Maplin MOSFET amp DC offset problem

[chip_mk]

It is the base to emitter differential that determines whether it is on or not. The base of TR1 can be grounded if desired, the DC conditions will remain correct as the emitters of the pair will be around 650 mv positive with respect to the base(s).

Agreed. Actually I should have said "With this base voltages...".

It's 67mV difference and it is enough to almost completely shut one branch judging by Figure 4 Differential amplifier - Wikipedia. This also means the input pair is not biased in the most linear region, leading to suboptimal THD figure.

 

[ivanlukic]

I wouldn't worry to much over the offset tbh, just pick a couple of transistors that get it nearer the mark... or leave it be 🙂

I think that I'll leave it be.🙂

 

[ivanlukic]

Hitachi/Maplin amp on the boards that I designed sounds excellent in spite of 100mV offset! No sign of any oscillation. Bias is 30mA. No active preamp is used, just a pot at the input. Sound is very detailed, almost bright but with low frequency authority and slam. Super fast and dynamic, bursting with life. For the moment I use regulated 30V lab supply but intend to build proper 50V supply.