Sound Skulptor forums

Build help and troubleshooting => MP573 Microphone preamplifier => Topic started by: rolly on October 07, 2023, 12:12:06 AM

Title: noise floor issue
Post by: rolly 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
A2.5 V 2.58 V
B4.0 V4.05 V
C22.0 V 22.4 V
D13.3 V13.5 V
E2.0 V2.02 V
F12,7 V12.88 V
G0.9 V0.896 V
H4.2 V4.11 V
I0.35 V0.342 V
J3.6 V3.56 V
K22.3 V22.6 V
L3.0 V3.00V after 30 mins. of warm-up


Any advise on how to fix this?


Regards
Oli

EDIT: typo
Title: Re: noise floor issue
Post by: JPK on October 09, 2023, 03:36:43 PM
Your measured voltages are good.
Can you measure the noise AC voltage level on the output?
Do you get the same noise after removing the jumper on CN1?
Do you confirm that the noise doesn't vary in level with the -/+10dB potentiometer?
Title: Re: noise floor issue
Post by: rolly on October 11, 2023, 02:05:10 AM
With CN1 plugged in:

gain pot. set to -10dB
p-p mean68,8 mV
RMS mean4,98 mV

gain pot. set to +10dB
p-p mean69,9 mV
RMS mean6,9 mV


after CN1 has been removed:

gain pot. set to -10dB
p-p mean68,8 mV
RMS mean4,98 mV


gain pot. set to +10dB
p-p mean71,1 mV
RMS mean7,25 mV


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


Title: Re: noise floor issue
Post by: JPK on October 11, 2023, 11:49:52 AM
Look for reversed electrolytics or tantalum between C24 and C29.
Is the V+ voltage correct?

Can you try the following:
Create a temporary short circuit between pins 2 and 3 of U2 (you can carefully use a small screw driver) and listen if it reduces the noise. If it does, then the noise source is U2 and it needs to be replaced.
(https://i.imgur.com/yFG154c.png)
Title: Re: noise floor issue
Post by: rolly 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.
Title: Re: noise floor issue
Post by: JPK on October 16, 2023, 08:35:41 AM
Try removing C24.
Try replacing C8 and C27 (for testing you can use any larger value).
Try replacing Q4.
Title: Re: noise floor issue
Post by: rolly 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. ;)
Title: Re: noise floor issue
Post by: rolly on October 16, 2023, 02:30:42 PM
What ESR value/range and tolerances for the caps do you recommend?
Title: Re: noise floor issue
Post by: JPK on October 17, 2023, 09:17:49 AM
It doesn't matter.
We are just testing in case a defective cap generates the noise.
You can use any cap with a voltage similar or higher and a value similar or higher.
Title: Re: noise floor issue
Post by: rolly 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?
Title: Re: noise floor issue
Post by: JPK on October 01, 2024, 09:37:32 AM
Did you try moving the noisy unit to a different slot, in case the noise is picked up by induction?
Title: Re: noise floor issue
Post by: rolly 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.
Title: Re: noise floor issue
Post by: JPK on October 03, 2024, 06:46:18 PM
I'm out of ideas, sorry.
Can you send it to us for a check?
Title: Re: noise floor issue
Post by: rolly 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.