• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Project: Add a DAC to Pete Millett's low voltage hybrid tube amp

Hi everyone,
After a few weeks of lurking and learning here and at diyforums.org, I'm ready to undertake my very first DIY audio project. My basic idea is a small, low voltage tube hybrid amplifier that primarily is a headphone amplifier connected to my PC. Because time and money are not unlimited, I've also added some other features which I believe make it more versatile to me in the future.

Here are my must-haves:

  • The ability to power small speakers as well as headphones.
  • Have tubes. I just really like them.
  • Be low voltage; ideally be powered off of a wall wart.
  • No output transformers either; I don't like the way the transformers bulge from the chassis. They can also really eat up a budget.
  • Connect to a computer via USB. That is where all my music is.
  • Have computer mute functionality. I hate scrambling for the mouse when the boss comes over to talk.
Here are the optional features I'll add if I have time:

  • Have computer volume up and down functionality.
  • Have play/pause/stop/next/previous track functionality.
  • Have a subwoofer out for use with a powered sub.
I plan to borrow the basic amplifier layout from Pete Millett's "Hybrid amp with adjustable damping", and the "GrubDAC" USB computer interface from Erik Soosalu et. al. Because of this open source heritage, I will also be posting schematics and gerbers made in KiCad.

The existing GrubDAC uses TI's PCM2706C as the audio USB converter, but does not bring out the mute or volume up/down pins. I plan to add that functionality, and also the play/pause, stop, next, and previous track functionality if I can. See page 6 of the datasheet to see how changing the FSEL bit might mess things up.

Next, I plan to upgrade the Wolfson WM8524 DAC used in the GrubDAC with a better DAC if I can. I'd like your suggestions on an upgrade, as long as it has single-ended outputs. I want to keep things simple, and feel that adding a differential output DAC that requires dedicated opamps will complicate the design.

Regarding Pete's Hybrid Amplifier, I plan on keeping the basic amplification circuit. I'm up in the air about the adjustable damping section, mostly because I'm not sure what it does or if I need it. I will probably upgrade the OPA548 power opamp with one with better specifications from TI. Again, I'm looking for help from this community for that.

Finally, I think I'll replace Pete's power on/off delay with something simpler, possibly this circuit from the MOSFET-MAX originally designed by Pete Millett and modified by Colin Toole.

I look forward to your constructive ideas and suggestions. Please keep them civil and positive for everyone involved.

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