OptiMOS amplifer build issues, help needed please - diyAudio
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Old 13th February 2010, 03:15 PM   #1
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Default OptiMOS amplifer build issues, help needed please

I am having some problems with a second OptiMOS amp and need some guidance in solving it. The design is taken from Randy Slone's book "Audiophile Project's Sourcebook" Figure 6.21. PCB's are self etched from the artwork taken from the back of the book. I built the 200W watt version using the single die MOSFETS.

Whereas the first amp i built worked pretty much first time, except for hum solved with proper ground wiring (thanks to this forum!), this second amp gets VERY hot VERY fast after power on. The odour was enough for me to call it a day and switch it off immediately. Note that i do hear music through the amp (i combined a speaker with the dummy load to take small amount of the signal), but it is HEAVILY distorted. But there is at least some life there. I have attempted to fault find by comparing multimeter readings between the board and the correctly operating one, changing some transistors to no avail.

Having searched this forum i am beginning to think that this could be an oscillation issue? I cannot take any readings while the amp is powered for obvious reasons, so diagnosing it is quite difficult. Nick Walker's previous posts are very instructive, he say:

"elimating the oscillation was a lot harder. I tried all sorts of things, some still remain, however the eventual fix was simply to increase the size of R23 & R24 gate ressistors to 1k on the 2sk1058 MOSFETs" - Nick Walker

Has anyone here built a Slone Amp or a Tigersaurus?

As to why my first board was OK and this one is not i am baffled. Different transistors operating differently due to manufacturing tolerances?

I would be very interested to know of any "essential" or "recommended" mods to the component values as compared to those in the book, as it seems perhaps there are some recurring issues which have already been tackled by those long before me.

Thank you in advance.
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Old 15th February 2010, 01:19 AM   #2
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Join Date: May 2009
Issue resolved, illusive short at the edge of the board - sheesh did that take some finding!
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