active EQ

Here's the latest schematic for the 3-band active EQ.

As you can see, I have decided to move the effects send/return positions to "inside" the boundary of the tube circuitry.  I.e., the send comes after the first tube stage, and the return comes before the last two stages.

Originally, I had planned to locate the send/return outside of basically the entire circuit, i.e., "right next to" the bypass relay contacts.  This would have meant that the send was always-active, even when the relay was in bypass mode: generally, a preferable situation.  However, due to the higher signal levels of tube circuits, jacks placed in these positions would not have been directly compatible with normal line-level audio gear; an external interface unit would have been required, just like the "Dumbleator" unit which is required to use external effects with Dumble guitar amps.  It's easy to pad down the send signal, but the return signal needs active gain to bring it back up to "tube" levels, and then the issue of inverted phase comes into play.

The clean solution, without adding more active circuitry, is to give up the always-active send, and put the jacks as shown in the diagram above.  The send can be padded as desired, including with an adjustment pot as shown; the return has a fixed, open-loop gain when the plug is inserted (the NFB path to the Bax tone control is broken by the switching jack).  The send signal is thus not always-active; it is only active when the section is selected "in" and the LED is on.  When the section is bypassed, the send is muted, because the input of the EQ circuit is grounded.

Note that there is one fairly significant disadvantage to my current effects loop arrangement: the external effect sees an out-of-phase signal.  As long as the external unit is non-inverting, its output signal is then inverted at the return stage, and everything ends up in-phase.  There should be no audible difference.  However, for certain applications (such as using external test equipment like oscilloscopes), it might be inconvenient or confusing that the loop signal is inverted.

There are two identical copies of this EQ section within my amp, named "eq1" and "eq2": one before the distortion, one after.  This distortion-bracketed-by-double-EQ configuration is a crucial part of my sonic concept; along with the nature of the EQ itself, i.e., active-Bax to give the capability to boost as well as to cut, and shiftable mid frequency to permit production of precisely-controlled formants.