FusesJoin Our CommunitySubscribe to Paul's PostsI thought this would add some perspective as it comes from a very talented amp builder.February 29, 2016 by Paul McGowan
Here’s a subject that simply drives some people nuts. Fuses.
Change the power fuse in a DAC or preamp and the sound changes, depending on the type of fuse you change to.
I first became aware of fuses and their sonic differences in the 1980s. We were working on releasing the 200C power amplifier, designed by Bob Odell. This 200 watt per channel amplifier was the best sounding power amplifier we had ever produced and we labored long and hard polishing every part and decision to perfection. In those days we relied upon an output fuse to protect the loudspeaker and the amp from each other. Too much current passes through the amp and the fuse blows, disconnecting the power amp’s output.
The prototype amplifiers didn’t have output fuses. It wasn’t until we got to the production versions that we added them, and that’s when the trouble started. The production amplifier didn’t sound as good as the prototype: thinner, weaker, with less bloom and midbass strength, relative to the prototype. Why the two sounded so different was a real head scratcher.
When faced with such differences, you start removing any changes between the two until they sound the same. It didn’t take long before we discovered it was the damn output fuse. Short it with a clip lead and the fullness of the music returned.
This vexed us greatly because we wanted the sound of no fuse while enjoying the benefits of its protection. Different types of fuses sounded differently too. We gold plated the fuse and its holder to see if that would help. It did. But not a lot. We even tried bypassing it with a small capacitor. That helped to, but wasn’t a good idea. And neither solved the problem.
In the end we came up with a clever scheme. We took the feedback for the amplifier not from the amplifier’s output, but from the output of the fuse. Thus, the fuse was included in the amplifier’s corrective feedback loop, and the fullness returned to the music. (For those of you giving this some thought, we also added a 100Ω resistor in parallel with the fuse so if the fuse blew the amp would remain stable).
With the clarity of hindsight there are many explanations of why this mattered, damping factor changes not the least of them.
The point of the story is simple. Fuses matter. But why should they matter in the AC circuit? I don’t have a great answer handy. But we’ll look some more tomorrow.
All the best,
Nonoise