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Old 21st October 2006, 09:04 AM   #491
AndrewT is offline AndrewT  Scotland
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Hi Mark,
that large area of almost empty PCB under the driver collection seems to be an embarassment to fitting your excellent heatsink.

Does the extra empty PCB really need to be there?

What about re-routing the traces and connections at that end to make the drivers/heatsink an easier fit?

What are the blue resistors? They seem a bit bigger than the rest.
film, metal, WW, 500mW, 600mW, 1000mW?
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Old 21st October 2006, 09:34 AM   #492
PWatts is offline PWatts  South Africa
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AndrewT:

The area under the drivers is for the PRE-driver heatsink, not the driver.
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Old 21st October 2006, 09:34 AM   #493
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Actually I think the heatsink will for the most part fit just fine if I notch it for the bias pot. C-7 is the only problem and one ond of it could easily be turned down 45 degrees and the one trace re-routed. Back side mounting might be ok but as far as making it interchangable with the original Krell boards I don't kow what will fit in. There is no problem physically with R-37... just missing the silkscreen I.D.

I will post more pics today or tommrrow so you can all see how I intend to mount the sink. I have to do quite a bit of machining to them first.

The blue resistors are just surplus stuff... I think the larger ones are 1/2 watt and the smaller torquoise ones are 1/4 watt.

Mark
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Old 21st October 2006, 11:44 AM   #494
PWatts is offline PWatts  South Africa
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OK I already modified the board by shifting C7 and VR2 both below the drivers' pins. No modifications will be necessary to the driver heatsink then, but the predrivers will need some ingenuity. However, the predrivers don't need nearly the same size heatsinks as used on the original.

R37 has been placed with the software on the bottom side, so the silkscreen is there - it's just on the bottom where we're not going to use any silkscreen. But I've flipped that to the top as well.

As it is now only one resistor close to the bottom needs to be mounted on the bottom side.

The original's standoffs were long enough to leave quite a bit of room for anything mounted at the back.
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Old 21st October 2006, 01:18 PM   #495
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Quote:
As it is now only one resistor close to the bottom needs to be mounted on the bottom side.
Which one is that... I am pretty sure ALL of my resistors are on top... or did I miss one? Or are you talking about there being one below after these mods you just did?

Mark
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Old 21st October 2006, 02:46 PM   #496
PWatts is offline PWatts  South Africa
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Mark: There's one crucial resistor at the very bottom going to ground. I don't have the layout or schematic in front of me right now, but I'm fairly sure it's the 825ohm feedback resistor going to ground. You can see its one hole on the ground plane just above the signal input, and the other between the rightmost two blue 150R emitter resistors.

Without it the gain will be very low and quite possibly be unstable too.
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Old 21st October 2006, 02:47 PM   #497
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I also noticed that the order of VR-2 and R-37 are reversed in order fomr the schematic... Probably doesn't matter much though. Also R-36 silkscreen has been left off ... right between Q-11 and Q-12. I found R-16 near the bottom rear just as you mentioned.

All I need to do is attach one sink and then I can fire up the main board which I intend to to without the output stage at first as many of us did with the KSA-50.

Mark
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Old 21st October 2006, 03:18 PM   #498
PWatts is offline PWatts  South Africa
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R36 and R37 were intended to be mounted on the bottom side, hence the absence of the silkscreen and refdes.

Something's not entirely clear from your refdes's - if you're uncertain, check the connections with the schematic instead of the designators.

The resistor between Q11 and Q12 is R37, 7k5.
The other one in series with VR2 is R36, 1k5

The reversal doesn't matter; A+B = B+A.

Firing up the board without the output stage can be dangerous - follow the guidelines as in the Leach assembly guide to complete the feedback loop and disable the bias circuitry, or the amp will probably oscillate.
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Old 21st October 2006, 09:21 PM   #499
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If you use the original heatsink like Krell used they don't need to be mounted on the bottom. I also found some nifty heatsinks for the pre-drivers, tight fitting and we'll see how they do temperature wise but this is as compact as TO-220 heatsinks get....
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Old 21st October 2006, 09:22 PM   #500
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Side view...
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