Amp primer

The Tone Stack

The "tone stack" is the common name for the variable controls on an amplifier that control the frequency response. They usually have lables like : "Tone", "Bass", "Middle", "Treble". It is really part of the preamp but I decided to make a seperate section for it so that the "preamp" section wouldn't have too much information in it to digest at one time.

My goal in this section is to familiarize the reader with the basic terminology used when discussing tone stacks, and to give some examples of the typical ones that are used in our favorite amps via some cool links.

Passive vs. active

Most tone circuits in guitar amps are "passive", that is, the tonal shaping that is done by them is the result of attenuating certain frequencies in order to emphasize others. An "active" tone control circuit would be one that boosts certain frequencies to emphasize them.

Insertion loss

When passive tone controls are inserted into a circuit, there is a loss in signal level that occurs that is known as "insertion loss". Some tone circuits have more insertion loss than others, but they all have it to some degree. An amp circuit with a passive tone circuit will often have an extra gain stage to boost the signal up to where it would have been had there been no tone shaping circuit inserted.

Some cool tone-stack related links :

Adam's Amps Tone Stacks A good overview of a number of different classic tone stacks. I was going to write an article similar to Adam's but then I realized that it wouldn't be any better and might look like I plagarized his. That's why I decided to do this section with links.

Vox AC30 Topboost - Reasons & Origins Glen Lambert's informative article on the Vox AC30's Topboost circuit.

Explaination of Orange tone controls Article about the Baxandall tone stack that old Orange amps used.

Duncan's tone stack calculator The Tone Stack calculator is designed to help you check out the design and response curves of a variety of tone stacks used in popular guitar amplifiers.

This section is a work in progress and more content will be added as time permits.

Tube amps contain lethal voltages and should not be worked on by those unfamiliar with the proper safety precautions that should be taken when working on them.