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MOSFET Amplifier IRFP240/IRFP9240
MOSFET Amplifier IRFP240/IRFP9240
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Old 13th June 2019, 06:13 PM   #1281
still4given is offline still4given  United States
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MOSFET Amplifier IRFP240/IRFP9240
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Originally Posted by apexaudio View Post
I connect ground like in schematic from post =1266 and I use MJE340 for VAS Outputs was 2sk1529/2sj200 and irfp249/9240 in several pcbs there is no any bias or osccilation instability.
Do you think it makes a difference if the input ground comes from the spot on the pcb vs the input jack? I can very easily change out the VAS for MJE340. I have many of them. Did you build one with BD139 in bias spreader? Maybe that is also an issue.
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Old 13th June 2019, 06:44 PM   #1282
apexaudio is offline apexaudio  Serbia
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Originally Posted by still4given View Post
Do you think it makes a difference if the input ground comes from the spot on the pcb vs the input jack? I can very easily change out the VAS for MJE340. I have many of them. Did you build one with BD139 in bias spreader? Maybe that is also an issue.
Yes there is ground loop in your build... I use BC546 for bias.
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Old 13th June 2019, 07:05 PM   #1283
still4given is offline still4given  United States
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Yes there is ground loop in your build... I use BC546 for bias.
So you only built one for sub in class B. What about crossover distortion? Will this be noticeable in full range use?
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Old 13th June 2019, 10:47 PM   #1284
still4given is offline still4given  United States
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OK, I installed MJE340 in VAS. I hooked up the grounds per the schematic. Still the same issue. Alone they seem behaved but with both channels powered it goes very high bias.
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Old 13th June 2019, 11:03 PM   #1285
nigelwright7557 is offline nigelwright7557  United Kingdom
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Originally Posted by still4given View Post
OK, I installed MJE340 in VAS. I hooked up the grounds per the schematic. Still the same issue. Alone they seem behaved but with both channels powered it goes very high bias.
For complimentary outputs source resistors are usually added for stability.
I use 0r22 with my irfp240/9240 amps.

The VAS should have 100 to 220pf across BC.

The diode from top 240 gate to power rail is a bit odd.
I have seen reverse biased diodes act like capacitors and pass rail noise to gate of mosfet.

My last comment is regards Vbe multiplier. It is currently set to 7v6 which could be a little high.
I usually use a trimmer in my bias circuit and slowly turn up bias until cross over distortion goes.

A few things for you think about.....
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Old 14th June 2019, 07:38 AM   #1286
apexaudio is offline apexaudio  Serbia
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Originally Posted by still4given View Post
OK, I installed MJE340 in VAS. I hooked up the grounds per the schematic. Still the same issue. Alone they seem behaved but with both channels powered it goes very high bias.
Only difference to my builds is input ground connect to CT of 10000uF elkos under the PSU100 pcb.
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Old 14th June 2019, 07:50 AM   #1287
still4given is offline still4given  United States
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Only difference to my builds is input ground connect to CT of 10000uF elkos under the PSU100 pcb.
There has got to be more of a difference than that. I cannot believe that one inch of copper trace is the difference between oscillation and no oscillation. The only thing left to try is to replace the BD139 with BC546 and see if that works.
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Old 14th June 2019, 11:05 AM   #1288
thimios is online now thimios  Greece
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Stereo mode.

Last edited by thimios; 14th June 2019 at 11:15 AM.
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Old 14th June 2019, 04:17 PM   #1289
thimios is online now thimios  Greece
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Scope test.
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