I've been working out the circuitry of the 3-band v-mid EQ, of which there will be two copies in this amp (the pre and post EQs). I have been helped by several Internet "gurus" of tube-amp design, who I will credit in an upcoming post: there are a few excellent websites out there with tons of useful information for anyone working with tube audio. Also, many of the famous-brand guitar amps such as Fender, Marshall, Orange, Vox, etc., have complete documentation including schematics, publically available (either from the manufacturers or from contributed reverse-engineering efforts by enthusiasts). All of this has given me the confidence to design this amp, assembling the design from some directly borrowed circuits, and some created or heavily adapted by me. In the latter category is the active EQ.
The issue is that the typical guitar amp treble-middle-bass "tone stacks", as found in the "F+" and "M" gain modules in this amp, are not able to really produce a midrange boost. The passive tone stack circuit naturally produces a "mid-scooped" frequency response; twiddling the knobs changes the relative heights of the bass and treble peaks and the depth of the mid valley, but a basic scooped shape remains. These response curves are an essential part of the "Fender sound" and the "Marshall sound", which is why I preserve the T-M-B tone stacks in the gain modules. However, even amps with the scooped response in their electronics, will still ultimately tend to produce one or more peaks in the midrange. These peaks, known as "formants", are crucial to the voicing of the guitar tone. Various components in the signal chain, especially the speakers and cabinets, contribute to formant production, but in many cases these elements are hard to change; thus, a particular guitar and amp combination may have a distinct tonality, which persists despite any changes in control settings. If the tonality is a good one, then this is not necessarily a bad thing; but some of us wish for more tonal flexibility, for a single system which can produce a great number of varied tone colours, perhaps including both the "familiar favourites" and also electric guitar tones that have rarely been produced or heard in the past. To follow this latter path, for guitar, one quickly comes to desire variable midrange boost; and for this, active EQ is needed.
Studying the topology of various active EQ circuits on the Internet, both tube and opamp circuits, I've picked up quite a bit of useful information, though my own designs must be considered experimental and "naive" until I've done quite a bit more building and testing. So beware...
One major advantage of active EQs, beyond the ability to more easily produce a mid boost, is that the level controls naturally end up wanting to be linear pots, as opposed to audio taper, and the midpoint of the rotation ends up being the "flat" position. The basic characteristic of all the active EQ circuits, whether tube or solid-state/opamp, is that the level controls for each band select or "mix" between the input signal, and a negative feedback signal which is an out-of-phase image of the output signal. By contrast, the corresponding passive EQ circuits will have level pots which mix between the input signal, and ground. By considering the "virtual ground" principle, it can be seen that in the active EQ circuits, the midpoint of the level pot is where the circuit is in perfect balance, with the signal from the negative feedback taking whatever form it needs to, to completely cancel the input signal as seen at the wiper of the pot. If there were no RC frequency dependence, it would be a simple cancellation with the same amplitude of signal at opposite phase; but with the RC action in the loop, e.g. the output will contain more high frequency signal if the RC network removes some from the input signal.
So with my 3-band design, the three level controls (bass, mid, treble) will each have a center-flat position. The fourth knob will be mid frequency; now I am considering about a 20:1 range, of 200Hz - 4kHz. It will probably take some field testing to determine exactly what range is needed. The frequency knob will also have a pull-switch, "pull wide", which will widen the mid peak by changing one of the capacitors. I.e., in effect, a choice of two "Q" values, though both are pretty low.
I believe I have worked out a good way to implement this EQ using two 12AX7 tubes. There is nothing exactly like this on the Internet, but I've found enough confirmation for the various parts, that I think it will all work together as I am intending. The design starts from what's known as the "active Baxandall" circuit. This gives the bass and treble. Then, I found an elabouration which provided 3 fixed bands, bass-mid-treble. My contribution is to remove the capacitors from the midrange portion of this circuit, retaining the mid level pot and the fixed resistors at each end; thus, the overall resistance of the Baxandall circuit stays the same, which I hope will keep the bass and treble operating as they are supposed to. The mid level control becomes a plain pot, selecting between the in-phase and the out-of-phase sides of the Baxandall network. The output of the pot is buffered by another 12AX7 section, running as a cathode follower; this buffered signal drives a variable-frequency RC bandpass filter (Wien bridge circuit). The dual-ganged frequency pot changes the "R" value in two points of the circuit; the "pull wide" switch changes one of the two "C" values.
The output of the Wien bridge BPF passes through a summing resistor, and recombines with the output of the Baxandall circuit; this summing point is the grid of the output amplifier, which operates as an inverting amplifier, and its output becomes the drive for the out-of-phase side of the Baxandall circuit. Thus, as with opamp circuits, the summing point becomes a virtual ground, so the summing of the signals becomes "mathematically pure", and interaction between the signals is suppressed.
As I originally "borrowed" it, the active Baxandall and active fixed-freq 3-band EQ circuits were inverting, because they used a cathode follower at the input to drive the Bax circuit, and then the inverting-with-feedback stage on the output. I have already added another stage, the mid-driver cathode follower above. My additional changes are to add the available fourth stage as a cathode follower after the inverting stage at the output (i.e., a two-stage inverting amplifier, with better impedance characteristics). And finally, the input stage changes from cathode follower (gain of roughly +1), to an inverting stage with local negative feedback, producing a gain of -1. Thus, the overall circuit becomes non-inverting.
In my original design, I had planned to use a passive Baxandall circuit, and a buffered-but-passive midrange circuit, so overall there would be significant signal loss; thus, I planned to fit a gain-adjust trim pot at the input stage, to allow flat-gain to be trimmed to 1. However, with the fully-active (feedback-based) circuit described above, I believe the flat-gain will inherently equal 1, so no trimming should be needed.
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