Guitar Aplifier

[fikic]

I disconected preamplifier and power amplifier still oscilated. I connected 1k resistor in series with C9, like you said and oscilation stoped. I hear very quiet hum in speaker, with volume potentiometer set to 0. I must try connect preamplifier back and try put some signal on. I will report.

EDIT: Now it oscilates when I turn Volume potentiometer to maximum...

[audioguru]

I disconected preamplifier and power amplifier still oscilated. I connected 1k resistor in series with C9, like you said and oscilation stoped. I hear very quiet hum in speaker, with volume potentiometer set to 0. I must try connect preamplifier back and try put some signal on. I will report.

EDIT: Now it oscilates when I turn Volume potentiometer to maximum...

Then the volume control wiring or the input wire are not shielded and are picking up interference from the power supply wiring or from the output wire.
Or maybe you did not connect the grounds as a "star".

[fikic]

Yes I will shield everything when I make case from aluminium and put everything in. I will then make grounding on the case and I will insulate case to prevent it to electricute me. So you thing shielding will make this oscilating go?

[fikic]

Can you tell me something..How can I use star grounding with my PCB. I think, i can't, becouse I used a copper ground...I can't wire every ground to the star connection with different wire. I am confused now.

[audioguru]

I notice that the power amplifier is inverting so capacitive coupling through the air from the output to unshielded wires at the input cause negative feedback, not positive feedback. oscillation occurs with positive feedback so that is not the cause of the oscillation.

Since you have the input circuits disconnected then they cannot cause the oscillation.

The supply voltage to the power amplifier will bounce up and down a lot especially if you have a low current fuse. Then the supply voltage can capacitive-couple to the input if the input wiring is not shielded and cause feedback.

Star grounding makes everything connect to a single point, usually the main filter capacitor's ground.

[fikic]

Yes I know what star grounding do, but I don't know if I can use it with my PCB. I should separate grounds of preamplifier from grounds of power amplifier, am I right? In that case, I must make new PCB.

[audioguru]

The ground of the preamp can connect to the input ground of the power amplifier because the preamp uses a very low supply current.

[fikic]

But in this contribution it says, not to connect them together...Can you draw me, how to connect grounds with use of star grounding..I would very appreciate that.
Thank you

[fikic]

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 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)..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

[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 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°C/W, including any thermal pads used.

[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?

[audioguru]

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:

[audioguru]

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