discrete power dif amp

[vlad777]

**broken link removed**

Hello.

I made this amp from salvaged parts from PC monitor and PC PSU.
Beleve or not it works even though I didn't bias input.
Bass is good and it has power but distorsion is notisable.
I am hoping to stabilize q point just by two resistor feedback.

So how can I make it better?

Many thanks!


Edit: Sorry power is +30V -30V.

 

[Grossel]

Hi. Try following:

Remove 10k resistor in feedback loop. This one reduce the feedback, and effect should be faster voltage shift when output voltage swings around ground.

On real amplifiers you'll have resistors between output transistors that ensure a smooth transition when conducting transistor change.

 

[vlad777]

What would be my gain then? 1?

 

[audioguru]

The output transistors have no bias voltage so the crossover distortion will be horrible even when it has lots of negative feedback with a gain of only 1.
Nobody makes a class-B audio amplifier.

The circuit does not have a frequency compensation capacitor so it will probably oscillate with more negative feedback.

 

[vlad777]

**broken link removed**

Is Q5 and Q6 error in this shematics and if not how does it work?
I think this has frequency compensation so how do I calculate it?

 

[Nigel Goodwin]

Q5/6 are a constant current source (as are Q3/4), and it still seems to be lacking a miller capacitor between base and collector of Q7.

 

[audioguru]

I couldn't find the Mosfet amplifier at **broken link removed** .
It originated here: 25 Watt MosFet Audio Amplifier - RED - Page2 where there are some details about it.

 

[vlad777]

Yes I know it's a constant current source but I got confused how will the voltage change if
current is constant.
Why is this better then just R?

 

[Grossel]

Yes I know it's a constant current source but I got confused how will the voltage change if
current is constant.
Why is this better then just R?

Slightly more linear compared to a similar amplifier with just a resistor instead. It also keep the bias voltage between the gates constant.

 

[Nigel Goodwin]

Yes I know it's a constant current source but I got confused how will the voltage change if
current is constant.
Why is this better then just R?

I don't know why you suddenly leaped from a bipolar amp to an FET one, but for the bipolar one it's vitally important in order to provide sufficient drive on that polarity output (or you can bootstrap it with a capacitor).

 

[vlad777]

Hello again.

Why does, the first transistor in my compound transistor, keep dieing?

Many thanks.

 

[audioguru]

The BFQ252 PNP transistor is a high voltage, low current video transistor. Since your amplifier has negative feedback but has no frequency compensation capacitor then the amplifier is probably oscillating at a high frequency that kills the first transistor in your compound pair.

 

[vlad777]

Audioguru can you please tell me where this cap should go?

Is it millers cap between base and collector (of q7 equivalent on my sch) ,or cap between out and gnd ,or cap over resistor from out to inverting input?

 

[audioguru]

Audioguru can you please tell me where this cap should go?

Is it millers cap between base and collector (of q7 equivalent on my sch) ,or cap between out and gnd ,or cap over resistor from out to inverting input?

There are hundreds of amplifier schematics on the internet. Here is a good one that is similar to yours and it has a frequency compensation capacitor to keep it from oscillating.