Ultra fast diodes

[lapsteelman]

I have been checking out some chipamp sites and I noticed some of the builders use ultra fast diodes in the power supply. I think this is supposed to reduce noise. (not 100 % sure). Is this just hype or is there something there? I ask because I have been building and tearing apart guitar and hifi amps for years and they all seem to use pretty generic diodes in the power supply.
I am toying with the idea of putting a switch in the power supply to go back and forth between the two styles to see what kind of difference it makes. Has anyone ever done anything like this?
I am making this amp for my steel guitar and I want a CLEAN, CLEAN, CLEAN sound. A good steel amp is alot like a hifi amp. (with reverb!)
Thanks, this is a great forum.

[J M Fahey]

Hi Frank:
short answer: pure Mojo/voodoo/you name it.
In fact, I have used fast diodes sometimes when foolishly I run out of regular 1N5402 and don't have time to go buy some more before the customer arrives.
No difference.
I have a few of them in stock because sometimes I make SMPS 12V powered amplifiers for street musicians.
*There* they are needed, of course, at 20 to 50 KHz; definitely not at 50/60 Hz.

[gbono]

http://www.ixys.com/product_portfolio/products_link.asp?framepage=ixyscolorado_contact.htm&framepage1=http://www.ixysrf.com/products/diodes.html

Probably start to see these in tube amp power supplies soon

[teemuk]

Fast diodes that have small reverse recovery times can indeed reduce switching noise but so do a few cheap 10 - 100 nF caps connected in parallel with the diodes, a proper layout and noding, or using toroidal power transformers with low leakage inductance.

Is noise really even an issue in your amp or are you just preset on the idea that you need fast diodes because they are part of the generic mojo in high end audio electronics scene? Most of the DIY hifi buffs use those just because they can but there really aren't any comprehensive statistics showing how much benefit you actually get from using them. Theoretically they can reduce rectifier's switching noise but in essence there are also plenty of other potential noise sources and a good chance is that a properly designed power supply is rather noise-free even with plain, slow rectifier diodes in use.

All in all, that fast stuff was really designed for applications in which diodes would fail if the reverse recovery times were too high - such as SMPS operating on very high frequencies. The benefits of fast reverse recovery in case of noise reduction are much smaller, sometimes perhaps even non-existent.

[lapsteelman]

Thanks for the info.

I had never had any problem before with regular diodes.
(Once when making a ham radio reciever I had some noise from the power supply but that was caused by RF getting into the rectifier, not an issue here).