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Old 5th March 2014, 07:14 AM   #1
ishiru is offline ishiru  Indonesia
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Location: Jakarta, ID - Nagasaki, JP
Default Nakamichi PA200 upgrades

hello all..

i have Nakamichi PA-200.

previously, i changed all of the electrolytic w/ the better one (i suspected all of them was dried out).

they actually sounds pretty good but kinda dried from my thought.

so changed the pre-driver w/ a mosfet. (i previously changed the original w/ ISC A968/C2238)

the mosfet i used was IRFP240/9240 (i only have them in my shelf)

i was pleased w/ the vocal.they sounds better.but strange!.i heard some distortion on the highs.

i couldn't write how they sounds like but they're pretty audible.no.not pretty audible.but very audible!

is it caused by the mosfet?.does the mosfet was too big?..

oh.i forgot to mention this.originally this amp's bias was very low (i couldn't measure them).and then i re-biased them to 10mV (don't say "that's low!".this amp runs freakin hot after re-bias).

but now after i changed the driver to mosfet the bias back to unmeasured.is this causing the amp has distortion?.this amp before re-bias has no distortion even runs cold.



before i perform this change, i simulated this to LTSpice and got no problem w/ the sinewave

i recorded the video but don't know how to upload it here

Last edited by ishiru; 5th March 2014 at 08:12 AM.
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Old 5th March 2014, 08:33 PM   #2
ishiru is offline ishiru  Indonesia
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Location: Jakarta, ID - Nagasaki, JP
i figured it out..
i simulated the amp to LTSpice and i figured that when i use mosfet w/ the same bias resistor, the bias goes lower (2.1kOhm bjt was around 3W while mosfet was only 20nW).

i'll try to set the bias pots to around 500Ohm and let's see the bias (yes.when i simulated this amp using mosfet, 495Ohm gave closest result on 2.1kOhm on bjt)

idiot me being idiot
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