Wednesday, January 23, 2008

Passive Tone Control for Vacuum Tube Amps

This is a passive tone control circuit that I developed as part of a single-ended class A triode amp project I'm working on. I wanted bass and treble controls, but I didn't want to use a standard Baxandell circuit because doing so would violate the minimal-feedback philosophy of this project.

There are lots of designs for passive tone stacks on the web, but they typically have 20dB or more attenuation when flat, and I wanted a circuit with less than ten dB. I was willing to trade reduced boost and cut ranges for less attenuation, and the circuit below is the result. It features a loss of about eight dB, flatness better than +0.5dB from 20Hz to 20kHz with the controls centered, max boost of slightly less than five dB, max cut of about seven dB, and a very smooth and well behaved set of curves compared to most passive tone control designs.

(Click the image for a larger version)

This circuit requires linear taper pots, not logarithmic. Use high quality 1% tolerance components if possible. The source (driving) impedance should be 1k or less, and the load (shown as R5 in the schematic) should be 1M or higher. You can get away with a somewhat higher source or lower load impedance, but it will affect performance so try to stick close to my recommendations.

I designed this circuit by hand and then refined it using a great little free program called Tone Stack Calculator. Highly recommended, and although it doesn't claim to work under Windows XP I didn't have any problems. To make it easier to plug my design back into Tone Stack Calculator I kept the same part numbering convention. R4 is missing because it wasn't necessary (set it to 1 ohm to model the circuit), and RA and RB are added to control the response curves at max treble boost and cut. They're easily simulated in Tone Stack Calculator by setting R6 to 115k (100k + RA + RB), and only going from 7% to 93% on the treble control.

For a good review of tone controls check out this site, and for a nice technical discussion of passive tone stacks go here.

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