SKA 150watt monobloc build log. 3D designed chassis. - diyAudio
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Old 21st October 2013, 11:06 PM   #1
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Lightbulb SKA 150watt monobloc build log. 3D designed chassis.

Hi All

I'm new here so go easy!

I'm building a set of 150watt SKA monoblocs. 2 for now, and then some more in the future (hopefully a total of 5)

I'm from a background of VFX, as well as custom computer building. so construckion and visualisations are not really a problem for me; the electronics, however, are. I really am learning as I go here, and I'm just trying to get used to some of the terms used etc. Luckily I can solder PCB's to instructions. and I can make things, and I know good sound from bad - I just don't exactly know how it works That's a bit of a bold statement around here, I know.



Thats' why I've chosen Greg's modules - I'm going for the 150watt amp modules, and the 300watt PSU modules (47,000uf) on his advice to drive my B&W CM9 speakers which drop to 3ohms and should be rated at 4 ohms. I'm giving each amp a 300va dual 33vac toroidal transformer which I've ordered from Airlink transformers. I added steel bands tot he order too which they are fitting.

The chassis will be nearly all aluminium except for some steel shield walls inside and some stainless and titanium bolts. The chassis will be constructed from 3mm aluminium which will be lasercut and sent to me in jigsaw form. I'll be brazing aluminium bars to the walls to keep it all together.


'RB-power' MK-I
seeing as I've come here late on I thoght I'd best start with a bit of history:

Click the image to open in full size.
Click the image to open in full size.
quick mock-up of what the MK-I amps would have looked like in-situ.

this was the first version of the chassis. I had my heart set on a design which kept the heatsink hidden away and not poking out of the back. to cut a very long story short; this design was scrapped in favour of easier and cheaper construction and vastly better ergonomics. I've also made the Mk-II chassis larger to accomodate the bigger PSU and transformer.


'RB-power' MK-II

MK-II has seen the chassis design really revised and refined - starting to think about the actual methods of fixing and constructing the case; how it's going to stack with more amps on top of it and rethinking a lot of the design and asthetic choices.


here are the main design features:

-stackable design.
each amp will be able to stack on top of another. Each amp will have spiked feet and the lits will be held on with titanium tapered cap bolts. The bolts will fix the lid aswell as providing a locator for the feet of the amp above it to sit in. the weight bearing will be taken by 15x15x86mm aluminium bars which you will see in the renders below. The spikes will be screwed into one end and the lid retaining bolts into the other; making the weight-taking abilities of the stackable amps very good indeed.

-no visible fixings
okay so not strictly true... there will be the caps on the top and screws in the back; but the front and sides will have no fixings on show. 15mm square stock will be drilled, tapped and then brazed onto the sides and front panels of the case. This is going to be tricky - I know. but I'm ordering enough bits to have a couple of goes at it.

-Sayagata grille
I love the Sayagata pattern and it had to be featured in my design; I understand that the slots may have a bad effect on EMI, but truth be told I don't really know what that is, or why it is a problem. I *think* I can get around it by putting a finer mesh below the main grille but I'd like to talk about this if anyone has any ideas on it and if it will be a problem.


-slightly suspended design
cluching at straws here a little but the idea of the built in feet was that the amps would only just sit above the ground. with only a few mm between the flat bottoms of the chassis and the ground - being kept up with spikes. hope that makes sense!

-finish:
The finish of the amps will be an abrasive-blasted finish. Quite which one I'm not sure of yet. Think G-tech drives (if you know what they look like) I'll be getting the entire case blasted in one go. I'm yet to find a place to do it though I have not really looked.


MK-II renders

Click the image to open in full size.

Click the image to open in full size.
detail of the back of the chassis; hopefully you can see the spikes working here to stack the amps

Click the image to open in full size.
two of the MK-II amps stacked



Click the image to open in full size.
Some advice regarding the layout here would be great. It's all amout as spaced out as it can be but I can move the outputs one above the other etc. Also please note that the IEC plug is going to be replaced with a version that has an intergrated switch and fuse - I just have not got around to modeling it yet.

Click the image to open in full size.


