Really Warm Gainclone

[rampage101]

At the moment my LM3886 chips are mounted on a piece of 3mm 50mm wide Angle Alloy. But the alloy is getting really really warm, to the point where I can't leave my hand on the alloy. Is this normal? Do I need better heatsinking? Or could this be oscilating?

Specs:
2x18vac 300va Transformer,
Carlosfm Snubberized PSU MkII (2.2k -> 10,000uf - > 100nF - > 1R + 100nF).
LM3886 with 1500uF 35v Electro's at psu pins.
Original BrianGT kit values (220Ohm, 22k, 22k, 680ohm) plus 10k mute.

So, any ideas? Sound is great, I would never complain of the quality. And whilst the amp was getting hot its only started getting this hot tonight since doing the better psu instead of just 6x1500uF (3x per rail)

[pinkmouse]

Oscillation probably.

Can you put it on a scope? What happens if you disconnect the new PSU? Does it still got hot with the input shorted to ground?

[carlosfm]

Originally posted by rampage101
And whilst the amp was getting hot its only started getting this hot tonight since doing the better psu instead of just 6x1500uF (3x per rail)

PSU voltage is more stable now, and the chip dissipates more heat.
At idle it should make no difference, but when playing music, yes.
Those heatsinks are small

[pinkmouse]

Originally posted by carlosfm
PSU voltage is more stable now, and the chip dissipates more heat.

Could you explain in more detail?

BTW I have a 3886 running right next to me now on an 18V traffo on similar heatsinks and it's perfectly cool.

[carlosfm]

Originally posted by pinkmouse
Could you explain in more detail?

BTW I have a 3886 running right next to me now on an 18V traffo on similar heatsinks and it's perfectly cool.

On the more difficult/demanding musical transients the PSU voltage sags, with lower capacitance.
As the PSU voltage drops, the chip has less heat to dissipate.

Are you sure you have similar heatsinks?
3mm is very thin, specially without fins.
And 50mm wide is two inches, have you noticed that?
Unless I didn't understand and those 50mm is just the size of the L shaped bar and he is not specifying the width.

[pinkmouse]

Thanks Carlos, I'm not sure how much effect that would have at normal listening levels though.

Mine is a stereo amp, with seperate supply, and the bottom of the case is used as a heatsink. It's 3mm thick ally plate, roughly 80mm by 60mm.

[rampage101]

The angle is 50mm, so 50mm heigh, 50mm wide, 3mm thick, and about 250mm long altogether.

[carlosfm]

Originally posted by rampage101
The angle is 50mm, so 50mm heigh, 50mm wide, 3mm thick, and about 250mm long altogether.

Still, 3mm is thin. It makes a 'hot-spot' on the back of each chip, heat doesn't spread properly.
Results depend on your speakers.
If you have difficult speakers, it can get quite hot, even at that voltage.

[Cobra2]

All my clones, single, paralelled & bridged have had ocillation-problems when first tested...
Even the BG kit ones. And all done "by the book".
So, now I always make an output inductor, 10-12 rounds of 0,8mm trafo-wire around an "AAA" battery, paralell with a 8-10 ohm 3W resistor. This have cured all my osc. problems.

Arne K

[rampage101]

Well in this case carlos, assuming its my speakers are too difficult, how do I get 12v from my 25v line to run a fan? I don't want another trafo in there.