Transformer input and resonance

[mashaffer]

I bet SY knows the answer to this (Heretical preamp and all). I understand the need for proper loading of input transformers but how does one deal with the likely resonance on the input side generated by the primary inductance in series with the output coupling capacitor of the source?

 

[SY]

Mike, are you referring to the sharpness of the LF rolloff formed from the output cap of the source and the transformer primary?

I don't have Spice, but someone who does could figure it out pretty quickly for a few likely values of C. There's no primary inductance spec on the Jensens I used in the Heretical (nor the Cinemags I used in the ImPasse), but I'd guess it to be in the range of several hundred Henries. That inductance is in series with 1k5 DCR of the winding and shunted by 10k of the reflected load. The Q is not likely to be very high.

 

[DF96]

Is the OP talking about the series resonance of the coupling capacitor and the transformer inductance? If so, no problem as the inductance is heavily loaded by the transformer load so the transformer presents a mainly resistive impedance - so no series resonance.

 

[SY]

Yes, that's how I interpreted the question and my answer was a somewhat more colorful version of yours.

 

[mashaffer]

Yes, you guys understood me. Not having any specs (it is amazing how little information even mainstream manufacturers provide) on a tranny I am planning on using I just ran my spice simulation with 25H on pri and each secondary. In simulation driven by a CF with one or two uF coupling cap I got a 3-5dB peak followed by what looked like a 2nd order roll off. Spice indicated that it could be managed by a resistance across the primary (or less effectively in series with it). My concern was that even with the transformer specs in hand one would not know what capacitance and series resistance any particular source would have. What I am hearing is the the inductance and series resistance in an actual transformer typically swamps that out.

I have the transformer in hand now so getting primary resistance will be easy enough. Measuring the inductance will be a bit tougher but just building the circuit and measuring the response may be good enough.

Thanks for the input guys (no pun intended).

 

[kavermei]

It's late at night and I might be blundering but wouldn't you only need to consider the leakage inductance?