Basic Push/Pull amplifier question

[synth-dude]

Hi,

I'm fixing up this Audio amp for my brother, symptoms are distorted output, I've worked out via the pre-outs the pre-amp sections are working ok, problem lies in the power amps.

I've downloaded the schematic (see below), and it has a push pull power amp stage, with a few filters and things in the final stage.

This question isn't really about this amp per se, but rather all push pull amps, why do they have a phase inverters (differential pairs) on the input?  I'm guessing this isn't a simple answer, I've looked everywhere online (Googled many different sources) but I cant find a good explanation.  Talking Electronics has a great discussion on transistors, and even has a 60W push/pull amp with inverter in their examples, but sadly no explanation what the inverter does, and how it works.

Any ideas?

Cheers,
Chris

[KhronX]

What you call "inverter", and what is actually / indeed a differential pair, is one of the main building blocks of amplifiers and operational amplifiers.

I'm no expert on the topic, but that stage is what compares the input signal with the signal fed back from the output.

https://en.wikipedia.org/wiki/Operational_amplifier#Closed_loop

https://en.wikipedia.org/wiki/Differential_amplifier#Long-tailed_pair

Douglas Self has a pretty extensive description and explanation in his "Audio Power Amplifier Handbook".
I hope it won't be terribly illegal or harmful to add a few quotes here.

"The input stage of an amplifi er performs the critical duty of subtracting the feedback signal
from the input, to generate the error signal that drives the output. It is almost invariably a
differential transconductance stage; a voltage-difference input results in a current output that is
essentially insensitive to the voltage at the output port."

"Using a single input transistor ( Figure 4.3a ) may seem attractive, where the amplifi er is capacitor-
coupled or has a separate DC servo; it at least promises strict economy. However, any cost
saving would be trivial, and the snag is that this singleton confi guration has no way to cancel
the second harmonics generated in copious quantities by its strongly curved exponential V in / I out
characteristic [1] . The result is shown in Figure 4.2 , curve B, where the distortion is much higher,
though rising at the slower rate of 12 dB/octave."

[Kleinstein]

Most audio amps have the differential pair at the input, because this is the obvious choice to amplify the difference from input and feedback. However there are a few amps that don't use it, but use a circuit more like a current feedback OP. So have the feedback to the emitter.