Spino
Well-known member
Hi,
I'm attaching the very first part of the 169 Studer mic preamp. it's the LPF and the trafo.
I am trying to understand better what happens. The Studer manual states that the LPF has a corner freq of 20k. A friend measured L1a and L1b and each has an inductance around 240mH. Ferrite core is reported as 3E4, but no ideas about how many windings.
Help me see if I get it right: the inductor L1a-L1b in differential mode cancels out its inductance, so it has little to no effect, at the same time the two R5 and R6 get bypassed. When in common mode though, inductance rises, and L1 + R5 R6 + C5 C6 form an LPF. Any RC filter calculator for 10kohm 1nF would give me around 15kHz. Is the L part raising this frequency to around 20kHz? And how? Has the trafo impedance any effect on the circuit (trafo specs from the site: Impedance ratio 200:3.2k; Source/load impedance 200/10k Ω)?
But on the practical side: if one were to reproduce the circuit, any common mode choke with around 240mH would be enough? Or should I look also for other specs?
More on the practical: right now mouser lists 240mH common mode chokes as SMD only. If one were to choose a through hole component, would you go for a higher or lower inductance?
Thank you very much!
spino
I'm attaching the very first part of the 169 Studer mic preamp. it's the LPF and the trafo.
I am trying to understand better what happens. The Studer manual states that the LPF has a corner freq of 20k. A friend measured L1a and L1b and each has an inductance around 240mH. Ferrite core is reported as 3E4, but no ideas about how many windings.
Help me see if I get it right: the inductor L1a-L1b in differential mode cancels out its inductance, so it has little to no effect, at the same time the two R5 and R6 get bypassed. When in common mode though, inductance rises, and L1 + R5 R6 + C5 C6 form an LPF. Any RC filter calculator for 10kohm 1nF would give me around 15kHz. Is the L part raising this frequency to around 20kHz? And how? Has the trafo impedance any effect on the circuit (trafo specs from the site: Impedance ratio 200:3.2k; Source/load impedance 200/10k Ω)?
But on the practical side: if one were to reproduce the circuit, any common mode choke with around 240mH would be enough? Or should I look also for other specs?
More on the practical: right now mouser lists 240mH common mode chokes as SMD only. If one were to choose a through hole component, would you go for a higher or lower inductance?
Thank you very much!
spino