Click the image to open in full size.
purple: GB300S PSU with 47,000uf
salmon: GB soft-start module
green: GB150D 150watt amp module
blue: airlink 300va, 2*33Vac secondries + steel band.

those walls around the transformer are steel - Can I have them that close? (almost touching)





Click the image to open in full size.


Click the image to open in full size.

exterior renders with grille. Advice on if I should put mesh behind this grill would be nice. I can make the open spaces in te grill smaller too but I'd rather not screw with the proportions of the pattern.


Thanks for looking
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Old 22nd October 2013, 04:33 AM   #2
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Impressed...!!!



Great work RB

I like the rendering of the Grill on the top cover. Fantastic
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Old 22nd October 2013, 06:36 AM   #3
Stuey is offline Stuey  Australia
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Nice!

Isn't 'brazing' a term specifically for when you use brass rod? I assume you're not actually brazing the side bits on. Or are you?

The steel around the transformer will be fine, so long as no metal such as a top panel touches the middle bolt/nut of the transformer (or the retaining flange under the nut). This causes a 'shorted turn' and is a major problem.

Last edited by Stuey; 22nd October 2013 at 06:41 AM.
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Old 22nd October 2013, 08:56 AM   #4
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Quote:
Originally Posted by Stuey View Post
Nice!

Isn't 'brazing' a term specifically for when you use brass rod? I assume you're not actually brazing the side bits on. Or are you?

The steel around the transformer will be fine, so long as no metal such as a top panel touches the middle bolt/nut of the transformer (or the retaining flange under the nut). This causes a 'shorted turn' and is a major problem.
thanks for the advice about the transformer. I'll make sure to follow that and have been reading up on shorted turns. There's plenty of headroom in the case above the transformer, and it can be raised / isolated from the base too. once the actual units are delivered I'll be able to finalise this part of the design based on actual measurements rather than just specs.


re: brazing; yes most often it refers to brass and copper. but it can apply to other metals too. essentially it's low temp welding, or high-temp soldering and there are a few products on the market to 'weld' aluminium with a torch. it's not a 'true weld' but it will be good enough. This is the only part of the build I still have hesitations about and need to test it out first before I commit to it. countersunk bolts would absolutly be the easiest way around it but I just don't want them to be seen.

excuse the almost infomercial-like levels of selling in this video:

https://www.youtube.com/watch?v=yqkceLrH9Ao
I've found equivalent products to this in the UK (ebay!) which I am going to try out. the biggest question is if it will work when the two pieces are clamped together; as in every demo they are just kind of free-standing.


Quote:
Originally Posted by Workhorse View Post
Impressed...!!!

Great work RB

I like the rendering of the Grill on the top cover. Fantastic
Thanks
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Old 22nd February 2014, 10:26 AM   #5
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*blows the dust off the thread*

Hello all,

This is still happening! I had to put it back on the back back burner for a bit but as of today all of the sheet and stock metal has been ordered! in fact it seems I have a lot more bits and pieces than I did last time I posted here.
I have all of the electronics, except a few 4700uf caps which popped in transit (still need to get onto Greg about these!) I have most of the hardware except the spikes and the titanium lid bolts; but these are not vital until I have enough amps built to need to stack them!

the last few weeks have mostly been about doing the drawings in AutoCad. My day job lends it's self to learning all kinds of software to do with most things 3D and CG; but not AutoCad! I used it in the distant past to design and laser cut panels for custom PC's (also a day job) but that was ages ago and I had forgotten completely how to use it.

this amp requires the same thing though; 2D drawing to be sent to a laser cutter (the same laser cutter that I used to use years ago!) so it was time to get back on it. This is where I was so glad I had made the model in Maya before the 2D drawing. Maya is not famed for accuracy - it's output can't really be translated to CAD, but as a visual tool to spin around and check it was very handy.

Cut 1: Aluminium
[img]

This took a while as I was learning as I went along. my google history is now almost entirely autocad related! everything has been test-fitted in 3D so I *should have no issues* the only issue that I will have is the tolerances on the material. I know that will be an issue but at the same time I was not sure how to deal with it so I have hit everything on the nose based on 3.0000000000mm sheet. In particular I'm proud of the speaker terminal outputs with the notches cut in
Aluminium Drawing:
Click the image to open in full size.

