OPamp + Darlington Will it work?
 

Hi


I need a cheap simple amplifier to a computer system . Is the schematic is correct?
What are your comments?

Look at sound.westhost.com for a simple opamp based amplifier.

Even simpler is an amplifier based around two LM1875 or two LM3875/3886.

Yes, but I do not like chip-amp :-)

yes that will work, in fact there was a wellknown italian design "Puccini" based on the same principle. It even has some tricks to maximise output power, by shifting the opamps supplies with the signal.

To be honest, if you do not like "chip amp" sound, then you probably won't like this circuit either. They are very similar.

Looks like a lot of fun, definitely worth building. I don't believe in 'chip amp' sound and would think you have quite a bit of scope to change the 'sound' of this design through component choices, some tweaks and changes, different op-amps etc.

Not sure why you need D1 as part of the Vbe multiplier unless you are trying to get a better match in the temperature compensation somehow.

Are you really using batteries or is that just a symbol for the schematic - because if this is a mains derived power supply with rectifier and filter caps etc. I'd be nervous about the PSRR of this design, the base junctions of the output transistors are very closely coupled to the power rails. Me thinks you should add resistor + capacitor filters of a decent size to both power rails to isolate the 'front end' of this amp.

The op-amp will want dedicated power supply decoupling capacitors.

I'd add a capacitor in series with R4, something around 100uF. This is to ensure that the inverting input of the opamp is able to 'see' the dc voltage at the amplifier output - and correct for dc offset. As part of a low dc offset I'd add a 22k resistor at the non-inverting input to match R3 when using bipolar input op-amps.

Lastly, I might consider a high quality capacitor (e.g. 0.47uF) in line with the input signal to provide a measure of safety from any dc on the source signal.

Hi

Attached diagram is only a concept developed to simulate in the OrCAD :-)

In the practice of V1 V2, of course, will be replaced with a transformer with rectifier and 2*6800µF

If you included the output stage in the feedback loop it would reduce the output stage distortions. Though it would be a little more complicated than just changing the take off point to the output, the blocking caps would cause havoc with the output DC offset. But I dont see much of a problem removing them and attaching the OA out directly to the base of Q1. And I also dont see the reason for D1, which is a hassle because it should be mounted on or near the darlingtons along with Q3 for temp. stability.

Not sure this is neccesary. Why will it not "see" the DC voltage thru the R3/R4 divider?

this would kill the benefits of the sound quality
do not apply gnfb;)

This is the schematic with your comments and PCB design