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Old 18th December 2007, 04:56 PM   #1
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Default Discrete Super Symmetric(I think) Opamp for I/V Etc...

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.

Cheers!
Russ
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File Type: pdf symmdrive.pdf (36.1 KB, 1363 views)
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Old 18th December 2007, 04:57 PM   #2
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Default Simulation files for those so inclined...

For LTSpice:
Attached Files
File Type: zip symmdrive.zip (5.6 KB, 200 views)
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Old 18th December 2007, 05:08 PM   #3
AndrewT is offline AndrewT  Scotland
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Default Re: Discrete Super Symmetric(I think) Opamp for I/V Etc...

Quote:
Originally posted by Russ White
I am a complete amateur, but I
show copyright on the schematic.
Just in case I want a feed a few lawyers to argue over whether someone has copied my creation?
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Old 18th December 2007, 05:54 PM   #4
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Default Re: Re: Discrete Super Symmetric(I think) Opamp for I/V Etc...

Quote:
Originally posted by AndrewT
show copyright on the schematic.
Just in case I want a feed a few lawyers to argue over whether someone has copied my creation?

I only put the notice to recognize my very real work. Nothing more. Honestly I have not invented anything to be sure.
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Old 18th December 2007, 06:42 PM   #5
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Default Re: Discrete Super Symmetric(I think) Opamp for I/V Etc...

Quote:
Originally posted by Russ White
I have been debating with myself if this circuit is super symmetrical or not.
If you apply symmetrical feedback to Q1 and Q2, then the
answer is yes.

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Old 18th December 2007, 06:53 PM   #6
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Default Re: Re: Discrete Super Symmetric(I think) Opamp for I/V Etc...

Quote:
Originally posted by Nelson Pass


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?

Cheers!
Russ
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Old 18th December 2007, 07:40 PM   #7
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Why don't you build it and find out?

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Old 18th December 2007, 07:43 PM   #8
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Quote:
Originally posted by Nelson Pass
Why don't you build it and find out?


I have already built a prototype but with different devices(2N5550 and 2N5401) and a slightly different error amp section it was very similar. It worked well, but I only did one channel as a sanity check.

I will build this circuit and report my results.

Thanks for your encouragement.
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Old 20th December 2007, 09:27 PM   #9
regal is offline regal  United States
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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.
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Old 20th December 2007, 10:14 PM   #10
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Looks nice, Russ
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