The Stereo 80 was the little brother of the Stereo 120. The Stereo 80 and 120 had similar designs. The main differences were:
Stereo 120 had an electronically regulated power supply and larger power transformer
Stereo 120 had larger heat sinks
Stereo 120 put out 60 Watts per channel, versus 40 Watts per channel for the Stereo 80
Stereo 120 had a larger chassis with protected binding posts, Stereo 80 had screw terminals for the speakers
The 120's regulated supply maintains the power supply output at 72 volts until the current limit is reached. The 80's unregulated
power supply output voltage sags as output power increases. Owing to the regulated power supply, it's my belief that the 120 should
measure and sound better than the 80, but there are a wide variety of opinions on this subject.
Almost 50 Watts per channel into 8 Ohms at 1 kHz, single channel driven
A bit more than 80 watts per channel into 4 Ohms at 1 kHz single channel driven
THD + Noise below 0.01%, with 0.002% typically measured (1 KHz, 40 Watts, 8 Ohms)
Restoring a Stereo 80 with the PWRAMP80 Kit
This gallery shows how I started with a particularly dirty Stereo 80, and ended up with a nice sounding amp. At the end of the day, there
are still some cosmetic problems. The Mother's Chrome polish did a good job, but the chrome part of the chassis was so heavily pitted that a
lot of rust damage remains. Still, the amp sounds and measures great! Then too, it only cost $40 on Ebay!
Stereo 80 Capacitor Kit
The most usual failure mode in the stereo 80 is that the big can capacitors lose capacitance. We listed them above, but there's an
easier way to go. Just order the Stereo 80 Capacitor Kit. It bundles the following items:
C7X2 kit - a dynamite capacitor arrangement that bumps the output capacitor value from 5000 uF to 9900 uF, improving bass power and
lowering bass distortion. A single C7X2 kit replaces both the left and right C7 output capacitors.The C7X2 kit also includes a 100K resistor
that you don't use in the Stereo 80 application.
Half of a PSRC - a single 3900 uF 100 Volt capacitor which replaces C11, which was a 1000 uF capacitor. This increase also decreases
noise and distortion by better filtering the driver supply rail.
SCA80C9 kit - replaces the original 5000 uF C9 bulk filter with 9900 uF in a dynamite capacitor arrangement. Once again, this increase
in capacitance lowers distortion and allows the ST80 to produce more power, and in particular, more power at low frequencies.
Updated Stereo 80, the Hard way!
Long before the PWRAMP80 kit was available, one of my customers, Tim, did the first such conversion. For that conversion, he ordered:
A special toroidal transformer (send me an email, and we can work it out)
Special power supply capacitors (send me an email, and we can work it out)
Tim is a handy guy, and he fabricated his own version of super heatsinks, along with a special super heat sink for the power supply.
He arranged them in the U-shaped form shown in the following pictures. That shape allowed him to fit (essentially) the same transformer
as we use in the GT-101 power amplifier, making the power supply brawny enough to support the higher output power.
You'll note that Tim has retro-fit:
gold binding posts for the speaker
isolated RCA input jacks
a 3-wire IEC style power connector
I don't think he put a blue-light kit on the amp, but hey, nobody's perfect :)
The end result is 120 Watts (60 Watts/channel) is the smaller footprint of a Stereo 80!