Skip to main content

Adjusting AMEK 9098 mic preamps CMRR trimpots and capacitor?

Dear Friends

Please forgive me, i'm new to this forum, sol maybe this thread discussed before.
I'm experimenting with a diy mic preamp, based on AMEK 9098 mic preamp schematic.
I'd like to know, is there a good/reasonable method to set the input variable capacitor and the two trimpots(LOW/HIGH CMRR TRIM)?
On the attached schematic the parts: VC1,VR14,VR15.

I heard, that the adjusment must be done at mid gain setting (30-36dB) and the trimpots are for mid frequency, and the VC1 is for high freq.
I imagine to short the two inputs, fed with some sinusoidal sign, and with an oscilloscope, monitor the outputs to have a minimal or zero sign between the high and low differential outputs (XLR pins: 2 and 3)
Is that correct?

Thank You in advance!

Capacitor Shootout

I recently did some comparisons of some popular capacitors for the electric guitar and I thought I would share the results with you. I tested poly film caps which are the little green rectangles that have been the standard on fender strats for over twenty years. I also tested ceramic discs which are a bit more modern or so I'm told. I got a bunch of orange drops and even threw in a monolithic cap too, just for fun :D

Gear Used:
Guitar: Jackson RR3
Pickup: DiMarzio PAF PRO in the neck position
Amp: Tube Works 100w Head through Crate 4x10 cab
Mic: SM57 positioned at speaker edge pointed at center of speaker cone. Some may call this my signature position :D

Things to know before listening:
The differences aren't going to be as obvious as a microphone shootout. Don't expect it to sound like a different guitar playing each part, because it isn't going to. Each capacitor has it's own 'flare' if you will, accenting certain frequencies along the spectrum. You may notice that some make the sound relatively flat while others give the sound some depth. You may also notice that some sound warm while others are a bit harsh. That said, the differences are subtle but through a nice system and with a good pair of ears you should be able to detect the inherent differences.

Now without further ado, the files. They are in WAV format but I compressed them for upload, so I'm not sure if that harmed the file's quality at all but the compressed package takes up about half the space as the uncompressed files.

short video

The cap names were too long for the file so here is what each letter is.

A - Polypropylene 400v Orange Drop .022
B - Polypropylene 400v Orange Drop .047
C - Polyester 400v Orange Drop .022
D - Polyester 400v Orange Drop .047
E - Monolithic .1
F - Ceramic Disc 50v .022
G - Ceramic Disc 50v .047
H - Poly Film 100v .022
I - Poly Film 100v .047
J - Polyester 100v Orange Drop .022
K - Polyester 100v Orange Drop .047

I may do the same test on distorted guitar in the near future if this gets good reviews 8-)