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

[Russ White]

Pretty cool stuff!

Still there are some design features that this amp delivers...

1) It nullifies(very close to zero) common mode DC offset. This can actually be tweaked to bring to zero.
2) It features symmetrical folded cascodes which improves the open loop gain a lot.
3) I designed it myself.

Cheers!
Russ

 

[regal]

Pretty cool stuff!

Still there are some design features that this amp delivers...

1) It nullifies(very close to zero) common mode DC offset. This can actually be tweaked to bring to zero.
2) It features symmetrical folded cascodes which improves the open loop gain a lot.
3) I designed it myself.

Cheers!
Russ

#1 is the key. The key to finally reaping the benefits of the PCM179x Dac's. A discrete solution has not been publicly shared in the DIY world.

This is a big deal.

How do I get on the waiting list?

 

[ejaouen]

I am also on the waiting list :
discrete totally balanced (differential ins and outs) I/V stage for the COD dac (pcm1794) will be awesome I am sure !
Also is there a way (resistor value choice) in this circuit to select output voltage range ? I need 1 to 2 Vrms max balanced output range depending on the power of the following power amp (active 3 way system).
waiting for that I/V stage to happen.

 

[Russ White]

It is in the works. Will have something I am hoping to have it laid out this week.

I am laying out the first of two versions, both I have high hopes for (at least they simulate well even with very nasty loads). They are both folded cascode circuits, but one is quite a lot more complex.

Cheers!
Russ

 

[ejaouen]

Any news on this side of discrete active balanced I/V stages ?
Also do they include anti-image low pass filter solutions ?

thanks

 

[Russ White]

Any news on this side of discrete active balanced I/V stages ?
Also do they include anti-image low pass filter solutions ?

thanks

It definitely coming along.

Yes, the PCB will support optional low pass filter caps.

 

[ejaouen]

Thanks for the reply
Did you already hear some of your discrete active I/V prototypes (with the cod) ?
If so do you think this discrete way is going to be worth it over the ths4131 I/V solution ?

 

[PigletsDad]

My take on a supersymmetric discrete op-amp has been up at http://www.battletonphoenix.co.uk/linebal.pdf for some months.

Pros and cons:
1) My circuit does not have complementary symmetry. I reasoned that once you introduce one sort of symmetry, and cancel all the even harmonics, additional forms of symmetry offer minor benefits, but put the parts count up.
2) Limited output drive in my version - it doesn't have emitter follower stages.

I have some simulation results somewhere but not to hand. I will look them out to compare with Russ; I expect his circuit will have much lower distortion, but mine is low enough.

I suspect they are at different points on the complexity versus simulated performance trade-off curve.

 

[Russ White]

Thanks for the reply
Did you already hear some of your discrete active I/V prototypes (with the cod) ?
If so do you think this discrete way is going to be worth it over the ths4131 I/V solution ?

I have tested it on protoboard, it does work very well, it just a matter of finalizing the PCB.

It is very comparable to the THS4131(it is a very similar circuit) in terms of performance, and actually a bit better at driving low impedance loads. It actually makes a wonderful headphone amplifier even for 16-25ohm cans.

 

[Russ White]

My take on a supersymmetric discrete op-amp has been up at http://www.battletonphoenix.co.uk/linebal.pdf for some months.

Pros and cons:
1) My circuit does not have complementary symmetry. I reasoned that once you introduce one sort of symmetry, and cancel all the even harmonics, additional forms of symmetry offer minor benefits, but put the parts count up.
2) Limited output drive in my version - it doesn't have emitter follower stages.

I have some simulation results somewhere but not to hand. I will look them out to compare with Russ; I expect his circuit will have much lower distortion, but mine is low enough.

I suspect they are at different points on the complexity versus simulated performance trade-off curve.

That is a simple elegant circuit for sure. Nice work.

Mine is quite a lot different though in many respects.

In practice I personally have found my circuit not very useful without the output stages. But i have thought about several options here, including ICs such as NS new buffer chip, or one of TIs buffer chips. Wrap those in the feedback and you should have a great amp.