Cut 2: Steel
The next process is steel. I had originally planned to have these made of steel but put it on the list of less importance. These pieces have been revised 100's of times before deciding on this design. These parts make up a double wall of shielding to shield the transformer and the soft-start / AC from everything else. This is pretty much guess work but I can't see it doing any harm so I went with it. Steel is cheaper and easier to cut so these pieces were a bargain really.

Drawing for steel:
Click the image to open in full size.

cut 3: aluminium square bar

These pieces have kept me up at night. they need to be tapped and drilled in the ends... not looking forward to it! I asked my laser cutter if he could do them and he said yes, but he apologetically said they just can't be competitive on price, which was fair enough... I figured what it was going to cost me to have htem cut, drilled and tapped I could invest in tools to do it my self.

I had a really good quite months back for just the plain bars cut to accurate length; I took them up on the offer andordered 36 bars and had an ebay trip to find some suitable taps. all I need now is avice for my drill press!

Here's what I sent off for a quote - no dimensions in this drawing, just a hole count and quantity.
Click the image to open in full size.

This is where I lost sleep... how was I going to find the centre lines for all of these pieces? Since I was getting a steel process done anyway I thought I'd design a quick template for drilling them. I designed it in my sleep and it's quite a simple affair:

the gaps will fit an aluminium bar in and the holes line up with holes in my drawing. quite happy with that although I did have to get up early and draw these before work to get them sent in time. It pushed the price up a little but it's going to be so worth it.

Click the image to open in full size.



Here's a rough render / model of what the steel parts are for. there's one missing though - there's a piece with a slot in it which goes on the back with cutouts for the speaker posts and AC that I have not modeled in 3D yet. not sure if I ever will :S The darker bits are steel, everything else is aluminium. The section which joins the transformer wall tot he rear panel will just drop in into mating slots
Click the image to open in full size.



So yea, there's a little update! I'll post back when it all starts arriving from the varies people and so on.

Rick

Last edited by R B Custom; 22nd February 2014 at 10:33 AM.
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Old 22nd February 2014, 10:37 AM   #6
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Sorry forgot all about the moderator approval system on here and may have tried to post the same thing more than once. I messed a few things up in that post and can't edit

Here's the render that was meant to be at the bottom:
Click the image to open in full size.
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Old 27th February 2014, 10:21 PM   #7
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Hey Guys,

No real updates yet. I'm waiting for the aluminium and steel to be ready / elivered. I'm really hoping something shows up tomorrow so I can have the weekend to work on it.

I'm continuing to tool-up for when everything arrives. I've been doing a lot of research lately on tapping aluminium and it seems there are loads of people saying one thing and loads of people saying the opposite... In the end I've chased down these kind of bits which are spiral flute taps with a yellow band (specified for softer metals) They were pretty cheap on eBay, and arrived the next day. I've tested them on the only bit of scrap aluminium I can find and they seem to work great.

Next was the purchase of centre drills. I'd never heard of these before and really wish I knew about them earlier as they seem to be the defacto solution to starting an accurate pilot hole. They have very thick shafts relative to the cutting bit so they do not bend and skate when starting a hole (like a small drill bit would) I've tested them and the smaller ones seem to be ideal. I think they can also be used to countersink too. I'm not positive what they are actually meant to be used for, as I have read about 'spotting drills' too but they seem to work for what I need them for so far. 6.90 delivered for this set so cant complain.
Click the image to open in full size.
Recent bit purchases^^

Also while waiting I've been trying to get my head around the wiring diagrams for the amps. wiring diagrams are not native to me... I'm still having a hard time reading them so the best thing I can do is draw my own 'RickCad' diagrams on paper to traces wires where I think they must go and then model them into my 3D scene so that I can feel like I'm wiring it up but not actually kill anything. My main problem is the terminology associated with star grounds, ground planes, earth's etc It's loosing me completely!

