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-   -   -290 dB Distortion? (https://www.diyaudio.com/forums/analog-line-level/338635-290-db-distortion.html)

hellokitty123 1st June 2019 02:04 PM

-290 dB Distortion?
 
Quote:

Originally Posted by Bonsai (https://www.diyaudio.com/forums/analog-line-level/337866-samuel-groners-super-opamp-post5799084.html#post5799084)
I agree that the -170dB THD is pie in the sky - LTspice fantasy.

If you ran A1 with a heavy load, the distortion would go up significantly, so I would say what this does is deliver the distortion performance of A1 when driving a high load so for a good opamp 1-2ppm. The ppb I mentioned is 'theoretical' of course.

You could tweak this design for improved performance - e.g. lower gain setting resistors (4562/797 easily drive 1k at ppm levels) and maybe run A1 inverting to remove CM distortion etc etc

I achieved -290 db distortion about a year ago using a design I came up with. The distortion doesn't change much with load until the parts are literally melting off the board.

How did I measure it? Well the design is a bit like an error correction system so I used an amplifier with a known -150db distortion and drove it out of spec until it reached to 0db distortion and then applied the EC. Using a QA401 and Cordell's distortion magnifier with quality oscillators I can get -150db measurement on the QA401 itself. Giving me a total measurement range of -300db. So I take the result of the post EC measurement and add it to the amplifiers original distortion number. If this is somehow an invalid testing method let me know.
Viewable results here
I got -290 dB Distortion

Edit: Hey mod, could you name the thread something a little less boasty and errogant? Kinda not my style. Especially since it's not confirmed yet. I kind of didn't want to advertise the fact either.

scott wurcer 1st June 2019 02:43 PM

Quote:

Originally Posted by hellokitty123 (https://www.diyaudio.com/forums/analog-line-level/338635-290-db-distortion-post5809311.html#post5809311)
I achieved -290 db distortion about a year ago using a design I came up with.

If this is somehow an invalid testing method let me know.

You are probably fooling yourself. Any pictures of the distortion spectra and noise floor that you are interpreting to be at this level?

Using Bob's distortion magnifier the noise floor is unlikely to be below 4 or 5nV so the equivalent BW of the measurement needed would be ~.0000006Hz or about 450 hours of data. This would also require some kind of synchronous averaging or auto-correlating instrument.

hellokitty123 1st June 2019 02:56 PM

I didn't take any pictures because it was being fed through the distortion magnifier in an odd set-up so it didn't mean much to anyone but me. I was over driving the amp by pushing the output voltage swing very close to the rails so there was distortion on every harmonic, mostly on the odd harmonics if I recall. I tweaked the gain until the distortion peaks hit 0db. Then I applied the EC and measured again to measure the improvement of linearity above 0db that the EC provided. This effectively gave me a -300db distortion measurement range. Noise floor is -130 +/- 5db on the QA401 without the distortion magnifier. I didn't really care about the noise floor measurement since I couldn't see it on the scope with a 100x magnifier so pretty inconsequential I think.

diyralf 1st June 2019 02:59 PM

Quote:

Originally Posted by hellokitty123 (https://www.diyaudio.com/forums/analog-line-level/338635-290-db-distortion-post5809311.html#post5809311)
I achieved -290 db distortion about a year ago using a design I came up with.

Any resistor has more noise. These are phantom measurements.

hellokitty123 1st June 2019 03:05 PM

Well if you read what I said I set the distortion of the main amp to 0db so I only needed to have -150db measurement capability to reach -300db effective measurement range. Of course in a practical scenario you will never hear anything below the noise floor.

syn08 1st June 2019 03:07 PM

Quote:

Originally Posted by hellokitty123 (https://www.diyaudio.com/forums/analog-line-level/338635-290-db-distortion-post5809311.html#post5809311)
I achieved -290 db distortion about a year ago using a design I came up with.

What??? -290dB distortion means a total of 3 femto volts harmonics to discriminate from noise. That means you either kept the DUT close to 0 Kelvin, or you spent years in auto correlating the signal from noise.

hellokitty123 1st June 2019 03:12 PM

I'm either really dumb or people aren't reading my post.
You don't need that much SNR capability to measure -150db.

jan.didden 1st June 2019 03:33 PM

I think what they are saying is that -290dB is a physical impossibility so that means your measurement method and/or conclusion is somehow not correct.

It's hard to understand exactly what you did from your description.
'I set the distortion to 0dB' what does that mean?

Jan

hellokitty123 1st June 2019 03:56 PM

Okay I'll try to explain it once more as clear as I can. I have a power amplifier that I know measures at about -150db distortion. I also have an EC system that I will apply to this amplifier at a later point.

So, let's call the -150db amplifier A1, and the EC circuit A2.

Step 1: I applied a load to A1 and increased the gain until the voltage swing at the output was close enough to the rails to put A1 into a non linear state, very very close to the point of clipping. I tweaked this gain until the harmonics on the distortion analyzer peaked at 0db.

So what we have right here now is an amplifier that originally had a distortion figure of -150db and has been overdriven to now have a 0db distortion figure.

Step 2: I applied A2 to A1 in this newly overdriven state and measured the distortion. Since A1 is now 0 db, any distortion I measure when A2 is attached to A1 is going to be the improvement that A2 provides to the entire system in this overdriven state.

In other words, If we measured an improvement of -140db and we now stop driving the amp out of spec, A1 will return to its original distortion figure of -150db and and A2 will provide an addition bonus performance of -140db making the total distortion performance equal to roughly -290db.

Hopefully that clears up any misunderstandings. I'd like to hear your opinions.

syn08 1st June 2019 04:14 PM

Quote:

Originally Posted by hellokitty123 (https://www.diyaudio.com/forums/analog-line-level/338635-290-db-distortion-post5809393.html#post5809393)
Hopefully that clears up any misunderstandings. I'd like to hear your opinions.

It did.

- To start with, a power amplifier with -150dB distortions is a big question mark by itself.

- Secondly, what is an "EC System" and how did you switch it in/out?

- Third, you need to understand that not all distortion types are correctable by a feedback loop. It all depends on the distortion type

- Fourth, you need to better understand the distortion reduction mechanisms. There is no LGM or daemon driving out distortions, it's all about negative feedback (in your case).

- Fifth, you need to better understand how distortions are added. It's not that simple as adding dB numbers

- Sixth, you need to better understand what an EC circuit does and what are it's limitations and cost (in terms of stability). Any EC circuit driving distortions down -150dB is highly suspicious.

I'll stop here...


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