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DIY Super Simple 6DJ8 amp hum issues.

post #1 of 10
10/30/10 at 11:44am
Thread Starter 

I have been posting in the Super Simple thread but I feel like that thread is only checked by a few regulars and while they have been and continue to be extremely helpful I figured it wouldn't hurt to ask the rest of the DIY community for some input.  

 

I finished the amp up on Thursday but my case has not arrived yet. The amp sounds fantastic but I am getting a nasty hum in a few situations. When the amp is on there is a low level hum that is dependent of volume but it doesn't go away when volume pot is at 0. The hum is worse when no source is plugged in to the RCAs. This is some what minimized by touching the sleeve of the DC jack or the sleeve of the headphone jack. Touching the body of the pot also gives a terrible terrible buzz.

 

I have the RCAs tied together and grounded to the PCB. I don't have a too terribly much wire running around and the wire I do have is either braided or twisted pair except for the RCA ground.  I have tried switching the polarity of the 2 prong wall wart this did nothing. It might be that I don't have an enclosure yet but I would like to figure out and minimize the hum issue before casing it up if possible. 

 

To show my wiring I made this crappy picture in paint. I only showed the left input but obviously the right is wired the same. I did show the right ground tied to the left ground then to PCB ground. 

 

Any suggestions would be appreciated.

 

6dj8.jpg

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post #2 of 10
10/30/10 at 12:57pm

Your wall wart may have ripple on it's dc output due to poor filtering. I would try adding a electrolytic cap in parallel with the power input and see if it reduces the level of hum. A 470 to 1000 mfd should be enough to tell you if it is helping. I suspect some of the rest of the hum you notice when touching jacks etc. will go away when installed in a chassis (presuming you are using a metal chassis). The metal shield of the volume pot is not grounded right now and makes an easy place for hum to get injected when you touch it.


Edited by Budgie - 10/30/10 at 1:01pm
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post #3 of 10
10/30/10 at 1:08pm
Thread Starter 

This is the pot that I have, link. Should the outer metal casing be grounded to PCB or case? Also do I ground both section of the casing?

 

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post #4 of 10
10/31/10 at 8:07am
Quote:
Originally Posted by yellowjeep View Post
 Touching the body of the pot also gives a terrible terrible buzz.

 


 


 



Pot's body must be grounded, it work like antena.

Try simply screw with pot nut pice of wire and solder other end to ground. Probably it can solve your other hum issues to.

If you plan to use non metal enclosure, better way is soldering ground wire to pot's body.

If you use metal enclosure problem solve itself by grounding enclosure.

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post #5 of 10
10/31/10 at 8:10am
Quote:
Originally Posted by yellowjeep View Post Also do I ground both section of the casing?

 



Both casings are connected together, check for sure with multimeter.

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post #6 of 10
10/31/10 at 9:42am
Thread Starter 

Awesome, thanks!

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post #7 of 10
11/1/10 at 12:02am
Thread Starter 

Grounding the pot solved 90% of the issue. I am hopping the case will solve the rest. Thanks for the help!

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post #8 of 10
11/3/10 at 7:47pm
Thread Starter 

I still am having issues. Putting a cap in parallel with power didn't do anything. It is 60Hz hum or close to it compared to a pure tone which based on what I have been reading is a PSU issue. I'm stumped. I suppose I should start looking for non 6 dollar PSU.

 

I guess I also have all of the panel mounted components grounded to the chassis including the DC jack, from what I have read this could also be a problem because I am tying two different ground voltages to together. Would just isolating the DC jack help? 

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post #9 of 10
11/4/10 at 4:24pm

Which panel mount components are grounded?

Most important is RCA isolated from casing. Output phone jack better isolate too. Than you can leave DC jack grounded, it work like casing ground to PCB ground, no additional ground wire between board and casing required. If pot is mounted on casing, it is grounded this way, additional wire not required too.

If you use isolated DC socket too, than connect PCB's ground with casing with one short wire.

 

Once I build Moran Jones headphone amp in old PC PS enclosure. Amp was silent with all kind of headphones, but wen I use it as preamp, there was small hum in speakers (small existing hum was not noticeable in phone, but poweramp with gain 20 show it).

Problem was in ground loop - amp grounded to box with wire and grounded, open stile headphone jack. Wen I replace headphone jack with plastic one, hum disappear, even in preamp mode.

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post #10 of 10
11/4/10 at 5:15pm
Thread Starter 

I was reading last on tangent's site that the RCA should be grounded and nothing else so thats what I tried before reading your post. I disconnected the HP, DC and pot from the case and I was getting weird distortion from the pot even though its grounded to the board. I reconnected the pot to the case leaving the HP and DC and no hum!

 

I got my stepped drill bit today and I didn't realize that my HP jack is supposed to be isolated anyway so I just have to make the hole a little bit bigger. The DC is a bigger issue but I'll get it figured out. Just glad that I've got this solved.

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