1970s Design Indulgence

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   Just with diode-drops as the base spreader - that's three MUR120's and one 1N4148 - it's enough to cause 44mA quiescent current to flow in the output stage.

Now, what you have are emitters with resistors and odd values at that, and the odd values gave the best distortion readings, so they're staying odd.

And the reason for this oddness is because the bottom 8 volts of the 77 volt DC power supply is used up in the voltage amp (you'll get it eventually).

So, the DC operating point is (77/2) + 8 = 46.5V, so 77 - 46.5 = 30.5V.

And that's the same ratio as the oddness of the emitter resistors, so quiescently, the power dissipated in the upper and lower halves are equal.

But, Self's idea is that emitter resistors are equal, which is OK in a symmetrical DC coupled amp, but not this antique design.

Therefore, the quiescent emitter voltages are not equal.

But, seriously, does this matter?

We learn from Cordell that the output transistors, although matched in many ways, are not matched in other ways, and how could they ever be matched when you consider that in NPN's electrons swap places, and PNP's the holes left by the electrons swap places.

Looking into Cordell's SPICE models we can see that there is base resistance and that base resistance differs between the NPN and the PNP transistors, which are otherwise well-matched.

Their emitter resistances are in fact, their base resistances divided by their beta (hFE), and so there is inbuilt emitter resistance other than the thermal resistance all transistors have.

Therefore, Self's equal emitter resistors are based on misplaced theory (maybe)?

Then Oliver's assumptions, even though based on multiple computer calculations, might also be misleading.

We then must examine how beta changes with current. Put simply, beta increases with current - not by a fantastic amount - but it changes.

Therefore the invisible base resistance (Rb) / beta emitter resistance isn't a constant.

Also, this situation says that the invisible emitter resistance won't be matched either.

So, at 44mA, the upper emitter resistance drops 9.5mV and the lower 14.5mV, but that's not including the part, Rb/beta.

Therefore, if we add that, then maybe the 9.5mV becomes a few percent larger, and the same goes for 14.5mV, but being dynamic - beta changes with current - then we can only guess as to what the emitter voltage drops really are.

The idea surrounding crossover distortion (zero-crossing distortion) is that the emitter resistance needs to stay within the limits 13mV to 26mV to prevent gm doubling.

The gm (mutual conductance) (of a silicon transistor) is 40...

40 what? I am not going into that right now, but it's caused by the thermal voltage of 26mV (at 25C).

So, if the total emitter resistance is at, or exceeds 26mV, there is gm doubling.

26 + 26 = 52, and 52/26 = 2. The 2 indicating double.

Once it reaches double and then starts to exceed it, zero-crossing distortion increases at low volumes - at low output power - and that is the failing of solid-state amps.

Likewise, if it goes below 13mV, then 26/13 is also 2, and zero-crossing distortion at low volumes rises again.

So, how loud do we actually listen?

My SPL meter tells me that comfortable is 70dB and getting on for loud is 80dB. Working backwards and using speaker sensitivity specs and decibels, we can calculate that it's less than a watt.

But that's an average, and I'm going to guess that it might just be 1/3rd of a watt, and 8 watts on transients, which means the smallest sound we hear is 0.3mW!!!

Therefore, it might appear that we really need minimal zero-crossing distortion and so the real and imaginary parts of emitter resistor voltage do need to fall in-between 13mV and 26mV (making Oliver right).

The voltage on the upper emitter resistor being only 9.5mV, is actually only the real part. The imaginary part is x, the value of x is unknown.

The value of x can be estimated, which I'll try to do next.

1970s Design Indulgence

Cadman Enterprises Limited
7 Kirkstall Close
South Anston
Sheffield S25 5BA
England

+44 (0) 1909 568739
+44 (0) 7947 107188

john at gspaudio.co.uk

1970s Design Indulgence

Cadman Enterprises Limited7 Kirkstall CloseSouth AnstonSheffield S25 5BAEngland

+44 (0) 1909 568739+44 (0) 7947 107188

john at gspaudio.co.uk

Cadman Enterprises Limited
7 Kirkstall Close
South Anston
Sheffield S25 5BA
England

+44 (0) 1909 568739
+44 (0) 7947 107188