10 watt output quasi design

Thread Starter

simpsonss

Joined Jul 8, 2008
173
thanks. another info i wanna share is, my 1Amp fius just broken after i on for maybe 6 to 7 songs. the only component i change is the R9.i change from 4K9 to 10K. And i can feel that the heat sink is really HOT!

thanks.
 

Thread Starter

simpsonss

Joined Jul 8, 2008
173
ok. because i want to learn the operation of a quasi-complementary amplifier.So i google it and then i found this circuit.then i build it and this happen.

thanks,
 

R!f@@

Joined Apr 2, 2009
9,918
Ur circuit does not have a current source and the output stage is not properly biased.
I can give you a better circuit to study of the same quasi output stage
 

Audioguru

Joined Dec 20, 2007
11,248
R9 is a voltage divider for the input signal. When it is set to max then the signal level to the amplifier's input is max. When it is set to halfway then the signal level to the amplifier's input is 1/10th if the pot is a logarithmic audio taper volume control. When it is set to zero then the input of the amplifier is grounded and has no signal.
But your volume control does not work like this so maybe it is connected wrong.

R10 sets the amount of idle current in the output transistors to eliminate crossover distortion. Without an input signal then the DC voltage across one of the emitter resistors (R7) is measured and R10 is adjusted for 12mvDC which is when the current is 25.5mA. If R10 is turned up too high then the current will be extremely high and the output transistors will be extremely hot. Then the fuse will blow.
Since maybe you connected the volume control wrong then maybe you also connected R10 wrong.

The 10W amplifier heats its output transistors to about 9W when it plays continuously at full power. Music is not full power all the time so the average power to the speaker is 1W and the average heating is about 2W.
 

Thread Starter

simpsonss

Joined Jul 8, 2008
173
But your volume control does not work like this so maybe it is connected wrong.
what if when i change the R9 from 4k7 to 10k potentiometer.i can now tune the volume to a more lower sound. Does this mean my connection is correct? Or maybe my connection to the potentiometer is correct just because i set the bias too high and make the output also too high?

thanks.
 

Audioguru

Joined Dec 20, 2007
11,248
If your music source is a low impedance then changing the value of the R9 volume control from 4.7k to 10k will make no difference. If the pot is connected correctly then both pots will make the output level go down to zero when they are turned to zero.
One terminal of the volume control is connected to the amplifier's ground and when the knob is turned so the slider wipes the grounded terminal then the signal feed to the input of the amplifier is zero.

The bias of the output transistors affects how much crossover distortion or how much heat they make, not output level.
 

Thread Starter

simpsonss

Joined Jul 8, 2008
173
ok. can i say R10 works as a voltage divider too? By refer to the voltage divider formula, if resistance between base & collector of Q1 is higher than the resistance between base and emitter then the voltage at base will be lower.Am i right? so to avoid too much heat i should set the POT to make the resistance between B & C more higher?

thanks.
 

Thread Starter

simpsonss

Joined Jul 8, 2008
173
regarding the connection of the POT.i connect it in this way.the middle pin of the POT which is the wiper to 10uF E-cap, then the 1st pin to input and the 3rd pin to GND. anything wrong?
 

Thread Starter

simpsonss

Joined Jul 8, 2008
173
R10 sets the amount of idle current in the output transistors to eliminate crossover distortion. Without an input signal then the DC voltage across one of the emitter resistors (R7) is measured and R10 is adjusted for 12mvDC which is when the current is 25.5mA.
Do u mean without any input,i measure the DC voltage across R7 and then tune R10 until the DC voltage across R7 shows 12mV DC?

thanks.
 

R!f@@

Joined Apr 2, 2009
9,918
Guys, I did simulated the OP circuit, and the bias readings are all wrong, there is no way OP can get a clear sound from that amp.

simpsonss
I am waiting for an email confirmation from the author who owns the schematics, as I talked to him about them when I built my amps. I can recommend his site to you but I don't think he has time to help novice builders as I have seen, pretty confident ones tackle his diagrams. He answers only to really impossible problems. If you try his method and you come with a problem, I don't think he will answer to simple problem's that arise from limited knowledge on amplifier topology.
This board is u'r only solution to study this and If you start posting his schematics without his permission I think you will be kicked out and you will lose everyone who is wiling to help.
So wait till I receive a reply and keep checking the thread for updates
 

Thread Starter

simpsonss

Joined Jul 8, 2008
173
how can i tune the bias? it works like a voltage divider at the base of Q1. So should be the b-c resistance higher than b-e? thanks.
 

R!f@@

Joined Apr 2, 2009
9,918
It's not that simple. Biasing should be stable and should not vary with input music or voltage fluctuation or with temperature.

designing Bias stages is not for beginners. Better to study a working one.
 
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