Bongiorno's Sumo 9 and 9+ were major steps forward at the time but could only be realized with bipolar junction transistors because of the requirement that an emitter base current flow from the input to the output of the power device for the closing of the current loop. The base current multiplied by the current gain hFE of the BJT defines the quiescent current of the power deivce. BJTs have an unwanted behavior: with increasing output current, the current gain hFE is decreasing. That means that the ratio between input (=base) and output (=emitter) current is decreasing for increasing output power. This descrease is non-linear.
Current gain variations of BJTs: Increase in temperature will cause thermal problems in the Sumo 9/9+ amp structure; decrease with collector current causes non-linear distortions. MJL21193/21194 are state of the art "Total Harmonic Distortion Qualified" pairs but note the differences between pnp and npn transistors!


In the circuit of the Sumo 9+, there are 2 diodes and the 150-ohm resistor connected to ground, which should counteract the decreasing current gain effect. If the positive output voltage level reaches the conducting voltage of the diode (about 0.7V), an additional current starts flowing to ground via the 150-ohm resistor. This additional current should deliver the extra necessary base current for higher output power. This circuit has two disadvantages: it compensates for the nonlinear base-current function only partially and adds a sudden switching behavior to the amp. When the output power is fairly low, unwanted noise or distortion become eminently annoying. This must be corrected for by the global feedback loop. The reliability or thermal runaway of the complete amp is dependent on the absolute value of the current amplifying factor of the massively paralleled output transistors. This factor increases with heat (see above) and therefore increases the quiescent current of the output stage. It's not foolproof either. If only one speaker terminal is accidentally shorted to ground (no short circuit!) and music is playing, this amp explodes. The Sumo 9 and 9+ were well known for their blown output transistors. Repair was nightmarish because of the requirement for perfect transistor matching. Hence very few of these amps survived. It's also clear that such a fragile design was only capable of delivering less than 100 watts. Increasing the output power would have meant far greater unreliability in practice.


The disadvantages of the Sumo circuit compared to mine are:

  • only bipolar junction transistors may be used in the output stage
  • switching artifacts, nonlinear open loop transfer function of the power output stage
  • only operation amplifiers (OPVs) in the voltage amplifier stage
  • no tubes possible for driver stage (must deliver high base current)
  • massively paralleled output transistors are a prerequisite to limit the non-linear behavior of current gain hFE (and hFE across the entire working and temperature range must be the same to require extremely precise matching of the output transistors)
  • no reliable protection against thermal runaway
  • amp blows up if just one output connector touches ground
  • output power is limited to less than 100 watts