mic input transformer question

[fatty]

is there any difference (in terms of fidelity/quality) between either of the 2 signals coming into the mic input transformer? theoretically, they should be exactly the same but just opposite polarity. i've hooked up a mic to the transformer and can hear no discernable differenence when i swap the 2 incoming leads.
does it even make any difference (other than the obvious polarity flip)?

[fatty]

does that question make sense? i'm talking about the input transformer on a mic pre.

[Geoff_T]

Originally posted by fatty
does that question make sense? i'm talking about the input transformer on a mic pre.

Hi

It kinda makes sense!

The problem with phase is that it depends on loads of issues with the recording environment, surface reflections and mic placings. You could step back from the mic a minute distance and change the phase at that particular instant

You may not notice any change with the way you are using the mic.

[fatty]

thanks geoff.

i really didn't word my question very well at all. let me try again.

pin 1 on an xlr connection is audio hot (in phase). this is connected to the in phase input on the input transformer. pin3 on an xlr is audio cold (out of phase connection). this is connected to the out of phase input on the input transformer.

my problem is that i do not know the pinout of my transformers, therefore i do not know which pin is hot and which pin is cold. i do not have any test equipment other than a multimeter.

so here's my question: if i hook up my transformers and get the connections "ass about", will i suffer any degradation to the signal, apart form the obvious polarity reversal??

[Geoff_T]

Originally posted by fatty
thanks geoff.

i really didn't word my question very well at all. let me try again.

pin 1 on an xlr connection is audio hot (in phase). this is connected to the in phase input on the input transformer. pin3 on an xlr is audio cold (out of phase connection). this is connected to the out of phase input on the input transformer.

my problem is that i do not know the pinout of my transformers, therefore i do not know which pin is hot and which pin is cold. i do not have any test equipment other than a multimeter.

so here's my question: if i hook up my transformers and get the connections "ass about", will i suffer any degradation to the signal, apart form the obvious polarity reversal??

Hi

First point is that pin 1 of an XLR is ground and should, strictly speaking, play no part in the audio path. It's the shield of the cable.

Your audio is across pins 2 and 3 and, provided you wire them consistantly (same pin for hot in as for hot out) it shouldn't really matter how you wire it.

If the worse that happens is a phase reversal I'm not sure if you'd notice it because I don't know if this is for a single unit or multi channels.

Can you not find out the transformer connections from somewhere?

[fatty]

oops i got the pins mixed up, but you nailed it on the head. it is for a multi channel set up. i will make sure to keep the wiring consistent between channels.

the problem with finding out what the connections are is that i have no idea what these transformers are! all i know is that they are out of a 6 channel rack mounted preamp/mixer built in the early 70's in the u.k. i can't remember the manufacturer's name off the top of my head, but i did a fair bit of digging when i got them initially, and could come up with no clues as to the origin of the transformers.

[Geoff_T]

Hi

Do the transformers have numbers on them?

Are they round or square?

Who made the amplifiers? Is there a name on the track side of the PCB's?

Do they look like the wedge sections in this photo? Each section has six times six channel mixers...

http://www.auroraaudio.net/aa_html/n...oles/A246.html

[fatty]

they are round transformers. they had some red numbers stamped on them but they have totally faded off over the years. now unreadable. one of them has a little round sticker on it with the word "mic" printed in black. they definately didn't come out of a neve or anything remotely sexy. i think the unit was used in school p.a. or something of that nature. i'll have a look at the pcb's tonight and see if i can answer any of your other questions. thanks a lot geoff, you've been a great help.

[dale116dot7]

In theory, there should be no difference if you wire the transformer either way (though always check the phase of the preamp to make sure it's consistent - pin 2 hot both in and out).

In reality, there is a difference in performance because the capacitance between the shield and the ends of the windings are different. That makes for more overshoot (a high-frequency resonance) when connected one way than the other - or possibly more undershoot (high-frequency loss). If you have a square-wave signal generator, you can connect it to the transformer and check how close to square the output is. Make sure there's no DC in the test signal, and use a low enough level so you don't saturate the transformer.

-Dale

[fatty]

thanks dale, thats good information.

anyway geoff, (or anyone else), time to put your detective hat on! the transformers came out of a unit manufactured by Millbank Electronics, Sussex. They are mounted on a pcb marked PC00012 (copyright symbol)1973.
As mentioned earlier. one has a "MIC" sticker. Another has a "074/A" sticker. They all appear to have had a red number stamped on them. i cannot get a complete reading from one transformer, but taking different numbers from different transformers i think the number is 35032.

[Geoff_T]

Originally posted by fatty
thanks dale, thats good information.

anyway geoff, (or anyone else), time to put your detective hat on! the transformers came out of a unit manufactured by Millbank Electronics, Sussex. They are mounted on a pcb marked PC00012 (copyright symbol)1973.
As mentioned earlier. one has a "MIC" sticker. Another has a "074/A" sticker. They all appear to have had a red number stamped on them. i cannot get a complete reading from one transformer, but taking different numbers from different transformers i think the number is 35032.

Hi

Millbank Electronics are still very much in existance but are more concerned with PA's and alarm systems for buildings. They made amplifiers and mixer amps back in the ol' days.

You could email them for info and I'm sure they'd be happy to help.

Here's a link to their history and you can work back to their home page

http://www.millbank.co.uk/History.htm

[fatty]

thanks geoff.