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Class D Switching Power Amplifiers and Power D/A conversion

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Old 9th December 2008, 01:58 AM   #201
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if the middle section is separate from the rest, then easy enough to have a couple of vent holes... have some top and bottom and the hot air will naturally flow out the top and cool air in the bottom. or add a little cpu fan to extract the hot air. or do the diy plate amp thing. plenty of options.
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Old 9th December 2008, 02:14 AM   #202
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Tiny practical problem with that arrangement. The entire top is a large 15Watt solar panel. So you can't have holes in the top. The bottom is recommend to place directly on ground or the tuning will be off (note to get correct tuning indoors you put it on stands at least 1m off the ground and with at least 40cm to the backwall) so holes in the bottom will not produce any useful resuslt. There are cooling holes already built-in as you can see on the picture but the air circulation is minimal. But as I said you could have a cooling fan that is either termally controlled or just kicks in whenever the voltage rises above 13.5V, ie. either form of charging is in progress.

I didn't include a fan because I don't think it's crucial and the lowest power fan I could find with suitable control circuitry uses about a 1.3 watts of power which is about a third of what the amp uses continously at maximum power, so adding one limit battery time too severely for my taste.
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Old 9th December 2008, 02:57 AM   #203
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ahh ok. in the photos it shows solar panels just on the ends.

1.3 watts is 1/3 of what the amp uses at max power? that doesnt sound quite right... you must be using more than 3.9W! the amp6 basic is 25W per channel for a total max power of 50W. Tim is using an Amp9, for up to 4x100W per channel - i know he probably wont use a fraction of that power, but even so i think some cooling arrangement is wise.

have you measured the current draw from the battery when it is running loudly? i'd be curious to know what that was.

the smaller cpu heatsink fans i have are rated at 60mA at 12V = 0.72W

having it on a thermal switch as you suggest would mean it only came on when things got hot enough to need it.

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Old 9th December 2008, 03:05 AM   #204
tsnell is offline tsnell  Australia
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Col, Iíve never thought about reusing old PC heat sinks before (thereís about three old PC boxes sitting in the garage at the moment) for this project or not itís a good idea to have in mind. Thanks for the idea and the offer of the spare.

Saturnus, I can see what you mean, there will be very little room for air to move in the middle electronics compartment. Keeping the operating temp as low as possible will be very important (especially at festivals like falls, we had 40-45 deg C for three days last year). Thanks for the drawing you attached. I have a few comments/questions:
1. I can see what you did with the grooves in the middle. Very smart idea. The middle section is a lot slimmer than I would have assumed from the photos in the first post.
2. It looks like the woofer and breather hole cut outs are tapered, are they or is it just the view of the model? Is this necessary or purely aesthetic?
3. Two things are still unclear, the ports and the bracing in the bass reflex enclosures. Would I be able to get a copy of one of the drawings or CAD files that did survive?

By the way if anybody is interested, I have found the Aluminium enclosures here:
http://www.jaycar.com.au/productView...=&SUBCATID=481

And the batteries here:
http://www.jaycar.com.au/productView...=&SUBCATID=250
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Old 9th December 2008, 03:34 AM   #205
col is offline col  Australia
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Saturnus, I'm interested in looking at the box response in my software. Could I have the bass reflex box dimensions and the port sizes?

edit: It's OK, just found this "Qbox of 1.1 tuned at 82 Hz" I can work it out from that.

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Old 9th December 2008, 10:49 AM   #206
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I can't remember the exact size actually. I know it's between 22 and 24 liters. And I can remember that I worked with either 22.65 liters or 23.45 liters but I can't remember which it was settled on.

Port size is 3407mm2, defined as 120mm total width and 30mm total height rounded to a 30mm circle in both ends. Length is 35mm, or 3 layered 12mm birchply which actually actual size is 11.65mm.

Here is the CAD files. It's a Google SketchUp file so you need to download that first. Please note that once it was imported there was some small changes for practical reasons. Such as making the port mounting slightly larger, not the actual port size but the wood pieces that make up the port.

The cutout in the top was removed and replaced with another cut out in one of the speaker enclosure size. This is because I first thought to stack 3 batteries which needed that cutout but in went with 2 batteries as it gives long enough battery time and it was also before I found that perfect amp6basic enclosure. The cutout in the top in one of the speaker enclosures is for the small electronics box that is on the backside of the solar panels.

I also changed the "breather"/access holes on either side of the middle section from being tapered to being rounded (3mm rounding) instead. The tapering on the woofers is essential in my mind as they are mounted from the inside but a heavy rounding (9mm rounding just as the tapering) could be done instead.

http://www.youshare.com/Guest/39078183761ead6c.zip.html

If you don't access to CNC machinery, I strongly suggest you look at making some changes that will enable you to make it using handtools. Such as reducing the number of interlocking routings which would be very hard to make accurately by hand.
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Old 9th December 2008, 02:29 PM   #207
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About the power consumption.

The amp6basic has a power diode built in to protect it against reverse voltage so that drops the nominal voltage from 12 to 11.4 volts.

So the total maximum output power before clipping for both channels combined is (11.4^2/4*0.6) 19.5 watts on a nominal 12 volts. But that only what the peak output is because recorded music is a series of peaks and lows the average power comsumption is much lower. In fact most recorded music follow the RIAA standard dictating that the average sound level is mixed to 12dB below maximum peak level (0dB). And since we know that -12dB equal dividing with 4, the average output power before clipping is (19.5/4) 4.875 watts.

The effeciency of the amp6 is 81% in 4 Ohms so total power consumption at maximum power before clipping is (4.875/0.81) 6 watts precisely theoretically. Under real life condition I have almost never measured above 350 mA drain on a 12 volt supply equalling 4.2 watts when measured over several hours. That's probably because of silence between numbers and intro/outro fades etc.

Please note that the voltage will vary with the charge level of the batteries and so will the output power and consequently the power consumption as well.
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Old 9th December 2008, 11:10 PM   #208
col is offline col  Australia
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Default Boominator box response

just stuck the TS parms in WinISD and run the numbers:

http://pix.minirig.org.au/main.php?g...serialNumber=1

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Old 9th December 2008, 11:26 PM   #209
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Looks about right. 3dB peak @ 2 times the box tuning frequency. Yup, that's a perfect outdoor speaker box tuning.
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Old 10th December 2008, 09:16 AM   #210
tsnell is offline tsnell  Australia
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Sorry for slow reply, work has kept me too busy.

Thanks for the files Saturnus. Yes will definitely be eliminating some of the routed interlocking grooves, it would offer a very neat finish but is way out of my league with hand tools.

Cheers for modelling that col.

One question Saturnus, the components are all shown as individual panels and I am still not really clear how the bracing fits into the box?
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