Tube modified CD player with TDA1543

[wa2ise]

I modified my old Magnavox CDB600 (really a Philips machine) CD player. It uses a TDA1543 "twin DAC" DAC chip, and it used to feed op-amp I/V converter circuits. But I found that this DAC chip seems quite happy to feed a resistive load for the I/V circuit. This DAC chip has, for each channel, a constant source current generator, and a varying (to the music) a sink current generator. The difference current between these two goes into/from the resistor load. It wants a bias of about 2.2V for this resistor load (going directly to ground with the resistor didn't work, sounds awful like severe clipping). This 2.2V bias needs to source or sink current. A conventional voltage regulator chip would only source current but not sink current, so that won't work. But an easy way to create this is to use a voltage divider network, 3k resistor from +5V, 2.4K to ground, to create a thevanin equivalent of a 1.3k resistor going to 2.2V. The DAC develops something like 1Vp-p of audio, which feeds a cathode follower triode grid. I used existing power supply voltages inside this machine to get about 40V for this tube (not a lot, but it seems happy).

Posted this over in "Digital" yesterday, but I figured tube people would be interested as well.