In electronics, the most fascinating constraint is that of reactance.  I have long wanted to build an EQ amplifier using one of those big, heavy, and expensive inductors revered in old text books. 

This is the 'Picture Frame' EQ.  It boats sliding HP and LP filters moving from full-spectrum to an LP -3bD at 1kHz and a HP filter that first cuts up 100 Hz then presses the power up into a nice resonant 'bloom' at 150 Hz.

The design is based entirely around the Hammond 156C inductor, at 150 H.  The somewhat bizarre design of the CF outputs on each of the internal amps was done to intentionally weaken the buffer between the stages.  Both the filter capacitor and the filter inductor voltages are controlled by the feedback of their respective amplifiers, but because of the CF outputs of both stages, some stresses are fed back through the entire amplifier.  The idea is to create a fluid LRC interaction.

Overall, this prototype has been pretty challenging.  The S.P.I.C.E. model allowed a critical error to slip through which I only found during the build, and my first attempt at an old school pen-and-tape circuit board was less than easy to do.  All that said, the circuit is working great!   Here's how it went...

The sketched schematic pictured above is a representation of how the circuit finally ended up working, but it all started with a S.P.I.C.E. simulation of a slightly different layout.  Once the simulation was running nicely it was time to start the prototype.  I happened to have mostly all of the necessary components in  stock, but still needed something to make a circuit board out of.  

Next up, transfer the grid from my paper layout to some masking tape on the copper foil...

Next, the entire schematic gets drawn, and the holes and traces get blacked out with a marker...

After the holes are drilled, it's helpful to thicken the lines which will leave bigger and more durable copper traces once the foil gets cut.  This is pretty important because the soldering process will melt the spray glue for sure.  Not much will stick to copper, and what's really nice with spray glue is that once you melt a trace off de-soldering some eventual mistake you can just press it back on and it'll stick again!

Once everything looks good, all of the excess copper is cut and peeled away using a razor.  This process is time-consuming, frustrating, and difficult.  You'll be scope testing a real circuit before your Gerber would have been ready to mill though!

I buried it in catalyzed black lacquer, then unmasked...and ready to go. 

Skipping forward to test #1:  After some difficulty fixing all of the 'hiccups' from the computer simulation during the initial construction, the scope testing shows a very nice base audio amplifier from which to work from.