Lunchbox Magnetic Field / PSU issue?

[Craig32768]

I think I'm getting a stray magnetic field in my Lunchbox 6b too. I just put in a SoundSkulptor MP573 and when cranked up noticed a noticeable buzz / faint hum. I took it out and attached it to an extender cable - problem gone. Whilst still attached to the cable I slid it into the lunchbox the correct orientation - buzz came back. Rotated it whilst inside the lunch box 90 deg - 180 deg buzz gone..... The buzz is only there when correctly vertical. Doesn't matter which slot I put it in.

So.... I'll contact JP from SoundSkulptor to see what he thinks - but has anyone come across this issue / have any suggestions. The lunchbox is in warranty also so I could drop it down to the dealer and test another I guess.

 

[pucho812]

what slot did you put it in? can you try it in another slot? what happens if it's in slot 1 as far from the PSU as you can get?

 

[Craig32768]

what slot did you put it in? can you try it in another slot? what happens if it's in slot 1 as far from the PSU as you can get?

Yeah - tried it in slot one. I also got an email back from Jean-Pierre saying that it's an induction issues with the PSU in the API 6 space Lunchbox. I'm a little disappointed if that's the case as it does not mention that on the site. I'll take my lunchbox to the dealer and try another incase it's just the particular unit though. It sounds amazing - if it wasn't for the noise  I'll update the post with my findings incase anyone comes across the same issue.

 

[mitsos]

Does this happen with other modules?  One of the reasons we added a full metal jacket to the EPS-1S5 was to avoid this kind of interference from lunchbox PSUs, in the slot furthest to the right.  We found that having another module to the right of the open-faced EQ had the same effect as a full enclosure on the EQ itself.  Does the buzz go away if there is another module between the 573 and the PSU? 

 

[Craig32768]

Does this happen with other modules?  One of the reasons we added a full metal jacket to the EPS-1S5 was to avoid this kind of interference from lunchbox PSUs, in the slot furthest to the right.  We found that having another module to the right of the open-faced EQ had the same effect as a full enclosure on the EQ itself.  Does the buzz go away if there is another module between the 573 and the PSU?

It's the only module in there at the moment as I leant my VP28s to a friend. I'm getting them back on Monday so i can put them in slots 2 and 3 and see if that solves the issue I guess. I'll try on Monday and update the thread with what happens.

 

[pucho812]

Does this happen with other modules?... We found that having another module to the right of the open-faced EQ had the same effect as a full enclosure on the EQ itself.  Does the buzz go away if there is another module between the 573 and the PSU?

That was going to be my second suggestion. but wasn't sure if there were more modules lying around. Other open modules do that to and not just on the 6 space lunch boxes, they also do it on the 11 space lunch boxes that have an internal PSU. I forget when they were made like that.

 

[Craig32768]

Does this happen with other modules?... We found that having another module to the right of the open-faced EQ had the same effect as a full enclosure on the EQ itself.  Does the buzz go away if there is another module between the 573 and the PSU?

That was going to be my second suggestion. but wasn't sure if there were more modules lying around. Other open modules do that to and not just on the 6 space lunch boxes, they also do it on the 11 space lunch boxes that have an internal PSU. I forget when they were made like that.

I'm wondering how easy it would be to get a thin sheet of copper or something and close the MP573 in and if that would make any real difference.

Not sure why manufacturers wouldn't make the modules more enclosed if this is a common issue.

Also wondering if adding a sheet of lead / Copper to the  side of the PSU would solve this if there is space.

 

[mitsos]

I'm wondering how easy it would be to get a thin sheet of copper or something and close the MP573 in and if that would make any real difference.

Easy enough, but you'll have better luck with steel if it's transformer hum.

Not sure why manufacturers wouldn't make the modules more enclosed if this is a common issue.

Not sure either, but I think it depends on the circuit. I've had loads of issues with Neve circuits, the gain switch is horrible for picking stuff up. For comparison, API 512's don't have a cover, neither do the VP28's I think, and I'd bet they don't pick up as much noise.  I wonder if anyone has had noise from the other 500 series neve stuff?  I'd talk to JP, maybe your unit has a problem?  His stuff seems pretty well thought out. 

Also wondering if adding a sheet of lead / Copper to the  side of the PSU would solve this if there is space.

Easy enough to test, but again, you want a ferrous metal. I'd wait to test with the VP28's shielding the 573 and decide if it's worth it.

 

[Craig32768]

I'm wondering how easy it would be to get a thin sheet of copper or something and close the MP573 in and if that would make any real difference.

Easy enough, but you'll have better luck with steel if it's transformer hum.

Not sure why manufacturers wouldn't make the modules more enclosed if this is a common issue.

Not sure either, but I think it depends on the circuit. I've had loads of issues with Neve circuits, the gain switch is horrible for picking stuff up. For comparison, API 512's don't have a cover, neither do the VP28's I think, and I'd bet they don't pick up as much noise.  I wonder if anyone has had noise from the other 500 series neve stuff?  I'd talk to JP, maybe your unit has a problem?  His stuff seems pretty well thought out. 

Also wondering if adding a sheet of lead / Copper to the  side of the PSU would solve this if there is space.

