Sunday, December 13, 2020

Velleman K4040, take three

This is the last of three posts on Velleman K4040. Here are the links to the first and second posts.

Over 10 years ago, I built a Velleman K4040 power amplifier from an (expensive) kit. The amplifier still looks quite impressive:

Not satisfied with the out-of-the-box performance, I modified the amplifier (see my previous post for details). The result was a dramatic improvement in sound. However, without its global feedback, the amplifier had higher measured distortion, higher input sensitivity and more hum.

Ten years later, I (slightly) revised the amplifier. I replaced the resistors in the signal path with mil-spec metal film from Vishay, tidied up the wiring of the phase splitter and added a global feedback loop that encloses the input stage. With it, the amplifier has 0.015% distortion at 1W (that is, one-sixth of its original specification), the hum is much reduced, and the input sensitivity is in line with the output voltage of today's signal sources.

The final schematic:

Measurement results compare well to those of tube power amplifiers made by major brands and priced at up to $10,000, as measured by Stereophile. The modified Vellemn K4040 offers respectable measured performance typical of a classic tube design.

More photos (click for higher resolution):