Discrete Super Symmetric(I think) Opamp for I/V Etc...

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I am a complete amateur, but I am excited to show what I think is a very good amplifier. It is designed as an operational amplifier, and can be scaled upward and downward for both current and voltage. I opened this thread because I thought it might be educational to look at the design along with its challenges. I have learned a lot from such design threads, and I want to start some conversation on the approach.

I have been debating with myself if this circuit is super symmetrical or not. To be honest, I am not really sure. I would ask Nelson to please clarify for me if it is or not. :) I do believe it is fully symmetrical, and that the summed balanced output has significantly less distortion than each individual output when referenced to GND (though even that is quite low).

Let me explain the motivation for the circuit. There are two major reasons I designed it.

1) I wanted a discrete I/V for my balanced PCM1794 DAC currently going by "COD".

2) I wanted a fully symmetrical op amp which could swing high enough voltage to directly drive power amps such as Nelson's F4.

Here are some key features:

1) The output common mode voltage can be nulled to zero via the error amp section. R34/R35 are a 1K pot used to set the null point. The input common mode voltage is not present on the output.

2) The balanced output distortion should be very low and noise as well if suitable devices are used.

3) It should work equally well with current source or voltage source inputs.

4) It actually works. :) I have tested it.

5) Its scalable. You could easily produce a Class A or Class A/B power amplifier with the same general scheme.

So please Nelson or anyone please give me some feedback on the design and its potential merits and pitfalls.

I hope someone gets something useful out of it.



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Re: Re: Discrete Super Symmetric(I think) Opamp for I/V Etc...

Nelson Pass said:

If you apply symmetrical feedback to Q1 and Q2, then the
answer is yes.


Well that settles that. :) Thank you Nelson. Well in this respect then it clearly falls under your SuSy patent and I will give you due credit. :)

Now the most important question, does it seem to be any good? :cool:

I will be following this closely. There is a huge void in decent analog stages for these new damned differential out DAC's. IC opamps have been the only solution in the DIY world
Really only PS Audio as a discrete stage for the PCM179x and it isn't the best.

Thankyou for working on this.
Thanks folks.

I have been playing with the circuit today. I have tried a few things:

1) CFPs on the buffers instead of emitter followers.
2) Discrete darlingtons using BD139/MJL4281A(x 2) to scale it up to 100W output or so. :)

Some nagging issues I still need answers to.

1) Will the diamond buffer type output stage stay thermally stable? Or do I risk thermal runaway using BJT output devices.

2) Is there a more effective way to compensate the amp than I have chosen?

3) The error amp section is a bit tricky, especially as you scale the amp up. I think I have a decent solution that.

I will post some schematics/simulations soon.

Nelson Pass said:
Considering that you are looking at this for I/V, you might want
to consider JFETs for the gain devices. My experience is that
they are much more immune to digital noise.


Thanks, I had thought about that too, but was wondering if a JFET front end would limit my ability to scale the amp voltage swing upwards more than I want to. I suppose I could cascode them?

Russ, not sure if you've seen this. Check out this hread at headwize.com and this suport site. It's a discrete diamond buffer design with JFET input stage and BJT output stage developed by Ti Kan (AMB) and several DIYers. Maybe you can get some ideas from it for your I/V opamp.

Regarding your design, when you do the circuit board. I would provide pads to bipass the VAS stage and use it only as a buffer. This would be good to buffer loop outs or phono stages. Think about it.

simulation results of abover circuit

.step f=1000
Fourier components of V(out+,out-)
DC component:-3.27687e-010

Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 1.000e+03 4.000e+00 1.000e+00 -32.39° 0.00°
2 2.000e+03 1.342e-09 3.354e-10 95.94° 128.33°
3 3.000e+03 4.704e-07 1.176e-07 -99.82° -67.43°
4 4.000e+03 1.200e-09 3.000e-10 -112.95° -80.56°
5 5.000e+03 4.092e-09 1.023e-09 15.99° 48.38°
6 6.000e+03 1.012e-10 2.529e-11 125.34° 157.73°
7 7.000e+03 8.622e-09 2.156e-09 141.21° 173.60°
8 8.000e+03 7.298e-10 1.825e-10 -45.90° -13.51°
9 9.000e+03 7.516e-09 1.879e-09 79.39° 111.78°
Total Harmonic Distortion: 0.000012%

.step f=5000
Fourier components of V(out+,out-)
DC component:-6.20175e-010

Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 5.000e+03 4.000e+00 1.000e+00 -161.95° 0.00°
2 1.000e+04 6.723e-10 1.681e-10 -129.45° 32.50°
3 1.500e+04 4.544e-07 1.136e-07 -139.38° 22.57°
4 2.000e+04 8.950e-10 2.238e-10 4.50° 166.45°
5 2.500e+04 1.143e-08 2.857e-09 -64.66° 97.29°
6 3.000e+04 1.257e-09 3.142e-10 -147.05° 14.90°
7 3.500e+04 1.071e-08 2.678e-09 12.83° 174.78°
8 4.000e+04 1.754e-09 4.385e-10 3.39° 165.34°
9 4.500e+04 3.851e-09 9.628e-10 39.78° 201.74°
Total Harmonic Distortion: 0.000011%

.step f=10000
Fourier components of V(out+,out-)
DC component:-9.36479e-011

Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 1.000e+04 4.000e+00 1.000e+00 36.10° 0.00°
2 2.000e+04 1.998e-09 4.996e-10 -41.23° -77.33°
3 3.000e+04 4.448e-07 1.112e-07 80.07° 43.97°
4 4.000e+04 1.483e-09 3.707e-10 132.02° 95.92°
5 5.000e+04 1.637e-08 4.094e-09 -163.38° -199.47°
6 6.000e+04 5.747e-09 1.437e-09 -119.47° -155.57°
7 7.000e+04 2.426e-08 6.064e-09 -39.62° -75.71°
8 8.000e+04 7.066e-09 1.767e-09 -80.97° -117.07°
9 9.000e+04 4.998e-09 1.250e-09 -4.83° -40.93°
Total Harmonic Distortion: 0.000011%

.step f=20000
Fourier components of V(out+,out-)
DC component:1.97193e-009

Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 2.000e+04 4.000e+00 1.000e+00 72.20° 0.00°
2 4.000e+04 1.220e-08 3.051e-09 139.37° 67.17°
3 6.000e+04 4.752e-07 1.188e-07 155.16° 82.97°
4 8.000e+04 1.762e-08 4.406e-09 -87.29° -159.49°
5 1.000e+05 4.120e-08 1.030e-08 13.55° -58.65°
6 1.200e+05 3.689e-08 9.223e-09 72.96° 0.76°
7 1.400e+05 4.863e-08 1.216e-08 -167.45° -239.64°
8 1.600e+05 4.430e-08 1.108e-08 -145.67° -217.86°
9 1.800e+05 4.560e-08 1.140e-08 -47.79° -119.98°
Total Harmonic Distortion: 0.000012%
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