sahib said:
So, the negative value input bias is due to the NPN pair. It would be positive for PNP.
Do we know what the offset is in that circuit?
I was stung by PRR to do a full offset analysis to prove I ken kont en reed en rite .
Assuming Vbe about 0V6 rather than 0V7, hfe TR1 220, TR2 200, TR4 100. dVbe = 25mV ln(Ic1/Ic2)
I get -24.3mV if working (new) and -1.658V if C6 is SC (after some on/off cycles).
Nasty but no Holy Smoke so I grovel at Guru PRR's feet in shame.
I suspect old fogeys like PRR , JR, Guru Baxandall & I have mass graves of Power Devices in their gardens. This urge to pontificate on Power Amps is catharsis for our acts of ethnic cleansing. But rather than re-design Abe's circuit component by component, why not point him in the direction of a simple circuit which will work or at least won't blow up.
My suggestion is Douglas Self. I have criticisms but I'm carping. You can get PCBs for some of his circuits. Some of his Power Amp book is at
http://douglas-self.com/ampins/dipa/dipa.htm You DO want to buy the book. Much stuff which I'd always wanted to test but never found time to do.
PRR, there's a great chapter on thermal stability which discusses the pros & cons of where to place the sensor, time constants, idle, bias after burst of power etc. But after pretending to think long & hard about this , I'm still undecided of the best thing to do. Instead, my approach is to have a feedback structure which make the choice of bias less critical. eg No visible artifacts at 25mA/device, and then bias to 50mA. Sensor on the H/S is then as good as any. I don't like his use of insulating pads to twiddle the thermal paths.
One Holy Smoke issue.
http://douglas-self.com/ampins/dipa/dpafig33.gif isn't a full working circuit. If you use his "Beta Enhance" TR12, you MUST have a resistor (eg 5k1) in its collector. Otherwise, when the amp overloads, TR12 will die & kill TR4, 8, 9 and your speakers. Then TR6, 7 will probably die in sympathy.
Don't forget some protection for your speakers too. At least a fuse or PTC resistor though the protection is marginal. Loadsa amps have DC sensing on the O/P to open a speaker relay if the amp dies.
Abe, I second Guru's PRR & JR's advice on loadsa Power Transistors & big H/S. Self has good recommendations on devices. These are MUCH better and more robust than the old 2n3055/2955 stuff which I started killing many years ago. The UN team will be coming to dig up your garden.