Wow, real batteries make a huge difference. Even the turn on thump is louder and brighter-sounding, lol. Still not enough to hurt my ears or headphones, thankfully. And now its a bona fide hand-warmer too. Certainly not hot, but now I finally notice that it does warm up.
After charging, the new batteries were each measuring 9.18V. When I first powered up the amp, I measured 15V DC bias and 6.5V AC signal at the output! This is more than double what I was averaging with those dollar store carbon-zinc cells. Maybe a tad too high for my needs, even?
EDIT: I was measuring incorrectly. Bias is actually perfect at 6.5V DC. See post #187. I'm still not sure how to measure AC signal...
I switched back and forth between old and new batteries a bit. The cheapies (which yielded a modest 6.5 DC bias and 3.5V AC signal at last measurement) are definitely more laid-back sounding, while the new ones sound brighter (not forward or fatiguing in the least, mind you), bringing the amp closer to neutral, to my ears. New batteries are also a big win in terms of overall excitement, detail retrieval (which is incredibly good now), dynamic power, and perhaps even bass quantity, too. It sounds like THD (especially H2, let's hope!) has risen along with the voltage. I quite like the change in the treble, but my initial impression is that the bass seems a tad less transparent than what I was hearing at lower voltages...leaning just a bit towards buzzy...my guess is that the increased harmonics are obscuring the fundamental notes and natural texture a bit? I've learned recently that capacitor distortion rises along with voltage. Overall, running this amp hot is more fun, and it sounds mind-blowingly good. If I had to sum up the improvements in a word, I would say my amp has become "snappier." It really grabs your attention now. Does the "sweet spot" for me lie with slightly lower voltage set points? Maybe, especially if I could get more battery life from the trade-off.
EDIT: After further listening, my impression of bass texture improved. See post #189.
Charging the batteries inside the tin is a partial success. I made some big (not so pretty but oh so functional) cutouts in the tin using a hole punch to start, followed by wire cutters and pliers, so that my cables could connect there, and hot glued my batteries in place in the tin. Unfortunately the batteries must sit about 1/4" away from the wall due to the curve of the tin corner, so the holes had to be big enough for not just the actual micro USB connector had to fit inside, but the entire plastic head of the cable too, in order for the connectors to reach the batteries inside.
There are tiny holes in the end of the tin (smallest hole punch from my 3 piece set) to reveal the charge indicator lights of one cell. From an angle it looks like the light doesn't line up with the hole, but it does. It's just that the battery is set back a bit from the tin wall due to curving tin corners.
But the big letdown is that I can't use the Y cable I bought (double micro USB) to charge them both simultaneously from a single USB charger. Well I did it at first actually...it just won't work while the batteries are connected to the amp. I
can charge the batteries in situ if I use two separate chargers with two separate cables. I think this is because of a shared ground in the cable, which must complete a circuit between the two batteries when they are snapped into the amp's battery connectors. This situation seems to always send one of the two batteries into some sort of limbo state. Voltage drops instantaneously to 0.3V (which results in a high pitched noise coming through the headphones when the amp is powered back on.) The battery comes out of limbo when charged alone with its own cable for a few seconds, and then behaves normally again.
X, I have a somewhat convoluted question for you, oh electronics guru. Before I ever saw this weird charging behavior, I was listening to the amp and poking around in the amp with my multimeter, and I stuck the probes on both the negative pins of the battery connector on the board (don't ask me why, I'm an artist, not an engineer!). Oops...Instantly music volume dropped drastically and I heard a high-pitched hum. I thought I had fried my amp or something at first. Then I measured the new batteries and found one had dropped to near zero voltage. I charged it for a few seconds and it was fine again, back above 9V. Now it works great except for in the charging-in-amp-with-Y-cable situation, when it returns to this same limbo state. Did I cause ALL this limbo behavior by damaging the battery with my multimeter, or did I just trigger some sort of protection circuitry with my probes, which is now just recurring with my flawed charging setup? The same finnicky battery makes an audible high-pitch whine when I measure it with my multimeter. Not through the headphones! Like something in the battery itself is oscillating while the voltage is read by the multimeter. I'm not sure if it did this before my initial probe mishap or not, but only one of the batteries does this, so I wonder. As I said, both batteries perform flawlessly in the amp, no noise.
Sorry that was long...it was hard to explain. My hope: If it is a protection circuit, Possibly the finnicky one is just slightly more sensitive so it always trips first, leaving the other one appearing not to have the behavior at all.
