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Old 8th April 2009, 07:33 AM   #1
Atilla is offline Atilla  Norway
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Join Date: Jul 2008
Default Debugging a PIMETA headphone amp

So I've gone and made myself one of those:
http://tangentsoft.net/audio/pimeta/

It's a pretty fun board to work with, I played around with the configuration for a while and figured out a nice wooden case to put it in.

However, I seem to have done something to it lately, since I have an issue that I didn't have before, or at least didn't notice before - there is some amount of noise or oscillation, when I crank up the volume pot with no signal present. I'm fairly certain that when I first tested the amp I didn't notice such a thing. But then again, I was mostly checking it with a signal present.

It's powered from a supply I made myself - a board with a good input filter and an EI transformer, which I cut out from an old broken modem (yeah literally - with a dremel). I rectify this with a TREAD board from Tangent, which puts out 22-24volt. Right now it's set at 22, since otherwise I don't have almost any regulation left. The supply is rock-solid, I wouldn't suspect it.

I'm using the standard schematic values for all resistors and I've a total of 4000mF in Elna caps of some sort in the PSU. The buffers are OPA634 and the opamps are OPA134 for ground and OPA2134 or 2xOPA627 for the L/R channel. The issue is present with both opamps, I got a spare one just to swap and see if I've any stability issues. The buffers are in high-badwidth mode, with 330ohm resistors IIRC and the opamps are biased into class A.

The rails are almost perfectly split in half, my output offsets are about 11mV for both channels, with source connected. All in all - nothing too much to worry about.

I can't understand where the issue is exactly. Here are the detailed symptoms:

When plugged into my sound card, there's a small amount of white noise that increases with volume. There's also some distinct whining sound at the higher volume settings (oscilation?). Changing the volume from the PC doesn't make a difference. Touching the pot sometimes removes the whining, that's why I've got it grounded now, so touching it has no effect.

When plugged into my phone, there's no noise. That makes sense, because I can sometimes hear the noise from the sound card with only my headphones connected - makes sense the amp would amplify that as well. However - the whining sound is present. It also reduces, when I touch the pot (it's still grounded). That hints towards some weird grounding-related issue.

There's a very distinct thump sound when I flip the on/off switch. I'm fairly certain it didn't use to be there when I first tried the amp. There's a small, similar issue at turn-on, I'm not sure if it appeared after the class-A bias or was there all along.

If I short the inputs, none of these effects is present, at all. If I short only one, it stays silent - the other channel experiences all of the above.


Sorry for the long post, but I've been at this for a while and I'm totally clueless if it should be happening, why could it be happening and what on earth to try next.

So - what have I done to the thing and does anyone have any clue what I should try next, or I should just put the paint and lacquer on the wood, nail it all together and just sit back and listen. This baby does magic to the sound otherwise and I'm only trying it with Sennheiser 212s.
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Old 8th April 2009, 09:13 AM   #2
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Join Date: Jan 2008
I've noticed exactly the same effect, but I don't think it has anything to do with the topology, as I'm able to repeat it with the simple cmoy I use as a portable amp.

I've always assumed that it's something to do with the high input impedance of opamps making them susceptible to picking up any and all sorts of interference.

I'm currently working on given the whole thing a decent ground. Currently, it's only grounded when it's connected to a grounded source. There's an issue with that hypothesis, which is that it's silent when connected to my Sony MP3 player, which is just as floating as the the amp alone.

So - you're not going mad, but I don't know either.
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Old 8th April 2009, 09:59 AM   #3
Atilla is offline Atilla  Norway
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Join Date: Jul 2008
Haha, it's good to know that I'm not alone in my hunt for the problem. The question is if there's any known cause and solution to that.

BTW - populating the R8 positions with different values do absolutely nothing for me.
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Old 23rd April 2009, 05:12 PM   #4
Atilla is offline Atilla  Norway
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Join Date: Jul 2008
Well I seem to have managed to figure it out to some extent.

Apparently this system, as arranged, is a little too powerful for my whimpy 32ohm Sennheisers. I had to actually give it something stronger to feed. As adding R8 didn't do anything significant, I figured I should try adding the resistors in series with the output, as suggested. 47 to 100 ohm were supposed to be put inline so I pulled out a few 68R resistors, matched them and soldered them, sticking out right of the output pads.

I tried one at first, to be able to make out the difference. Whine without source connected - gone. Hiss without signal - gone. Volume - lower (obviously).

I fixed both channels, fired it up and all seems to be perfectly fine. There's any hint of hiss only at max volume and that's pretty much dangerous. I can move the pot a little more freely now, which is also OK in my book. The 627 seems to like to be fed a little more input as well, since it seems to be in better control that way. The highs are a little clearer that way, normal, given that there isn't any background hiss.

I also swapped my original 680uF cap for a lower, but still acceptable, tolerance one, that has a 2200uF value instead. This didn't do anything, which is understandable, since the supply is regulated well and the PIMETA itself has a total of 4000uF on the board (heh).

I'll stack a pair of buffers on top of the current ones at a point and leave it alone. I suspect that raising the output resistors a tad higher will actually get rid of any leftover side effects at max volume, but I see no reason to mess with them anymore, especially when I might plug some higher impedance headphones in as well, who knows.
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