Knoppson RIAA-Amplifier, KRA-92D
Data per mono unit [drawings]:
Rectifying tube: EB91 (Twin RF Diode)
Amplifying tube: ECC83 (Twin RF Triode)
Filter options: RIAA/flat
Gain options: MM/MC
MM output: 0,50V@1kHz/5mV
MC output: 0,25V@1kHz/0,5mV
Hum: <2mV/50Hz
While the internal filter is selectable by a switch,
these amps work for microphones too. This
concept is f.i used in my guitar amplifier
but may also be used in studios.
This is my fourth attempt in building a commersial RIAA-Amplifier. I have put lots of energy into making them look
nice. I really like the copper plate which has been both polished and coated to keep it's brilliance but most of
all I like the fact that it both uses Edison bias, a rectifying tube for the high voltage and that the heater is
AC-fed. The use of a rectifying tube is far from a disadvantage because it prevents high voltages to build up
across f.i capacitors before the amplifying tube is heated up. It is also quite simple to get very low hum with
the aid of simple RC-filtering. AC-heating of the amplifying tube is however a slight disadvantage with regard
to hum but it is remarkable how little hum that is actually generated at output with these simple methodes.
