There are couple of avenues you might want to consider. Firstly although this circuit resembles some of Self's designs there are a couple of significant differences.
It appears the first of these is to make an amplifier that would have higher input sensitivity so it would be capable of working with a selector switch and a volume control at the input.
The closed loop gain of a system using a preamp would be around 23 (Self), however in this instance the figure has been hiked to 31.3 -hence more of the gain capability is being devoted in this sphere leaving less for negative feedback.
This means a reduction in the closed loop frequency response and the value of the dominant pole capacitor in the Vas will need to be reviewed.
That leaves an option of a reverse "fix" by changing the resistors in the feedback dividing network to 22k and 1k which may align with 100pF which is already in place - it should be closer to the mark.
The other thing to consider is to increase the values of R23 and R24 to 100R which will reduce the standing current through Tr10 and Tr11 by half to around 6 m.a.
If you compare this circuit to Self's invariant load amplifier you will see due pairs of power transistors of the type supplied in this kit for each output circuit half. The values for the equivalents to R23 and R24 are 100R.
Suffice to say common emitter amplifiers are more linear with higher collector loads e.g. Tr10 and Tr11.
Self's design is not the only circuit I have seen with the same values for R23 and R24.
mjona. Thanks for your suggestions. I'll continue to play around with the circuit along the lines you've identified, although the PCB is very compact and as well, some of the pads are starting to suffer with my continued attentions!
