BA Issue

Support for JLM Audio Kits

Moderator: Joe Malone

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Paul
Posts: 30
Joined: Sat Mar 24, 2007 5:16 pm

BA Issue

Post by Paul »

morning all,

one of my BA channels is acting a little strange. it seems to work until i put the gain up and make a loud sound and the pre amp start making a thumping sound that is repetitious that continues until i turn the gain down and im not sure what the cause is. This pre is using the OEP transformer and the OPA2604AP op amp.

This BA is build on the PCB board marked 09/07 so im not sure if this is the root cause and that the fix to correct the hum issue would solve this as i have not implemented this yet (also have another BA with jlm transformer and jlm99v op amp which has a little bit of hum however not the issue as described with the repetitious noise above that needs the fix in place too).

any ideas?

thanks

paul.

chrisp
Posts: 184
Joined: Sun Mar 04, 2007 11:46 am
Location: Sydney, Australia

Re: BA Issue

Post by chrisp »

Are you saying you have not implemented any part of the "hum fix"? It addresses two issues - one relates to C16 (an electro DC blocker, hence "hum" fix) where you cut a trace and put a link to 0V, but the other is to mount the transformer's load capacitor (CL) on the underside of the PCB. If you have done neither, I would try it out as the first step - especially the CL cap - to see if that helps.

If not, either post or email Joe with a good photo of the PCB top and bottom, and we'll all look further.
Chris P
---------------------------------------------------------
I do lots of things. I believe eclectic skills are best.

Paul
Posts: 30
Joined: Sat Mar 24, 2007 5:16 pm

Re: BA Issue

Post by Paul »

thanks chris. i am planning to do those fixes this weekend, hopefully it sorts out the issue i am experiencing; just ensuring the issue im having wasnt related to something else.

Just to clarify on the fix before i go destroying something

What do you mean by cut a trace? Does it mean to cut the pcb to remove that connection on that small horizontal line in the pic? And then i simply use a wire to join the 0v to the cap.

Now i think the kit came with an extra cap (CL) as i put the other one in already which is not used.

Image

chrisp
Posts: 184
Joined: Sun Mar 04, 2007 11:46 am
Location: Sydney, Australia

Re: BA Issue

Post by chrisp »

That's what the instructions say - my BA boards pre-date this, so I've never done it, but yes, you need to physically cut the pcb where indicated by that small horizontal yellow line to break the trace (when I've had to do this for other projects, I have used a sharp, small bladed knife or a very small jewellers screwdriver), and use an insulated wire to connect the capacitor pin to 0V as indicated.

FWIW I would do the CL thing first and see if that fixes your problem - you will need to then do the rest of the fix at some point in time, but at least the CL thing is non-destructive so you don't need to sweat it.

Joe, step in here if this is going wrong or have other suggestions.
Chris P
---------------------------------------------------------
I do lots of things. I believe eclectic skills are best.

Paul
Posts: 30
Joined: Sat Mar 24, 2007 5:16 pm

Re: BA Issue

Post by Paul »

thanks chris, that's how i read them too, just making sure im heading in the right direction.

Paul
Posts: 30
Joined: Sat Mar 24, 2007 5:16 pm

Re: BA Issue

Post by Paul »

CL didnt do anything for me.

I checked all my voltages and they seemed quite ok. it only seems that this occurs when gain is up really high so if need be i can get away with not using excessive gain nearly at max level so its not a show stopper.

i'll email joe and see what he can tell me.

thanks for the help chris.

Paul
Posts: 30
Joined: Sat Mar 24, 2007 5:16 pm

Re: BA Issue

Post by Paul »

and i can confirm it was all in the 09/07 hum fix. woo!

chrisp
Posts: 184
Joined: Sun Mar 04, 2007 11:46 am
Location: Sydney, Australia

Re: BA Issue

Post by chrisp »

Cool!

Good to know they are now working well.
Chris P
---------------------------------------------------------
I do lots of things. I believe eclectic skills are best.

Paul
Posts: 30
Joined: Sat Mar 24, 2007 5:16 pm

Re: BA Issue

Post by Paul »

well one of the channels i did the hum fix on doesnt work with its gain pot for some reason.. grrr!! but i'll get to the bottom of this!

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