input lpf/choke on studer 169 pre

[Spino]

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

 

[abbey road d enfer]

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?

The calculation for LC filters gives F= 1/2.pi.sqrt(LC). The resistor modifies the shape of the response; the lower it is the milder the response (damping factor).

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)?

It does react somewhat with the LC filter but seeing how the secondary is properly terminated with a Zobel network that linearizes the impedance to make it pretty much resistive, the actual response is certainly very close to what basic calculations would show.

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?

Size and composition of the core may affect the high-level response so you should try to match that with the original, if information is available.

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?

There is no reason to change the value when changing technology from TH to SM, but in any case, if the actual 240mH value is not available, I would recommend a lower value since a higher value would restrict the HF response too much. The 20kHz corner frequency comes from broadcast regulations that you don't necessarily have to comply with.

 

[Spino]

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thank you abbey! very useful and clarifying infos!

sp