best input transformer for OPA627

nonsqtr

Member
2010-12-17 4:28 am
That OPA627 is sweet Sweet SWEET! "No servo needed". Less than 0.1 mV drift at the OUTPUT with an 18v supply and a gain of 1000.

My question for y'all is: what would be the best input transformer to use with this chip? I'm looking specifically for a mic transformer, my application is a mic preamp (special purpose though, I'm measuring rumble in iMax rooms with B&K mics, hence the need for DC coupling, ha ha).

Should I be looking for something with very low secondary impedance (say, a Jensen JT-16a), or something in the usual bipolar input range (~5k, so say a nice Marinair 10468, yuk), or since this chip seems to like high input impedances, how about a nice 10 or 15:1 tube-type transformer like a Jensen JT-115-K (which is one of my favorite transformers of all time - a little noisy perhaps, but I mostly seem not to mind very much).

Details: the chip datasheet talks about a noise leg right around 2k of input impedance. Do we know of any mic transformers that come in "just under" that? Maybe some Beyer versions?

And finally, are there any "sleepers" I should be looking into? Trannies that maybe are your favorites but maybe you have to get 'em from old military surplus gear or something?
 
transformers "reflect" the loading on one side to the other.

If you have a 1:2 step up transformer then the "reflection" factor is 2^2 = 4

Load the input of the transformer with a 100ohm mic and the pre-amp sees 400ohm as the source impedance.

Load the transformer output with the 2k input impedance of the pre-amp and the mic sees a 500r load.

In the meantime the 1:2 (turns ratio) tells you that 1mVac applied to the input becomes a 2mVac emf at the output.

find what load the mic wants to see.
find what source impedance the pre-amp wants to see.
select a transformer that is wound to roughly match those impedances.
select a turns ratio that gives you the gain you want.
Then do the reflected load calculations to see that preferred source and load impedances are OK for both the mic and the pre-amp.

Finally transformers are designed differently for different impedances, but they are also designed differently depending on their location. At the input of a pre-amp uses a different transformer than at the output of the mic, even when the turns ratio is the same and the design impedance is the same.
I think you are looking for an input transformer to be located at the pre-amp input.

Don't accept my post as gospel. Get corroboration, but use my comments as a guide to what questions you should be asking.
 

nonsqtr

Member
2010-12-17 4:28 am
Thanks for your reply AndrewT. My question was a practical one, not a theoretical one. There are two philosophies when it comes to impedances, one is to "match", the other is to "bridge". The latter involves making an op-amp's input impedance as high as possible so affect the circuit being "bridged" as little as posible. So, if you're briding across a transformer that has a matching Zobel network, your op-amp will not further "load down" the transformer secondary. The problem with this approach is that high impedances generally equate with higher noise, and how much noise you can afford (or tolerate) depends on your application.

The other approach is to try to use as low impedances as posible across the board, in theory this minimizes voltage noise, and audio op amps like the AD797 and 990c take advantage of this but in turn they require low impedance input transformers (like, for instance, the Jensen JT-16 which is just a 2:1 transformer, so you pretty much rely on your op amp for all the necessary gain). A higher impedance transformer doesn't sound real good with an AD797, but on the other hand a Jensen JT-115 sounds GREAT with an OPA604 or OPA627 (both very high input impedance chips).

So, I was asking in a practical sense, if anyone had a favorite transformer they liked to use in these high-impedance environments (and specifically with the OPA627, but more generally with any high impedance op amp).