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Added a note on crossover distortion
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frr
frr
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Another thought: if you say that you get distortion above 1 kHz, in general this could be a sign of "crossover distortion". This does get worse at higher frequencies. And, it may be a sign that your Darlington power totem is not properly biased = that its quiescent current is zero. But: it can only "almost vanish" at low frequencies IF: if you have a strong negative feedback, from PA output to its inverting input. I cannot see that in your schematic.

Historically I'd get into thisa sort of feedback loopey trouble (in the bass / subbass range), if my input amp stage had some degree of coupling of the power rail voltage back into input. Such as, using a DC-biasing resistive divider between 0 and +Vcc in a single-supply topology, serving to keep the DC potential of my (AC) signal input halfway between the power rail and GND. That is why, I strongly prefer dual=symmetrical power supply from the very first stage, which allows me to have an inherent solid signal reference ground "somewhere in the center" between the power supply rails. The ref.gnd potential is inherently within the operational range of all the silicon in the circuit, and I can reference the input signal source, and any feedback networks, and the output, to that very center ground. If I wanted to be 100% sure that I don't get a feedback through the PSU, I'd use a pair of linear stabilizers for the preamp (consider 7809/7909 for your assignment) if you need to use the same mains PSU for both the preamp and the power output.

Historically I'd get into this sort of feedback loopey trouble (in the bass / subbass range), if my input amp stage had some degree of coupling of the power rail voltage back into input. Such as, using a DC-biasing resistive divider between 0 and +Vcc in a single-supply topology, serving to keep the DC potential of my (AC) signal input halfway between the power rail and GND. That is why, I strongly prefer dual=symmetrical power supply from the very first stage, which allows me to have an inherent solid signal reference ground "somewhere in the center" between the power supply rails. The ref.gnd potential is inherently within the operational range of all the silicon in the circuit, and I can reference the input signal source, and any feedback networks, and the output, to that very center ground. If I wanted to be 100% sure that I don't get a feedback through the PSU, I'd use a pair of linear stabilizers for the preamp (consider 7809/7909 for your assignment) if you need to use the same mains PSU for both the preamp and the power output.

Another thought: if you say that you get distortion above 1 kHz, in general this could be a sign of "crossover distortion". This does get worse at higher frequencies. And, it may be a sign that your Darlington power totem is not properly biased = that its quiescent current is zero. But: it can only "almost vanish" at low frequencies IF: if you have a strong negative feedback, from PA output to its inverting input. I cannot see that in your schematic.

Historically I'd get into a sort of feedback loopey trouble (in the bass / subbass range), if my input amp stage had some degree of coupling of the power rail voltage back into input. Such as, using a DC-biasing resistive divider between 0 and +Vcc in a single-supply topology, serving to keep the DC potential of my (AC) signal input halfway between the power rail and GND. That is why, I strongly prefer dual=symmetrical power supply from the very first stage, which allows me to have an inherent solid signal reference ground "somewhere in the center" between the power supply rails. The ref.gnd potential is inherently within the operational range of all the silicon in the circuit, and I can reference the input signal source, and any feedback networks, and the output, to that very center ground. If I wanted to be 100% sure that I don't get a feedback through the PSU, I'd use a pair of linear stabilizers for the preamp (consider 7809/7909 for your assignment) if you need to use the same mains PSU for both the preamp and the power output.

Added a note on older design approaches
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frr
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EDIT:

with respect to the note about using a transformer in the PA, if the lecturer really wants you to dwell on the very basic principles, how audio used to be built like a century ago, maybe try a query into google images along these lines. Note that these old topologies, with a minimum of valves and lots of transformers in the signal path, initially were adapted for / adopted in transistorized designs. In those days, discrete transistors cost an arm and a leg. What was that - sixties? Maybe still into early seventies? Guess what the ratio is today, comparing the cost of a transistor to the cost of a custom transformer. Especially when prototyping.

Next came transformer-less audio PA designs, initially with a single-transistor CE N-BJT feed-forward input and a quasi-complementary output totem. Back in the day, the quasi-complementary output totem was popular for cost reasons, and remains in use in on-chip integrated designs where the process favours N-polarity transistors (P-polarity are difficult or impossible to achieve, or their parameters lack luster).

