By the time the Stereo 150 came out (around 1975), the venerable Stereo 120 had been in production a relatively long time. To modern eyes, the Stereo 120 would probably look a bit baroque. The Stereo 150 is a design that bears a strong resemblance to current day designs. Even so, certain common features of current day designs are missing.

- PC-36 versions used a resistive load and bootstrap capacitor in the voltage amplifier stage
- PC-43 versions used the more modern single transistor current source as the load for the voltage amplifier stage
- Neither version used a current mirror to turn-around the input differential stage
- Neither version used any input stage degeneration resistors, which gives it a slow (by modern standards) 5V/us slew rate

- That there are two versions, distinguished by whether the amplifier board is either the PC-36 or the PC-43
- The amplifier channels could be paralleled to produce a more powerful single channel amplifier. The manual quotes 150 Watts into 8 Ohms. At first this was confusing, because based on the +/-50 Volt rails, output power would max out at a theoretical 156.25 Watts into 8 Ohms. One of my customers, Mitchell, was kind enough to correct me on this point. A careful re-reading of the manual shows that upon converting to parallel mode, the power supply is also strapped to increase the B+ and B- from +/-50 to +/-63 Volts DC. That's enough to get to 150 Watts into 8 Ohms with some room to spare.
- Speaker fuses were a part of the output protection scheme
- It used a thermistor on the input that would decrease the amp's sensitivity if the output heatsinks got too warm

- Complete Assembly Manual, this early version covers the PC-36 (large download, 27 Megs)
- Schematics and BOM for later version, with PC-43
- Complete Assembly Manual, this later version covers the PC-43 (large download, 20 Megs) (Thanks to Jacques)
- Pullout wiring diagram, this later version covers the PC-43 (Thanks to Jacques, again!)

Through the miracle of Ebay, and a very reasonable "Buy It Now" price, I now (11/30/2013) have my very own Stereo 150. Here are some quick thoughts and observations.

- The big filter caps are 10,000 uF @ 80 Volts; They are 2 inches in diameter, and their cases are about 4.125" tall.
- My amp is the earlier one, based on the PC-36 version of the amplifier PC board.
- It arrived functional, and in good shape. It put out about 27 Volts RMS into 8 Ohms before clipping, about 91 Watts per channel (1 channel driven)
- At idle, the rail voltage was +/- 49.5 Volts, very near the nominal +/-50 called out in the manual
- At idle, the ripple was about 0.2 Volts peak to peak
- With one channel putting out 90 Watts at 2 kHz, the ripple was 1.1 Volts peak to peak, dropping in about 6 milliseconds
- 27 Volts RMS=>27*1.414 volts peak, or 27*1.414/8=4.77 Amp peaks from the positive supply rail. That translates to 1/pi times that amount of DC current, or 1.52 Amps.
- Assume the total current drain is about 1.6 Amps DC with one channel driven and the other idle. Then using Cdv/dt=I, we can solve for C=Idt/dv=1.6*0.006/1.1=8700 uF. Granted, that's a bit of a crude estimate, but the caps may have lost a little capacitance, or perhaps just had a wide tolerance
- Original caps have screw terminals. Replacement screw terminal caps are quite expensive. You could do a dynamite capacitor arrangement with three 22 mm diameter caps (aggregate diameter of 22mm*2.1547=1.866 inches diameter, or 25mm*2.1547=2.12 inch diameter, which is a touch big, being 120 mils more than the nominal 2"