Originally I planned on building an integrated amp and subwoofer, and I even built the enclosure. But a few things happened that made me rethink that plan… My living room layout and furniture changed, making me realize how large and impractical the size and location of the sub would be. The preamp I ended up with didn’t put out as much voltage swing as I originally expected. The subwoofer I intended on using fell off a 6ft shelf onto a concrete floor, bending the frame possibly beyond repair. And some testing indicated that the speaker/cabinet resonance was likely to cause mechanical feedback with the tubes.
Taking a step back and looking at the problem and the parts I had, I realized I wasn’t taking advantage of the most unique and special feature of my donor amp – it was a PA amp designed to drive speakers over long distances, allowing me to keep the amp with the control equipment and place the speaker wherever it works best.
Back to the drawing board, I came up with a new circuit design more closely related to what I did for the main channels. In order to do everything possible to keep 60hz hum under control I added a 1:1 stereo input transformer, with the two windings connected in reverse so they’d be in-phase but cancel any inductive hum, like a humbucker pickup on a guitar. After the first gain stage is a volume control, and after the second stage is an adjustable 2nd-order low pass filter that can be tuned between 50hz and 200hz. To compensate for the insertion loss of the low-pass filter I switched from a long-tail phase inverter to a gain stage + cathodyne arrangement. This is similar to the donor amp’s original topology, allowing me to reuse the negative feedback circuit. It inserts into the cathode of the last gain stage before the PI. The output and power sections are also directly pulled from the donor amp, but I added DC filament supply for the first two gain stages.
With the circuit worked out I started working on a layout that would let me build this chassis to match my 5-channel amp. They will sit side-by-side underneath the TV for maximum visual effect. That amp is 12.25″ deep by 18″ wide by 2.75″ tall, covered in African Padouk wood over a homemade steel chassis.