D.I.Y. Tube Bass Preamp/DI #1

Email: th@thallenbeck.com.

After the massive worldwide success of the Sortaflex preamp, I wanted to try something more like a ‘desktop’ unit that would function both as a tube bass preamp and a DI box. It uses an AC input and a single 12AU7 tube. This one has no EQ, but I like the way it sounds. On the other hand, since there isn’t any passive EQ stage to reduce the gain like in the Sortaflex, this one’s output is probably too hot to be used as a DI, and I had to use the input pad when I plugged the pre-transformer unbalanced output into a bass amp.

For this one, I burned my own circuit board. It’s just a simple two-stage amplifier with a 1 MegOhm pot controlling the input to the second stage.

Here is an action-packed video of this thing:

This is the first time I’ve done an audio circuit with a ‘desktop’ layout instead of a pedalboard-type configuration. I was able to cannibalize a PCB, a power transformer, and a switch assembly from an older Sortaflex project, but there are a few things I didn’t think through too well, like the positioning of the LED. It’s running from a voltage divider ensconced on the rectifier board (square green one) way back by the power transformer. That’s fine for a pedal-type enclosure, but I wound up having to run the wires all the way up to the ‘front’. If I do any more of these, I might try working the rectifier section and the main section on to a single board.

Schematic, part layout, and image for etching the PCB:

If you look very, very closely at the photos, you might (or might not) notice that some of the resistor values aren’t the same as they’re marked on the schematic. I experimented with shorting R6 and R11, and finally settled on 470 Ohms. I got a stronger low-frequency response by increasing R13 from 220 kOhms to 1 MegOhm, which makes sense because R13 is in parallel with whatever the input impedance is of the output transformer (I still haven’t found that out). As I recall, everything else is the same, except that I forgot to wire up the unbalanced (1/4″) output after R14 before installing the board, so I wussed out and ran the unbalanced output from the junction of R13 and R14 because it was easier to reach.

From left, the controls are ‘pad’ (switch), 1/4″ input, LED, gain, XLR out, ground lift (switch, top), and unbalanced out (1/4″, bottom). The switch at the right looks like it has something to do with the 1/4″ output below it but it doesn’t – it’s connected to the XLR output. Something else I didn’t think through.

For more about the transformer, the ground wiring, and the components, see the Sortaflex project (scroll to the bottom). It’s mostly the same stuff except for the enclosure, which is a Hammond 1455B1601BK and costs a little more than the Hammond 1590D I used for the Sortaflex.

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