1970s Design Indulgence

Most commercial Vbe multipliers have some resistance, but it's small and usually around 20 ohms plus the transistor's emitter resistance.

Here, because of the addition of a fourth diode, the trimmer is 50 ohms, and when adjusted for the correct Vq, is nearer to 30 ohms, and so closer to a commercial Vbe multiplier than with the 120 ohm "padding resistor."

With the 120 ohms in series with the trimmer the sound was likened to overshoot, and had poor bass - even though the measurements were no different.

Such resistance is sometimes bypassed with a capacitor, but it would need to be a rather large capacitor value to cover the lowest bass. An electrolytic would be needed, and the leakage current would extend set-up time to hours before the correct Vq settled.

I therefore prefer a little resistance as long as it only contributes a tiny fraction to transient non-linearity. The negative feedback should easily polish it off.

Considering that this resistance can cause a difference in subjective sound quality, I feel justified in making the Vbe stretcher passive. Consider that a transistor used for that purpose can suffer instability due to it having beta, whereas diodes don't.

The downside is in having to fine-adjust Vq yourself, requiring an additional potentiometer and meter on each channel - but, you can be confident your Vq is what it should be, rather than blindly hoping it is.

It had one major drawback; you had to parallel pairs of output transistors to match the number of drivers, plus output; so in a 'double', as here, you have a driver and an output transistor, making two Vbe drops, and so you needed two sets of output transistors in parallel to obtain the two diode drops. A 'triple' you needed three sets of output transistors in parallel to obtain the three diode drops.

In this (and many other designs), just one pair of output transistors is required and can easily reach 100 watts into 8 ohms, given sufficient voltage. I don't want to parallel another pair, and even if I did, the drivers would suffer the additional load and twice the capacitance!

The result must have been poor sales, and so the thermal track went out of fashion as quickly as it came. It had, however, shone a light on the thermal feedback imperfection of relying on heatsink temperature to maintain a 'healthy' bias. The best place to monitor the temperature had always been the transistor itself, but gluing a diode to an output transistor would have been messy in mass production. Here, the amateur can score over the manufacturers by having the time to stick the diode to the most appropriate part of the transistor.

It then leaves the need for two other diodes to match the pair of drivers. This used to be done with one diode and a trimming resistor - the way I had it - but in my earlier post, using the equivalent to the thermal track diode, I found I needed two.

If I left the trimming resistor out to leave just the four diode chain, I would have virtually no resistance in the Vbe stretcher and zero non-linearity in this part of the VAS load. The inventors of the thermal track must have understood that this was a 'pinch point' for distortion. The thermal track hedges its bets by under biasing, even though it claims not to. It might provide perfect class-B bias, but I'm running it in class-AB, at the optimum before gm-doubling occurs. That requires a tiny bit more bias, hence the need for a trimming resistor to 'pad it out.

The extra diode and smaller value trimming resistor approximately quartered non-linearity in this part of the VAS load, compared to the one diode and larger trimming resistor.

The action of the two diodes simply sat on the board not only satisfies the Vbe stretcher voltage drop but will also act on temperature stability by being subjected to the ambient temperature inside the case.

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