PM1000 - 50Hz/60Hz Hum on EQ?

Hi all,

First off, thankyou to everyone for the huge amount of PM1000 module racking info. I think I posted one power supply related question several months ago but since actually getting stuck into it I've been able to find everything I've needed on the boards without having to ask further questions. And this being my first DIY project, I've learnt a hell of a lot so big thanks again! I'm really enjoying it.

So after much labour, they're finally up and running. I was apprehensive on powering up for the first time but everything went smoothly - I think I got it right first go! Clean and quiet (much quieter than expected) signal passing through, phantom working properly etc.

Except When I turn up the EQs, especially the bass, I'm getting a large degree of 50Hz (Oz) hum/inteferance in the audio (or higher harmonics of, with the mids and highs). Likewise, the HPF switch is causing the same problem and both channels are behaving identically. I think a previous poster had this identical problem but fried his power supply before it could be solved.

I've uploaded a file to:

http://www.secondworldstudios.com/stuffz/pm1000 eq tests.mp3

Note that there's a fair amount of 50Hz present even when the EQ is flat but this file has been normalized with a lot of gain added. As mentioned, I was quite impressed with how quiet the preamps are.

The file plays in order -

1) Flat
1) HPF to 40Hz
2) HPF to 80Hz
3) HPF back to 40HZ
4) Flat
5) Bass boosted to 10
6) Bass cut to -10
7) Mids at 1k, boosted to 10
8) Mids to 2k, boosted to 10
9) Mids to 4k, boosted to 10
10) Mids back to 2k, boosted to 10
11) Mids back to 1k, boosted to 10
12) Mids at 1k, cut to -10
13) Mids to 2k, cut to -10
14) Mids to 4k, cut to -10
15) Mids back to 2k, cut to -10
16) Mids back to 1k, cut to -10
17) Flat


After reading previous threads and researching elsewhere on the board, I came to the conclusion that the hum is being caused by EMI from the power supply transformer. And if you see where I've placed the power supply this makes a lot of sense. Live and learn...



Another (short) file -

http://www.secondworldstudios.com/stuffz/pm1000 shield test.mp3

- is me moving a 3mm alluminium panel in and out between the power supply and the channel strips, while the bass is boosted to 10. Not doing a lot of course but clearly seeming to knock off some of the higher harmonics.

So I wired up another power supply outside the chassis, about 1m away and problem solved...no hum whatsoever. So an external supply is certainly one easy solution to my problem.

Then I moved the power supply in the case to the far right corner (where it should have been in the first place)...and it made a sizable difference (about 10-15db) but the interference is still much too loud to make the EQ usable.

The thing is, I've seen photos of several people's units in similar sized chassis with identical power supplies (Scodiddly, KHStudio etc.) and they don't seem to be suffering the same problem.

What I'm wondering is:

1) AFAIK I've grounded everything properly, adhering to advice in all previous posts and checked this very carefully, but is it possible I've missed something and could this be causing the 50Hz hum?

2) I never recapped either of the channels - could this in any way be causing the problem?

3) My DCV out of the power supply is 48.3V and post the diodes is 43V - one too many diodes! The circuit still seems to be operating fine but is there any way this could have anything to do with it?

Just to quickly eliminate number 3, i removed a diode...now at 43.8V and no change in hum. I couldn't really imagine this was the cause but thought it might be worth mentioning anyway.

the only problems I have ever had with PM1K modules in a rack were all grounding related. Make sure to play around with a few different grounding schemes until you find the quietest one. I would try that before even moving the PSU around

I don't think you'll ever get it quiet with the large power transformer that close to the channel strips. I'd move the PS out of the box if it's not a hassle for you.

If you want to keep the PS in the box, you could always get/make a smaller PSU with a torroidal transformer as these strips only draw about 50mA each (I think I measured 48mA on the last ones I racked).

Cheers,
--
Don

I’m not sure If this would cause the problem on not, but did you disconnect any of the grounding from the grounding tabs of the EQ pots?

I can't tell from the photo, but on each of the EQ pots (or at least on mine PM1000's) there’s a grounding tab for each pot to go to ground. The pots grounding tabs are tied together and then tied to a grounding point on the board.

Since it seem like you just cut the chassis down, I can't imagine the tabs being disconnected, but I thought I would just throw the idea it out there.

Elk

Thanks guys.

I've played around with the grounding some more but no success.

1) Input shields connected to chassis.
2) Output shields connected to circuit ground.
3) Circuit ground connected to - of power supply
4) Circuit ground connected once to chassis (when this is disconnected, no connection between circuit ground and chassis can be measured)

Elk - I thought this might be it as well but the eq ground tabs are still connected as they were originally.

idylldon - I've got a console of strips (16) to rack up. I'd like to keep at least 8 of them permanently in the rack and for those, I will use an external power supply. But for this pair and a few others it would be great if they could be all-in-1-box for portability.

I've connected the other power supply outside the case again. When I move it to about 1ft or so from the circuit boards the hum appears...so I'm really thinking this has to be it. It's true 50Hz hum with a second spike at 200Hz, so I'm guessing it has to be originating from the primary winding of the power transformer - and isn't noise from improper grounding on the DC side of things. Does this ring true? Also for this testing, the AC ground is disconnected and I'm just being very careful not to short anything.

I'm still intrigued as to how others are using this same power supply successfully in a small chassis? It's the Power-One HB48-0.5-A, that is pretty common to use for this project.

I wondered if the transformer might behave differently when jumpered for 240V. This seems to connect the 2 120V windings together to make 1 long 240V winding, rather than the 2 120V windings being connected in parallel. I thought maybe something smart might be happening when jumpered for 120V, whereby the 2 primary windings are wired in a way to cancel each other's interference out - not sure if this is even possible - but I jumpered it for 120V and plugged in via a 240/120V stepdown transformer - and absolutely no difference...

Regarding using a toroidal transformer, I am very interested in this. The current transformer has dual secondaries - which appear to be at 56V and 12V. I'm assuming the 12V might be to power the regulating circuit? I was wondering if it would be possible to retain the existing regulating circuit and simply replace the current laminated transformer with 1 or even 2 toroidals to provide the 56V and 12V?

Sorry for these long posts...I have trouble condensing what I want to get across

That looks like the same PSU that I've used a couple times with no problems. Maybe the issue is that it's close to the EQ inductors... in the ones I racked I stuffed it as far into the corner (right rear corner) as possible.

yeah, that's what I figured...but it's pretty much just as bad in the right rear corner. I literally have to get it over a foot away from the rear of the chassis before the hum goes away. A tiny bit closer and I can rotate 90 degrees and it's ok...but inside the case this makes minimal difference.

could there be a component problem on the boards that's making them more susceptible?

When I used the Power-One psu, I also ran a ground wire from the psu chassis to ground point on the rack chassis, or if you have it screwed to the chassis, make sure that the paint is removed, etc so its a good ground. couldn't tell from your pictures.

Mike

Hey,

    Did you ever solve this? I'm having the exact same problem. 60hz hum most noticeable when I engage the HP filter (medium loud at 40, twice as loud at 80hz), and also when I turn up the bass eq.

    I've been moving the grounds around like a mad man, but with no luck.

It's much more likely proximity of the toroid/psu to the inductors than earthing issues.  Can you safely move/rotate the toroidal transformer hile the unit is on and listen for minimum hum?

Nailed it!    Thank you so much, you just saved me from cutting my whole pm1000 grounding setup apart. I went ahead and took the supply out of the rack. They are so much quieter and I like them a lot better now. There wasn't much room to move it around in there anyway. Now I just need a place to hide all these external power supplies!