mic input transformer help ---tube mic pre

[StephenBOmaha]

I bought these xfmrs pretty cheap and and am wondering if anyone knows what they are/ has data sheets available.

One person on ebay is selling them as "beyer 71107 A", but beyer themselves said they were unaware of this xfmr "71107 A".

I am currently using them as mic input transformers for a "Roberts Mod Tube Pre"

i have a picture if that helps.

thanks!!!!

[RodC]

Welcome to the board.

I have never seen that model before, did the seller mention what they came out of?

Many transformer makers will make different models for other companies, and will label them with non standard part numbers.

[lassoharp]

Taking a set of DC resistance readings for the pri and sec will get you enough info to get you in the ballpark.

For a tube pre input you'll want a 1:7 or higher turns ratio for best sig to noise specs.

Transformer should be well shielded - preferably in a mu can. Otherwise noise pickup might be a problem.

[StephenBOmaha]

thanks for the posts

I ll look into the dc resistance.... I hooked them up "trial and error" style until the ground hum and signal cancellations were solved.... i need to investigate a little more.

On the "ef86 pre" I think I had some high end clipping.... didn't sound bad, just looked funny in the digital realm . . . the tops cut off the waveform. (not even close to digital clipping)

I built a little box to try different xfmrs. Basically every transformer I have is of unknown origin... the other ones look similar to the shure m67 transformers, I seem to get more gain out of them but also a little harsher mid range than the "beyer" ones....


Thanks again for the posts and I'll report back w/ my findings!

[The Scum]

If you can generate a signal and look at the resulting waveforms (I use a test oscillator and oscilloscope, but a DAW can do it), you can figure out relative winding ratios and winding polarity. Apply signal across one pair, and see what comes out the other pairs.

Sometimes transformers have a number of parallel primary and secondary windings, so you can wire them in series or parallel for various impedance/gain ratios.

[quote didn't sound bad, just looked funny in the digital realm . . . the tops cut off the waveform. (not even close to digital clipping) [/quote]

Sounds like it could be core saturation...what sort of waveform were you using to test? Are there little "shark-fins" of overshoot at the start of the clipping? They Beyer mic transformers I've seen are about the size of a thimble...so the cores couldn't be very big, and will likely saturate at line levels. But not a problem for most mic levels.