Originally Posted by tent
still the difference between IG and V+ is 3.00V and IG and V- is -4.71V with my 7.2VNiMH 9V battery.. still do you think it's ok, Warren?
Nope. You shouldn't measure more than about a tenth of a volt off of ideal.
volume seems to me quite low.. even at full pot turned up
Also a good problem indicator. The stock PIMETA v2 gain, while lower than that of the CMoy, is still fairly high as headphone amps go.
(Why? So that uncompensated op-amps like the OPA637 will work. If not using uncompensated op-amps, you probably want something lower.)
LED1 is bright when I turn on BIAS and is very dimmed when I turn off the BIAS.
The brightness of LED1 should only change as a function of supply voltage changes.
This suggests that your problem may be a dead short across the battery, causing its voltage to drop.
So, one test is to be measuring voltage across the battery terminals while you flip the switch. It shouldn't change much, if at all. If it does, your amp is drawing more than the battery is intended to supply, but with NiMH you can pull 1C continuously without problems. That's on the order of 4-5x the expected supply current of a PIMETA v2. One 7.2V NiMH "9V" datasheet I just looked at fully characterizes the battery up to 3C. (900 mA!)
You can try measuring current directly instead, but be sure to use your DMM's high current range first. It's quite possible to get multiple amps out of a NiMH battery for short periods, which will blow the fuse (or the meter, if there is no fuse) on low current ranges.
Another test is to measure resistance from the top of LED1 to V- while the amp's power is switched off, or the battery is removed. I get 56 kΩ here on one of my PIMETA v2s. If you get something much lower, especially near zero, you have a short or you've got something connected across the rails that you shouldn't have.
RBLIM is 0.653V I actually cannot see even LED1 powering on (unless of course I turn off BIAS)
I can't be sure that's wrong without knowing the Vf of your LEDs.
With a 7.2 V battery, you shouldn't be using 3.7 V LEDs as shown in the schematic. To a first approximation, you can add the two Vf values and get the minimum supply voltage. The transistor behaviors change that a bit. At these low voltages, they may well change it more than 10%, which is why I say it's only a first approximation.
I use two 3.7 V white LEDs in one of my PIMETA v2s (the red one in the docs) but it has never been tried with less than an 8-cell (9.6V) NiMH battery.
Edited by tangent - 2/7/13 at 2:14am