3v radio, but only 3.3v or 2.7v supply available - which to use?

[Delta]

I've got a cheapo AM/FM portable radio (a transistor radio as my grandparents would say 😀 ) that runs off two AA batteries.  I want to bodge it so I can run it off a 5v USB plugpack, but I only have 3.3v linear regulators on hand.

So should I run it at 3.3v and assume there's nothing in it that can't tolerate .3v overvoltage, or stick a diode in series and assume it should work fine at 2.7v? (which I'm sure it will, if it was designed to take rechargables...)

Cheers!

[Augustus]

A device, designed to operate on two AA batteries in series is very unlikely to get toasted, if hooked up to a 3.3v supply, given the fact that fresh batteries are in the range of a healthy ~1.6v per cell, depending on used chemistry. I think it's a safe bet to assume that this thing will quite happily run on 3.3v... 

[Delta]

Cheers for that mate.  I've stuck in a 3.3v PNP LDO, with 10uF across each side, and it seems quite happy.  The radio says 100mA max, so I've worked out that the LDO will not be dissipating more than 170mW - that should be fine for a TO92 package, yes?

Incidentally, I've found that Apple USB chargers (the small ones for iPhones) are ridiculously noisy - AM reception was impossible!  It's fine though with an Amazon one though (which came with a Kindle Fire - which is a piece of shit but at least has a clean plugpack!)

Thanks again. 

[cpt.charlie]

If you need a little vdrop, you can use an shottky diode in series with the load, BTW the noise on the apple charger could be normal if the charger is a fake Chinese unit.

[Delta]

The charger is genuine apple - it came with the missus's iphone!

[flynwill]

Well I wouldn't blame Apple...   The charger wasn't design with powering an AM radio in mind, and the AM radio was designed to run from batteries and not from a switching power supply.  It wouldn't surprise me if the switching in the little Apple converter landed smack dab on top of the radio's 455 kHz IF, or somewhere in the AM band.