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Microphone transformer specs
Microphone transformer specs
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Old 4th June 2019, 08:29 AM   #1
Mottlefeeder is offline Mottlefeeder  United Kingdom
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Default Microphone transformer specs

I have a couple of ex-BBC 'microphone transformers' labelled as LL/76 MSC.

I would like to use one to convert an active bass guitar signal (-10dB?) from unbalanced to balanced.

Does anyone know the signal level at which this transformer core will saturate, or does anyone know of a site with a spec sheet for ex-BBC spec transformers?

The transformers have two 57 ohm windings and are soldered into cylindrical containers 25 mm diameter and 25 mm long, excluding the mounting bush.

Thanks

David
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Old 4th June 2019, 11:19 AM   #2
simon7000 is offline simon7000  United States
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A transformer that size would probably do -10dB down to around 50 Hz.

But of course measuring the resistance with an ohmmeter would probably damage them by magnetizing the core.

Probably takes less time to try one than to wait for an answer based on measured data around here.

Of course that is my opinion and around here opinions aren't worth much

What is good news is that with matched resistances you can probably use either side as the primary. I assume you know the tip and sleeve would go to the input and the output would go to pins 2 & 3 if the XLR connector. You can leave the shield open at the transformer end. But if really fussy use a .01 uF capacitor shield to the transformer case.
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Old 4th June 2019, 07:44 PM   #3
Mottlefeeder is offline Mottlefeeder  United Kingdom
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Quote:
Originally Posted by simon7000 View Post
A transformer that size would probably do -10dB down to around 50 Hz.

But of course measuring the resistance with an ohmmeter would probably damage them by magnetizing the core.

Probably takes less time to try one than to wait for an answer based on measured data around here.

Of course that is my opinion and around here opinions aren't worth much

What is good news is that with matched resistances you can probably use either side as the primary. I assume you know the tip and sleeve would go to the input and the output would go to pins 2 & 3 if the XLR connector. You can leave the shield open at the transformer end. But if really fussy use a .01 uF capacitor shield to the transformer case.
Thanks for the information - I'll give it a try.
If I have a magnetised core, could I demagnetise it by connecting it to an amplifier and pumping 50 Hz into it, reducing the volume to zero over a few seconds - similar to the way I used to demagnetise tape heads by drawing the demagnetiser away from them?
David
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Old 4th June 2019, 09:09 PM   #4
JMFahey is offline JMFahey  Argentina
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I understand you have 2 identical windings, floating from each other so they can be conn ected in various ways?
And you measured 57 ohm DCR?
If so they are probably 600 ohm : 600 ohm impedance.

I would not worry *that* much about magnetization, transformer iron is "soft" magnetic and designed to not keep much of it, and in any case you should measure what you have today; if it can be eventually improved, so much the better.

The simplest practical experiment I can imagine is to start by feeding, say, 1kHz signal to primary, secondary loaded with a high impedance, say 10k resistor, and first check how much of my primary signal (say, 200mV RMS) reaches seconday.
I would expect some 170/180 mV or better.

Then add a 220 ohm resistor in series with primary and repeat test.
Would expect at least 140/150mV or better.

If everything fine, then I would sweep frequency down until I find where secondary level drops by 3dB or more.

IF 42Hz or lower, itīs perfectly usable as is and shows very high quality for the task; if -3B point is considerably higher, say 80/100Hz, Iīd add a highpass at that frequency between preamp/active instrument and transformer, based on "why feed it what it canīt handle anyway?"

After you performed this basic "usability" test, you may try to demagnetize it and see if that improves performance ... I wouldnīt worry much about that, itīs not a tape head or something similar.
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Old 4th June 2019, 10:55 PM   #5
simon7000 is offline simon7000  United States
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You could do that or I suspect just playing through the transformer for a bit.

Without being to rude, in your application I don't think much will matter. However these transformers are too low impedance to use directly from a guitar pickup.
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Old 5th June 2019, 06:04 PM   #6
Mottlefeeder is offline Mottlefeeder  United Kingdom
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Quote:
Originally Posted by simon7000 View Post
You could do that or I suspect just playing through the transformer for a bit.

Without being to rude, in your application I don't think much will matter. However these transformers are too low impedance to use directly from a guitar pickup.
Thanks for your replies - In summary, magnetisation is probably not a deal-breaker, and if I feed the transformer from an op-amp capable of driving into 600 ohms I should have a workable system?

David
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Old 10th June 2019, 11:03 AM   #7
simon7000 is offline simon7000  United States
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Yes as long as you keep the level low. Probably below 100 millivolts.
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