The Polybius preamp is our introductory product, offering a linestage preamp with remote control capabilities as well as a headphone output capable of driving high impedance headphones with a budget focused price point.

The circuit is quite simple with a 6J6/E90CC dual triode used as a grounded cathode amplifier feeding a pair of EL34 pentodes wired as triodes serving as cathode followers.  The EL34s run a substantial amount of current at relatively low plate voltage to drive up transconductance which keeps the output impedance very low at 100 ohms.  This allows the Polybius to drive headphones that are 150 Ohms or greater.  A constant current source load is used for both stages.  For the first stage, it is very handy to allow us to not need an excessive high voltage power supply to properly load the 6J6, and in the output stage we are able to have a small amount of cathode bias voltage and run lots of cathode current without a low value cathode bias resistor sucking down most of the available signal current.   The low output stage cathode voltage also permits us to use premium audio grade electrolytic capacitors in the output coupling position. 

Bandwidth: -3dB at 10Hz and 67kHz (worst case volume control position)

Output Impedance: 100 Ohms (EL34, higher with 6L6 equivalents)

Distortion: Less than 2% at 2V out with 1V in

Maximum Output: 7V RMS (1V RMS of source voltage required for these conditions)

Gain: 10.5dB