BAT VK-500

BAT VK-500 MOSFET Monster Amplifier Repair

My customer had an issue with a binding post on his amazing Balanced Audio Technology BAT VK-500 MOSFET amplifier. Read about the repair below.

Technically speaking, the BAT VK-500 MOSFET amplifier is an amazing design. It uses only MOSFET devices in the driver and output stages and a large bank of smaller, paralleled filter capacitors to create a power supply rails of exceptionally low impedance.

Featuring 24 N-channel MOSFET devices in the output stage, the VK-500 avoids inherent complementary pair mis-matching and theoretically creates a more transparent amplifier. The N-channel MOSFETs are identical and faster, allowing for a 300V/us slew rate, an extraordinary spec for a modern amplifier.

Specifications:

  • 250/450 Watts RMS per channel into 8/4 ohms
  • 1100 joules energy storage from 300,000 uF power supply capacitance
  • Zero global negative feedback design
  • MOSFET output devices
  • Balanced XLR inputs
  • Completely dual-mono topology (two transformers and two power cords)
  • 48cms (W) x 58cms (D) x 24cms (H) (Mass) 50kgs

More information about the VK-500 and comprehensive technical explanation can be found here.

The Repair

The binding posts fitted to the BAT VK-500 are pretty good, but the plastic used becomes brittle with age, making it susceptible to fracture. With this binding post, the plastic on the retaining side, underneath the retaining nut, had fractured and failed.

Repairing this required either a replacement of all four binding posts or a repair of the broken one. I prefer to repair parts like this, because it minimises wastage of good parts and helps maintain the originality of the equipment.

First I desoldered the broken binding post. This required a lot of heat, due to the heatsinking effect of the heavy metal parts and chassis. Once I had the binding post out of the chassis, I cleaned it and measured it up for repair.

I used some high-quality nylon washers to build a stronger retaining arrangement than the factory part. The washers fit perfectly and are much stronger than the original plastic parts. I was able to torque the retaining nut up onto the new nylon retainer and resolder the output wire.

The repair looks completely factory from the outside, something quite important to me when making repairs such as this. Maintaining the factory appearance and avoiding the need to modify the chassis to fit new parts are great results.

BAT VK-500
The awesome BAT VK-500, showing massive toroidal transformers, stunningly clean layout and mirror-image left and right-hand amplifier modules. Note that this amplifier is dual mono, from the twine power cables and mains sockets to the speaker binding posts.
BAT VK-500
This is the broken binding post, removed from the amplifier chassis. Damage is obvious…
BAT VK-500
This is where the broken binding post came from, enough room to work in here but only just.
BAT VK-500
View from the rear with binding post removed.
BAT VK-500
Binding post reinstalled, with nylon retaining washers in place. This is a very strong repair, the nylon is extremely tough and completely non-conductive.
BAT VK-500
Finally, binding post with connecting cable resoldered into place. This again required great heat, due to the heatsinking effect of all those metal parts!
BAT VK-500
Repair looks perfect!

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