Spice Simulation – Soundcraftsmen PCR 800

Spice – Soundcraftsmen PCR 800

Apart from the Phase Control unit in the Soundcraftsmen PCR 800, the power amplifier is a really basic circuit. In this post, I’ll take a closer look with Spice.

Fig 1 – DC Analysis

Fig 1 is the circuit of the PCR 800. I changed the outputs to BJT instead of using the Hitachi mosfets because Spice doesn’t have them. It won’t make any difference because what I’m interested in are the DC operating points.

First and foremost, this is the first time I’m using Spice for a full discrete design. In other words, no op-amp is used in the front end. Instead, two transistors form the input differential.

In this stage, a current sink guarantees a standing current of 1.77mA. However, they are not shared equally in the differentials. Ideally, both sides should be 0.885mA (1.77mA÷2) but on the left, it’s 408uA whereas the right is 1.3mA.. This is due to the loading caused by the VAS.

Another current sink in the VAS results in a standing current of 5.83mA flowing through it. The bulk of the voltage amplification is from this stage. After that, there are no further amplification. The next stages are concerned with current. The VAS drives the drivers which in turn drives the power transistors. Bias and thermal control are in the VAS.

Fig 2 – Transient Analysis

Fig 2 is the Transient analysis of the PCR 800. No anomalies are observed. The amplifier is able to drive an 8Ω load at the output.

Summary

There are lots of room for improvements in this circuit. Most glaring is the 101mV DC at the output. Even though it will not harm the loudspeaker, it is still too high for my liking. To bring it down to +/-10mV, a DC offset adjustment circuit needs to be installed.