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'Plate Amplifier' Design

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Muckerbarnes1 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Muckerbarnes1 Quote  Post ReplyReply Direct Link To This Post Posted: 22 July 2013 at 9:12am
Why did I put Z shape.. Erm.. A U shape of a few plates accurately bent in a vice to get the best surface contact and heat transfer should be a home diy possibility.
 
So sort of similar in shape to this one I found on Ebay;
 
 
Now imagine a flat plate to start, and then U shaped plates with a flat centre. Each bent different to make the shape. This of course only convects best in one plane, but it easy to make.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote odc04r Quote  Post ReplyReply Direct Link To This Post Posted: 22 July 2013 at 10:14am
I've got one just like that sitting on my work desk - came off a 50A thyristor module. Was considering making it into an amp for a DIY HT sub enclosure. Not necessarily a plate though.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote audiomik Quote  Post ReplyReply Direct Link To This Post Posted: 22 July 2013 at 10:33am
Malc
if I was intending to do a 'one-off' or very small run of these, then what you suggest could be a very likely way to proceed.

Now a 'U' shaped section with slots to have many vertical spines such as a 2" x 1" x 1/8" stock channel extrusion could achieve something like this but I can't really see many potential constructors having access to milling machines

Also have to look at how to thermally isolate the Heatsinks from the base-plate which will hold the whole assembly into the cab so as to minimise any thermal problems in the virtually enclosed space containing the PSU and other parts

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Post Options Post Options   Thanks (0) Thanks(0)   Quote imageoven Quote  Post ReplyReply Direct Link To This Post Posted: 22 July 2013 at 9:21pm
Originally posted by audiomik audiomik wrote:

Now a 'U' shaped section with slots to have many vertical spines such as a 2" x 1" x 1/8" stock channel extrusion could achieve something like this but I can't really see many potential constructors having access to milling machines
Mik


A thin flat plate bent into a U could have slots cut into it (on the bent up sections) with tin snips / nibbler / guillotine, these could then be twisted through 90 degrees to give airspaces between them.

Rather than a U shape this could be done on four sides (like a box fold).

multiple plates of reducing size could be stacked together.

An assembly like this would need protecting in some way though as it would be fragile (and potential be a dangerous mass of sharp edges) - it would be hard to make it look anything but ugly.
Keep pushing on, things are gonna get better.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Muckerbarnes1 Quote  Post ReplyReply Direct Link To This Post Posted: 23 July 2013 at 10:16am
Mik, can I assume the o/p devices will be mounted via a ali (or similar) angle or direct mounted?
 
Depends on o/p devices of course. A large conductive film and clamp on devices would be easy, especially with plastic encapsulated.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote odc04r Quote  Post ReplyReply Direct Link To This Post Posted: 23 July 2013 at 10:23am
I think you're going to have trouble producing a standard heatsink design that anyone can copy mostly due to variation in tools and ability as mentioned. Probably best off designing the circuit and layout whilst reserving areas for a couple of standard heat sink sizes? Then letting the end user mount whatever they can get from old parts, ebay etc to meet a spec. of xC/W?

There seem to be a couple of more standard heatsink designs out there that could be designed for, something like 200x200x25mm which gives these options at RS Type 1 and Type 2. Maybe they don't meet your spec. but I'd definitely include off the shelf and then perhaps a custom cooling solution in my design. The most important part to me would be to specify the area available and then let the builder meet the C/W spec anyway they like.


Edited by odc04r - 23 July 2013 at 10:24am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote audiomik Quote  Post ReplyReply Direct Link To This Post Posted: 23 July 2013 at 11:19am
Malc
Re: "can I assume the o/p devices will be mounted via a ali (or similar) angle..."

Most probably but not yet decided upon.
Original Output Transistors for the Power Amplifier Circuit I'm thinking of using were high power TO3 beasts - 200 Vce, 20A Ic, fast switching with a very good SOA.
A 'BU' something type - but as is always the way these are no longer manufactured and are very difficult to source!
Might be one FET in there somewhere, perhaps in the Limiter?

Usual very time consuming search through Manufacturers spec sheets needed, then try samples to see which works best against cost

Mik   
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Post Options Post Options   Thanks (0) Thanks(0)   Quote audiomik Quote  Post ReplyReply Direct Link To This Post Posted: 23 July 2013 at 11:54am
Originally posted by odc04r odc04r wrote:

I think you're going to have trouble producing a standard heatsink design that anyone can copy mostly due to variation in tools and ability as mentioned. Probably best off designing the circuit and layout whilst reserving areas for a couple of standard heat sink sizes? Then letting the end user mount whatever they can get from old parts, ebay etc to meet a spec. of xC/W?

