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February 6, 2017 / consort3

Darlington Part 4, more thoughts

I developed the amp a few years ago and was surprised how good the transient performance of such a simple design was. I think that performance is partly due to the current drive of the output stage.

The concept makes a virtue of a potential problem. Baxandall pointed out that Transistor Cbc  would slow the response. In this case it acts as a free Cdom stabilising the Vas, which is the output stage.

I needed 800 ohm base stopper resistors to make the simulated design work, but they were not needed for the real world (except for the anomalous ST TIP142).  I suspect that this is why the real world high frequency distortion was better than 0.1%, whereas the simulated distortion was 0.5%. More work needed to match the simulated to the real results but I leave that to the experts.

It was revealing to look at the op-amp output when the amplifier was giving a full output 20kHz triangle wave. There is a dead band which transitions in 1 to 2uS so you will be blissfully unaware of any problem. The ramps show that the darlingtons have good linearity.

However I was not 100% happy with the voltage regulators as a band-aid fix for the quiescent current with supply voltage variation problem. Hence the design never made it to a dedicated PCB layout from the Veroboard prototype. Nor did I formally post it to Diyaudio. I developed it until it was good enough for my own domestic use, but would not regard it as a “production” design. However the prototype has been happily working for several years so I offer the following ideas which I have not tried for you to experiment with.

The main issue was the quiescent current variation with supply volts. A way around this is to have 2 current setting pots, one for each of the upper and lower halves of the output stage.  The current defining resistor is eliminated. Also the voltage regulators are removed. When setting the currents a resistive load is required. R13,14,30 and 31 to be 250 ohm preset pots. R10 and R28 are removed as well as the regulator components.

The current stability would also be improved by defining the op amp voltages with zeners. To lower the self heating effects of the NE5534, the voltage of these would be 12 rather than 15V. R2,3,21 and 22 are now 12V zeners. Also R4,9 ,23,and 27 are 330 ohm.

The thermal feedback would be more consistent if the Thermistors were bonded to solder tags and the tags used as the power transistor mounting washers. Nelson Pass uses tagged thermistors in his designs which gave me the idea.

Lastly a R-C filter at the input to limit the bandwidth both for TIM and radio interference reasons would be worth fitting. Put 470pF caps across R1 and R20

I just discovered the reason for the anomalous ST TIP142. The zeptobars website has photos of the silicon dies used for various devices. One of these for the 2STD1665 gave a clue that the device would be fast (too fast for owner!) If you look up the spec for the device it is fast. Curiously ST do not make speed claims for the similar planar base island TIP142

https://zeptobars.com/en/read/ST-2STD1665-NPN-BJT

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