Alesis RA150

Like her predecessor the RA100, the Alesis RA150 is also a convection cooled power amplifier. Her rated Power output is somewhat lower at 75W into 4Ω and 45W into 8Ω. She made her appearance in about 2010~2012. This is about 10 years after the RA100.

The RA150 adopts a different typology from the RA100. She now sports a mirror image circuit (Fig 1) as opposed to a double differential. Constant current sources ensure the differentials are biased at 1mA per leg. Collector load is at 1kΩ. The VAS is biased to healthy 12mA.

Alesis RA150Fig 1 – Alesis RA150 Voltage Amp Schematic

The Red line in Fig 1 is the signal path in the RA150. This is only the Voltage amplifier. What that means is this is where the voltage amplitude is magnified. Fig 2 below is what is called the Current stage. No voltage amplification is involved here.

Fig 2 – Alesis RA150 Current Amp Schematic

In this second part, we are interested in Current. Over here, the VAS drives a pre-driver (Q212) which then drives the main driver (Q216). This arrangement is called a Darlington. One of the properties of Darlingtons is High Current Gain can be achieved with very small base current. It is a bit of a luxury to use a Darlington here because there is ample current in the VAS. Using a single driver will not “load” the VAS.

In this newer series, they have abandoned the Sanken MT-200 transistors in favor of the 2SD1047 and the 2SB817. These are smaller packages (TO-3P) and two pairs are used for the power outputs. They have also changed the Compound Feedback used in the RA100 to a more conventional Emitter followers.

Summary

The Alesis RA150 is the least powerful amplifier in the series.One grade up is the RA300 and finally the RA500. All of them use the same circuit as the RA150.

As with the RA100, the RA150 is ideal for retrofitting with another power amplifier module. Due to her lower power, it seems the LM3886TF can be used.