The Beyer audio input transformer catalog points out the importance of terminating impedance

The excellent Beyer Dynamic audio input transformer catalog can be found by searching on Google with these words.


http://www.ka-electronics.com/images/pdf/Beyer_Audio_Input_Transformers.pdf


It has excellent information and graphs. And the great part reference code.


What is rare to find in other documents, one can see the importance of not only resistive but capacitive termination to the secondary of the input mic transformer too.


With no capacitive termination the peak reasonance at very high frequency just flies up.


(Having said that, if a transformer is unterminated or terminated with a high load, the reasonance may lead to an interesting sound. Would 40KHz boost lead to more air on a vocal recording as the 20KHz would be lifted up too etc. The Goldfrapp Felt Mountain album used according to Dave in Bristol the Decca EQ which could boost at ultrasonic frefquencies, 40KHz?)


On a previous euroelectron example on recording with ribbon mics stepped up and direct to an electron tube reel to reel recorder the very smooth cymbal sound could be further explained...


The signal pathe is:


Grampian GR1/L 50 Ω mic - Sowter 1:20 step up transformer - 1/2 or so meter of unbalanced cable - Ferrograph series 6 valve reel to reel tape recorder


By necessity the mic input transformers had to be a bit further from the Ferrograph to avoid hum induced to them from the Ferrograph power supply transformers and motors. This small length of terminating cable is a significant capacitance in picoFarads and should have added to the smoothness and excellent treble of the cymbals possibly by taming the ultrasonic reasonance since the input resistance of the Ferrograph is quite high at 1 Megohm.


Caution should be exercised on using the Pleiades K117 JFET prepreamp which has a very very high in Megohm input impedance. The reasonance can be very clearly heard on many transformers unless they are terminated.


The Pleiades V series electron tube prepreamps operating with just a few volts, typical less than 6V, do not have this problem. The electron accelerating resistor from anode to grid (of the order of megohms) lowers the input impedance of the stage to a typical 100KΩ. So great, natural and smooth sound is already there. Low noise too!