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Old 4th January 2011, 05:57 PM   #1
Join Date: Jan 2005
Default Little mini amp

After the failed mosfet amp i startedf wondering if the board was even working so i replaced some komponents and reconfigured the drivers to become outpur devices and hooked up a small low voltage psu and after fixing a broken trace i found around one of the drivers, the thing jumped to life.


Driven by this:


The drivers are BD139 and BD140 and the rails are about 15VDC unloaded.

It gives a few watts in 4 ohms and sounds pretty good for beeing biased into class B.

I was suprised the circuit works at all below 35V rails since this was designed for minimum 30V rails.
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Old 4th January 2011, 07:06 PM   #2
jaycee is offline jaycee  United Kingdom
diyAudio Member
Join Date: Apr 2004
Location: Norwich, UK
Use it for headphones
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Old 4th January 2011, 07:56 PM   #3
Join Date: Jan 2005
Im wondering if that broken trace on the base of the low side driver could have caused the 0-5V oscillation and lack of response on the input.

I remember that on the first powerup when i had miswired the vbe multiplier it did something before my current shunt died from the 10+ A that was flowing, afterwards it did the oscillation and nothing else.

I tried replicating this scenario in simulation but it just removes the negative half of the sine and adds some ringing, though simulation doesent include the stray inductance and stray capacitance of a typical board, wires and components needed to make an amp oscillate if something is wrong.
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