Mic input transformer, HF bump at low input how to solve

[andre tchmil]

I need to replace a  broken Haufe input transformer in an unbranded old preamp.
I found a good replacement with same ratios, unfortunately there is no type number and such.
fits perfectly and sounds good but at low input I have a serious HF rise , which becomes more flat at higher input.
I'm interested to learn about this phenomenon, and most of all , is there a way to solve this ?
attached is a  response curve.

 

[RuudNL]

How did you measure this? Can you also hear this effect?
There is no reason to assume that the frequency response of a transformer is level dependent, anyway not for HF.
(If so, you may have invented a passive HF compressor!)


Edit: "frequency" should be "level"

 

[radardoug]

Are you sure its not something wrong with your measurement?

 

[andre tchmil]

How did you measure this? Can you also hear this effect?
There is no reason to assume that the frequency response of a transformer is frequency dependent, anyway not for HF.
(If so, you may have invented a passive HF compressor!)

yes I can clearly hear this effect.

The preamp is a V72 copy , more or less, with a U pad in front.
I'm stuck

 

[CJ]

interesting,

could be a bi-filar wound primary, lots of capacitance.

seen it in the Peerless K214D at about 10K to 12K, which is a bi-fi coil,

tried termination resistor or network?

do you have a cap checker that will do pico-farads?
if so, try checking pri to sec capacitance, one lead of pri to one lead of sec,

is there a shield that needs grounding?

also make sure that the polarity of you test leads stays the same when doing both tests, but this effect from the measuring equipment leads usually shows up at higher freqs,

has nothing to do with the core as it is out of the equation by the time you hit about 5 KC,

if you can remove bell covers or pop the lid on the can you can sometimes see the winding structure,

 

[analogguru]

Without any further information about the transformer and the following circuit its hard to say anything final but the response is pretty normal and the HF-bump probably comes from the resonance of the (unloaded ?) transformer, which will have a peak (up to 10db) at about 80kHz.  You need a proper load resistor (matching to the secondary winding impedance [probably around 10kOhm]) to tame this effect.  If you like you can use a trimmer with a capacitor (220p - 1n) from the slider to ground (should be adjusted for best square wave).

 

[andre tchmil]

guys, don't hit me.
While reading all your replies and exploring the preamp, I discovered a little crack in the network cap ( 20 pf).
Replaced it and coincidence or not , problem solved  8)
Still a great forum after all those years.