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Sapphire Headphone Amplifier rev. 2.0

Posted 22nd November 2013 at 12:45 AM by rjm
Updated 5th January 2014 at 09:17 AM by rjm (update schematic to 20f4)

Update: I've ordered parts for small number of Sapphire 2.0 kits. The normal price will be $125, but as an introductory offer the first batch will be available for $100. Kit includes a set of boards and all the parts for the board. You need to supply the transformers and diodes, as well as a volume control, and the chassis hardware.

Update: boards are in stock, see photo.

Original here, diyaudio thread here.

November. That time of year for finally getting around to advancing some of my audio projects a little.

The Sapphire has remained in "rev 1+" for some time now, partly because of time constraints, partly because of the lack of popularity, and partly because it was already a re-spin of the beta version and worked just fine.

There were a few housekeeping things I wanted to add though, which have been included in the 2.0 revision.

- added a dedicated ground (GND) pad to connect to chassis
- added the ceramic bypass caps next to the transistors and op amp
- added the optional Zobel filter
- renumbered the parts to something more logical
- added option for more filter capacitance in the primary bank (now 3x1000uF per rail per channel)
- rearranged the diamond buffer transistors so the thermal coupling is between an output and its driver
- improved the heatsinking capacity and flexibility

The main change, though, is the addition of small bias resistors (main schematic R9 R10) that increase the bias current in the output transistors. Use a wire instead, and the circuit devolves back to the original diamond buffer.

I've tried various settings, both higher and lower bias currents, and settled on the middle ground at 28.5 mA, obtained with R9,10 at 4.7 ohms and R11,12 at 1 ohms.

The negative changes relative to the first board are that the resistor spacing is reduced to 10 mm from 12 mm, and the option for adding the buffer into the op amp feedback loop has been removed. (Only one person ever got that to work properly, and they said the open loop configuration sounded better anyway.)

The new boards will be drop-in replacements for the old s14 version.
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Attached Files
File Type: asc Sapphire 20.asc (3.0 KB, 320 views)
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Total Comments 3


  1. Old Comment
    jacques antoine's Avatar
    Hi RJM,

    Are new PCB boards available? Thnks.

    Posted 7th December 2013 at 02:59 PM by jacques antoine jacques antoine is offline
  2. Old Comment
    rjm's Avatar

    I expect them to arrive before Christmas.

    Posted 7th December 2013 at 11:02 PM by rjm rjm is offline
  3. Old Comment
    Hi RJM,

    Just wanted to point out that using these ltspice models from Cordell you can simulate your Sapphire buffers with BD139/BD140. The distorsions are a bit higher by the way and quite different with the third order becoming dominant. Here's a look:
    Click the image to open in full size.
    Posted 8th April 2014 at 08:46 AM by SunRa SunRa is offline

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