Guitar Aplifier

[Hero999]

Did you use the same transistors or did you replace them all?

It sounds like there's a problem with one of the transistors.

Your speaker is quite old, Yugoslavia no longer exists. ;D

 

[fikic]

I used same transistors. Which one should I suspect?
Hehe I am quite young..I was born, when yugoslavia died ..Then we got Slovenia

 

[Hero999]

Try replacing Q5 and Q6 first, then Q3 and Q4.

 

[fikic]

What if I should put this thing in iron case and ground everything and use shielding like this guy

 

[Hero999]

Yes, that might help.

 

[fikic]

But how do I have to connect things? And for connection like this I would aquire supply circuit by itself, not togheter with preamp and power amp...

 

[Hero999]

Doryy, I don't know wht you're talking about.

 

[fikic]

I am saying, that I don't know, how to connect everything with the chassis. I also don't know, how to shield inputs. Can you tell me that? Beside that, I have power supply circuit on the same PCB than preamplifier and power amplifier

 

[Hero999]

The 0V supply rail should be connected to the case which should also be connected to safety earth/ground.

The inputs should be connected to whatever you're connecting the amplifier to with screened cabe.

 

[fikic]

i did that. I connected 0V rail to radiator. What is screened cabe? In Slovenian forum, I got advice, to remove C10, becouse this capacitor causes many unwanted effects. It drives signal from power amplifier back to preamp and cause oscilating. Amplification without this capacitor is much lower, but reduces oscilating. What about that?

 

[audioguru2]

Screened cable is shielded audio cable that is used to connect stereo components together.
It stops interference and feedback. The volume control should be connected with it and the input from the guitar should use it.

The power amplifier should be disconnected from the input circuit to see if the feedback comes from the power amplifier (it is simply 3 transistors with a 4th one as a variable couple of diodes).
Do not remove C10 because it is a "bootstrap" capacitor that increases the positive-going output swing of the amplifier which increases its max power to 10W into 8 ohms.
If the power amplifier still oscillates when the volume control is turned down then add a 1k resistor in series with C9.

Here is the extremely simple power amplifier parts:

View attachment 40668

 

[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... ???

 

[audioguru2]

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.

 

[audioguru2]

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.

 

[audioguru2]

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 ;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)..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