Overall, class AB with a differential input stage (excess open-loop gain and a negative feedback), is like eighties to nineties. About a decade ago, if not two decades, there's been a massive exodus towards class D in audio PA. Even consumer radio of any kind (long wave to WiFi to 4G/5G) hardly has any near-discrete analog front end anymore. Pretty much everything is done by single-chip solutions using broadband sampling (rather than superhet mixing). You do meet some LNA's and mixing in TV antenna technology (satellite and terrestrial), but that's a swan song.

EDIT:

with respect to the note about using a transformer in the PA, if the lecturer really wants you to dwell on the very basic principles, how audio used to be built like a century ago, maybe try a query into google images along these lines. Note that these old topologies, with a minimum of valves and lots of transformers in the signal path, initially were adapted for / adopted in transistorized designs. In those days, discrete transistors cost an arm and a leg. What was that - sixties? Maybe still into early seventies? Guess what the ratio is today, comparing the cost of a transistor to the cost of a custom transformer. Especially when prototyping.

Next came transformer-less audio PA designs, initially with a single-transistor CE N-BJT feed-forward input and a quasi-complementary output totem. Back in the day, the quasi-complementary output totem was popular for cost reasons, and remains in use in on-chip integrated designs where the process favours N-polarity transistors (P-polarity are difficult or impossible to achieve, or their parameters lack luster).

Overall, class AB with a differential input stage (excess open-loop gain and a negative feedback), is like eighties to nineties. About a decade ago, if not two decades, there's been a massive exodus towards class D in audio PA. Even consumer radio of any kind (long wave to WiFi to 4G/5G) hardly has any near-discrete analog front end anymore. Pretty much everything is done by single-chip solutions using broadband sampling (rather than superhet mixing). You do meet some LNA's and mixing in TV antenna technology (satellite and terrestrial), but that's a swan song.

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frr
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I'm wondering what level a course this is, and what the lecturer seeks to find out about the apprentices. If there's one particular desired solution in the curriculum, or rather, if the lecturer would like to learn what the folks are up to, how advanced topologies they would venture, how far they'd fetch their designs, how many of the classic hurdles they'd be able to overcome / see in advance (based on study of the topic) etc.

Do I understand correctly, that you've actually tried building some of the circuits? Or are you still working with a simulation only?

In my practical experience: "motorboating" may be a sign of a feedback loop coupled via the power supply. Do you have a potentiometer for volume control, likely someplace between the preamp and the power amp? The "motorboating" starts when you turn the potentiometer some way up, thus increasing the overall gain of the amp cascade, thus possibly getting into unstable territory (phase and gain vs. frequency, see the Nyquist's stability criterion. Explanation available.)

Having your prototype as a physical rats-nest makes undesired coupling stronger / more difficult to avoid. Keep wires short where they're necessary, and prefer to work with a breadboard (a dedicated PCB design would be impractical at this stage, I guess). Put PSU blocking capacitors on the breadboard or close to it.

Historically I'd get into this sort of feedback loopey trouble (in the bass / subbass range), if my input amp stage had some degree of coupling of the power rail voltage back into input. Such as, using a DC-biasing resistive divider between 0 and +Vcc in a single-supply topology, serving to keep the DC potential of my (AC) signal input halfway between the power rail and GND. That is why, I strongly prefer dual=symmetrical power supply from the very first stage, which allows me to have an inherent solid signal reference ground "somewhere in the center" between the power supply rails. The ref.gnd potential is inherently within the operational range of all the silicon in the circuit, and I can reference the input signal source, and any feedback networks, and the output, to that very center ground. If I wanted to be 100% sure that I don't get a feedback through the PSU, I'd use a pair of linear stabilizers for the preamp (consider 7809/7909 for your assignment) if you need to use the same mains PSU for both the preamp and the power output.

Hmm. This symmetrical (split) supply really only makes sense for circuits using the op-amp concepts, i.e. differential input etc. The very basic feed-forward "common emitter and friends" tend to rely on single supply, where GND serves as the supply (-) and a signal reference ground at the same time. Which in turn gives you a "not soo good" PSRR.

@periblepsis has suggested a handful of neat ideas, including a nice topology for a simple feed-forward preamp. I can see it containing some constant current sources... looks like a really nice circuit, albeit marginally incomprehensible to me in all the details (I'm too young, and a hobbyist).

