rackmonkey

A bunch of transformer freq response test curves
« on: June 04, 2017, 12:35:14 PM »
I've taken an awful lot from this forum over the years without giving much back, so hopefully this will find some use with someone.

EDIT: Bottom line: if you are tempted to/must use oddball transformers for projects (PA, broadcast, scarce or little known manufacturer, etc), then you might find these useful. Skip the dissertation and go straight to link below.

I don't know how many audio transformers I've accumulated over a 15+ year period but it's a lot. In addition to the usual suspects, my congenitally cheap nature drives me to pull from old PA equipment (think Bogen, TOA, later Altec, etc) that I get for less than the price of a single UTC ouncer, and quite a number of these have proven surprisingly good in projects.

To aid in buying decisions for this sort of stuff I wanted to have a quick and dirty reference to remind me whether a particular transformer or unit is worth dropping the change or not. The result is a Google Drive folder with dozens of freq response curves for various input and output transformers under varying primary/secondary source/load impedances. Also helps when I'm trying to decide whether a particular transformer might be workable in a given project.

The folder is here, accessible to anyone with the link: https://drive.google.com/drive/folders/0B__Vr0ik2GcgLVlidXRjd0c2b1U?usp=sharing

The big caveat is that I'm also congenitally lazy about tests like this. So the measurements were done with - *gasp* - white noise rather than freq sweeps. This saved me the time of having to trim the image of the curves at the start/end of the sweep. I'd call that pretty lazy. But I found that I got similar results whether I used a traditional sweep or WN. So I just stuck with the WN approach.

I also just limited the curves to 20 - 20k. But that has served my purpose well enough of just helping me make a buy/no buy decision or a try/don't bother decision for a given project. When selecting for a project, more extensive tests would then be performed.

I guess it brings up the question of the value of WN for this sort of thing vs a sweep. Haven't seen this discussed here before, but I might have just missed it. Again, whatever flaws there may be to this method, it's served my purposes well enough.

Some might be interested in the test setup, so here it is. I used REW for the RTA Fourier analysis. A cheap chinese noise generator was used for the test signal. Cheap, but as you can see from the direct (no xformer) freq response plot, it's dead flat from 20 - 20k. The unit has a 50ohm output impedance. Depending on the transformer/application I wanted to test, I put a series resistor in to get the source impedance to where I wanted it (e.g., 50ohm output Z + 150ohm series R = 200). This unit only outputs 5V RMS - fine for most mic/line input transformers, but inadequate for anything else. So when required, I ran this signal through a cheap DC amplifier (100v p-p, 1A, DC - 50k +/- 0.5dB). This unit also has a 50ohm output impedance, so I used the same approach to source impedance as with the generator.  Always use this with output transformers and it seems to excite the cores adequately.

I ran the transformer secondary into a GenRad decade resistance box to set the load impedance and from there the signal went into the PC sound card. I didn't mess with Zobel networks or add an output capacitor to the setup. Obvious caveat on the latter, but remember the purpose was quick and dirty, "is it worth even buying/messing with in project X".

My soundcard has an obvious slight response dip from about 16k - 19k, then comes back up to flat at 20k. That's consistent across all these tests, so draw an imaginary line across that dip for a more accurate response picture. I'm currently building a Pete Millett SC interface that will hopefully improve things in this regard.

Oh, and don't bother trying to glean anything from the levels in REW, other than relative response levels. I constantly use that program and am always fiddling with the SC input sensitivity which affects the levels REW RTA displays. So those are all over the map. No, I didn't test mic input transformers at +30dBm or anything like that. Signals into mic input transformers were generally around 50 - 70mV, which would be around the high end of what a modern condenser mic puts out (based on my tests w/a microvoltmeter, anyway). Sometimes I tested w/lower levels depending on the transformer and mic type it (appeared) to be designed for. Line input transformers varied by experimentation in the 1.25V ballpark and up. Output transformers were subjected to varying signal levels depending on either specs (for known transformers) or experimentation (unknown) at up to about +26 - +30dBu (tops - not all were tested to these levels. Just depended on what the RTA showed). Sorry these are not documented in the tests. Again, L.A.Z.Y. But the levels were  at least "appropriate" enough to give a decent enough picture. Project results bear that out. Remember the purpose.

Anyway, hope someone can use this stuff. I've found it very handy and it's saved me quite a bit of time and (I assume) money in buying/picking xformers for my projects.

I've got a backlog on transformers I still need to test. But I've largely skipped over my rather sizable collection of off-the-shelf UTC transformers since the old catalogs have plenty of info on those. There are some tests on those, but I probably have 20 or so A-x, O-x and LS-x's I'll ultimately test that I haven't yet. I did test all the custom UTCs I've acquired though. Some nice surprises there.

(BTW I didn't bother to detail how I come up with impedances for unknown transformers. In short, DCR tests on the various pin/wiring combinations gives the roadmap. Then (if needed) test for winding ratios. From there, terminate with common resistor values via GenRad decade box and test opposite windings with a little Peak Atlas LCR45 set at a 1kHz signal. Read magnitude and phase of impedance from there. Works surprisingly well and matches up with nominal specs for known transformers incredibly close. Never had major problems with using this approach in projects.)

Anyway, enjoy (or scoff, if you prefer). Thanks for all that this forum gives to us trying not to electrocute ourselves on a daily basis.

BT

« Last Edit: June 05, 2017, 12:01:32 PM by rackmonkey »
Whether you think you can or you think you can't, you're probably right.


rackmonkey

Re: A bunch of transformer freq response test curves
« Reply #1 on: June 07, 2017, 02:30:57 PM »
Added:

Carnhill VTB2280 (600:600 and 600:150)
ADC 114C Input/Splitter 200:600 (250 nominal on primary)
Sescom MI-44 Input/Splitter 200:600
Whether you think you can or you think you can't, you're probably right.

warpie

Re: A bunch of transformer freq response test curves
« Reply #2 on: June 07, 2017, 04:38:33 PM »
Thanks for doing that!  :)

rackmonkey

Re: A bunch of transformer freq response test curves
« Reply #3 on: June 09, 2017, 04:57:24 PM »
Added:

UTC A-11 (200:50k and 500:50k)
UTC A-22 (500:125 and 500:500)
Whether you think you can or you think you can't, you're probably right.

rackmonkey

Re: A bunch of transformer freq response test curves
« Reply #4 on: June 13, 2017, 02:07:08 AM »
Added:

Akai M8/Roberts 770x Output Transformer (6.5k - 8.5k: 8ohm)
RCA 12301-A Input Transformer (~30 - 50ohm: 50k (probably for dynamic, crystal or carbon mic (?) - grid); tested multiple primary configs at 200:50k and 50:50k.
Whether you think you can or you think you can't, you're probably right.

rackmonkey

Re: A bunch of transformer freq response test curves
« Reply #5 on: June 14, 2017, 01:58:17 PM »
Last post of just a couple of transformers. I'll save up and post batches of several in the future to keep this thread from jumping up top so frequently. Wanted to get these out though because the Ampex is so common and the Protec shows up on Ebay quite often as part of a line splitter card (or something like that).

Added:

Ampex 4580200-01 Bridging input Transformer (Green base, 20k-20k)

Ampex 4580200-01 (Brown base, also 20k-20k, just to show there's no differences between base colors alone. The 4580200-02 is a step up input transformer (line, I think) and has both green and brown base versions as well. No difference between those, either. Will test again and post when I have a chance)

Protec RÜP 1401M Input Transformer
Whether you think you can or you think you can't, you're probably right.