Tuesday, December 20, 2011

Two Stage Tube Phono Preamp

Tubecad, in a tutorial article, described a number of tube phono preamps. This is one of the simpler Tubecad designs and uses two 12AX7s in a two stage, passive RIAA equalisation, configuration. Golden Dragon tubes and quality tube rings are used here. The preamp employs a 24VAC wall wart which is step up to 260V and then well filtered. The HT filter is in two stages using a C-R-C-R-C network. The heater voltage is also rectified and filtered. There is a MM/MC switch to switch between 47K and 100 ohm loading. Gain is high.

All construction is on veroboard (strip board) and silver soldered. The four coupling capacitors are the Russian paper-in-oil (PIO) and the RIAA equalisation circuit uses Sprague “orange drops”. The enclosures are Hammond anodized Al cases. A separate power supply and substantial filtering helps keep noise out of the sensitive preamp cct. The sound is bright and very liquid. The preamp appears to work extremely well with pop, rock and jazz and most classical music.

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