Design description
Let’s start with background. The HPF-Pre uses design elements that are well known, but combined in a
novel way. Here are some online sources that deserve particular credit:
http://en.wikipedia.org/wiki/Butterworth_filter
: Design concept for active Butterworth filter
with equal resistors.
http://www.jensen-transformers.com/as/as098.pdf
: This is the “starting point” for my active
filter design, a nice 2-transistor source feedback preamp.
http://www.ciphersbyritter.com/RADELECT/PREJFET/JFETPRE.HTM
: More information
about simple discrete preamp circuits, including source feedback.
The preamp circuit is based on a classic active filter design, using discrete transistors instead of op amps.
While I have nothing against op amps, the discrete design turned out to be simpler and more efficient. With
half the transistors compared to op amps, current consumption is cut in half, and noise is reduced by 3 dB.
Also, a Class-A design with minimal open loop gain will exhibit asymmetrical soft clipping – a musically
forgiving overload response behavior that is shared with tube preamps. (Note that I am stopping short of
claiming “tube sound.”) Here is a schematic of the circuit.
Q1 is a classic JFET phase inverter circuit. Since negligible current flows through the gate of Q1, equal
currents flow through R3 and R4. The signals at the source and drain of Q1 must be of equal amplitude but
opposite phase. A SPDT switch selects one of these signals – it is the phase switch. C2 and R5 hold the DC
offset of these signals equal, so there is minimal "pop" when you flip the switch.
While C1 and R2 form a highpass filter, the cutoff is 0.5 Hz, not musically useful. I wanted a high value of
C1 to allow for the lowest possible noise with pickups that have high capacitance. R1 limits current into Q1
if the input is temporarily overloaded, thus providing a little bit of protection.
A shunt capacitor at the front end of an audio preamp is unorthodox. However, the circuit is designed for
piezo pickups. C2 provides some measure of protection for the gate of Q1, eating voltage spikes. Its low
capacitance means that it has a negligible effect on the response of capacitive pickups.
The second active stage is the classic Butterworth active filter design. (Compare to the Wikipedia
reference). Q2 and Q3 form a two-transistor source feedback amplifier – think of it as a crude op amp. The