It should be said that I'm not 100% sure if this is correct, and the paths / routes are certainly not as they will be on the day, but from studying the diagrams this is how I thought it goes - I'm checking with Greg from Ska before I wire anything up for real though:
Click the image to open in full size.
( hi res: http://i.imgur.com/4bVitQG.jpg )

You may notice I've taken the steel divider out of this model - I've made a bit of a ****-up and I will actually have to drill my own holes in that part when it arrives (I need a hole to take wires from the soft-start that I didn't know were there - unless I have it wrong :S). I've just removed it for now though but you can see the slots in which it will drop into. and you can even see the transformer brace under the honey-comb mesh All of the parts on this model are made from the CAD drawings, so it's all exact (except the square pillars are not alined properly if you can see that?)

hopefully will have an actual update soon!
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Old 28th February 2014, 08:01 AM   #8
Bonsai is offline Bonsai  Taiwan
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How are you holding the external panels in place? I don't see any screw holes?

Nice design by the way - looking forward to seeing it in the flesh!
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Old 28th February 2014, 08:42 AM   #9
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Originally Posted by Bonsai View Post
How are you holding the external panels in place? I don't see any screw holes?

Nice design by the way - looking forward to seeing it in the flesh!
Well spotted!
This will be subject to testing, but basically after looking into various forms of brazing, soldering, welding etc I was getting pointed more and more in the direction of just using JB weld. If this fails then I will just drill some neat countersunk holes and use a countersunk hex bolt - I don't want to do this though as external fixings will spoil the look.

On the plus side the sides bear no weight as all of the weight is carried through the four square posts in the corners, and then carried to the floor / rack / table through spiked feet. I only think and hope this is going to work! but it's not the end of the world if it doesn't; If the front and side panels were to just fall off one day the rest of the amp would still be bolted together (bolts on back, top, bottom and interior shields)

hope that makes sense. I'll make an image of the rigid parts tonight to show where mechanical connections are made.

The delivery of the square aluminium bar is due today by the way! exciting
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Old 28th February 2014, 10:15 PM   #10
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Originally Posted by R B Custom View Post
hope that makes sense. I'll make an image of the rigid parts tonight to show where mechanical connections are made.
Just for giggles:
Click the image to open in full size.
This is an image of all the parts that are fixed mechanically; and the ones that are not are on the floor. the front and sides will be J-welded to the square bars (3 bars per side)



I had a delivery today!
Click the image to open in full size.
36 square aluminium bars! wow!
They're meant to be 86mm, I asked for the tightest tolerance possible but they all seem to be around the 86.75 - 86.95 mark. that shouldn't matter as long as they're all in that range.

I set about doing a test drill one one piece (a lot of those are spare!)..

Last weekend I had to option of buying a cross vice for my drill for about 35 more tha the one I bought... I thought a regular drill press vice would do but this one is utter rubbish! the jaws jack-knife ad you tighten it which throws the work off and to be honest, lining up the bar central to the bit is a total pain. it took me quite a while to get the centre drill centered and managed to plug a perfect pilot hole into the piece... I had to reset the vice though to change to the 5mm bit. somehow this went wrong and I ended up drilling the hole off centre by quite a margin. I'm still a little bit puzzled how I did this since the centre hole seemed to be spot on; I'm guessing it just wasn't aligned properly.

A cross vice would have been ideal to fine tune the positioning of the work before drilling it. At the moment I need to loosen the drill vice and knock it into place with a hammer - back and forward until it's correct. As a result I've ordered one online for 30 delivered (using a discount code) I'm very aware that it's just another cheap tool, but I can't justify spending big bucks on a better one so I'm hoping it's going to be okay!

Click the image to open in full size.
stupid cheap drill press vice buckles and skews the work

Click the image to open in full size.
Resulting in an off centre hole. I'm only after a result a little bit better than that really. a little breathing room in the holes in the sheet metal will allow for a little bit of off-centre drilling, but hopefully not that much.


On the plus side the depth of the hole worked out as planned and it was pretty easy to thread! I've tested it with a bolt and I'm quite pleased with it.
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