There seem to be a couple of more standard heatsink designs out there that could be designed for, something like 200x200x25mm which gives these options at RS Type 1 and Type 2. Maybe they don't meet your spec. but I'd definitely include off the shelf and then perhaps a custom cooling solution in my design. The most important part to me would be to specify the area available and then let the builder meet the C/W spec anyway they like.




For delivering 300 Watts RMS into a nominal 8 Ohms load ('Low' Amplifier), dissipation would be 150 Watts for a Class AB type.
The allowance for T-Ambient of 25º and Output Transistor Tj Max (for the time being ignoring Tj-case conductivity) of 200ºC then indicates needing at least 0.5ºC overall thermal capacity for the Heatsinks of this 'Low' Amplifier.
In actual fact this will likely be a Bridged implementation so a minimum 1ºC/Watt twice.
The 'High' Amplifier Channel would be single ended so also would require a third 1ºC/Watt heatsink.
In each case, each heatsink would rise in temperature to 100ºC in 'resistive load' situations, further increasing above this should there be an adverse Load Power Factor and/or restricted airflow. Probable Temp switches at 85ºC to retain a safety margin?

Now given the target parts cost of £125 for a bi-amplified design in an earlier post, using 3 'stock extrusion types' with vat included would be a third of this budget!

With this, a realistic convection cooled design is very much on it's limit, larger output powers then will no doubt need forced cooling

hope this answers you
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Post Options Post Options   Thanks (0) Thanks(0)   Quote odc04r Quote  Post ReplyReply Direct Link To This Post Posted: 23 July 2013 at 2:56pm
It's a very tight spec! Depends what you are designing for I guess, I got the impression that you'd like to make this a powered design for lots of other users to build into their own cabs as a versatile solution. In which case I'd keep the layout and the heat sinking separate in the design brief and let users implement their own alongside an off the shelf solution in which perhaps budget is less of an option. Ebay finds, old kit, there are heatsinks out there to be had!

I agree that if you want an off the shelf solution to hit within budget for a ~400W total of AB amplification, there is going to have to be forced cooling involved. I've been thinking about this recently using a plate mounted amp and small 1U fans of the sort that you see in PKN's designs.

How about this for an idea - buy a length of rectangular aluminium hollow tubing, of the right size to mount 2x1U fans at each end. Position flat pack transistors down its length with circuit board on top in a similar manner to Rod Eliott's design for his P68 sub amplifier board. This forced air cooled tube can then be directly mounted on a plate which in turn is mounted into the enclosure. Air would be pulled through grills in the plate, then forced down the tube, then exhausted at the top of the cabinet. If the cab design was good enough then the fans exhausting air could be pushed up to meet a routed section of the cab so the wood edge protects them while the plate of the amplifier is recessed into the back of the cab.

Drawbacks of the idea are larger internal amounts of volume lost, but the heatsinking is not exposed on the back of the plate and the plate itself should run cooler. You could add fins to the centre of the hollow rectangular tubing as well to improve heat transfer to air enclosed in it.

I think I'm going to give this a go when I build my future HT sub.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post Posted: 23 July 2013 at 4:57pm
Mik:

The price of a fan will save on the (large) amount of ali you´d need to run anything high power without forced air cooling. Amps like the Yamaha 2200 come to mind. Of course, the other alternative is to go Class D...

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Muckerbarnes1 Quote  Post ReplyReply Direct Link To This Post Posted: 23 July 2013 at 5:04pm
Class D JC... tut tut, such language to us anal ogue blokes.
 
I'd still FET it Mik, but then I'm a FET anorak.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote audiomik Quote  Post ReplyReply Direct Link To This Post Posted: 23 July 2013 at 5:39pm
Worth starting with a tight specification so later it can be 'moderated' to suit other user's needs methinks.

Well, this afternoon's expedition into Town by Bus (deffo not recommended in this weather!) has located about 42" x 6" of 10SWG (3.3mm) Ali sheet and four 4" x 2" x 1/2" Ali blocks with milled ends.
The Ali blocks are chosen to be a suitable size for mounting Power Transistors as well as initially trying them as spacers.
Cost about £12 including cutting/milling and should make two heatsink blocks.
Am expecting a cost reduction as I don't think the spacer between plates needs to be 2" x 1/2", probably 1/2" x 1/2" will be more than adequate and cheaper.

So can now build a couple of sample heatsinks, each with two 6" x 10" plates, Power device mounting and spacer block.
Then attach a metalclad power resistor, apply some known DC currents and do some temperature measurements. If it gets too hot, then can simply add an extra spacer block and plate, or larger plates, then test again

Mik

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