I can almost remember the first time that I read about the 'old guys' taking a signal of 60Hz (probably 50Hz) from the mains transformer on a recording setup, and gate it into the mix!!!  The idea is that the xfmr provides what is essentially a convenient 'tank' of 60 cycles which, when triggered by the kick drum signal, adds a thick punch of power.

Fascinated by the concept for well over a decade, in late 2022 I built this version of the circuit using in-stock components as per usual.  It was designed, built, tested, and then shelved.  All in a period of about 2 weeks.  I've pulled it back out to do some documented testing of the unit. 

There are a couple of interesting sections to the design of this gate and I'll explain each section with drawings and finish up with some scope shots.  This type of circuit represents the absolute bare-bones of any sort of motor-controller circuit. This one is for audio signals though...let's get into it!

Power Supply Section

The power supply is about as simple as it gets.  The mains transformer (10V; 5VA; Class2) lacks a center tap, and I needed a dual supply, but for this circuit a half-wave rectifier will perform just fine.  I used 2x diodes and 2x filter caps then grounded one end of the secondary and let the other end of the secondary flip back and forth about ground.  The goal was to use a minimum of components.

The 60Hz signal voltage is taken from the secondary before the rectifier diodes.  The two resistors form a simple voltage divider halving the signal from 10VAC to 5VAC, and greatly increasing the output impedance preparing the signal for the Op amps coming later on.

The Gate Circuit

This is how I built the gate circuit.  It consists of a timer and a switch.

The pass element is constructed from 2x 2N5401 PNP transistors wired back-to-back with the bases connected together.  When the collector of the NPN switch transistor is 'closed', no base current can flow in the pass element and the gate is 'closed'.  Once the NPN transistor turns on, current can flow in both directions through the pass element.  Neat!

The timer starts when the audio signal pulls the base voltage of the current source transistor low enough to charge the timing capacitor.  This, again, is a very basic version of a gate circuit.  It works really well and uses a small number of components.  The only real downside of this type of gate is that the attack/charge time constants rely on each other.  For example, with a long release time very little charge current is needed to open the gate however if the release time is fast then a huge amount of charge current will be needed to flip the switch.

The Output Stage

(A good idea that worked..... yet still pointless....)

This project was going to use up the last of my nice commercial grade fiberglass protoboard blanks, so I figured I'd put a C.A.P.S. (constant amplitude phase shift) module right on the final output amplifier in order to maximize the build value.  I thought I'd be clever and make the circuit such that I could adjust the phase of the sample to that of the audio it will eventually be mixed back into.  

After getting  the circuitry to function beautifully, I realized that the phase of the tank will never end up correctly phased with multiple trigger hits in real program material!!!   A cool proof of concept, but utterly pointless in this application!

Scope Testing

Here are some screen shots from the oscilloscope during the function testing of the circuit...

-On the top in YELLOW is the trigger signal. 

-On the bottom in PINK is the 60Hz continuous wave from the transformer.

-The blue trace in the center is the gated 60Hz, and is now ready to be used as a triggered sample!

I just couldn't help myself but to run a little EDM (electronic dance music) into the input and see what it would do........INTRO...

...and with some of the regular program material...

It's pretty interesting to see how consistently the gate triggers and fires when the audio program material was also super consistent.  In the second instance, with much more dynamic program material, another really interesting gated output.  Maybe some day we'll even try listening to it...