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Very simple quasi complimentary MOSFET amplifier
Very simple quasi complimentary MOSFET amplifier
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Old 26th May 2018, 10:17 PM   #2181
Bix is offline Bix  Australia
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Very simple quasi complimentary MOSFET amplifier
Nice, looking forward to your review of the sound. Love the safety precautions, I get super nervous with first power up. Recently I had a cap reversed, luckily it was low voltage. Took a while to realise what I had done wrong. Now Iím super careful with cap orientation.
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Old 27th May 2018, 01:06 PM   #2182
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Very simple quasi complimentary MOSFET amplifier
I found something interesting while testing the boards tonight.

I set the bias to 100mA (33mV R20) but observed it was fluctuating some.

I realised that if I put a finger near or touched C5 (33P Mica), the bias current would increase.

Just bringing a finger near and touching will get the bias to about 140mA, but if I bring my finger close quickly and hold it there, the bias can sit at around 240mA.

Wonder if anyone has come across that kind of behaviour / if it is something normal. It won't really matter in the end because the amp will be boxed up, but I'm curious anyway.
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Old 27th May 2018, 01:47 PM   #2183
avtech23 is online now avtech23  Australia
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Very simple quasi complimentary MOSFET amplifier
Got the first 2 boards mounted as a module in preparation for boxing up.

Will probably not use the 14+ year old computer fan for the real deal though...
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Old 27th May 2018, 01:58 PM   #2184
AKSA is offline AKSA  Australia
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This is HF oscillation; you can verify with the Zobel 10R at the output.
If the Zobel resistor never heats during use however the amp is quite stable.

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Old 28th May 2018, 08:15 PM   #2185
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I am thinking of trying IRF140 N channel fet with bootstrap on the high side and unity gain PNP darlington config on low side and test the performance of this with approx 4.7v of bias accross them.
This is primarily to avoid an elevated B- supply to get rail to rail output.
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Old 28th May 2018, 08:40 PM   #2186
xrk971 is offline xrk971  United States
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Very simple quasi complimentary MOSFET amplifier
Click the image to open in full size.

Nice tunnel cooler. Check out some large 120mm PWM fans and PWM controllers. Set to 10% and they are dead quiet. Built in temp sensor to speed up motor and also audible alarm.

Noctua NF-P12 redux-1700 PWM 9010018100365 | eBay

12V PWM PC CPU Fan Temperature Control Speed Controller Module High-Temp Alarm | eBay
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Old 28th May 2018, 11:15 PM   #2187
avtech23 is online now avtech23  Australia
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Thanks X.

They look pretty good. I'm looking at using the smaller fan for this build at about 50% voltage just because the chassis I intend to use isn't much higher than the fan. I'll probably get some for another build in a bigger enclosure though.
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Old 28th May 2018, 11:32 PM   #2188
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Strangely, the smoke came out of board 2 last night :-/

I put a 200mV 1KHz signal in but the output across the 8ohm dummy load looked horribly distorted. In under a minute, the PSU went into protection mode and the dreaded smell appeared soon after...

Appears the IRFP250 oscillated to death :-/ Although the signal generator is new to me so perhaps it was that..

Nothing else seems to have been harmed but I'll check through the current path to the FET to be sure.
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Old 28th May 2018, 11:36 PM   #2189
xrk971 is offline xrk971  United States
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Very simple quasi complimentary MOSFET amplifier
That’s the first VSQ to die a horrible oscillating death that I know of. Maybe 47pF instead of 33pF? Did you have the 1.2uH parallel 4R7 Thiele network in between the amp and speaker?
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Old 28th May 2018, 11:42 PM   #2190
Bix is offline Bix  Australia
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Ah nuts, bummer dude. What PSU are you using?
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