Ruby amp fizzes out on battery power

[Buran1997]

I recently built a Ruby amp. When connected to a 9V wall wart, the amp works as expected. On battery power, however, the amp will work alright for about 30 seconds before it starts losing headroom and eventually fizzes out over about 15 seconds, passing no signal. The LED power indicator will also dim when this happens. If I turn the amp off for a few minutes, the battery seems to reset and the amp will work normally for another 30 seconds before fizzing out.

I'm not using a completely fresh battery—I tested it and read 7.5V before I put it in. I followed the schematic and layout from runoffgroove.com:



To this schematic I added five modifications: an additional 47Ω resistor between the 9V terminal and the board, a 1N4007 diode parallel with the 100μ cap for reverse polarity protection (I've verified that it's oriented correctly, anode to ground), a B2K gain pot, and a headphone jack wired as in the third image with a 5K trimpot in place of the 10Ω resistor (the sleeve is isolated from the enclosure).

At what battery voltage does a Ruby amp normally begin to fizz out? Do I just need a new battery?

[antonis]

Yes to the very last question mark..!! 

[willienillie]

^ Yes.  A battery that reads 7.5V out of circuit is a dead dog.

[davent]

I built this little 9v battery tester years ago, surprising how often i use it and surprising how a load will bottom out the voltage on a spent battery that unloaded, reads okay.

https://paulinthelab.blogspot.com/2013/02/self-powered-9-volt-bar-led-battery.html

dave

[anotherjim]

Why not measure the battery voltage when its powering the circuit? The voltage of a battery off-load is no indication of its condition. A pp3 type battery isn't really much good for powering an amp that has to drive a speaker. It can be ok with headphones because that only needs 100mW average power, but not the full 1W the speaker can take. The size of the cells in a pp3 are small and it only takes one of them to go bad -  and it does what yours does!