[Marzie]

Hello all.

I have been using my Pimeta for a few months now and just realized that with the volume high (even with no source connected) there is a loud hum. This can get better or get worse when touching the input jacks or volume pot shaft. I had the volume pot grounded and when I took away the ground strap, thats when the buzzing seemed to get worse when touching the pot. The rest of the time it seemed to get better but not go away.

I started by pulling out all the jacks. I am going to recable the input jacks and wire the output jack directly (I was using crimp on terminals before) although I doubt this second issue is the culprit. I am using the hammond enclosure with the plastic side panels. Also, I had some problems when first testing the amp: two of the chips I used (can't remember which off hand) didn't have all of the leads soldered onto the brown dog adapter at first. I heard the oscillation and felt the heat and fixed one, retested, then found and fixed the other. Should I try ordering different and/or the same opamps and buffers? I am using LMH6321 for the buffers and AD8620 and AD8610 for the opamps.

 

[joneeboi]

The loud hum could be from the power supply. What are you using to power your PIMETA? I'm running mine from a TREAD, and even before that, I didn't get any hum. At the very least, I think you should go back to grounding your pot. And if you fixed the opamps, I don't see a problem in keeping the ones you have. Try jumpering the buffer positions to see if that's causing the problem. If you have different wallwarts, you may want to try those too. And for your ground, are you connecting the signal ground to the case in any way? Is your case fully plastic?

You may also want to give this site a looksee: Basic Troubleshooting for Headphone Amplifiers

 

[n_maher]

Without a source attached it's perfectly normal to get buzz/hum. If it happens with a source connected you've got a ground loop.

 

[Marzie]

I am using a tread as well. I will be grounding the pot once I get the input and ouputs rewired. I didn't see any way for the signal to get grounded to the case... The case is a metal hammond with the plastic end panels. I covered the bottom of the case with cardboard and then that with a layer of electrical tape to prevent any shorts.

No other wallwarts and I have read the baisic troubleshooting a few times, but didn't start back at the beginning once I had this "new" problem. Once I get it back up and running, if the problem still exists, I will start at the top.

I had considered adding the "noise" resistor at R8. I may try that as well.

I first noticed it using a sony ps1 as a source, I don't know too much about dc offset on the source but I will check that tomorrow. I know that the power supply on this is two pronged, no ground connection.

 

[joneeboi]

You can solder a wire from a GND connection, strip the other end and let it fly in the air until it rubs against the case when you close it. I did that with a perfboarded amp in a Hammond 1455C801 even though the pot was already grounded. Can't tell you how much it helped, but I did it at the other end of the case for a little RFI protection like in the Mini^3. I was enjoying some music through my PIMETA+TREAD last night, and with the source removed, I don't get any noise at all. I did get some noise, however, when I had a bad buffer. I have one of Sijosae's discrete buffers in my L and R buffer slots, and I've made a few bad ones that gave me buzzing when nothing was connected. Did you try jumpering the buffer positions yet?

 

[n_maher]

If you've got the plastic case you almost definitely need to ground the amp to the chassis. Also, you'll need to ground the pot shaft or hum/buzz is pretty much guaranteed.

 

[Marzie]

Which GND would you recommend connecting to the chassis, Input or output? I will tape or otherwise secure this floating wire to the side of the chassis. I will definately ground the pot as well.

I imagine the reason I don't have this on my new CKKIII is the pot is connected to the star ground and the star ground is touching the chassis (just a screw touching the bottom, I don't have a hole drilled yet), so there is no hum even at high volumes.

As far as jumpering the buffers, if the grounding does not work, I will try that, but I have a feeling this will work. Do you know offhand which pins should be jumpered? I can check the datasheet if you are unsure.

 

[heatmizer]

Input ground only to chassis. Grounding the pot is always good idea.

 

[n_maher]

Quote:

Originally Posted by Marzie /img/forum/go_quote.gif Which GND would you recommend connecting to the chassis, Input or output? I will tape or otherwise secure this floating wire to the side of the chassis. I will definately ground the pot as well.

Unless I'm remembering something very incorrectly grounding the output would be a BAD idea. I'm 99% sure you want to ground IG.

Also, be aware that the coating on the Hammond cases is pretty non-conductive and that to ground things properly you're probably going to have to expose some bare metal.

