SoundSkulptor MP573 - grounding problem

[Thompa]

Hi,

I just built my first kit, the MP573 preamp.  It works fine with the DI and a dynamic mic without phantom power. Sounds nice! But, I have a problem:

When I use a condenser mic with 48V tuned ON, I get a humming sound (sounds like 50Hz hum) and sometimes I get a sort of crackling sound. As soon as i touch the chassi of the lunchbox or my audio interface it stops. So some kind of grounding problem when using 48V it seems.

I have tried different slots in the lunchbox and different cables. I also checked solder joints around where the +48V comes in. Any idea what could be the problem? The lunchbox is Fredenstein Bento 6S (the new S model is supposed to be very quiet).

JP from SoundSkulptor suggests "Try grounding the lunchbox case." But I don't understand... isn't that allready grounded via the powercable into the wall socket? Like all modern electronic gear.

Schematic:
http://www.soundskulptor.com/docs/mp573-schematic-02.pdf

 

[[silent:arts]]

Before looking at the MP573, do you have another MicPre to test in that Lunchbox?

Up to now I only had one customer with noise problems in a OV560 module,
after he changed the Rack from Fredenstein to another brand all was good.

 

[Thompa]

Unfortunately the mp573 is the only module I have. I bought the Bento 6 to get started with 500 series. So you say that the lunchbox itself could be defective in some way?

 

[[silent:arts]]

Doesn't need to be defective. Some Racks and PSUs are just less compatible than others.
Your Fredenstein 6S has a Ground Lift Switch on the back, does this change anything?

 

[Audio1Man]

Hi Thompa
In the schematics I only see two connections to CHASSIS GROUND and no connections to the signal grounds. The mic has no phantom power return path.  Not all of the schematics are posted.  You need some path the connects the signal & chassis grounds.

Duke

 

[[silent:arts]]

In the schematic posted I see pin5 and pin13 connected to the phantom power relevant ground (page 2)

 

[Thompa]

Doesn't need to be defective. Some Racks and PSUs are just less compatible than others.
Your Fredenstein 6S has a Ground Lift Switch on the back, does this change anything?

Thanks guys. I am a novice at audio electronics so I really appreciate that your trying to help me. No, the ground lift don't seam to do anything (a bit more noise).  It's strange... sometimes when I turn the Bento on there is almost no hum/noise and other times there are. It kind of comes and goes.

Hi Thompa
In the schematics I only see two connections to CHASSIS GROUND and no connections to the signal grounds. The mic has no phantom power return path.  Not all of the schematics are posted.  You need some path the connects the signal & chassis grounds.

Duke

Thanks. I'm not vey good at reading schematics, but did you see there are 4 pages total?

I'm attaching a pic of the Bentos pin description. I assume these follow some kind of standard and are the same in all lunch boxes. Anyway here it is:

EDIT: Ok, so I did a continuity test of the pins on the connector of the pre. Pin 5 (audio ground) is not connected to pin 1 (chassi ground). Pin 13 (PSU ground) is not connected to pin 1 (chassi). But Pin 5 and 13 are connected. Is that correct? In my vey basic understanding I thought that all "grounds" should be connected together? But I may be totally wrong... 

 

[[silent:arts]]

EDIT: Ok, so I did a continuity test of the pins on the connector of the pre. Pin 5 (audio ground) is not connected to pin 1 (chassi ground). Pin 13 (PSU ground) is not connected to pin 1 (chassi). But Pin 5 and 13 are connected. Is that correct? In my vey basic understanding I thought that all "grounds" should be connected together? But I may be totally wrong... 

This is correct. Chassis is not really a ground, and should connect to "the other grounds" in the PSU of your lunchbox only.
I'm sure it is the PSU of the Bento.

 

[Thompa]

I got a chance to try it in another lunch box today. No problems! So yes, it seems like the Bento is the problem then. I will try to return it. Thanks guys!

 

[[silent:arts]]

My taiwanese customer brought his Bento back to the (taiwanese ) Manufacture.
They said they can only use the "green mode power" PSUs anymore, and "some old design 500 module will have this wired noice".
So they know the problem.

 

[JAY X]

Hi!

Last week a friend got the same problem with some soundskulptor preamps: MP512 and MP573. He has a fredenstein bento 10 rack, loaded with 8 preamps. 4 MP573 and 4 MP512. The bento rack phantom supply was noisy. He also got a small MIDAS rack. He tested all the preamps and the phantom supply and worked well on the Midas rack. No phantom noise.  Except one MP573 preamp.

The MP573 phantom schematic is simple. I checked all grounds and resistor values... all are ok. ¿Could it be the electrolytic capacitor? ¿Anyone with experience with this preamp?

Thank you for your advise!

Jay x

 

[JPK]

Does the noise follow the unit if he swaps 2 MP573 ?

 

[JAY X]

Hi JPK!

To be more precise, I think that the phantom supply on that unit doesn't work anymore. On the other units it works well. Phantom noise/ripple was related to the fredenstein rack. But on this single unit it does not work anymore. We tested condenser mics. Dynamic mics work ok, no problem.  if we had the 500 extender cable we could do some testing, but we had not one..

Te phantom circuit, after the schematics, seems to be ok, solderings... the switch circuits, etc... Maybe it is the electrolytic capacitor?..

Jay x

 

[JPK]

Check if the 48V gets to the input XLR pins 2 & 3 (measured between the pins and the lunchbox ground).

 

[JAY X]

Hi!

Ok! I will test ASAP!!

Jay x