As mentioned, I'm past the stage that "trivial is good". I'd suggest using a handful of discrete BJT's to implement a minimalistic op-amp topology. Gives you quite a bit of self-adjustment of the various operating points, a negative feedback to help you set the closed-loop gain and frequency response, and quite a bit of "power supply rejection" (immunity against feedback and EMI ingress via the PSU).

I'd use one op-amp-like circuit for the preamp, and another one for the power amp - to stick to your assignment, to follow the usual "macroscopic modular arrangement of gain blocks", and to have some room to tilt the gain this or that way.

There's a free online PDF book that I'd like to recommend: Designing Analog Chips by Hans Camenzind. Mentioned to me before, by others here at SE. You can probably skip all the silicon doping and etching for the moment. I am referring specifically to chapters explaining these topological building blocks:

  1. the long tailed pair
  2. the constant current source
  3. the current mirror

This will allow you to study the most basic op-amp internal topologies and come up with discrete-based circuits along those lines of your own, as a starting point. See for instance this simplified topology or this practical example, allegedly a TL082 When trying this with discretes, you may need to throw in a resistor here and there - although surprisingly the self-biasing circuits often need very little in the way of extra resistors. To prevent premature release of smoke, exercise in your simulators.

In the preamp block, the output stage should work just fine in class A = open emitter into a pull-up resistor, or into a constant current source, or some such. The required output impedance and amplitude mean that neither heat nor current gain are too much of a concern.

As for the power amp: The complementary class AB output stage that you have suggested - looks nice in principle. Next, you need a voltage gain stage to drive it. The simplest would possibly be a single N-BJT connected with common emitter, working into a P-BJT constant current source. Or, rather than a stand-alone CE N-BJT, I'd suggest to use the open collector output of a differential input stage = combine a long-tailed pair, with a current mirror to couple its output to the bases of your Darlington power stage (you can flip N- and P-BJT polarities to correspond to your previous stages).

Also, the cascade of diodes between the darlington bases (to bias the quiescent current of the power stage) seems like a bit of an overkill... Are they schottky? Principally, each B-E junction (two of them in series in each Darlington) should correspond to a somewhat open Si diode, so like 4 Si diodes (1N4148 ?) should have a forward voltage drop roughly on par with your power transies.

I've seen a design where this cascade of diodes was replaced by a single N-BJT connected in "common emitter" basic topology, with a trimpot between C/E, the adjustable pin connected to the Base - allowing you to adjust the quiescent current of the power Dalington totem. (Use a multi-turn trimpot, and start with the Base pulled all the way to the Collector, for minimal voltage between the Dalington bases.) If you thermocouple this "bias setting helper BJT" to the power darlingtons, you get automatic quiescent current adjustment to prevent thermal runaway.

Further sources of inspiration: 1 2 3 - ordered from the simplest to the most comprehensive. This is not to humiliate you, or to suggest that this is the way to go - I mean to suggest that you try to find repeating concepts, circuit stages and tricks in each of those amps. All of them are your basic class AB with a diff input, just with a different "depth of attention to detail".

Note that the Darlingtons have some forward voltage drop from B to E, especially when you drive them to seriously open. And you need some more "voltage headroom" for the stage that's driving the Darlington bases. You should strive to squeeze the most out of that little "headroom near the rails" - by using a current mirror against an active constant current source, to drive the Darlington bases. Even that way, the inevitable voltage drops down the cascade of the B-E junctions in the very output, principally limit the efficiency of this style of a power amp. If you have +/-12V to play with, and you need 5V RMS = approx. 7V peak on the output, I believe that this is achieavable without additional radical tuning. (There are designs with CE-connected power output, able to drive "rail to rail", that are however more sensitive to setting the quiescent current.)

Does it feel way too complicated?

Based on my historical early experiments with "simple" circuits, just individual transitors (up to 3 per amplifier block), all I can say is that reducing the transistor count ends up bringing more headache than design clarity. Those "simple" circuits easily end up requiring more tweaking (based on experiment and measurement), and get you stuck in a swamp of compromise and inefficiency, compared to a circuit making use of the "three friends of op-amp magic" listed above.