Sonic Impact T-amp + RS auto jumper

[Decker]

I did a quick search and didn't see anyone else posting this. Yesterday, while I was charging the Radio Shack auto battery jump starter that I keep in my pickup ($40), I looked at my T-amp, which was running on an AC battery charger into a Pi filter then to the amp. Hmmmm, says I. I used a cigarette lighter socket cord (about $5) and connected the two. Great sound, says I. Very low noise, of course. Still needs very efficient speakers, which I have. Seems to be an efficient turnkey way to power a T-amp on batteries.

[!]

Yes it'll work but be careful about not charging it while the amp is turned on. Max voltage for that SI chip is 13.nV and most chargers reach closer to 14V or so. Earlier revisions of the T-amp boards had a solder slug sinking the chip to the PCB which helped keep it cooler but later revisions didn't so you could have an overheating potential in some situations as mentioned.

What you've done will do well for a portable battery powered amp, but there's not so much benefit from "very low noise", it is a myth using batteries on most semi-recent vintage integrated solid state amps for sonic benefit. Chipamps, perhaps even moreso the digital ones, have great PSRR and don't need nor benefit from ultra low noise battery power like a tube or discrete amp might. All they really need is for the noise to stay out of high frequency domain which is done acceptibly on the circuit board and to have a relatively low impedance which can't be had with a battery - whether battery or ac-dc, it just means a moderate sized capacitor before the t-amp board or replacing the stock 'lytic on the board.

The battery serves a different role though, portable power.

[Decker]

Thanks for replying! I haven't had to recharge the battery yet after several hours of use, but I will be sure to unplug the t-amp when I do. I do think I have a little less low frequency noise with the battery, so maybe I had a ground loop issue?

[!]

Noise from an AC-DC supply depends on exactly what that supply is like. A typical wall-wart transformer is isolated from (earth) ground, but there is no guarantee "all" are. Instead it might be AC line hum 50/60Hz type but this is pretty easilly rejected by putting a linear regulator (LM317 set for 13.xV (see the chip datasheet for max, I forget what "X" is) or LM7812, though the closer you get to the max chip voltage the higher the output potential, but also the higher the heat which may become significant if yours is a latter T-amp w/o chip rear soldered onto the PCB- it is easy enough to tell just looking, whether there are a lot of large vias filled with solder on the PCB side opposite the chip (meaning you may not see the vias at all, just a solid soldered area, or unfilled vias).

Once you resolve "X" above, you could also use a linear regulation stage to limit the peak voltage to the amp-battery combo, allowing use while the battery is charging but you practically have to use an adjustable regulator set to the max chip voltage to have a high enough voltage to charge the battery reasonably, and even then some batteries might not reach 100% charge. Another option might be putting a diode or two in series between the battery and amp so the voltage is dropped, but still a little higher to charge the battery. It does take away a slight bit of output power though just as a 12V LM7812 regulator would for the prior idea. On most amps a loss of 0.6-1.4V would be so significant but the T-amp already has marginal output for some scenarios.