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Old 22nd October 2006, 07:09 AM   #521
AndrewT is offline AndrewT  Scotland
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Hi Mark,
what have you done to the heatsink?
It looks like you machined away 70% of it's dissipation capability.
All you needed to do was machine off two fins underneath and the sink would have fitted the extended PCB.

I still don't understand why the PCB needs to be so long. Is it a compatibilty issue?
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Old 22nd October 2006, 08:09 AM   #522
PWatts is offline PWatts  South Africa
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Phew a lot has happenned since I went to sleep last night!

Correct about the feedback loop; I kinda forgot the KSA's are already completed

Regarding the issue of the heatsink shorting out the top layer: the original had the same problem, and it was also fixed by manually drilling removing and drilling out the copper. Otherwise you can always rely on the anodizing of the heatsink
The options are difficult - production houses charge more if you change the holes that they're not through-hole plated since they have to drill them out themself. I can change the pad to a via and thus it will be covered by solder mask, but that will happen on the bottom layer too, and the idea is for the mounting screw to provide a solid connection between the rails and the corresponding collectors. I can minimize the pad size so that it will be easier to scratch off with a sharp knife, but with thick copper on proper boards it will be difficult.

All in all the best solution would be a piece of teflon, silicon or whatever's available.

AndrewT: initially the board was much smaller, hence the large open spaces at the sides too (which I stuffed with decoupling caps afterwards). The size increase was to make it a 100% dop-in replacement for both commercial Mk1 and Mk2's.

Not much I can do about the 62pF's position to avoid pushing them down - Personally I prefer a single piece of sheet aluminium for all four predrivers.

I think Mark has shortened the heatsink to make it like the original. Also, it cannot extend much higher than the board or it will touch the chassis cover.

Regarding final changes for the board prior to the group buy:

1) I've shifted VR2, C7 and R36 below the driver transistors.
2) Reversed the bottom side text.
3) Added polarity indicator for the SMD LED's.
4) Increased the mounting holes to 4mm for compatibility with the original's mounting standoffs.
5) Minor changes to silkscreen.

Mark, do you have any plans before heading off to the Group Buy? At least we have to check the necessary heatsink sizes at full power and if possible analysis of the three different FET types. Maybe it's also time to get the wiki up and running in addition to the board orders.
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Old 22nd October 2006, 08:31 AM   #523
AndrewT is offline AndrewT  Scotland
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Hi,
Quote:
it cannot extend much higher than the board or it will touch the chassis cover
agreed.

But there is space to the side (end) of the vertically mounted PCB to take a WIDER heatsink. More like the shape of the unaltered sink pic that Mark posted, with just two fins removed from the underside to allow the big sink to fit the PCB.
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Old 22nd October 2006, 12:27 PM   #524
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Andrew,


I am attempting to keep it as close to the original as possible. Do a comparison and you'll see that I still have considerably more than Krell did with theirs. Mine is also thicker. The one thing I had to do though was to machine the base of it thinner that I wanted to do. This is because of the short length of the devices leads which limits how far above the board they can actually be mounted. I am about 1/4 of the way done weeding out the old Cerwin Vega amp to test it in. Fortunately the Cerwin Vega amp is modular and has an optput device board that I have easily modified to work with the KSA-100 driver board.

Mark
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Old 22nd October 2006, 12:29 PM   #525
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I just have to get some insulated standoffs to mount the driver board to the output board then I can wire it up. I aso have to flip the output device board around so the + and - devices are on the correct sides!
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Old 22nd October 2006, 12:36 PM   #526
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This is obviously way too small a sink to run it in class A without any fan so I am going to bias it into class AB for now... perhaps you all could suggest a bias level for me to set it at to approximately come out in AB mode. The normal class A level is 575 mv across the emitter resistors, K-Amps does have a bias calculator some place though

PWatts...

Am trying to get this going so I don't leave those interested in this hanging for the better of a week as I'm off to a convention in Florida for the moast of next week. We can certainly try the other devices as time goes on but it may be a few weeks before I aquire the parts and get around to actually doing it. On your friends KSA-100 Mk-2 there is supposed to be a network across the output terminals according to the schematic... can you find out the value of those parts for me???

Thanks!
Mark
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Old 22nd October 2006, 12:43 PM   #527
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FWIW: The amp I gutted is an old Cerwin Vega A-2200 Sensurround power amp circa about 1974 or 5. It was filthy and was sitting for years. I have completely stripped it down and cleaned it up though and the chassis is actually in pretty good shape. This is strictly meant as a test bed to proove that the boards function properly but I di plan on making it a stereo amp... when I get back from the trip I will assemble the other channel.. When I crank it into Class A I will definately need a 4" fan running at each end of the amp .

Lets get the group buy set up and runnning when I get back from the trip....


BillWW,

Hang in there I'll give you a call when both channels are up and running. LuckyLyndy doesn't seem to interested in this amp for some reason or other.....


Mark
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Old 22nd October 2006, 01:14 PM   #528
PWatts is offline PWatts  South Africa
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The Mk2's snubber filter on the output is shrouded inside a plastic cover so I don't have access to the node to measure the resistance, but on the Mk1 it was a 5R6 and 100nF. Progress looking good.

Since everybody is free to experiment with the FET's of their choice, I think we may just then sort out the issue of heatsink sizes before the Group Buy.

Mark, what's the status on the driver heatsink group buy? Separate ones that are nicely anodized will give a so much nicer appearance and I'm sure most wouldn't mind the extra cost. A blue board with blue heatsinks (and blue LED's hehe) will look so cool
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Old 22nd October 2006, 01:39 PM   #529
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Have not done anythng on any group buty at this point in time. Too busy with work and getting this proto up and running. I sonder of Floedstrom has done anything else with the tranny group but...?

Mark
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Old 22nd October 2006, 10:06 PM   #530
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Have been listening to one channel of the amp now for several hours. PWatts is right. this is a dynamyte amp! I will almost go so far as saying that one KSA-100 channel is better than a stereo KSA-50!

Bias is set at 300mv across the emitter resistors for now. Runs very hhot but not hot enough to trip the thermal sensors on the amp chassis.

Mark
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