ruffrecords
Well-known member
The OEP A187A15C is a 1:10 mic step up transformer. I am using it in the poor man's tube mixer because it is low cost. I am also using it in one version of the Mark 3 channel module. However, over the last couple of days of testing I have found some unexpected behaviour.
When driven from a low impedance generator (less than 50 ohms) it exhibits the usual peaking response above 20Khz that you typically get with this 1:10 mic input transformers (I have seen it in Jensen, Sowter and Cinemags transformers).
When I insert my normal 20dB pad consisting of a pair of 680 ohm resistors in series with the hot and cold legs and a parallel 150 ohm resistor, the response flattens out right out to 40KHz (it does with Sowter, Jensen and Cinemag :10 ratio transformers). However, with this OEP transformer the response is actually down nearly a dB at 20KHz and nearly 5dB down at 40KHz. I spent a lot of time trying to find the cause for this but eventually tracked it down to the 20dB pad. The normal design with a 150 ohm parallel resistor works fine for all the other transformers. But with this OEP I found I needed to reduce it to 75 ohms to get the response flat past 20KHz. What concerns me is this implies the response will be less than ideal with regular 150 ohm mics.
Cheers
Ian
When driven from a low impedance generator (less than 50 ohms) it exhibits the usual peaking response above 20Khz that you typically get with this 1:10 mic input transformers (I have seen it in Jensen, Sowter and Cinemags transformers).
When I insert my normal 20dB pad consisting of a pair of 680 ohm resistors in series with the hot and cold legs and a parallel 150 ohm resistor, the response flattens out right out to 40KHz (it does with Sowter, Jensen and Cinemag :10 ratio transformers). However, with this OEP transformer the response is actually down nearly a dB at 20KHz and nearly 5dB down at 40KHz. I spent a lot of time trying to find the cause for this but eventually tracked it down to the 20dB pad. The normal design with a 150 ohm parallel resistor works fine for all the other transformers. But with this OEP I found I needed to reduce it to 75 ohms to get the response flat past 20KHz. What concerns me is this implies the response will be less than ideal with regular 150 ohm mics.
Cheers
Ian