Our first hybrid power amplifier was constructed by Frits Savelkoul in 1988. At that time tube amplifiers and more specific single ended ones were very popular among audiophile listeners. Tube amps though have one big disadvantage: output impedance. Current loudspeakers are relatively complicated and are a bad match for those amps (they are all designed to be used with a transistor amp with very low output impedance).
The young designer tried to construct a very special amp. It had 16 (!) power fet transistors at the output. Further more the high voltage was stabilized, delayed and applied after the warming up of the tube cathodes. At the input there was an "SRPP" amplifier followed by a "Lofton-White" cathode follower. Each channel had three PCB's with various electronics on it. All together it was a complicated design and the component cost was extremely high.
In 1991 another student, Joep Remkes, together with John van der Sluis designed the basic A-50. We then thought it not longer necessary to delay the high voltage. The input circuitry was the same as in the A-80 and at the output we had 8 fet power transistors. The power transformer was 500 VA (=Watts) instead of 1000 VA in the A-80.
Over the last ten years there were several changes in the design, mostly by choosing better components. The basic circuitry still is the best thing we can think of.
But there are other considerations. A major advantage nowadays is that it is relatively simple and cheap to use double sided PCB's (circuit boards). So we have a better way to design the "grounding" paths. Also we found that vibrations are tricky in audio. So it's a good thing to absorb and/or suppress vibrations. Finally we thought it a good idea to increase the power.
Advantages of the new A-60
(when compared with our former A-50)
The toroid power transformer now can deliver 650 VA at a slightly higher voltage. Also the two windings each have its own rectifier. (A technique very common in medical electronics!) Between these windings and the rectifiers resistors are added to suppress "spikes" originating from those rectifiers.
The circuit in principle is the same with a few changes. The local feed back capacitor in the cathode follower now is an "audiophile" type.
The couple capacitors between the cathode follower and the fets also is an "audiophile" type and the same goes for the capacitor at the output of the high voltage stabilisation.
All metal parts have absorbing pads on them suppressing the influence of vibrations. The bottom has "Sorbo pads" isolating the cabinet from the panel or construction where it's placed on.
All together we have a nicer amplifier delivering more "musicality" then any earlier design.
Stereo imaging is improved so you get a better "feel" of the ambiance where the music was recorded.
Specifications
maximum power at 8 Ohm = >60 Watt
maximum power at 8 Ohm//2 µF= >60 Watt
maximum power at 4 Ohm = >120 Watt
maximum power at 4 Ohm//2 µF = >120 Watt
maximum power at 2 Ohm = >200 Watt
maximum power at 2 Ohm//2 µF = >200 Watt
maximum power at 1 Ohm = >250 Watt
maximum distortion at full power -3 dB = <0,5 % (20 Hz - 50 kHz)
overshoot at 8 Ohm//2 µF = 20 %
overshoot at 4 Ohm//2 µF = 16 %
bandwith open loop at full power -3 dB = >1 Hz - 400 kHz
bandwith with input filter at full power -3 dB = >1Hz - 190 kHz
slew rate = >13 V/µs
transfer time = 0,3 µs
decrease at 1 Watt vs. 8 Ohm:
at 4 Ohm = 0,3 dB
at 2 Ohm = 1,0 dB
at 1 Ohm = 2,0 dB
distortion at 1 Watt - 8 Ohm = <0,1 % (20 Hz - 50 kHz)
sensitivity = 800 milliVolt
input impedance = 100 kOhm
DC-offset = <5 mV
All measurements at 10 kHz unless otherwise mentioned.
Distortion mainly 2nd harmonics
