About a month ago I had an exchange with a reader talking about The Purpleheart Signal Meters and modifying the project to provide multiple inputs and some gain balancing. I’ve been giving this a lot of thought in the last couple of weeks because I have the same needs. So I think this is going to be my next major project.
This project will have some similarities with the Purpleheart Signal Meters but will support more functionality. This functionality will include six sets of inputs for lots of equipment versatility, buffered outputs capable of driving amplifiers with input impedances from the megohm range down to about 10kΩ (i.e. both vacuum tube and solid state amplifiers), a gain balancing control to adjust the selected input signal level to a desired output level, and, of course, signal meters showing the output level.
The physical form of this project will also be a little different from most of my projects. I want to put this unit in my office on the stand with my Cambridge AXC35 CD player. Because of this it will need to “stack” with other audio equipment. This means everything will be internal including power transformers, rectifier tube, gain adjustment tube, and output buffer. The base plate will tie into the wood chassis (as I typically do in my builds) but the top plate will be held in position with machine screws into threaded brackets. This will allow easy access for tube swaps and any other maintenance and trouble shooting.
I have already been playing with some gain and buffer stages. And I think I’ve decided on the rough internal layout as well. An interesting thing about this build is that it will have two power transformers. One Hammond GK88VG for all the tubes and signal circuitry and a separate 12VAC transformer for the signal meter driver board. The reason for this is simple, I already have both transformers on my parts shelf. If feels good to start a project with all the iron already in hand.
For the physical build, I’ve decided to closely match the footprint of my CD player which is the widest piece of equipment I currently have. The final unit (I’m still not sure what to call it) will be 17" wide, 3-1/2" tall, and 11-3/4" deep. Currently the 6V6 “Lacewood” amplifier is sitting on top of the CD player and this project will probably be added to that stack. Probably on the bottom.
While I’m working on the schematic and layouts, I need to find some time to dive into my stack of various hardwoods and fine a nice piece of woof for this chassis. I have some nice pieces of Bloodwood, Wenge, Bubinga, and Padauk I was considering for the 6AS7 “Purpleheart” so maybe I’ll use one of those. However, there are lots of other options in the shop including curly maple, walnut, mahogany, madrona, and apple. This choice may take some thinking.