Spice Simulation – Ampeg SVT-4 Pro BJT Mod

Spice – Ampeg SVT-4 Pro BJT Mod

Having spiced the SVT-4 Pro, I decided to see how well this circuit will perform with BJT outputs. One of the main reasons is to see how she sounds like for HiFi use. For this purpose, I’ve lowered the supply rails to 55Vdc. This will result in approximately 125W into 8Ω and 200W into 4Ω. In my view, this is more than enough for home use.

Fig 1 – DC Analysis

Fig 1 is the modified circuit of the SVT-4 Pro BJT Mod. Disregard the TIP power transistors at the output. I only have them as a reference. When I prototype this BJT Mod version, I plan to use the Toshiba 2SC5200/ 2SA1943 or the Sanken 2SC2922/2SA1216. I’m curious to hear how they sound like.

Fig 2 – Transient Analysis

Fig 2 is the Transient analysis of the SVT-4 Pro BJT Mod. There is no problem swinging to almost full power with an 8Ω load. Input signal is slightly on the high side, at 1.5V.

Summary

One of the advantages of an op-amp front end is there’s no necessity for any DC offset adjustment. This reduces manufacturing cost. But using an op-amp at the front has their own problems. Nonetheless, if properly done, it has it’s benefits.