Altec P518a / Coke Bottle mic strange issue

[snoestudio]

hey guys,

I recently got an Altec 150a/21d capsule and P518a psu. I recapped the PSU and the mic is working with good sound and relatively low noise. However I'm running into something strange with the low frequency cut-off switch on the PSU.
The switch is a 3-position cut off at 20 (ie no cut off), 40 and 120hz. However, I noticed that I am getting considerably more low end at the 120hz position. I have checked the switch and the two caps that go with it to make the circuit, and I am totally confident that I have it wired exactly as the schematic; I've also checked the working of the switch and can see that it is doing what it's supposed to do (20hz position=no caps in circuit; 40hz=1 cap in circuit; 120hz= both caps in parallel).
There is definitely more low end at the 120hz position, and a hair more gain (maybe 1db more gain). The mic doesn't sound thin at the 20hz position, it's just that it seems bigger/bassier at the 120hz position.
I'm attaching the schematic here; can anyone with more experience than myself have a look at it? Is it possible that there is an error on the schematic??
thanks in advance for any help!

 

[mjrippe]

What output impedance is the transformer strapped for?  It could be that the secondary is not properly terminated by the mic pre and it is creating a resonant filter circuit causing a bump up at 120Hz.

 

[snoestudio]

thanks for the response. the Altec is strapped for 250 ohms output. I'm using a Studer transformer balanced mic pre with an input impedance of 1.2k. I never thought to try it with a different mic pre... I will do that and report back.

 

[PRR]

Switching caps on the transformer primary will be VERY load-sensitive.

Bypass that switch and bring your full-range signal in verbatim. You surely have better filters in your console and DAW.

 

[ruffrecords]

In your post you said the 120Hz position has both caps in parallel. They should be in series as per the schematic you posted.

Cheers

Ian

 

[snoestudio]

ah, sorry my mistake. the caps are in series not parallel.

 

[snoestudio]

ok mjrippe you were spot on the money. I installed 250 ohm terminating resistors and the low cut filter now works exactly as it should. thanks everyone for your help!

 

[mjrippe]

ok mjrippe you were spot on the money. I installed 250 ohm terminating resistors and the low cut filter now works exactly as it should. thanks everyone for your help!

And thank *you* for reporting back!  I'm sure it will help someone with the same problem in the future.