PIMETA question
Nov 18, 2004 at 6:59 AM Thread Starter Post #1 of 11

aeriyn

Headphoneus Supremus
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Oi, with my PIMETA, the volume seems to go up rather rapidly when turning the pot upward. Slight adjustments give big volume increases, but the sound doesn't sound crackly.

Not sure if its oscilliating or not (because of the volume increase, I think it might be, but the volume doesn't sound crackly) or just that it has a lot of power and my headphones (Beyerdynamic DT440) are only 32 ohm and are rather efficient in any case.

Any ideas? And if it is oscilliating, does anyone know how I can fix it?
 
Nov 18, 2004 at 7:10 AM Post #2 of 11
Your Pimeta's voltage gain is probably too high for your sensitive headphones. Try reducing R4's value from 10K to something like 3.3K. This should allow you to use a more reasonable range on the volume knob.
 
Nov 18, 2004 at 7:11 AM Post #3 of 11
Thank you very much. ^^ I'll give that a shot. I was so worried about faults and DC offset when testing that I didn't think that maybe the default gain would be too high for the DT440s.
 
Nov 18, 2004 at 8:08 AM Post #4 of 11
I put a 3.3K resistor in each of the R4 positions, and it helped a little. I still only use perhaps the bottom 1/5th of the pot's range.

I'm just wondering, if something is wrong with the amp that could damage it and/or my headphones.
 
Nov 18, 2004 at 2:02 PM Post #6 of 11
If you're using unity gain stable op-amps, you can lower R4 still further. You can go as low as making it equal to R3, which will give you a gain of 2. You can go even lower than that, but at that point you might as well rework it to be a buffer -- gain = 1.
 
Nov 18, 2004 at 4:22 PM Post #7 of 11
I'm using the pana evj pot.

Tangent, the opamp I used is the AD8620 in OPALR and AD8610 in OPAG.
 
Nov 18, 2004 at 10:10 PM Post #8 of 11
Quote:

Originally Posted by aeriyn
I'm using the pana evj pot.


Is it the exact one recommended (Digikey P2U4503)? The reason I ask is the B curve, which would be P2R1503 or something like that, is a linear curve, instead of the "D" curve of the P2U4xxx.
 
Nov 19, 2004 at 3:56 AM Post #9 of 11
I copied and pasted the digikey part number from Tangent's part list directly to the order form for Digikey.
 
Nov 19, 2004 at 5:13 AM Post #10 of 11
Quote:

Originally Posted by aeriyn
I copied and pasted the digikey part number from Tangent's part list directly to the order form for Digikey.


Alright, assuming the pot is a proper log pot, then the amp still has too much gain for your headphones. As tangent sez, maybe make the amp unity gain. To do that, just remove R3.
 
Nov 19, 2004 at 8:10 AM Post #11 of 11
Okay, I've done as Tangent suggested... it's not to unity gain, but it's at a gain of 2, and it's much more accurate per these headphones now.
 

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