was the cause but I don't feel guilty :wink: , as added at the time it was a remark of the IIRIC-kind.

Regards,

Peter

PS
This is going to be a nice interface indeed !
 

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