1

MP573 Microphone preamplifier / Re: noise floor issue

« on: October 04, 2024, 08:37:11 PM »

OK...i've plenty of time to recheck the amp this weekend and do a side by side comparison of the two units i have.
If this does not lead to a result, I will send it over to you for inspection.

2

MP573 Microphone preamplifier / Re: noise floor issue

« on: October 03, 2024, 12:43:30 AM »

Yes, that was the first thing i've tried. Same behavior in another slot and in your 501 as well.

3

MP573 Microphone preamplifier / Re: noise floor issue

« on: September 28, 2024, 01:40:01 AM »

Hi,

sorry for the huge delay, but sometimes things don't go as expected.
Finally i found some time to work on this issue again.


I have done what you suggested step by step, but unfortunately the issue still persists.

- removed C24 -> no change
- replaced C8 and C27 -> no change
- replaced Q4 -> no change either

What else can I try?

4

501 Single Host / unable to entirely close switcher shield

« on: November 14, 2023, 06:24:33 PM »

Hi,

finally i found some time to build the 501, but i'm not able to get the switcher PCB properly shieled.
Inductors have been installed as close as possible to the PCB but the coil's wire ends are causing a gap of 2-3mm, making it impossible to entirely close the switcher shield on this side.

How critical is this? Can i continue troubleshooting my noisy 573 and sort this out later?


regards
Oli

5

MP573 Microphone preamplifier / Re: noise floor issue

« on: October 16, 2023, 02:30:42 PM »

What ESR value/range and tolerances for the caps do you recommend?

6

MP573 Microphone preamplifier / Re: noise floor issue

« on: October 16, 2023, 09:05:14 AM »

OK, then I have to order and build a 501 first to be able to dig deeper into the troubleshooting on my desk with all necessary tools in easy reach, instead of sitting on the floor in front of a rack.

7

MP573 Microphone preamplifier / Re: noise floor issue

« on: October 14, 2023, 11:15:02 PM »

I checked all electrolytics and tantalum caps between C24 and C29 as suggested, none of them is reversed.

TP2 -> V+ = 24,2 V
TP 0V -> V+ = 15,2 V
TP 0V -> TP2 = -9.0 V

Short-circuit between pins 2 and 3 of U2 does nothing except giving a short peak signal to the output.

8

MP573 Microphone preamplifier / Re: noise floor issue

« on: October 11, 2023, 02:05:10 AM »

With CN1 plugged in:

gain pot. set to -10dB

p-p mean	68,8 mV
RMS mean	4,98 mV

gain pot. set to +10dB

p-p mean	69,9 mV
RMS mean	6,9 mV

after CN1 has been removed:

gain pot. set to -10dB

p-p mean	68,8 mV
RMS mean	4,98 mV

gain pot. set to +10dB

p-p mean	71,1 mV
RMS mean	7,25 mV

Turning up the gain pot makes the noise a bit louder, and removing CN1 doesn't seem to make any noticeable difference.

9

MP573 Microphone preamplifier / noise floor issue

« on: October 07, 2023, 12:12:06 AM »

Hi,

i've built two MP573's a few months ago, and they sound(ed) absolute amazing.

After initial tests and recordings i mainly keept using a single amp for vocals, leaving the second one untouched for a while.
Two weeks ago i wanted to do some stereo recording, but the second one now outputs an audible white noise. It's completely independent of any switch setting except the mute position of the polarity switch. Mute is working as intended and the amp itself does what it's supposed to do.

Voltages are looking good to me:

TP	ref.	measured
A	2.5 V	2.58 V
B	4.0 V	4.05 V
C	22.0 V	22.4 V
D	13.3 V	13.5 V
E	2.0 V	2.02 V
F	12,7 V	12.88 V
G	0.9 V	0.896 V
H	4.2 V	4.11 V
I	0.35 V	0.342 V
J	3.6 V	3.56 V
K	22.3 V	22.6 V
L	3.0 V	3.00V after 30 mins. of warm-up

Any advise on how to fix this?


Regards
Oli

EDIT: typo