BTW, I probably will not use complementary symmetry, that was just a whim. In reality it does not yield enough benefit for the added complexity/cost.

I will post some details of the amp soon.

Cheers!
Russ

 

[PigletsDad]

I suppose I should add that I assume that Nelson's patent applies to my circuit as well, in those territories where the patent has been granted.

It was certainly designed with Nelson's ideas very firmly in mind, as the simplest discrete circuit I could think of that gave DC accuracy and the supersymmetry principle.

 

[Russ White]

Current circuit

Here is the circuit as I am laying it out now.

It has short circuit and SOA protection.

It is thermally stable thanks to the CFP output.

The distortion should be very very very low.

The PDF is of the simulation, reference designators and such are not finalized.

It is a simulation of the circuit in I/V mode with filter caps for a Fc of about 200khz.

The circuit as shown would work with PCm1794A and output 2VRMS peak output.

Cheers!
Russ

 

[Russ White]

here are the simulated THD number into the 300R/300pf load:

The first is the balanced THD, The second is the THD of a single output to GND.

Heightened Def Con from 0.000256019 to 0.000256019
Fourier components of V(out+,out-)
DC component:8.29446e-010

Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 2.000e+04 2.770e+00 1.000e+00 -5.68° 0.00°
2 4.000e+04 7.463e-10 2.694e-10 -20.80° -15.12°
3 6.000e+04 5.798e-08 2.093e-08 69.54° 75.23°
4 8.000e+04 7.130e-10 2.574e-10 -126.70° -121.01°
5 1.000e+05 1.230e-08 4.440e-09 149.26° 154.95°
6 1.200e+05 1.214e-09 4.382e-10 -63.77° -58.09°
7 1.400e+05 8.478e-09 3.061e-09 -50.92° -45.24°
8 1.600e+05 1.409e-09 5.087e-10 50.66° 56.34°
9 1.800e+05 3.645e-09 1.316e-09 -29.03° -23.35°
Total Harmonic Distortion: 0.000002%


Fourier components of V(out+)
DC component:-0.000220695

Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 2.000e+04 1.385e+00 1.000e+00 -5.68° 0.00°
2 4.000e+04 6.406e-07 4.626e-07 -15.68° -9.99°
3 6.000e+04 2.908e-08 2.100e-08 70.81° 76.49°
4 8.000e+04 1.410e-09 1.018e-09 139.62° 145.31°
5 1.000e+05 5.837e-09 4.215e-09 152.53° 158.21°
6 1.200e+05 1.175e-09 8.487e-10 -79.13° -73.44°
7 1.400e+05 4.244e-09 3.064e-09 -43.23° -37.55°
8 1.600e+05 1.138e-09 8.217e-10 14.40° 20.09°
9 1.800e+05 1.647e-09 1.190e-09 -40.27° -34.58°
Total Harmonic Distortion: 0.000046%

 

[Russ White]

simulation FFT

20 khz, same load

Green trace is balanced output.

Blue is SE.

 

[ejaouen]

Are these discrete balanced I/V (and LPF) stages going to be available soon ?
From your trials of these : are they worth it over THS4131 opamps IVY stages ?

I will place an order of COD modules and wonder if discrete balanced I/V stage is coming.
thanks in advance

 

[Russ White]

Are these discrete balanced I/V (and LPF) stages going to be available soon ?
From your trials of these : are they worth it over THS4131 opamps IVY stages ?

I will place an order of COD modules and wonder if discrete balanced I/V stage is coming.
thanks in advance

Hello,

Yes they are coming at some point. We just have a lot to do right now.

Measurement wise I doubt it will be much if any better than the THS4131, it may actually not be quite as good, but it will have more load current capacity, and it is discrete which will make some people happy.

I will have one measured and report when the project is finished.

Cheers!
Russ

 

[Blitz]

Hi Russ, very nice indeed...any progress on this one ?

 

[Russ White]

Thanks Blitz,

I am testing a proof of concept output stage which I plan on applying to this circuit as well. If that goes well then I will proceed with this amplifier.

Thanks for your interest.

Cheers!
Russ