Samuel Groner's super opamp

Can you tell us more about the DAC?
Sigma Delta or Successive Approximation?

Nothing too innovative or exciting. It's largely a platform for evaluating different ideas.

The DIR is a WM8804, plus some active conditioning of the S/PDIF input signal prior to the DIR.

The DAC chip is a PCM1794A, which is a differential current output, 6-bit sigma-delta unit. Although, my design treats the differential outputs as single ended. Meaning, I take only one of the differential phases.

The I/V is passive via 15 ohm resistor, plus a band-limiting capacitor in parallel.

The first active analog stage (following the passive I/V) is a low-pass, multiple-feedback 2nd order Chebychev filter for final signal reconstruction. This stage also provides around +21dB of gain. The second analog active stage is a variable gain Baxandall style volume control. So, the DAC will have an integrated analog volume feature.

Both analog stages are based around implementations of the Goner/Polak composite super op-amp. Since each channel of the design require two op-amp stages, my implementation utilizes dual versions of both the OPA211 (OPA2211), and the THS4031 (THS4032) for compactness.
 
It could work, but probably wouldn't be nearly as good by the time you're done getting it stable.

In terms of getting it stable and ended up with a better result when stable would it be better to use a lower bandwidth opamp at the input/first position? Such as perhaps the AD823 (16 MHz low distortion FET) at the input and then use the LM4562 (55 MHz) for the output/second position?
 
I can't think of any reason why that would be advantageous - you can always get rid of the extra loop gain by putting more attenuation between the stages, and then you can make more effective zeros (better lead compensation) with C1 and C2.

The circuit of post #235 should work with an LM4562 when you change R3 to 18 kohm and R4 to 1 kohm, or at least it works in my pole-zero extractions.
 
I wanted to look into it halfway 2019, but never got further than a basic functional check of this OPA2134 circuit:

https://www.diyaudio.com/forums/ana...amuel-groners-super-opamp-24.html#post5819098
That was an interesting approach and I played with LTspice to extend the idea, target being to exploit the full Aol @ 20kHz for master and slave opamp, running at low noise gain. I also breadboarded it and as expected it didn't work that well... at least I managed it stop being an Oscillator at some point.

In the Bode plot, top to bottom @ 10Hz:
- composite open-loop gain (red)
- composite loop gain (green)
- master OpAmp Aol (brown)
- slave OpAmp Acl (cyan)
- noise gain (blue)

Btw, the sim was also double-checked with Tian loop gain probe and it showed little difference, basically only slight changes above 10Mhz.

Basically, if all is under full control and can be trimmed and extensively tested this might be a workable circuit for experimental purposes. Then again, low gain inverting isn't just that rewarding after all. Non-inverting is... if we manage to control CM distortion (eg by rail-shifting the master OpAmp with the signal).
 

Attachments

  • comp-bode.gif
    comp-bode.gif
    29.3 KB · Views: 457
  • comp-circuit.gif
    comp-circuit.gif
    11.2 KB · Views: 472
AX tech editor
Joined 2002
Paid Member
I've got some I don't need anymore. This is a very small PCB, and it takes a regular DIL08 spaced adapter to fit a DIL08 footprint. See attached.
Tiny components - you need good dexterity and a fine solder point.

Jan
 

Attachments

  • groner cct.pdf
    81.6 KB · Views: 209
  • groner pcb.PNG
    groner pcb.PNG
    15.3 KB · Views: 178
Last edited: