Soundcraftsmen PCR800

The PCR800 was introduced in 1984 when the Hitachi 2SK135/2SJ50 came into the market. Four were used for a rated output of 324W into 4Ω and 231W into 8Ω.

In this post, I’m doing away with the Phase Control and just focus on the amplifier itself. Reason being that while the Phase Control do result in a regulated supply of +-70Vdc, I find it inefficient. Let me explain further.

The output of the power transformer after rectification is about 90Vdc. It then enters the PCR circuit for the thyristors to perform regulation down at 70Vdc. Basically, there is a 20V headroom for regulation. For me, this is a huge waste of energy. What if we just run the amplifier with unregulated rails of +-70Vdc. This is what almost all amplifiers do anyway. No wasted energy and added complexity with the PCR.

Soundcraftsmen PCR800Fig 1 – Soundcraftsmen PCR800 Amplifier Schematic

Fig 1 is the PCR800 amplifier proper without all the frills. The circuit is very basic. An input differential coupled to the VAS followed by a pair of drivers for the power mosfets. Technically, there’s nothing wrong. Sound wise, she can be improved tremendously by replacing all her transistors with more modern ones. The power mosfets, 2SK135/2SJ50 can be reused but if they are damaged, they can be replaced by 2SK1058/2SJ162.

Another power transformer will have to be used though. Personally, I would run the amplifier with rail voltages of +-56Vdc which means a transformer of 40-0-40Vac. With these voltages, the power output will be about 120W into 8Ω and 200W into 4Ω. The power mosfets will be more comfortable at this level.

If you want higher power for PA, go for unregulated +-90Vdc rails. This will mean you will be able to re-use the power transformer and the filter capacitors. The circuit needs to be re-worked for the new voltages. Running the Hitachi 2SK135/2SJ50 at such high voltages may stress them. On top of that, there need to be at least 10 mosfets because at +-90Vdc, we are looking at about 400W into 8Ω and 550W into 4Ω. A safer option is to use higher voltage BJT like MJ15024/25.

Fig 2 – Inside the PCR800

Fig 2 is the PCR800 with the top cover removed. Looks like the case cannot be re-used. It would be easier to find another chassis for the new PCR800.

Summary

The Soundcraftsmen PCR800 main marketing point is the Phase Regulation. Remove that, the amplifier is nothing special. If you want the sound of the PCR800, the circuit can be retained and re-worked with modern transistors. The essence of the PCR800 will still be there. Just without the Phase Control.