While visiting my brother earlier this month I noted how he is SO into music but really doesn't have anything decent for play back. Because of his situation he listens mostly with headphones so I decided to build him a handy dandy headphone amplifier. But, I needed a box or case to put it in, and to make a few decisions on functionality.
First, the functionality aspect of the design. The basic circuit can be adapted to serve many functions being set up as two gain stages separated by a volume control. I decided to make it more flexible than a straight amplifier by providing three inputs, a selector switch, defeat-able line outs for driving an amp (or powered speakers), a power switch and pilot light. A little less functionality than the 66-001 preamp; no balance control or mono switch, and less input options. With that figured out I could go shopping for parts, scavenging whatever I could from my spare parts collection (ever expanding I might add).
For the casework, I knew that the power transformers would be on-board (unlike the 66-001 series of products which have external regulated supplies), so I would need to find a box big enough to house everything. We have a local PC recycle place that has all sorts of discarded electronics for relatively cheap (though lately they have put ridiculous prices on some real junk) where I found an old Avid 888, an 8 channel 48K Pro-Tools interface from the '90's for $15.00. Perfect! Just need to drill a few holes, fill in a few more and do a custom faceplate.
The distortion readings shown are at 1kHz. The top trace is the sweep and shows the frequency response, the black trace is THD, the rest of the traces are 2nd, 3rd, and higher harmonics (see legend at bottom of graph). The right channel shows slightly more distortion; this is due to differences in the tubes, which are never matched perfectly and are the biggest variable in designing tube circuits (modern passive components are pretty stable and precise in value). But the overall distortion in the right channel is at .321% THD, well below the theoretical 1% threshold of audibility.
So now everything is finished, tested and tweaked, ready for the faceplate. Below is an image of what that will look like, it's about a week out as of today.