Some strange audio transformer response curves

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emrr

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Three, in fact. 

The two similar curves have the oddest sharp roll-off at the top I've ever seen, with the tiniest resonant point too.  Almost looks like they are purposefully wound with a LPF built in.  Any comments or ideas?

The crazy S curve:  Would this be indicative of shorted turns?  Many many shorted turns?  Any other theories?  I have another of the same that measures quite flat. 
 
gyraf said:
CJ said:
try it with more power.

they might be slightly magnetized.

Good point. Is there anything to do to restore to original specs in such case?

Jakob E.

A nice coincidence, Jim G./ZapNSpark (IIRC it was him) just posted a link at the micbuilders-list
to a Studer-article about (de)magnetizing TXs:  SwissSound18eFeb87LR.pdf

I'll post the link later; I only have the file here, but Google doesn't find it.

Regards,

  Peter


 
There are quite a lot of different makes of de-magnetizers. Perhaps these can be used. I have never done it before though.

I don't exactly remember where I read it but even the test equipments would magnetize the core, obviously depending on the size of the core and the test current used.
 
Drop seems like 12dB/oct, region around 8KHz indicates atleast 4 poles. What tx is this and how did you loaded it (and what are you driving it with)?

cheerz
ypow
 
Hi there, yes curves looks like a "narrow band" xformer as some were used in telephone carrier's pabx  :-[ Is the upper freq' response the same ? most of them just work in vocal band (300/3400Hz) . You can find nice UTC on ipay for cheap for this purpose  ;D mil-spec radios have the same specs !  ;) Cheers , Pierre
 
I should have said a bit more.  All 3 are 1930's units from the same manufacturer.  Loaded appropriately, tried with various alternate loadings.  Driven with a 200 ohm source with appropriate series build-outs or loads on both sides.  I always vary both to see what the variables introduce.  All are driven with level somewhat under max rated power; either capture system or transformer rating.   

The 2 with matching high roll-off are roughly the same type, and date to the early '30's; multiple pri 50-500 : 100Kishm rated 50-10K +/-1.  They do better than rated.  Sound great too.   These are really quite flat for this era; better than most by far.      I've never seen a LPF type roll-off of this nature.   Makes me think it's purposeful.    Same manufacturer made units with higher specs too.  Both of these are from different sources, one is NOS.   Both are rated for providing current to carbon mics, so 'phantom ready' as they say. 

The S-curve.  I have two, the other is perfectly flat.  Both NOS.   1937ish.   8K DC plate coupled output to 500 line.

Thanks for the input.  I hadn't considered magnetization.
 
if you run the transformers close to max. rating, the curves will show a combination of it's "own" R/L frequency response and the further low-end attenuation that comes from saturating the core. Remember, saturating reduces (dramatically) inductance, which in turn reduces low-end.

To verify this try a re-run at 30dB under the level used - should give you a much flatter low-cutoff. Also take a look at waveform at e.g.20Hz/full level - that should show clearly what happens..

Jakob E.

clintrubber said:
A nice coincidence, Jim G./ZapNSpark (IIRC it was him) just posted a link at the micbuilders-list
to a Studer-article about (de)magnetizing TXs:  SwissSound18eFeb87LR.pdf

I'll post the link later; I only have the file here, but Google doesn't find it.

..Please...!
 
Okay, I'll say it a different way.  I've plotted close to a hundred of audio transformers and never seen curves like these.  Big, small, low power, high power.  I run the level up and down.  The comment about running as hot as possible addresses CJ's post.  Hot as possible within reason.  I realize a lot of people don't have any reason, and that has to be addressed in any responses at this forum.  ;D 

These three stand out as oddities, and that is what I am addressing.  I'm hoping someone can humor me and assume the testing method is correct, as all other plots look normal, and I usually do these sorts of plots in batches; guaranteed I tested another 10 'looking' units on the same session.  I should revisit the two mic/line to grid types to be sure.  The S curve compares to another identical NOS unit that looks perfect, measured back to back on the same day.  All three types are built on cores/lams approaching Edcor or UTC LS series in size.
 
gyraf said:
clintrubber said:
A nice coincidence, Jim G./ZapNSpark (IIRC it was him) just posted a link at the micbuilders-list
to a Studer-article about (de)magnetizing TXs:  SwissSound18eFeb87LR.pdf

I'll post the link later; I only have the file here, but Google doesn't find it.

