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Old 13th March 2008, 09:03 AM   #1
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Default maplin mosfet GA28F upgrades?

hey-hey

iv got two home made monoblocks using the good old maplin GA28F mono boards

they have a 1kw dougnut and 6 BH or BHC? 15000uf 63volt capacitors each,
3 for the power supply and 3 for the output stage?

the GA28F's have the Exicon transistors 10p16 and 10n16

the two main capacitors on these boards [c11 & c9] are 63v 220uf(m) made by dubilier

is there a chance of upgrading any of this to improve sound?

the sound at the moment is fair but not as good as my sansui AU-919 which i know is a top amp but its 1979 technology afterall

im testing my amplifers with IMF tls80 monitors

thanks all
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Old 13th March 2008, 11:43 AM   #2
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Quote:
they have a 1kw dougnut and 6 BH or BHC? 15000uf 63volt capacitors each, 3 for the power supply and 3 for the output stage?
No, 3 for + rail and 3 for - rail!

Simple upgrade hints:
Polyester input capacitor + high-quality elko in the feedback + ceramic-NP0/mica compensation capacitors
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Old 13th March 2008, 12:57 PM   #3
jaycee is offline jaycee  United Kingdom
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The Maplin amp is basically a copy of the Hitachi sample note for the original output MOSFETS.

I'd say build a better amp. http://www.wnaudio.com/amos100.pdf would be ideal.
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Old 13th March 2008, 05:04 PM   #4
burbeck is offline burbeck  United Kingdom
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hi
a quick upgrade would be to replace the output mos fets with dual die parts, these are drop in replacements, equivelent to 2 parallel parts in one case

try BUZ900D and BUZ902D pairs
or ECF20N20 and ECF20P20 pairs
the Idle current will need need adjusting to double what it is now I would suggest 200mA. with either of above pairs.

with this module i recommend replacing the R15, 4R7 3 watt with a non inductive (non wirewound part) i use 8R2 2 watt.
this helps with hf stability.
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Old 14th March 2008, 03:13 AM   #5
jaycee is offline jaycee  United Kingdom
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I wouldnt recommend that. The MOSFETS are driven directly by the VAS stage in this design. Adding dual die outputs will load the VAS further, and distortion will rise.

You could try this with the White Noise amp I linked to, but to be honest one pair on 50V rails is plenty
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Old 17th March 2008, 06:34 PM   #6
burbeck is offline burbeck  United Kingdom
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i have used parallel mosfets in this circuit no problem, im sure im not the only one
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