Hi Pass On A Valve Mic

[Swedish Chef]

I have been trying to get my thick head around the maths for the Hi Pass filters such as on the C12a and C12VR schematics.
(Apologies for being un-able to find an online schem to link to and not having my camera with me!! Will get them up ASAP)

In the C12a circuit there is a tapped inductor which is switched in to be strapped across pin 2 and 3 (hot and cold) of the output XLR. There is no spec on the inductor for the C12a schem but on the C12VR it specs 520mH, although there are some odd looking caps in-line with the 'Cold'/ pin 3 line.
According to my calculations and experiments this gives rise to a rather high frequency for a bass roll off isn't it?

I am confused.

chef

EDIT

 

[Guest]

6 db/oct bass roll of. Do you really need that?

 

[Swedish Chef]

I am not sure WB!
That is what I am trying to find out! ;-)
What are other ways to do it?
The cross-line way obviously means there is no need to match parts...

chef

 

[rodabod]

I couldn't see it in this schematic:

Or am I blind?

But, from what you're describing, it's an inductor in parallel with ~200 Ohms?

 

[Swedish Chef]

[quote author="rodabod"]I couldn't see it in this schematic...[/quote]

Quite right. somebody must have pinched it... ha!

It is only on the C12a schem I have , not the C12 original, but yep the inductor in parallel with 200R...

Will attampt to get the schem up now...


chef

 

[Swedish Chef]

The best I can do is the VR, I'm afraid :-(
You get the idea though...

chef

 

[rodabod]

I can see this when it is switched in:

So, the transformer is really in parallel with an LRC filter. But, pin2 is tapped off after the cap rather than at the transformer secondary.

So, the potential seen between pins 2 and 3 is the Voltage sitting across the inductor and resistor? I think I'm confused.

 

[Swedish Chef]

[quote author="rodabod"]...I think I'm confused.[/quote]

Yep!

chef

 

[Swedish Chef]

I breadboarded the circuit and then ran some sweeps through different variations.
Makes interesting reading!
Sorry the image programme I used seems to have filled in the dots :-(
It's pretty easy to tell which is which.

chef

 

[Guest]

[quote author="Swedish Chef"]

I breadboarded the circuit and then ran some sweeps through different variations.
Makes interesting reading!
Sorry the image programme I used seems to have filled in the dots :-(
It's pretty easy to tell which is which.

chef[/quote]

All of them are useless. However, if you are going to mike too close it may help...

 

[Swedish Chef]

[quote author="Wavebourn"]All of them are useless. However, if you are going to mike too close it may help...[/quote]

Useless?

We have a bit of a theme then, don't we?

Don't tell me, tell AKG!

chef