three-band EQ with sweepable mid

As mentioned, one of the crucial components of my guitar amp design is a pair of identical EQs, one placed before the primary distortion-producing stages, and one placed after.  These EQs could have many forms; what is important is that they must have a well-defined "flat" position in the middle, from which they are able to boost or cut selected frequencies in a symmetrical manner.  Graphic EQs could certainly fit the bill, as could generalized parametric EQs.

The important thing is that it should be possible to establish a certain pre-emphasis curve going in to the distortion, to control the voicing of the distortion, and then apply the precise reciprocal of that curve on the output, so that the pre-emphasis applied to the distortion is cancelled afterwards, and therefore doesn't necessarily affect the final tonal balance of the output.  One clear example is if the pre-emphasis is very bass-heavy.  The distortion will take on a particular "sludgy" character, especially when low notes or chords are struck.  But then, the post EQ can be set to reciprocally cut the same bass frequencies, so that the final tone is bright and trebly; yet, it will still have that character of breaking up heavily on low notes and chords.  Of course, it's not necessary to set the post-EQ to the reciprocal curve of the pre-EQ: this is simply one tonal technique which is facilitated by having two identical EQs.

For various reasons, both technical and "emotional", I have decided to make this amp (and all of my amp projects) based 100% on tubes in the audio path.  Thus, full graphic EQs or generalized parametrics become a bit expensive in terms of tube count.  I have decided that the best EQ topology in terms of flexibility without excessive complexity, is three bands: fixed bass and treble, with sweepable-frequency midrange.  Each band has symmetric boost and cut, around a central flat position with the knob at 12:00.

I originally sought to design this EQ using only one twin-triode tube (12AX7), but the performance compromises required were, I think, too great.  Hence, I am now figuring on two tubes per EQ.  Given that, I believe I can rather readily take an existing solid-state design which is known to work, i.e., the channel EQ from the Carvin MX22 series mixers, and adapt it to tubes.  In this circuit, note that every opamp section has a grounded (+) input: therefore, they are simply inverting amplifiers, which are readily implemented with tube stages.  I believe I can at least get into the "ballpark" by simply multiplying all resistor values by 10, and dividing the capacitor values by 10.

project update as of April 2026

It's been a while since I posted in this blog.  Most of my work in recent years has been on the McGrath Mini amp, which has its own blog.

The work I started here has morphed considerably since I last posted.  The first product I plan to build, which will embody the circuit ideas developed here, will be called the McGrath Stereo Amp.  This will be, of course, a stereo tube guitar amp, in combo format, with two (probably) 8-inch or 10-inch speakers (maybe both, as options).  Power will probably consist of 4 x EL84, one push-pull pair per channel, cathode-biased.

The preamp will contain sections developed in the McGrath Mini project, but with generally more features and capabilities.  In particular, there will be dual three-band sweepable-mid EQs, one pre-distortion and one post.  This symmetrical pre and post EQ topology is one of my fundamental design concepts.

There may be a stereo spring reverb, i.e., separate tank for L and R channels (another of my design concepts).  However, this may be an optional component of the power amp section, with the power amp also available with no reverb.

The conversion from mono to stereo will take place via the fx loop.  There will be one mono send (perhaps with two isolated jacks for convenience), and then a stereo pair of return jacks.  Thus, when stereo effects are used such as reverb or delay, the signal will be transformed into stereo.  If no effects are enabled, and if the stereo spring reverb is bypassed or not present, then the amp will simply behave as a twin-speaker mono amp.

The electronics of the amp will be housed in at least two separate 19" rack units, which the cabinet will be designed to contain.  The preamp will be a 2U rack unit, and the power amp and optional spring reverb will be a 1U unit.  There may be one additional 1U space provided, for effects units of the user's choice.

This is the broad overview of where my conception of this project has evolved to, in the intervening years since I last posted here.  I will probably branch this work off into a new blog, specifically titled to reference the McGrath Stereo amp, when I actually begin work on that project in earnest.  For now, I am still mainly engaged with completing the McGrath Mini project.  In fact, I'm about to ship my first unit to a paying customer!