1

MP573 Microphone preamplifier / Re: 2 noisy preamps

« on: April 15, 2022, 01:14:23 PM »

Yes i could do that. How should i proceed ? 

2

MP573 Microphone preamplifier / Re: 2 noisy preamps

« on: April 14, 2022, 10:34:37 AM »

I did every of those solder joints again. No change...

3

MP573 Microphone preamplifier / Re: 2 noisy preamps

« on: April 13, 2022, 07:59:30 PM »

I took some pictures of the oscilloscope screen. The Volts/Div is on "5 mv" and the Time/Div is on "0.5 ms".

Here is the signal between pin 2&3 of the "noisy MP573 unit" output (same oscillation between the yellow wire and TP2) :




And here is the signal between pin 2&3 of the "healthy unit" :



The black pots of the two preamps are on 0 and the red switches on 10. The mute switch on "POL". 

4

MP573 Microphone preamplifier / Re: 2 noisy preamps

« on: April 12, 2022, 05:57:55 PM »

The noise between pin 1 & 3 has no meaning because pin 3 is floating when pin 2 is disconnected.

I see !
I'm not sure that my DMM is very precise.

I checked every solder joint, it seems to be in order...

I managed to get an oscilloscope, here it is :


Can you explain step by step how to check for a HF oscillation ?

5

MP573 Microphone preamplifier / Re: 2 noisy preamps

« on: April 09, 2022, 04:42:28 PM »

There is 0 mv between pins 2 & 3. 
About 50 mv between pins 1 & 3. 
Same results on my other 573 preamp.

6

MP573 Microphone preamplifier / Re: 2 noisy preamps

« on: April 09, 2022, 03:50:06 PM »

Hi !
I checked the preamp into another lunchbox this week, but there is the same output noise.

Can you measure the maximum noise voltage on the preamp output with your DMM in AC mode?

How can i do that ? Do i have to place the DMM between the ground pin and the hot pin, turning the gain pots at there maximum ?

I don't have any oscilloscope...

7

MP573 Microphone preamplifier / Re: 2 noisy preamps

« on: April 01, 2022, 07:56:28 PM »

You could try to bypass U1. This regulator is used to limit the inrush current at power up.
To do this you need to solder a small wire between the 2 extreme pins of U1. The wire can touch the center pin without a problem.

Just did, it seems that U1 is not guilty... The noise remains.

8

MP573 Microphone preamplifier / Re: 2 noisy preamps

« on: March 31, 2022, 11:44:55 PM »

I haven't installed another equipment close to the unit recently, and i just tested it alone in the lunchbox, far from any other equipment, the noise is still here. I could probably check the unit into another lunchbox next week, in another studio.

The noise is a high frequency hiss, and it disappear when the phase switch is on mute.
The level change is very slight. I recorded it turning the black pot from 0 to +10 and -10, with the red switch on 10: https://we.tl/t-xQT8kjy69q

9

MP573 Microphone preamplifier / Re: 2 noisy preamps

« on: March 30, 2022, 03:01:02 PM »

Hi !

I think i have the same kind of issue than Jgriff on one of my MP573 unit (i have two of those, rev 2.2 and 2.3). The most recent one (rev 2.3) worked perfectly for a year, then a constant output noise appeared on this unit. 
The noise level is not modified by the stepped gain switch (it's just more significant on the 20 position), but it is slightly modified by the little gain pot. 
The DC voltages are very close to the expected results (on the "in case your MP573 doesn't work as expected" page).
The noise is still here when i remove the jumper on CN1, and same thing when i bypass U2.

Any idea ?

Thanks,
Jules