Analysing Power Amplifier circuits-General discussion

[fantabulous68]

Voice your opinions on the operation of the power amplifier in the attached diagram. Anything useful will be appreciated.

 

[BrownOut]

What exactly do you want to know about it? It looks like a very typical amplifier with a class A-B output stage. It appears to be a schematic of an integrated circuit.

 

[audioguru]

This simple amplifier uses many resistors.
High quality low distortion amplifiers use constant current sinks and sources instead of many resistors.

 

[Nigel Goodwin]

This simple amplifier uses many resistors.
High quality low distortion amplifiers use constant current sinks and sources instead of many resistors.

Does it really matter?, the difference between 0.05% and 0.001% isn't very audible

More importantly it's not a real design, various bits are missing (such as emitter resistors, feedback decoupling capacitor, input coupling capacitor), and many of the values are completely wrong.

 

[mneary]

A 400W amplifier design is nothing with just a schematic. Thermal design is at least as important.

This schematic has no transistor part numbers so there's nothing to build. The writing is so tiny that resistor values are just guessing.

The output stage has no protection from thermal runaway.

 

[Nigel Goodwin]

A 400W amplifier design is nothing with just a schematic. Thermal design is at least as important.

This schematic has no transistor part numbers so there's nothing to build. The writing is so tiny that resistor values are just guessing.

Small, but clearly readable - R15 220K and R16 220K - bit on the high side.

The output stage has no protection from thermal runaway.

 

[Jony130]

Rin≈R17;
Voltage gain≈(1+R9/R10)
Q1, Q2 is a input differential stage.
Q8 is a main voltage amplifier stage with bootstrap load C2, R3 , R4.
Output stage is a complementary feedback pair ( Sziklai output).
Q4 vbe multiplier provides AB biasing and prevents from thermal runaway.
If Q4 is thermal coupled with Q4.

 

[tcmtech]

A 400W amplifier design is nothing with just a schematic. Thermal design is at least as important.

This schematic has no transistor part numbers so there's nothing to build. The writing is so tiny that resistor values are just guessing.

The output stage has no protection from thermal runaway.

Everything is there to build it. A good electronics wiz could easily reverse engineer the circuits to get the nessisary calculated range of allowable tolerances for the transistor component values and should be able to pick what fits the aplication from that. It might not be high quality or high performance but its doable.

The writing looks just fine on my 30 inch LCD monitors.

 

[audioguru]

I think the output transistors will melt at such a high power.
The circuit does not have frequency compensation so it will probably oscillate.

 

[fantabulous68]

What exactly do you want to know about it?

Distinguishing what the different stages in the circuit are. What the stages do. What components make up the various stages. Flaws, improvements & errors in the circuit. Even raising questions on power amplifiers based on the given circuit. I want to swamp myself with thoughts of power amplifiers, i want to get a feel for it when I see circuits. Any useful comments on the circuit will be appreciated. Thanks to all that are contributing

I especially like a response like jony 130 's.

 

[fantabulous68]

Q4 vbe multiplier provides AB biasing

How do you know its in class AB?

I know that in class AB the quiescent current is set just above cutoff when there is no input signal....






What do R6 & C1 do?

 

[audioguru]

R6 is the emitter resistor of the driver transistor with a value that is way too high.
C1 couples the signal to the lower output transistors.

The designer does not know how to show joined wires and wires that cross without being joined.

 

[Hero999]

I've added the junctions and zoomed in a bit so you can see the values.

For some reason the letter E is used to represent Ω, when it should be R

 

[fantabulous68]

Thanks Hero999. The circuit looks much better now.

More importantly it's not a real design, various bits are missing (such as emitter resistors, feedback decoupling capacitor, input coupling capacitor), and many of the values are completely wrong.

Care to share which component values are wrong and why you say so?

 

[Nigel Goodwin]

Thanks Hero999. The circuit looks much better now.




Care to share which component values are wrong and why you say so?

It needs throwing away and starting from scratch - there's a free guitar amp book on the foum, I would suggest you try downloading that and having a read.

 

[Nigel Goodwin]

For some reason the letter E is used to represent Ω, when it should be R

That's because it's not an English diagram, the Dutch magazine Elektor uses E instead of R (although they would never publish such a poor non-working circuit).

 

[fantabulous68]

there's a free guitar amp book on the foum, I would suggest you try downloading that and having a read.

directions to that link please? Theres alot of guitar amp topics when i search.........

 

[Nigel Goodwin]

No need to search, it's in the Electronic Books forum, top sticky.

 

[fantabulous68]

Thanks Nigel

 

[fantabulous68]

Power Amplifier circuit 2

Circuit 2 (in the attached diagram) is a standard circuit with single Vcc and complementary o/p stage as opposed to
Circuit 1 which was configured with Sziklai o/p and dual supply.....

I welcome any views on the circuit.





What is C3 for?