Ampeg SVT-4 Pro Bass Amplifier

The Ampeg SVT-4 PRO bass head is quite popular with guitarists. She is a 2-channel amplifier rated at 300W into 8Ω and 490W into 4Ω. Her power transistors are vertical mosfets, IRFP240 and IRFP9240. They are used in some professional amplifiers because of their robustness and power handling.

In this post, I will address the amplifier only, partly because that’s the one that breaks down most often.

Fig 1 – Ampeg SVT-4 PRO Voltage Amplifier

Fig 1 is the Voltage amplifier of the SVT-4 PRO. This is where the incoming signal is amplified before it reaches the power mosfets. The front end is not discrete but a 5532 op-amp. The incoming signal (Red line) enters the non-inverting input (Pin 3). The output (Pin 1) is amplified by transistor Q103 with a collector load of 1kΩ. The signal is further amplified by transistor Q102 in the VAS. It is in this stage that the power mosfets biasing mechanism and mosfets Gate drive are situated.

Fig 2 – Ampeg SVT-4 PRO Current Amplifier

Fig 2 is the current stage. The Red line drives the Gates of five power mosfets. The Pink line is the output of the amplifier. Negative feedback (NFB) is routed from this point back to the 5532 op-amp non-inverting input. All the power mosfets have 5W source resistors of 0.47Ω for current sharing. The power supply capacitors are four units of 8,200uF/100V, that is 16,400uF for the positive rails and 16,400uF for the negative. This is somewhat low to service 2-channels but is quite acceptable in a commercial product.

Fig 3 – Ampeg SVT-4 PRO

Fig 3 shows the layout of the SVT-4 PRO. The PSU caps are at the top right corner. The power transformer is a conventional EI type with EMI shielding. This will help greatly in reducing hum caused by EMI radiation from the transformer.

The power mosfets are mounted on two heatsinks, one for each channel. A fan is used to blow air into the heatsinks for cooling. It is imperative that the fan is working properly otherwise the power mosfets will overheat especially when the amp is driven hard. This will lead to the SVT-4 PRO breaking down.

Summary

Over the years, I’ve been asked to repair various power amplifiers and this SVT-4 PRO is no exception. However, I had to reject these requests as I’m not into the repair business. It’s not a question of abilities but I find it more challenging in identifying the strengths and weaknesses in amplifier circuits.

When I have the time in future, I will re-work the SVT-4 PRO circuit. There are things I plan to do differently that will enhance the reliability of this bass head.