I'm going to build out a three-way speaker, and an associated amplifier with integral crossover using Rod Elliott's P101 amplifier. Question: In the low-power version (which they all will be), would it be acceptable for the amp powering the tweeter to be heat-sinked only to the chassis (and not a finned heat sink)?
This would make chassis layout a bit simpler. Otherwise, due to the size of the boards, I'd need either a 3U, 400mm chassis for a single three-way, or a 4U 400mm for stereo 3-way... Huge! Attaching the tweeter amp to e.g. the amp faceplate would then only require a 4U, 300mm deep chassis for stereo, or 3U 300mm for mono.
This would make chassis layout a bit simpler. Otherwise, due to the size of the boards, I'd need either a 3U, 400mm chassis for a single three-way, or a 4U 400mm for stereo 3-way... Huge! Attaching the tweeter amp to e.g. the amp faceplate would then only require a 4U, 300mm deep chassis for stereo, or 3U 300mm for mono.
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Like all mosfet designs, most of the cooling needed is only to dissipate idling power. In total, that's quite a lot which doesn't vary so much per pair for any type of mosfet, whether for low or high power use. You'll need at least 100 mA bias for quality sound and if we assume +/- 50V supplies, that's going to need around 0.5-1°C/W rating of cooling per pair or amplifier - less for lower voltage supplies.
For six low power amplifiers, that's a total required of 0.1-0.2°C/W cooling - more than even a thick 4U plain front panel can accommodate, even if you don't mind burning your fingers on it.
As an alternative, I'd suggest using CFP or quasi-comp. design BJT amplifiers, which can run well on just a sniff of bias. Otherwise, just use multiple economy type heatsinks or a common fan and air tunnel exhausting to the rear. With convection heatsinks, more height is no great benefit, more width is much more efficient at cooling so you can use as small a rack case as necessary that still accommodates up to 6 heatsinks, either 3 per side or 6 fanning directly out to the rear, mounted from one edge on the rear panel.
For six low power amplifiers, that's a total required of 0.1-0.2°C/W cooling - more than even a thick 4U plain front panel can accommodate, even if you don't mind burning your fingers on it.
As an alternative, I'd suggest using CFP or quasi-comp. design BJT amplifiers, which can run well on just a sniff of bias. Otherwise, just use multiple economy type heatsinks or a common fan and air tunnel exhausting to the rear. With convection heatsinks, more height is no great benefit, more width is much more efficient at cooling so you can use as small a rack case as necessary that still accommodates up to 6 heatsinks, either 3 per side or 6 fanning directly out to the rear, mounted from one edge on the rear panel.
Thanks for the detailed response! To be clear, only the tweeter amps were to be mounted to the faceplate; not all six!
Given, though, that you're saying MOSFET amplifiers produce as much heat at idle as they do cranking out a lot of power, I'll put all channels on a proper heatsink.
The DIY Audio Store 4U 200mm heat sinks have a rating of 0.38 C/W. Is it correct to assume that, if I mounted two amps to that one heatsink, they'd effectively have 0.76 dissipation, each, and would thus have cooling within the recommended range?
Given you are using lower supply voltages and all amplifiers likely will have the same idle current, it makes sense to use a common heatsink for all of each channel's cooling requirements. The heat hogs will be the bass amps and the others will contribute little extra heat due to output power efficiencies of roughly 65%. 'May as well use the monsters rather than heat up the front panel for no good reason.
Theoretically, you could be increasing the heat by a further 35% of 3 x 120W/4R at full maximum power from all amplifiers but with music, that's quite impossible. I think those sinks will be quite adequate, given the idle load is only say, 3 x 9W = 24W. At 0.38°C/W, it contributes only 9° rise above ambient temperature.
With maximum possible music program, full continuous dissipation for all 3 amplifiers of say, 160W, might be more like 24W+60W = 84W for each heatsink which results in a max. temp. rise of 32°. With real music programme, that amount of heat won't even be approached. With 8 ohm loads, even less and they will remain relatively
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Theoretically, you could be increasing the heat by a further 35% of 3 x 120W/4R at full maximum power from all amplifiers but with music, that's quite impossible. I think those sinks will be quite adequate, given the idle load is only say, 3 x 9W = 24W. At 0.38°C/W, it contributes only 9° rise above ambient temperature.
With maximum possible music program, full continuous dissipation for all 3 amplifiers of say, 160W, might be more like 24W+60W = 84W for each heatsink which results in a max. temp. rise of 32°. With real music programme, that amount of heat won't even be approached. With 8 ohm loads, even less and they will remain relatively
.Hmm, given the dimensions of the amplifier boards, to have all three on one heat sink would require a 5U heat sink to stack them vertically. Given the DIY Audio Store chassis dimensions, and the amp board dimensions, I figure the following configs are possible:
- 1 chassis per speaker: 4U x 300mm depth; two boards (mid/high) stacked vertically on one side, one board (bass) on the other.
- 1 chassis for both speakers: 4U x 400mm depth; 2x 200mm heat sinks per side. Three boards in a line, front to back. One board spans two heat sinks, but transistors contact only one. Other boards are fully on one or the other heat sink.
- 1 chassis for both speakers: 5U x 300mm depth; three boards on each heatsink, one channel per side.
The first two options would not have all three boards on one heat sink; would that cause a problem in practical use? Or just a theoretical difference?
- 1 chassis per speaker: 4U x 300mm depth; two boards (mid/high) stacked vertically on one side, one board (bass) on the other.
- 1 chassis for both speakers: 4U x 400mm depth; 2x 200mm heat sinks per side. Three boards in a line, front to back. One board spans two heat sinks, but transistors contact only one. Other boards are fully on one or the other heat sink.
- 1 chassis for both speakers: 5U x 300mm depth; three boards on each heatsink, one channel per side.
The first two options would not have all three boards on one heat sink; would that cause a problem in practical use? Or just a theoretical difference?
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