Figure 1 - Basic Amplifier Schematic
Almost all amps of the era from which this circuit originated used the 2N3055 - this was the pre-eminent power transistor (NPN of course), and there were no vaguely equivalent PNP devices for less than about 5 times the price, and even these were highly inferior. As a result, the quasi-complementary output was very common, until decent PNP power devices became more readily available. Immediately, just about everyone started using NPN and PNP Darlington coupled devices for the output stages (as shown for Q3 and Q4) - the funny part is that it was demonstrated back in the mid 1970's that the full Darlington connection actually sounds (or at least measures) worse than quasi-complementary stages. Is not progress a wonderful thing?
So, I got to thinking about this (as I have done many times, but it never went anywhere), since the input stage of a current feedback amp is not subject to the phase problems of the long tailed pair, and amps with this input stage tend to be inherently stable. They do have a problem with DC offset (which was not a problem with capacitor coupled speakers), but this can be solved with a DC servo circuit using an opamp, or a simple bias offset can be used.
As shown, the gain for audio frequencies is 31 (30dB), which means an input sensitivity of 700mV for an output of 60W (near enough to 0dBm). This remains unchanged for the variations following.
A Theoretical Examination Of Improvements
Note that this article is, for the time being, a "theoretical study", in that the amp described has been simulated but not built. The output stage is completely conventional, using the complementary pair configuration which is now the standard for all designers who have ever read anything by Doug Self, Matti Otala, John Linsley Hood, myself or a myriad of others who have all denounced the Darlington as an inferior output stage in every significant respect.
Figure 2 shows the circuit of the amp in basic form, remaining fairly true to the original concept except for the dual power supply, direct-coupled speaker and a bias servo allowing lower value emitter resistors for the output stage. A DC offset control is mandatory here, and the LED is used as a stable voltage reference for the offset voltage. With a solid power supply (such as that described for the 60W Power Amp), this amp is perfectly capable of 60 to 70W into 8 Ohms. Additional output transistors can be connected in parallel to allow for 4 Ohm loads, where 100W should be readily achieved.
Figure 2 - The "New Improved" Version
The Class-A driver is perfectly normal, but can be improved by using a bootstrapped buffer transistor, and the use of a current sink load for the Class-A driver will improve gain and linearity. As shown, the Class-A driver load is still a bootstrap circuit. With a sufficiently large capacitor to allow for the lowest frequencies, good linearity is obtained, with the driver current remaining effectively constant for the full swing of the amp.
Even without the buffer on the Class-A amp stage, a simulation (admittedly using "ideal" transistors) of the input and Class-A stage shows a gain of about 100dB, or 100,000 with a current sink of 100k Ohms. This is approximately what can be expected from the bootstrap circuit due to the losses in the output stage.
A useful increase in gain may be achieved by increasing the current through Q1, by reducing the value of R4. There is a problem with this however, since the voltage across R5 becomes excessive, raising the input DC voltage on the base of Q1. One can reduce the value of R5 (the feedback resistor) but then the required capacitance of C4 becomes too high to be sensible because R12 must be reduced for the same audio gain.
Further Improvements
Figure 3 shows all the additional improvements possible while still retaining the input stage, and a simulation indicates that the open-loop gain of this configuration is over 150dB (or 30 Million) open loop - this is likely to be somewhat optimistic, but is a good indicator of the available gain one can achieve without the current mirrors and other accoutrements generally found in typical input circuits. With a gain as high as this, there is enough feedback for anyone - without getting more complex.
The input capacitor has been changed to a polyester (or similar) and with 1uF has a lower -3dB frequency of 7Hz. This may be made larger if your speakers can go lower than that. One thing you cannot do with this input stage is direct couple from a preamp. The voltage on the base of Q1 will be about 1.3V for 0 Volts at the speaker output. If the input were grounded, then there will be -1.3V across the speakers - this is generally considered to be a bad idea. It is only 200mW for an 8 Ohm load, but it should be avoided.
Speaking of feedback - because the input stage creates an inherently stable amp, there is no reason to expect that TIM (Transient Intermodulation Distortion) will be a problem (assuming it actually ever existed in a real amplifier operated under normal conditions), since feedback is simply applied to the emitter of the input amp, and little or no frequency "compensation" is needed. This is an area where some experimentation is needed, and it might be necessary to connect a low value (47pF ?) capacitor between collector and base of Q2 - it was not needed in the original, but this configuration has vastly more gain and today's transistors are a great deal faster.
Figure 3 - The "Even-Newer More-Improved" Version
I tend to like Figure 2, since it appeals to my KIS approach (Keep It Simple) towards all things electronic, while still maintaining a sensible attitude towards providing adequate feedback and other techniques to minimise distortion. Having said that, Figure 3 is probably going to be the better amp overall, since it will have better linearity before feedback is applied.
The astute reader will realise by now that the entire Class-A driver and output circuits are virtually identical to many of the high-end amp designs one can find on the Web and in magazines (etc). The only bit missing is the long tailed pair input stage, and its current source in the tail, and the current mirrors in the collector circuits. Oh, and the mandatory "Miller" capacitor to limit frequency response for stability and all the other stuff one finds in input circuits. In reality, it is almost certain that a small value cap will be of benefit to ensure stability with difficult loads.
In so many cases. it seems that the amp circuits one sees have been designed for the sole reason to use more components than any other on the planet, and the next one you see is even worse.
