Guitar Aplifier

[Hero999]

Just to report, I made an LC filter to reduce oscilating and brum..I connected it in series with C13 and to power amplifier...It works great, but I burned everything ;D I turned Volume potentiometer to full and played for 2 minutes really hard, when suddenly everything shuted. I checked PCB and saw broken connection between Q6 Collector and ground..(it was verry tinny connection).

As Q5 and Q6 carry a considerable current, the PCB traces need to be thick enough to carry the current, otherwise it'll behave like a fuse.

.I repair it and replace C6. When I turned circuit on, it started to smoke..As I assume, Q4, Q5 and Q6 are dead. One of them was dead before (Q4 or Q5), so it started to smoke...I am wondering, if I am using to small cooling rib or not too good contact from collectors to cooling rib. I am waiting for new parts. I will replace every transistor and check them all to prevent puting in a faulty component. I will report and I just wanted to say, that LC filter solve the problem of oscilating in huge rating..I will report

Of course the transistors need to be on a good heat sink.

If the amplifier is driving a 4Ω load, the heatsink should be rated to at least 4

 

[fikic]

I repaired this trace. I soldered 1mm wire instead of pcb trace on it..I will get new parts tommorow maybe. Or else latest in monday. I will report. But I am also afraid, that when I will play on full volume for too long, something will die..Even heatsink won't help. Oh, and about oscilations. Would it be good idea to connect 520pF capacitors between base and collector of Q5 and Q6?

 

[audioguru2]

C13 is the main filter capacitor for the power supply. You never want to connect anything in series with it.

Adding a 47pF capacitor from the collector of Q3 to ground (not to its base because the base is used for negative feedback) will reduce high frequencies which will reduce the possibility of the power amplifier from oscillating at a high frequency (you cannot hear the high frequency). Q3 has the capacitor because it is the only transistor in the power amplifier that has voltage gain. Q5 and Q6 have no voltage gain (they have current gain) so a capacitor at them will do nothing but cause trouble.

 

[fikic]

I connected it like this:

View attachment 40679

 

[audioguru2]

Why did you install the filter netwok? Audio amplifiers don't use one because they have a good power supply noise rejection ratio.

 

[fikic]

Why it worked then? I mean with LC filter. It reduced oscilating and noice brum..What is wrong with that connection?

 

[audioguru2]

You blew up the amplifier with a supply voltage that was double what it was supposed to be. It still has problems.

An audio amplifier does not use and does not need an LC filter.