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MOSFET Amplifier IRFP240/IRFP9240
MOSFET Amplifier IRFP240/IRFP9240
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Old 12th June 2019, 08:40 AM   #1271
apexaudio is offline apexaudio  Serbia
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Quote:
Originally Posted by still4given View Post
Hi Thimios,

Is that how you wired yours? I will try that but I don't think that should be necessary. This amp seems unstable to me even playing only one channel, it doesn't sound right.
You can try compensation... use 47pF from base to colector of VAS transistor...
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Old 12th June 2019, 10:08 PM   #1272
still4given is offline still4given  United States
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MOSFET Amplifier IRFP240/IRFP9240
Quote:
Originally Posted by apexaudio View Post
You can try compensation... use 47pF from base to colector of VAS transistor...
I tried this. Same problem. This is two separate sets of boards doing the same thing. I will go no further with this until I get another tone generator so I can look at the scope. At this point I am just chasing ghosts.

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Old 12th June 2019, 10:34 PM   #1273
stocktrader200 is offline stocktrader200  Canada
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55v rails are high for a single pair of outputs, 4 ohm loads are definitely going to be problematic.
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Old 12th June 2019, 11:11 PM   #1274
nigelwright7557 is offline nigelwright7557  United Kingdom
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Make sure there are no ground loops when connecting two amplifiers.
The only common connections should be at the smoothing caps at zero volts,55 volts and - 55 volts.
If you get noise in the ground line it can modulate the input signal which drives the amplifier which in turn modulates input signal more.

I designed a USB mixer that had ground problems and had 1VAC of signal on output with input shorted !
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Old 12th June 2019, 11:23 PM   #1275
still4given is offline still4given  United States
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MOSFET Amplifier IRFP240/IRFP9240
Here is a schematic with voltages.
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File Type: jpg Mos200 voltages.jpg (81.7 KB, 181 views)
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Old 12th June 2019, 11:58 PM   #1276
nigelwright7557 is offline nigelwright7557  United Kingdom
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Quote:
Originally Posted by still4given View Post
Here is a schematic with voltages.
My only criticism is no capacitor on VAS.
Q1 b-c should have 100pf across it.
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Old 13th June 2019, 07:08 AM   #1277
thimios is online now thimios  Greece
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I will try to run a scope test today if possible.
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Old 13th June 2019, 11:20 AM   #1278
thimios is online now thimios  Greece
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Yes Terry i can confirm that is an oscillating amplifier, unfortunately.
I haven't a scope test but i measure 35mV a.c 137kHz. at the speaker out.
This oscillate only when speaker or dummy load attachment. Without a load is ok.
You can kill oscillation if you short the input.

Last edited by thimios; 13th June 2019 at 11:23 AM.
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Old 13th June 2019, 03:26 PM   #1279
still4given is offline still4given  United States
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Quote:
Originally Posted by thimios View Post
Yes Terry i can confirm that is an oscillating amplifier, unfortunately.
I haven't a scope test but i measure 35mV a.c 137kHz. at the speaker out.
This oscillate only when speaker or dummy load attachment. Without a load is ok.
You can kill oscillation if you short the input.
Hi Thimios, I did see oscillation for the short time my signal generator was working. Now I can't get it to power up with both channels hooked up. Single channel plays but doesnt sound right. I put 47p on the vas but no improvement. Miles says many of these have been built and used but maybe with different transistors than I used.

Last edited by still4given; 13th June 2019 at 03:29 PM.
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Old 13th June 2019, 05:47 PM   #1280
apexaudio is offline apexaudio  Serbia
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Quote:
Originally Posted by still4given View Post
Hi Thimios, I did see oscillation for the short time my signal generator was working. Now I can't get it to power up with both channels hooked up. Single channel plays but doesnt sound right. I put 47p on the vas but no improvement. Miles says many of these have been built and used but maybe with different transistors than I used.
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.
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