1

MP573 Microphone preamplifier / Re: 2 noisy preamps

« on: March 19, 2022, 04:50:48 PM »

Hi JP

it seems U2 is the problem on both units. when bypassed noise is gone.

2

MP573 Microphone preamplifier / Re: 2 noisy preamps

« on: March 17, 2022, 02:05:51 AM »

wondering if it might be the IC opamps. I've had those go in other gear and come to think of it, the noise was similar.

is there a way to check to see if the IC is ok?

3

MP573 Microphone preamplifier / Re: 2 noisy preamps

« on: March 16, 2022, 03:25:55 PM »

Hi JP

The noise level is not modified by the stepped gain switch BUT it IS modified by the gain pot. (did not notice that before...just checked the switch)

Noise is still present if i remove jumper at CN1 (one of the pres was connected to an EQ573 at CN1 but have disconnected EQ for all tests)

both PCBs are REV2.3 which i think are the ones i bought about a year ago. 

4

MP573 Microphone preamplifier / Re: 2 noisy preamps

« on: March 15, 2022, 10:27:33 PM »

Here's the reading from one of the preamps. the other is almost identical

A: 2v6
B: 4v0
C: 22v6
D: 13v6
E: 2v0
F: 13v
G: 0v9
H: 4v0
I: 0v35
J: 3v5
K: 22v8
L: 3v0

5

MP573 Microphone preamplifier / 2 noisy preamps

« on: March 15, 2022, 10:06:23 PM »

I have 8 mp573s. been using them for about 5 or 6 years. 

2 that i built about a year ago are now outputting noise. They still are working fine...but there's just this noise that is now present all the time. I have tried the preamps in different slots on my 500 series box and they put out the noise on different channels as well...working pres do not output noise in any of the channels...so its not the box, and its not a cable. It is low volume (around -60 db  but sometimes gets louder) does not seem to get louder if i increase the gain.

I have checked the bias, and have removed the DI. noise is still present. Have checked for bad solder, but all seems good....and the exact same issue is happening on 2 of them.

I find it curious that both of the "new" mp563s started doing this at roughly the same time. maybe some bad components in that batch? Any ideas how to fix?