HUM PROBLEM --> JLM FetDI input jack

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AW_music
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Joined: Thu Nov 27, 2008 10:34 pm

HUM PROBLEM --> JLM FetDI input jack

Post by AW_music »

Hi everybody,
i need some help on this project.
I've finished my SSL pres and they are fantastic, so i've decided to add the JLM FET DI+ go between
http://www.jlmaudio.com/JLMDI.htm

The Phase, Phantom, Pad are OK, but when i plug a jack on the input DI a BIG HUM (i think 50Hz) goes through the signal and inside the PRE.
SO it seems impossible to use a guitar direct with this pres.

I have used the star ground scheme, so i can't understand what's the problem.
Maybe the JLM DI cannot be used without an audio input transformer ?

Someone can give me a clue ?

PS: i've posted the same problem on prodigy forum so it can help others from my mistakes
http://www.prodigy-pro.com/diy/index.php?topic=30605.0

Best regards

Image

conleycd
Posts: 31
Joined: Mon Aug 06, 2007 9:26 am

Re: HUM PROBLEM --> JLM FetDI input jack

Post by conleycd »

How are you powering the DIs? If you are powering them from the GoBetween's phantom power - then I might be at a loss for suggestions. If you are powering them separate from the GoBetween (like the connectors right on the DIs) then there are two wires you need to take out of the ribbon that connects the GoBetweens to the DIs (you need to disable the phantom power in the ribbon). I can't remember which ones they are but it is listed somewhere here on the site. I'm sure Joe could tell you too (or ChrisP who is always super helpful).

CC

chrisp
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Location: Sydney, Australia

Re: HUM PROBLEM --> JLM FetDI input jack

Post by chrisp »

Hi there Aw

ConleyCD's recommendation is heartwarming but misplaced - I'm just an average hack, believe me.

Its hard to diagnose over the 'net, but here's my thoughts. With no jack in, the DI board acts as a simple through, so its no surprise that it works OK. If anything, it just shows that you've got the switching jack properly wired to the PCB :-> When you plug in a jack, you get the hum, right?

I think to get the diagnosis going, you should disconnect the Go Between, and wire up the DI card by itself to the preamp, using the connectors on the back of the DI card. If this works fine, it means the problem lies in the 10-pin connectors (possibly mounted or wired back-to-front), but if its still a problem, its a short on the PCB. When troubleshooting, my general approach is to try to find this sort of either/or test that narrows down where I should be looking.

The other thing is that I don't know about the SSL preamp you're using. Can you talk us through the power supply arrangement you have in place?
Chris P
---------------------------------------------------------
I do lots of things. I believe eclectic skills are best.

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Joe Malone
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Re: HUM PROBLEM --> JLM FetDI input jack

Post by Joe Malone »

Ok I would think this will be a 48v power rail issue as the power supply kit you have is made to provide some phantom power for few mics from memory. The JLM FET DI runs on the 48v rail using 25 to 35mA each at idle. Is the 48v reg on a good heatsink? Does the 48v reg input pin have 55v unreg or more coming into the reg with everything plugged in?

Try unplugging 3 of the DI's so there is not as much load on the 48v rail to see if the hum goes away or is quieter in the one DI left plugged in.

Let me know what you find and we should be able to get you up and going quickly.
Joe :-)
JLM Audio
Capturing Audio without Injury

AW_music
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Joined: Thu Nov 27, 2008 10:34 pm

Re: HUM PROBLEM --> JLM FetDI input jack

Post by AW_music »

Ok, i have tried to workaround with the shield, but everything i do, i've got always the HUM.
The HUM is still present even if i disconnect the ground/shield connection.

I've done what JOE suggested, and when disconnecting the others DIs, leaving only one , the HUM is quieter in the one DI left plugged in.

I think this is the right direction, a "48v power rail issue" ! Wink

So what can i do now to eliminate it? Huh

conleycd
Posts: 31
Joined: Mon Aug 06, 2007 9:26 am

Re: HUM PROBLEM --> JLM FetDI input jack

Post by conleycd »

I'm not sure if you answered these suggestions already - but make sure you are not double powering the DI. Also, make sure pin1 on the gobetween ribbon is going to pin1 of the DI.

CC

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Joe Malone
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Re: HUM PROBLEM --> JLM FetDI input jack

Post by Joe Malone »

Let me know what AC volts the windings on the transformer are? And direct me to the circuit for the power supply you are using? And let me know if you made any changes to cap values etc? And I will do the math :)
Joe :-)
JLM Audio
Capturing Audio without Injury

AW_music
Posts: 3
Joined: Thu Nov 27, 2008 10:34 pm

Re: HUM PROBLEM --> JLM FetDI input jack

Post by AW_music »

Hi Jim,

finally i've found the time to give you all the infos you nedd.
I've used a 0-220 to 0 -18V 1.39A / 0-18V , 1,39A toroid .
and this is the schemo :
Image

I hope you can give me "the solution" ....

Many Thanks

:)

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Joe Malone
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Re: HUM PROBLEM --> JLM FetDI input jack

Post by Joe Malone »

Woops edit I missed the 18v - 0 - 18v which is to low. So you will need at least 20 - 0 - 20 to make enoguh voltage for the 48v rail to work properly. A 22 - 0 -22 from digikey would work perfect. 18 x 1.414 x 2 = 50.9 - 1.4 = 49.5v going into the 48v reg and it would need to be at least about 54v for the 48v reg to work properly.

If the 2 x 18vac measures a bit higher in reality then maybe change C3 and C4 to 1000uF 50v caps so they pump the voltage doubler for the 48v better and also C5 to a larger value like 470uF-1000uF 100v cap and that may fix it without the transformer change.

Our AC/DC has a 1000uF for all of these 3 cap values standard to hold the pump charge rail smoother for the 48v reg to work from.
Joe :-)
JLM Audio
Capturing Audio without Injury

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