Decided that I could work with the op-amp buried inside the DAC chip (PCM53-V) of an old Fisher Cd player. This is a DAC chip where you can't escape the op-amp. One DAC to feed into a pair of sample and holds to split the two channels apart. Hey, it's vintage 
Anyway... The summing node is avalaible but there is an internal feedback 10K resistor. But if I hook up a cathode follower stage etween the op-amp's output and then feedback the CF's cathode iutput back to the summing junction that would add some tube "flavor" to the audio. This new feedback path is another 10K resistor and I added a 22uF cap to block the DC offset the CF would create. The op-amp compensates to try to remove the tube's effect, as seen at the "virtual ground" aka summing junction. To a first order approximation, the total feedback signal is half of just the op-amp's output, and half of the CF's output. The CF's slight nonlinearity in the feedback path causes the tube "flavor" to be reflected at the op-amp's output. I tested this with a custom burned CDR with test tones, and captured a spectral plot (fisher-5906.gif)
showing that the 2nd harmonic about 48dB down, which is said to be "musical" . The tube used here, a 5906 submini pentode wired as a triode CF, has a 26V heater. This works out nicely as the CD player's power transformer provides 24V, close enough. Other than the transformer, this avoids loading the power supply. Schematic:
And of course what it looks like with a web cam:
The SMD's are the added 10K resistor and 22uF cap next to the DAC chip. I wanted to minimize the amount of stray capacitence hanging on the summing junction.
More at my web page of CD player mods
Anyway... The summing node is avalaible but there is an internal feedback 10K resistor. But if I hook up a cathode follower stage etween the op-amp's output and then feedback the CF's cathode iutput back to the summing junction that would add some tube "flavor" to the audio. This new feedback path is another 10K resistor and I added a 22uF cap to block the DC offset the CF would create. The op-amp compensates to try to remove the tube's effect, as seen at the "virtual ground" aka summing junction. To a first order approximation, the total feedback signal is half of just the op-amp's output, and half of the CF's output. The CF's slight nonlinearity in the feedback path causes the tube "flavor" to be reflected at the op-amp's output. I tested this with a custom burned CDR with test tones, and captured a spectral plot (fisher-5906.gif) An externally hosted image should be here but it was not working when we last tested it.
showing that the 2nd harmonic about 48dB down, which is said to be "musical"
. The tube used here, a 5906 submini pentode wired as a triode CF, has a 26V heater. This works out nicely as the CD player's power transformer provides 24V, close enough. Other than the transformer, this avoids loading the power supply. Schematic:An externally hosted image should be here but it was not working when we last tested it.
And of course what it looks like with a web cam:
An externally hosted image should be here but it was not working when we last tested it.
The SMD's are the added 10K resistor and 22uF cap next to the DAC chip. I wanted to minimize the amount of stray capacitence hanging on the summing junction.
More at my web page of CD player mods