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The Double-Diamond Amplifier (DDA)

Posted 13th June 2012 at 06:09 AM by rjm
Updated 16th June 2012 at 02:57 PM by rjm

This isn't my first attempt. It's been on my mind for a while: how to coax a diamond buffer into giving voltage gain, without resorting to fronting it with a op amp.

After reading a particularly gregarious thread over in the headphone forum, I'm more and more stoked on giving this a real shot.

Despite the (catchy) name I'm thinking pre-amplifier rather than amplifier applications.

update: I have have a quick and dirty sim up and running in ltspice. Curiously, the output distortion is 15 dB lower when the buffer runs open loop than when it is included inside the feedback loop. Intrigued. Currently under investigation.

update: refined the sim slightly, achieved -85 dB distortion levels at 0 dB / 1 kHz / 600 ohms running the output buffer open loop. Bandwidth is just under 1 MHz, adjusted by changing the feedback resistance. As before, performance sims out notably worse with the buffer
inside the feedback loop.

update: Distortion is far higher for time segments taken later in the simulation than it is for the first 10 ms. I don't know which result is closer to what might be expected for the real world performance. The behavior is independent of load, bias current, and feedback configuration.

update: LTSPICE file provided if anyone wants to play along at home.

update: Distortion issue traced to too few data points in the sampling window. Fix by manually setting the value of the maximum timestep variable in the tran simulation command. Take home message is that the original, low distortion FFT is the correct simulation result. (good, good...)

update: and.. we are back and can confirm -85 dB harmonics for 0 dB 1 kHz into 600 ohms.

Fun facts about this circuit:
  • Input impedance is 100 kohms.
  • Gain is 5.5 dB, adjustable 3-12 dB by changing R5.
  • Clipping at 4 V rms output.
  • Total current draw of 36 mA. Total circuit power <1 W.

The next step is to do up a layout and see if this power can be harnessed to real-world stability. Fortunately the bandwidth is not unworkably high, it's about 800 kHz, which should help keep oscillations and peaking under control.

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Attached Files
File Type: zip current feedback prototype 6 LTSPICE file.zip (1.4 KB, 236 views)
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Total Comments 2


  1. Old Comment
    Stee's Avatar


    this amp is inverting
    set R10/13 to have 450mV bias on Q15/16
    (output stage)
    drivers are BD139/140
    Posted 1st March 2014 at 03:33 PM by Stee Stee is offline
  2. Old Comment
    rjm's Avatar
    @Stee I'm not totally sure what you are aiming for with those revised circuits, but why not? Go for it!
    Posted 3rd March 2014 at 01:41 AM by rjm rjm is offline

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