What's been keeping me so busy lately

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gbyleveldt

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Finally I've got the prototype discrete I/V working on the Sabre DAC that I've been messing with. I'm not going to be posting a schematic, as it may or may not end up in a commercial design that I may or may not pursue.

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But for those that are interested, here's what I've done:

- Fully symmetrical common base transistor amplifier stages (4 in total) to do the I/V Conversion - feeding into,
- Current mirrors in order to bring distortion down (there's also a trick to ensure that the DC operating point can reliably be set, as current mirrors and symmetrical stages are generally a no-no),
- The other side of the current mirrors also have common base connected transistors, manipulated by,
- DC Servo's. It should be noted that the output of the DC servo never connects to the input signal, so is technically also not in the signal path. This is the reason for the current mirror complexity as well.
- These antiphase signals are fed into a non-feedback Class A Voltage stage feeding into,
- A non-feedback diamond current buffer.
- For the SE stage, I made a discrete LTP stage feeding into another non-feedback Class A stage.

As it stands, it's still a little temperamental. It turns out that I'll be forced to use Vbe multipliers on the Class A stages in order to reliably set the current through these stages. This is what you get when you too lazy to match the almost 90 odd transistors being used.

Sure, it's probably overkill and there's many clever folk here that'll have better ways of doing it but all said I'm rather pleased with the result. I need to do more comparative listening to the opamp based I/V, but what I'm already finding is that the aggressive bite that sometimes popped up in the opamp I/V is now gone. There's also a good sense fluidity to the music, more so than what I had.

Anyway, after all the swearing I've done over the last few months to get this working I just had to brag a little  :tongue:. Next up, I need to get done with the Jung Super regs and then hopefully the plankie can actually go into a proper box in order to make it less of an eye sore. I also need to get this done so I can finally start messing with my new speakers.
 
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