 

[Marzie]

OK, input ground it is. I will scratch away some of the coating before gluing the wire down and taping over the bare conductors. I'll let you know how it goes. Thanks everyone for your help so far.

 

[joneeboi]

Quote:

Originally Posted by Marzie /img/forum/go_quote.gif *snip*As far as jumpering the buffers, if the grounding does not work, I will try that, but I have a feeling this will work. Do you know offhand which pins should be jumpered? I can check the datasheet if you are unsure.

If grounding the pot doesn't help, jumper pin 3 and pin 6, so the third row of pins from the top.

 

[Marzie]

It worked. The buzz was bad, but I think a lot of it was because the two source components I used had a lot of noise. The first was the PS1. Now the noise on that is nonexistent until the last few degrees up to max volume. When I first heard the noise, I went to my pc and used a mini to rca out of the front panel connectors because my dac is plugged in to "air wired" rcas for my CKKIII until I finish the casing tomorrow (hopefully). That was and still is FULL of noise, pops clicks, intermittent buzzing and humming. But when the pimeta is plugged into my DVD player, there is no noise when the unit is paused and at max volume. It may have a muting circuit, but when at a DVD is playing I went to max volume and didn't notice anything.

Tonight, however, my CKKIII started getting intermittent static/cutting out in the right channel. I have a few thoughts on what it might be but I will wait until I have it properly cased before I pass judgment.

 

[Marzie]

CKKIII is cased up, sounds great. Fading problem solved. I noticed something about my pots. It appears I have the same pot, the alps RK27112, I bought one from Tangent and one from Amb. The one I bought from tangent takes a strong force to turn. The amb one seems really easy to turn. The inside stickers on the pots have different numbers, the tangent one reads 712G and amb reads 547G.

I fear that it is not a problem with the model, but an error I made while soldering the pot. I touched the side of the iron to it not once but twice and put two melted spots in the bottom of it. I am not sure how it is constructed internally, but could this be the cause of my problem? I am having no negative audio effects. Has anyone else noticed anything similar with alps pots or is this the cause of my haste and ill-lit room?

 

[Pars]

Quote:

Originally Posted by Marzie /img/forum/go_quote.gif CKKIII is cased up, sounds great. Fading problem solved. I noticed something about my pots. It appears I have the same pot, the alps RK27112, I bought one from Tangent and one from Amb. The one I bought from tangent takes a strong force to turn. The amb one seems really easy to turn. The inside stickers on the pots have different numbers, the tangent one reads 712G and amb reads 547G. I fear that it is not a problem with the model, but an error I made while soldering the pot. I touched the side of the iron to it not once but twice and put two melted spots in the bottom of it. I am not sure how it is constructed internally, but could this be the cause of my problem? I am having no negative audio effects. Has anyone else noticed anything similar with alps pots or is this the cause of my haste and ill-lit room?

That shouldn't bother them, unless you melted the piss out of them. You are saying that either the easier to turn or harder to turn pot is the one you hit with the iron, and wondering whether that is the cause of the diff in turning force, and/or possible damage? I had a couple of Alps blues, one from tangent, and one from amb, but didn't really directly compare them in terms of turning force, etc. I think it is probably just variance, and would bet the tight one loosens up some with use. The smoother one should just stay pretty much the same I think...

 

[Marzie]

That is exactly what I am saying. The Tangent pot is harder to turn and I have had it longer and used it quite a bit. The amb pot is brand new (other than being hit with the iron.) On the datasheet for the alps pots, they list the rotational torque as 8 to 35 and the push pull strength as 100N max. I am guessing that the torque is what I am interested in, and it looks like it can be a wide range. I am not sure what the push-pull strength means.

The solder info is listed as 350 Deg C. max. and 5s max. I may have left it on for more than 5 seconds when soldering a few pins... but if the "turn-resistant" element (that dictates the amount of torque necessary) is connected to the electrical resistance element, and I damaged this when soldering the pins, I should be hearing problems with the audio quality... at least by my understanding.

I guess I could just buy another pot from amb and see if it is the same, not that I will replace it unless there are any problems, it just couldn't hurt to have one around for the next project.

BTW, the reason I am asking is I think I prefer the hard to turn model because of its smoothness, the other just gives too easily.