Resistors: What's The Difference?
by Mike Marsh
I am always asked why my amp kits come with Carbon Composition resistors . The answer is that I am trying to accommodate the marketplace and they want carbon comp. resistors. I guess it’s the notion that the vintage amps they grew up with have the tone they want to replicate in their amplifier DIY projects. So is there really a difference? Is there mojo in the Carbon Comp??? Well actually the answer is yes and no. When signals are high enough through them they will tend to contribute to the second order harmonics which are very musical and pleasing to the ears. At lower signal levels however they exhibit none of the pleasing attributes and only tend to exhibit higher noise levels and value drift than Metal Film resistors . Metal film resistors tend to be less expensive, are less noisy and exhibit less value drift than Carbon Composition resistors. Then we have Carbon Film resistors which are the cheapest resistors that have less value drift than Carbon Comp. but are almost as noisy. Last but not least we have to mention the Metal Oxide resistors . These are typically used as the higher wattage resistor used for power rail circuits as they are flameproof resistors.
With all this in mind, you can see that there is a way to optimize a project by using all of these resistor types using each in the specific place in the amplifiers design that make the most cost effective use of the resistor type’s attributes. Think about Metal Film as plate and cathode resistors in first and second stage preamp circuits. Use Carbon Compositions for the plate resistors where signal levels are likely to be higher in a third gain stage or driver circuit. Consider Carbon film in non signal voltage divider circuits only and Metal Oxide resistors for your power rail voltage distribution circuits.
Further to this when I’m asked the difference between Xicon Carbon Comp. resistors and Allen Bradley’s, I tell them the AB’s have longer leads and cost a lot more!!! That’s it!!!
The above is meant to be informational and based on my own humble opinion.