Super Low Distortion Ultra Pure Audio Oscillators Revisited

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mediatechnology
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Re: Super Low Distortion Ultra Pure Audio Oscillators Revisited

Post by mediatechnology »

A poster at diyaudio.com provided the new link for "flip-flop world"

The oscillator link is here: http://flip-flop.world.coocan.jp/audio/ ... index.html
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mediatechnology
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Re: Super Low Distortion Ultra Pure Audio Oscillators Revisited

Post by mediatechnology »

I just built a 10 kHz test version of ULDO-Nacho.

I used Mouser's 5% 1 nF polystyrene caps because I wanted to try them.
I didn't scale the tuning resistors: They remained 15K9 and 15K9/2.

The polystyrene caps I had are the radial version and the long leads allowed me to tack solder them.

The caps ran about 5% high in value so I had to pad the tuning resistors to move it from about 9500 Hz to 10K.
At 9500 Hz "Nacho," the notch filter, couldn't be pulled far enough to get a good null.
Once I got the oscillator moved to 10 kHz I could get a deep null.

I expected the distortion to be about 20 dB higher than the 1 kHz version owing to the fact that the op amp open loop gain falls off at 20 dB/decade.
That proved to be the case with distortion measuring, at +20 dBu output, around -123 dBc or just under 0.7 ppm with a 48 kHz measurement bandwidth. (H2-H4, Fs=96k)

With a 0 dBu output feeding the Focusrite 2i2 A/D, distortion of the A/D measures 0.0002%.
This is far, far better than the 2i2 measures in loopback.

I'm impressed with the 2i2 A/D's distortion at that frequency and pleased with ULDO-Nacho's 10 kHz performance.
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mediatechnology
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Re: Super Low Distortion Ultra Pure Audio Oscillators Revisited

Post by mediatechnology »

Here's' the FFT results of the 10 kHZ Uldo-Nacho and the Focusrite 2i2:

Image

The sample rate is 96 kHz, 24 bit. FFT is a 1M pt 10 sample average.
The output is 0 dBu loaded only by the notch filter and A/D.
The red trace is the notch filter output. The measured values do not reflect the +40 dB gain of the post-notch amp.
(The 30 kHz spur is actually -125 dBc/dBu.)

The hash in the notch filter output is due to long capacitor leads, wrapped construction of the polystyrene caps and lack of sufficient shielding.
I paid no attention to "outer-foil-to-lowest-impedance" on the caps and the board is only shielded by a sandwich of cardboard-clad grounded aluminum foil.

The calculated H2...H4 HD of ULDO-Nacho is -119 dBc relative to 0 dBu or just over 1 ppm.
With the oscillator running at higher level and using a pad to obtain 0 dBu I expect it to measure slightly lower.

The blue trace shows the Focusrite 2i2 (Gen 3) A/D HD at 0.0002% or about -113 dBc.
Note that the H2 is introduced by the A/D.
Overall ULDO-Nacho is about 6 dB "better" than the A/D which says a lot for both of them particularly the 2i2.

I've got some 1 nF 1% stacked polypropylene Wima's in my Mouser cart for my next 10 kHz build.
I'm also going to try 100 Hz.
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