..Please...!

Oops, it wasn't Jim (but a gentleman from The Mastering Room AB,
Goteborg, Sweden), but here goes:

See the tutorial by Peter Frigo of Studer on page 5 of:

ftp://ftp.studer.ch/Public/SwissSound/SwissSound18eFeb87LR.pdf

Demagnetization of microphone transformers: "Optimum recording."

This article was also to be read in Studio Sound May 1987 Page 50.


What do the people here think of that proposed method ?
Sounds decent & practical to me, but haven't any experience with it yet.

Bye,

  Peter
 
Yes, the low-frequency wobblings on the two relatively flat plots are measurement artifacts; pink noise FFT.  The basic curves look right, only the high frequency roll-off is of interest.

The S-curve unit clearly has a problem.  The plot must suggest something to someone. (?)

Three total plots of three total transformers. 

Thanks
 
The plot must suggest something to someone.

The only time I've seen such behaviour was when someone left one secondary output floating, but that's not the case I guess. If the response is intentional then perhaps it is some sort of pre-emphasis..?

Samuel
 
Samuel Groner said:
The plot must suggest something to someone.

The only time I've seen such behaviour was when someone left one secondary output floating, but that's not the case I guess. If the response is intentional then perhaps it is some sort of pre-emphasis..?

Samuel

Yes it does look like something is floating.  But it's not, and it passes audio.
emrr said:
The S-curve.  I have two, the other is perfectly flat.  Both NOS. 
 
emrr said:
Yes, the low-frequency wobblings on the two relatively flat plots are measurement artifacts; pink noise FFT.  The basic curves look right, only the high frequency roll-off is of interest.

The S-curve unit clearly has a problem.   The plot must suggest something to someone. (?)

Three total plots of three total transformers. 

Thanks

I have seen a similiar response in what I believe was a shorted input line to grid tranny.
How do the dcr measurements compare with the others? Try a phase response chart.?

Bill
 
Regarding the nit with no low end, DCR looks the same as the 'good' unit.  Spectrafoo plots phase as well, and there was nothing obvious there so I didn't post it.  Phase always follows freq to a high degree, so when the freq looks whack the phase does too. 

Keep'em coming; thanks!
 
clintrubber said:
gyraf said:
CJ said:
try it with more power.
they might be slightly magnetized.
Good point. Is there anything to do to restore to original specs in such case?
Jakob E.
A nice coincidence, Jim G./ZapNSpark (IIRC it was him) just posted a link at the micbuilders-list
to a Studer-article about (de)magnetizing TXs:  SwissSound18eFeb87LR.pdf
I'll post the link later; I only have the file here, but Google doesn't find it.
Demagnetizing Microphone Input Transformers ....lifted out of my Studer 963 manual;
Symptoms;
If magnetized, you might experience microphonic noise, e.g., tapping the input module without any input source connected, you will audible modulation.

Residual magnetiation can accumulate over extended operating time. Can be caused by connection of unbalanced sources or the shorting of the + and - leads when the 48V phantom supply is activated

Solution;
Switch off mixer/unit.
Connect audio generator as follows;

    ____ Resistor 50 ohms _____  + Cap 1000uF 10V -______________________
   |                                                                         |                             |
Generator                                                               Resistor 600 ohms     Trans Primary
   |______________________________________________|__________________|

Set Generator to 32Hz, 0V output, slowly increase the generator output from 0V to 3V

Slowly decrease the generator output from 3V to 0V

Demagnetization completed.

Cheers
Michael
 

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