Audio Amplifier Design HELP (one op amp)

Thread Starter

Remix23

Joined Mar 11, 2016
17
This is what i am trying to achieve.


1575839478179.png

I am not sure if R15 is required but those resistors and capacitors need to be calculated. I need a supply from C6 as well. What you guys have posted above might be other ways but this is the way i have been told to design it.
 

Audioguru again

Joined Oct 21, 2019
542
Why do teachers post horrible old circuits like that??

Your circuit is missing a solder joint at the collector of Q1, R13, R14, D1 and D2.
It is also missing very important negative feedback.
It is also missing a very important supply voltage.
 

ronsimpson

Joined Oct 7, 2019
349
I don't like the DC bias.
Normally Q3,4 Emitters should be at 1/2 supply. To make that happen Q1 Collector should be at 1/2 supply. But that is not going to happen.
 

Thread Starter

Remix23

Joined Mar 11, 2016
17
Why do teachers post horrible old circuits like that??

Your circuit is missing a solder joint at the collector of Q1, R13, R14, D1 and D2.
It is also missing very important negative feedback.
It is also missing a very important supply voltage.
I dont know what the admins are doing, moving half of my post around. seriously?

This is the full circuit

1575851026098.png
 

TeeKay6

Joined Apr 20, 2019
532
Yes, my math is also bad. Then the total gain is 20 x 2.5 x 40= 2000 times, or +66dB.
Here is a similar power amplifier producing 18W into 8 ohms:
The new schematic you posted uses an appropriate low-noise, high-speed opamp and has overall negative feedback. Much more likely to work well.
 

Thread Starter

Remix23

Joined Mar 11, 2016
17
Hi, I am not too concerned about the noise as long as it meets the target and it works cause it's a temp thing.

From my circuit could you draw what's required in the transistor power stage and how I need to calculate each resistor and capacitor value.

Same goes to the picture posted with a new proposed transistor design.

Thanks
 

Audioguru again

Joined Oct 21, 2019
542
Your amplifier circuit is missing driver transistors so the peak current from the opamp must be about 56mA to drive the bases of your output transistors, plus another 56mA to drive your R8 and R12 base resistors. But the minimum output current from a lousy old 741 opamp is only 5mA when it has a voltage loss of 5V. Then its peak output of 5mA is 2.5mA into the bases of the output transistors, plus 2.5mA more into R8 and R12. You can use Ohm's law to calculate the value of R8 and R12.

2.5mA as a peak base current for the output transistors results in a minimum emitter current of 125mA. Then the RMS output current is 125mA x 0.707= 88mA and the output power with very bad distortion will be only 88mA squared x 8 ohms= 0.06W, far less than the 20W you want.

With driver transistors added, their minimum peak base current is about 2.1mA, plus another 2.1mA for the base resistors which can easily be provided by a modern opamp. Then the output power at a very low distortion will be 18W.
 

Audioguru again

Joined Oct 21, 2019
542
Q1 is not wanted since it will add additional open loop voltage gain which will probably cause the opamp to oscillate unless the high frequencies are filtered out. Q1 is supposed to be a pair of driver emitter-followers or Sziklai pairs as in the Red Circuit amplifier.
 

ronsimpson

Joined Oct 7, 2019
349
The out put stage needs to be AC coupled. Add a large cap.

There are two "1/2 supply" nodes. R7,8 and R13,15. If there is a difference in voltages it will be amplified by 20 and sent to the transistors. With 5% resistors the error could be as large as 20%. I would either use the same voltage for U2(+) and U3(+) OR ac couple U3 using a cap like C4.

The high frequencies are rolled off by R6C2. You can get the same effect by putting a cap across R14.
1575923204226.png
 
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