Alesis RA100

The Alesis RA100 is a convection cooled power amplifier with a rated power output of 100W into 4Ω and 75W into 8Ω. She was introduced sometime in the 90s. Since no fans are used, she was ideal in home recording studios and domestic HiFi.

Alesis RA100Fig 1 – Alesis RA100 Amplifier Schematic

The RA100 uses a proven double differential typology in her circuit (Fig 1). There’s an input differential followed by another differential for the VAS. The current stage is a bit unusual in that instead of the drivers’ emitters driving the power transistors, they have opted for a compound feedback arrangement. This is also known as a Sziklai pair, after the inventor George Clifford Sziklai. Compound feedback is known for higher gain and improved stability. However, I question the resistors from the collectors. Usually, they are connected to the emitters.

Summary

I am surprised that the RA100, being a budget amplifier, has chosen to use the Sankens MT-200 2SC3858 and SA1494 for the power transistors. They are expensive and usually seen in high-end HiFi amplifiers but not in budget ones.

Regarding the sound, I would do away with the MPSA transistors that are used in the input differential and the VAS. Those transistors are more suited for PA amps. With proper tweaking, this RA100 can sound superb.

As for the chassis, the RA100 is ideal for retrofitting. The heatsinks, transformer and case can be re-purposed. A new power amplifier module can then replace the original ones as what I did with the AB Precedent 400.