DRM Direct Mixer Using an EF95/6AK5
This hybrid DRM receiver with a single valve and a single transistor
features good large-signal stability. The EP95 (US equivalent: 6AK5)
acts as a mixer, with the oscillator signal being injected via the
screen grid. The crystal oscillator is built around a single transistor.
The entire circuit operates from a 6-V supply. The receiver achieves a
signal-to-noise ratio of up to 24 dB for DRM signals. That means the
valve can hold its own against an NE612 IC mixer. The component values
shown in the schematic have been selected for the RTL2 DRM channel at
5990 kHz. That allows an inexpensive 6-MHz crystal to be used. The input
circuit is built using a fixed inductor. Two trimmer capacitors allow
the antenna matching to be optimized. The operating point is set by the
value of the cathode resistor. The grid bias and input impedance can be
increased by increasing the value of the cathode resistor. However, good
results can also be achieved with the cathode connected directly to
ground.
