[Jgriff]

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?

[Jgriff]

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

[JPK]

Is the noise level modified by the gain switch, by the gain pot?
Is it present if you remove the jumper on the CN1 connector?
What is the PCB revision number of these units?

[Jgriff]

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. 

[Jgriff]

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?

[JPK]

The IC's are not used in the audio path, only used for the clip LED.
I would try to bypass temporarily U2, by connecting its metal tab to TP2. This will bring the supply voltage from 24V to 32V so do not leave it too long. Check if the noise remains. If not replace it.

Next I would try to swap sequentially Q4, Q5, Q6 with a noiseless unit to find out if one of these is the responsible.

[Jgriff]

Hi JP

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

[JPK]

Contact me here to get replacements as they are becoming difficult to find.

[JulesMart]

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

[JPK]

Have you installed another equipment (audio or light) close to the unit recently which may radiate?
Check that it doesn't come from the lunchbox PSU itself.
What kind of noise is it? Hiss, buzzing, crackling, high or low frequency?
Is it disappearing when the phase switch is on mute?

but it is slightly modified by the little gain pot.
How much is the level change?

[JulesMart]

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

[JPK]

I could probably check the unit into another lunchbox next week, in another studio.
This would be helpful.

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.

[JulesMart]

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.

[JPK]

Can you measure the maximum noise voltage on the preamp output with your DMM in AC mode?
I would recommend to try the unit in a different lunchbox first.
If you can get access to an oscilloscope, check the output on TP4 for an HF oscillation. This would result from a failed C25, C26 or C28.
If there is no oscillation, start replacing sequentially C24, C27, C29, Q4, Q5.

[JulesMart]

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...

[JPK]

How can i do that ?
You must connect the DMM, in millivolts mode, to the output XLR, between pins 2&3.

[JulesMart]

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

[JPK]

The typical output noise of the MP573 with gain maxed is 4-5mV.
So there may be a sensitivity issue with your meter?
The noise between pin 1 & 3 has no meaning because pin 3 is floating when pin 2 is disconnected.

Sometimes the noise may come from a HF oscillation.
Check you haven't a bad solder joint on C25, C26, C28 for the output stage.
And C15, C16, C17, C21 for the input stage.

[JulesMart]

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 ?

[JPK]

With no signal in, place the scope probe on the transformer yellow wire and the ground clip on TP2.
Scope input switch on AC.
The display should be an horizontal line.
Increase the scope gain (VOLTS/DIV) until you can see something. If there is an oscillation you will see it. Adjust the sweep speed (TIME/DIV) to display it properly.
You can also connect the scope to the output XLR (pins 2 & 3) you might be able to see the noise.

[JulesMart]

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". 

[JPK]

Did you check you haven't a bad solder joint on C25, C26, C28 for the output stage?
And C15, C16, C17, C21 for the input stage?

[JPK]

Also check the solder joints of C2, C3, C4.

[JulesMart]

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

[JPK]

I am short of ideas. Maybe you could send it to us for a review ?