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.

Circuit

Circuit diagram

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