Easy enough to test, but again, you want a ferrous metal. I'd wait to test with the VP28's shielding the 573 and decide if it's worth it.

I wonder if there's an market for 500 module covers / shields to cover open modules? These would need to be ferrous though - maybe thin galvanised steel? I get my VP28s back tomorrow so I'll put them in and see. I'm also completing my JLM LA500 tomorrow too so I'll through that one in too.

 

[Craig32768]

Just out of interest - I noticed that there seems to be a mumetal shield on the side of the PSU. I tested for continuity between this and pin 1 / earth on the mains connector and there was none. Should there be a connection from the mumetal shield to earth?

 

[JPK]

I just made the test myself with a MP573 in an API 6 slots.
I do get some hum increase in the rightmost slot (about +3dB). The hum comes in through the input transformer which is notoriously sensitive.

But I get no hum at all in the leftmost slot, same thing with the extender cable, when the module is outside the lunchbox.

I can see 2 possibilities:
1. Your MP573 has a problem, and it is not simply hum you are picking,
2. Your Lunchbox transformer is radiating too much.

It would be a good starting point to check with another Lunchbox as you mentioned before.

If the problem persists in a different lunchbox, it would help help doing the following measurements:

1. Max -no clip- output
Place the MP573 in the leftmost slot of the API Lunchbox,
inject a 1KHz, 0.5 volts AC, sine on input,
(you can loop this audio file),
adjust the output for maximum level without the clip LED turning red,
measure the output AC volts (betwen pins 2 & 3 of the XLR)

2. Noise
disconnect input,
set gain switch and pot to max,
measure the output AC volts

 

[Craig32768]

I just made the test myself with a MP573 in an API 6 slots.
I do get some hum increase in the rightmost slot (about +3dB). The hum comes in through the input transformer which is notoriously sensitive.

But I get no hum at all in the leftmost slot, same thing with the extender cable, when the module is outside the lunchbox.

I can see 2 possibilities:
1. Your MP573 has a problem, and it is not simply hum you are picking,
2. Your Lunchbox transformer is radiating too much.

It would be a good starting point to check with another Lunchbox as you mentioned before.

If the problem persists in a different lunchbox, it would help help doing the following measurements:

1. Max -no clip- output
Place the MP573 in the leftmost slot of the API Lunchbox,
inject a 1KHz, 0.5 volts AC, sine on input,
(you can loop this audio file),
adjust the output for maximum level without the clip LED turning red,
measure the output AC volts (betwen pins 2 & 3 of the XLR)

2. Noise
disconnect input,
set gain switch and pot to max,
measure the output AC volts

Thanks for your support JP.

I get no hum / buzz when the MP573 is on the extender and all the set-up voltage checks / bias seem all fine so I think the unit is fine. In the setup test and bias setting the voltage tests without the output transformer plugged in was about 25v and around 24.8v with the transformer plugged in.

I'm going to see a friend with another lunchbox tomorrow and try it in his before contacting the dealer. If I get the same issue in his, then I'll carry out the two other tests you suggest.

I just want to take a moment and thank people like yourself who make these DIY kits available. I suspect a lot of your time is spent in looking into technical issues and your support has been great - thank you.

I'll update the thread tomorrow.

Cheers
Craig

 

[Craig32768]

Hi JP - Here's the results from your TEST 1 - Non-clipping. I've attached an image for reference.

0.5v 1k tone. Gain to 42db before red clipping LED. 13.2v on output.

On TEST 2 - Input unplugged gain full I get almost 0v on output.

I'm going into Auckland tomorrow to try out another lunchbox.

Cheers
Craig

 

[Spiritworks]

How did you ever make out with this issue?

 

[zayance]

Ah missed that topic.
Some time ago I had done a modular PSU that would feet the 51x rack for making it a 8 slot lunch box.
I had same magnetic field issus, even if the Toroidal was vertical and closest to to end of the rack (compared to Api design)
But this was especially obvious with inductor based EQ, but no prob on preamps tough, beside a little bit of hum on the slots close to the PSU, the latest ams-neve 500 stuff is having a full my metal enclosure, especially the EQ iirc, and I think it's a nice add for anyone using a lunch box.
I was able to tame those stray fields by using some Ultraperm 80  mumetal shielding found on EBay, got it down a pretty good amount, just one piece of foil evenly distributed on the Toroidal Side of my PSU.
But my PSU design is maybe not optimum, can't say I have years of experience in that domain, I did my best, but Toroidal that close will surely do some harm anyway...
Api lunchbox has a thin mumetal foil iirc, and their transformer is a high profile one, pretty high actually compared to the standard height we generally use in DIY (space was their concern obviously)
In a lunch box configuration a well designed switching PSU seems more appropriate, as seen on the latest Lindell Audio lunch box, and the well known Purple Audio product.

 

[JohnRoberts]

Just out of interest - I noticed that there seems to be a mumetal shield on the side of the PSU. I tested for continuity between this and pin 1 / earth on the mains connector and there was none. Should there be a connection from the mumetal shield to earth?

I believe mu metal mainly shorts out the magnetic field in the plane of the metal layer, so AFAIK it doesn't have to be grounded. What happens if you try grounding it?

JR