Monday, January 4, 2021

Krell KSA-5 Clone - Review of the Original Schematic

In the last post, I demonstrated the measured performance of my KSA-5 clone before the upgrade, which was not so great. Let's see why it distorts.

KSA-5 is designed along the lines of "moderate feedback", that is, it uses very little to no global feedback but lots of local degeneration.

The pair of input JFET buffers (Q1, red box on the schematic above) run independently of each other and outside of the global feedback loop. With low loop gain, they see very different signal levels, so the differential stage downstream doesn't cancel their distortion. (BTW, because of this JFETs need not be matched. Also, the expensive and hard-to-find JFETs can be easily replaced here with BJTs.) Having said that, a JFET follower loaded by a current source has 100% degeneration and low distortion, at least at low signal levels, so the buffers are not the biggest problem.

The pair of differential stages (Q2+Q3, Q7+Q8, orange box) is heavily degenerated by 680ohm emitter resistors and have R10/(R1+R2) = 2 = 6dB of amplification.

The pair of common emitter stages (Q12, Q13, purple box) is also heavily generated by 402ohm emitter resistors and, with the low load of R23 and R24, provides R23/R16 = 9 = 19dB of amplification.

Since the output stage (blue box) is a double emitter follower with approximately unity gain, the total open loop gain of KSA-5 is 2x9 = 18 = 25dB. The feedback divider (R45-R47) attenuates the output signal by a factor of 9 (19dB), which leaves 18/9 =2 (6dB) of global feedback. That is, the global feedback loop attenuates the distortion of the output stage by a small factor of 1+2 = 3.

The output stage, meanwhile, is the biggest source of distortion. Although Krell claimed that KSA-5 runs in "pure Class A", in reality it is Class AB. The output pairs run at only 50mA of quiescent current each and leave Class A (that is, one half of the output stage stops conducting current) when the output current reaches 200mA. The driver quads (Q15-Q22) also run in Class AB (R37 and R38 are connected to the output), which means they stop conducting at that point, too. With a 100ohm load, it would happen at 20V peak output voltage, so the amp never leaves Class A with such a load. However, with 32ohm, KSA-5 leaves Class A at 6.4V peak; with 8ohm, at 1.6V - that's 320mW! No wonder the owner's manual warns not to connect this amplifier to any loudspeakers.

Even within Class A region, the output stage is not very linear, especially with low impedance loads. It uses paralleled transistors with relatively large emitter resistors to ensure current sharing. The dark side of large emitter resistors is that they make the output impedance of the emitter follower large and nonlinear in the crossover region (see e.g. Douglas Self and his "wingspread" diagrams). Since the output impedance forms a voltage divider with the load, its nonlinearity makes the gain of the emitter follower nonlinear, adding crossover distortion and negating the benefit of the large bias current.

Overall, KSA-5 has a not-so-linear output stage and a nice and linear frontend that doesn't help the output stage to stay linear.

In the following posts, I will show how I improved the distortion performance of KSA-5.