Project Motorola Hi-Fi Amplifier

[Shahriar]

Hi
I was Looking in Audio Project section and I encountered with this Project (Motorola Hi-Fi Power Amplifier)
http://www.electronics-lab.com/projects/audio/041/index.html
Something is strang to me. The output voltage is always equal to OpAmp Output. The Maximum
output Swing for Opamp is limited with R3 and R4 to (Vcc/2 - 0.7V) . So the output power will be 1/4 of the
Rated powers in Table, which I think is calculated with respect to Maximum swing of Vcc.

1- Tell me if I am wrong.
2- What better OpAmp do you know with Maximum Vcc of +-30V (741 is a little bit Old)


ThanX
Shahriar

[audioguru]

Hi Shahriar,
The circuit is odd. It has nothing to control its current. Some opamps have an operating current much higher than others and this amplifier's output transistors amplify the variation. If the output transistors heat and draw more current then there is nothing to sense the excessive current and nothing to adjust it like "normal" amplifier circuits have.

A modern opamp might have trouble with the phase shift of the transistors causing high frequency oscillation.
I think the output swings a p-p voltage that is 5V or 6V less than the total supply.

I disagree with the higher powers as shown in the graph. 20W RMS into 8 ohms is possible with a 40V total supply. 25W and 35W RMS is not. 50W RMS is not possible into 8 ohms. 80W RMS is not possible into 4 ohms. With a 44V total supply I think it will provide 47w RMS into 4 ohms. 

[Shahriar]

Dear Audioguru

ThanX for your reply.
The Maximum Output swing of this opAmp is Vcc/2. i.e.  If +vcc = 22V the Maximum Swing will Be 11V Because of R3 & R4 WHich Are equal and provide a Voltage devider.

Am I right?

[audioguru]

The Maximum Output swing of this opAmp is Vcc/2. i.e.

[Kevin Weddle]

I have used two, parallel and opposite ,diodes on the opamp output to limit the output swing. An accidental high voltage swing can destroy the input to many devices.