Computer Audio Asylum: Re: next step by John Swenson

Hi Thermio, great job, you starting to hear what can be had with digital done right.

Another interesting output stage which you might want to try is a current mirror. See the thread below with Monkey for some discussion on solid state versions, but there is no reason why you cannot do this with tubes!

It would take a high transconductance pentode such as a E810F or D3A. Put a 150 to 200 ohm resistor from cathode to ground, a plate resistor in the 1k to 2.5k range (the higher the value the greater the output voltage) and a variable grid bias. The cathode wants to be in the 3-3.5V range so a grid voltage of 1-1.5V works well to get you into the 20mA range. The exact value is not important as long as its greater than 10mA and less than what burns up the tube. An LED bias or 1.5v battery or SS regulator and pot would all work. The grid does NOT handle any AC signal so it doesn't need a specific impedance, its purely a voltage reference. You will probably need a grid stopper though.

The output from the DAC chip gets connected to the cathode (yes the cathode!) No other caps or resistors. Dac output pin, cathode and 175ohm resistor to ground.

I would put an OB2 or some such on the screen. The output voltage is taken from the plate, you could parafeed that to your 1544A and get very good sound.

If you build this be ready to have your jaw scrape the floor, it will be a significant improvement over what you have now.

Have fun!

John S.