Heatsink selection help

I've decided to build a small amp to run in my truck and connect directly to the speaker wires (since my stereo was stolen) so I can play my mp3 player in the car. I seem to have narrowed it down to Toshiba's TA8251AH chip, it's a 4ch, 18W/Ch chip, and this site has them for cheap. My problem is I don't know much about heatsinking chips, so could anyone point me in the right direction on selecting one that will work for me? Any help would be appreciated, thanks.
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
> Do you mean specifically a P4 heatsink + fan, or just for a computer processor in general?

Any of the big hot chips. A proper 486DX2-66Mhz sink will work (though most 486DX2s were woefully undersinked). I've seen sinks on cool P-133s that were big enough. I just retired a dual Pentium-Pro 200MHz 1Meg cache mobo with a couple huge passive sinks that would easily shed 40 watts. But the market is full of P-4 and Athlon super-coolers as low as $7 with fan. Yes, you may not need the fan. For possible fan-less use, avoid the sinks with many tightly-spaced fins: they need a fan to scrub the dead air out of the tight spaces. You want a garden rake, not a lice-comb. (But since you have about 12V handy, if you don't mind the fan you can get super-cool and compact.)

If you would rather use scrap-metal: I estimate like 6"x6"x0.125" aluminum. However that seems large for what is obviously a dashboard radio chip. I know car radios couple heat into all their outside surfaces, but even so I suspect the chip is designed to work even when extremely hot by conventional standards.

Or look in yard-sales (or your own attic) for a dead, sick, or unloved 35W+35W stereo amplifier, and pull the heatsink out of it. Back in the 1980s, everybody had one of those. One issue is that a 35+35 will apply the heat in four places, and you just have one device applying the entire heat load. OTOH, your chip is probably tougher than the silicon of the past.
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
> That retailer has used the distortion specs for THREE watts per channel, not 18. At 18W it is ~10% distortion..:bigeyes:

Car-audio is "traditionally" rated at 10%. For about any recent chip, this is far into clipping. People really will crank-up to that point (my wife cranks her Miata-radio to :yikes: ).

FWIW: Toshiba wrote the 3 Watt spec, the retailer just copied it.

If you look in the sheet, THD is <0.05% from noise up to a point, and then THD goes straight up. The amp is very linear, until the low supply voltage forces it to clip. And 3W is not an unlikely listening level, when you are not wanting max-out.

A perfect bridge amplifier on a 14.14V supply (car with generator running) will make 10V RMS at the edge of clipping. 10V^2/4Ω=25 watts. In fact THD is well up at 15 Watts, or 11V peak per side, so the chip is wasting 14V-11V= 3V, say 1.5V per side. That's extremely good for a car-sound chip without bootstrap caps (which would need eight more pins!).

So in the hi-fi world, you might call it 15W/4Ω at ~0.1% THD on 14V supply. So it ain't 18 Watt, but it ain't a lot less to the ear.

THD is low at 1KHz, but shows a rise below 100Hz (may be capacitor slump, may be thermal feedback) and a classic rise above 1KHz due to limited bandwidth. Music power spectrum falls above 1KHz, so this is not necessarily bad. It would be smoother if the THD curve were flatter, but hey! You can't beat four channels of 15 watts in one blob for 1.5 bucks. That's less than 3 cents per Watt! Killer little mini-amp for PC, patio, boat, etc.

> this site has them for cheap
From the same site (save shipping), consider these sinks:
G14834 4" tall x 3"W x 1.24" SALE! $2.49 ea.
G2209 $2.98 4 7/8"L x 2 3/16"W x 1 1/4"Tall
G13270 $1.99 50mm..fan 136mmx50mmx25mm black anodized aluminum
G15412 $5.95 6.21"L x 6.2"W x 1.95"Thick. over 2 lbs.