Heatsink size adequate?

[mullardel34]

Allright, let's take stock of the situation. First, let's review the scaling factors for your power-amplifier, based on the implementation at hand. You've elected to use the LM3886 chip-amps with a +/- 36VDC power-supply configuration. Therefore, the maximum RMS output voltage will be approximately 25.5V, resulting in maximum RMS output power into 8-ohms of about 81 watts (the square of the voltage divided by the load impedance). As a result, with an 8-ohm load, the maximum RMS output current will be about 3.18A (4.5A peak). So, while the implementation is pushing the outer bounds of the LM3886 application notes, everything looks OK.

As to the thermal load that can be sustained by the LM3886, the chip itself has built-in protection for ensuring that the chip is only operated within its SOA (safe-operating area) parameters; this is the SPiKE (Self Peak Instantaneous Protection) feature, which safeguards against over-voltage, under-voltage, overloads, including shorts to the supplies, thermal runaway, and instantaneous temperature peaks.

The LM3886 is specified to be operational up to 85-degrees Celsius (185-degrees Farenheit) and will automatically shut itself down if this internal temperature threshold of the output-device junctions is exceeded. So, monitor the temperature of your power-amplfier's output stage under heavy load and see if you're getting close to prompting the built-in self-protection of the LM3886. If you find that you're flirting with temperature-based shutdown, you'll need to increase your heatsinking of the LM3886's.

BTW, a small, handheld infrared temperature probe is very useful in remotely monitoring temperature conditions within your DIY-audio projects; something like the following is what I use:

Digi-Key - 290-1427-ND (Manufacturer - 381)

[chris661]

Agreed - the case alone isn't likely to provide sufficient surface area to get shut of all that heat.
Get them properly ventilated (ensure good air flow), and think about adding a switchable fan, that is only used when the volume gets turned up.

How about a thermistor mounted on the heatsink, in series with a relay coil? When the temperature gets high enough, the relay clicks and the fans activate.

Chris

[chlorofille]

Hi rock,

I would use much larger heatsinks. At least 3 times the size.
Since you have so much space in the case, why not ?

[pacificblue]

Originally Posted by mullardel34

You've elected to use the LM3886 chip-amps with a +/- 36VDC power-supply configuration. Therefore, the maximum RMS output voltage will be approximately 25.5V, resulting in maximum RMS output power into 8-ohms of about 81 watts (the square of the voltage divided by the load impedance).

±36 V and 8 Ohm lead to an output voltage swing of ~32,2 V and an output power of ~65 W.

Originally Posted by mullardel34

the SPiKE (Self Peak Instantaneous Protection) feature, which safeguards against over-voltage, under-voltage, overloads, including shorts to the supplies, thermal runaway, and instantaneous temperature peaks.

Sounds perfects. However there are many blown ICs around that show the fallibility of that protecion system.

Originally Posted by mullardel34

The LM3886 is specified to be operational up to 85-degrees Celsius (185-degrees Farenheit)

It is specified for 150°C (302°F), although SPiKe starts to bother at 125°C (257°F). If you calculate derating for this application, the IC should be switched off long before reaching that temperature. Hopefully the ±36 V were measured at idle state.