Super Low Distortion Ultra Pure Audio Oscillators Revisited

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fredbloggstwo
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Re: Super Low Distortion Ultra Pure Audio Oscillators Revisited

Post by fredbloggstwo » Sun Jul 25, 2021 11:28 am

Hi again Wayne,

I tried the OPA 1656 chip in the notch filter and it works fine.

Here's some photos of the attenuator I built based on 'O' or 'Box' pads. I recommend this type of pad as it is 'ground free' and so does not have any grounding funnies. My fancy 'scope which also does Bode plots, shows that it is flat from d.c. up to 5MHz (-0.1dB and a fraction of a degree phase shift) and can't measure the insertion loss as it is so low. The case is lined with self adhesive copper plate except for the front panel which is done with 3M nickel screening sheet, and the interlinks are done with 0.4mm silver plated copper wire. Not a sign of any mains or RF pickup.

All pin 1s on the XLRs are star linked and floating with an option to connect them to the case screen on the back panel. The front panel legend is rudimentary but works for me. The leads coming off the back of the PCBs connect to 'finger' tracks between each pad to add to the screening.

A couple of questions for you. What is the "dBc" measurement that you use - that's a new one to me - what is the reference level?
Secondly, have you thought of doing a variable frequency version. A 15k/30k 1% wire wound pot with a gearing mechanism would give a good range variation in the first instance. Coupled with an output attenuator it might be a commercial opportunity there for you :)

IMG_0334.JPG
IMG_0333.JPG
IMG_0332.JPG
Cheers

Mike

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mediatechnology
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Re: Super Low Distortion Ultra Pure Audio Oscillators Revisited

Post by mediatechnology » Sun Jul 25, 2021 12:37 pm

Nice work Mike!

I would have used a floating attenuator if I didn't have the need for unbalanced outputs.

The ground on the one I have should be liftablle however since for balanced loads the net current in the ground is zero. That assumes perfect resistor matching so at the lower steps the error might be larger than unbalanced.

dBc is a ratio relative to the carrier level. If the measurement reads -120 dBu and the output level is +20 dBu the ratio is -140 dBc.

dBc is a little easier to follow with all those leading zeros in front of the percent sign. Somewhere below 0.000n% they become a blur and I have to count the decimal places. "Wait was that 4 or 5 leading zeros?"

I hadn't thought about a tunable one but do want to try 100 Hz and 10K fixed versions.
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Re: Super Low Distortion Ultra Pure Audio Oscillators Revisited

Post by mediatechnology » Wed Jul 28, 2021 5:04 pm

The Attenuator boards for ULDO-Nacho arrived:

Image

I used DC as a source and the DC dB capability of the Fluke 8050A meter to check attenuation accuracy prior to actually hooking it up to ULDO.

The attenuator had ground floating making it a fully-floating "O" pad and was loaded in 600Ω.
I calibrated the 0 dB step and found the step accuracy from about -40 dBu (DC) to 0 dBu to be about 0.07 dB
At -50 down it looked to be off by about 0.1 dB and at +10 dB it was 0.2 dB off.
-60 and -70 looked to be dead on but meter resolution is only 1 dB at that level and tenths aren't displayed.

I got the layout balance right...

I'm using ULDO's on-board 49R9 build-out resistors since there will be a short length of cable between it and the attenuator.
For that reason two resistors are linked.

Once I get it hooked up to ULDO I can better read the low-level accuracy.
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Re: Super Low Distortion Ultra Pure Audio Oscillators Revisited

Post by mediatechnology » Fri Jul 30, 2021 6:38 pm

Just got a new run of ULDO-Nacho boards in.

Assembly and Test Instructions for ULDO-Nacho PCB 20: https://proaudiodesignforum.com/images/ ... 1021_1.pdf

Image
ULDO-Nacho Ultra Low Distortion Oscillator and Notch Filter PC Board

The new boards have three improvements.

A second "Generator Monitor" Phoenix connector was added to keep from having to mult the output to both the load and notch filter input. The monitor ground connection is intended to be connected to a non-current-carrying shield.

I also added a 150KΩ + 10 pF series RC to ground at the VCA input to maintain an input impedance seen by the VCA to about 60K at HF.

D6 provides a clamp for the Ec voltage to about 600 mV to decrease the AGC loop recovery when the output is saturated or when J1 is opened for test.

I'll be putting these boards and boards plus THAT IC kits in stock in the next day or two.
THAT2180s are very hard to come by right now as are most ICs but I have some to bundle with boards.
The THAT340 and 1246 are available in stock at Mouser as well.

This is the current schematic for the oscillator portion of the board. The notch filter hasn't changed.

Image
ULDO Ultra Low Distortion Oscillator

Right click to reopen and enlarge.

I forgot to mention that the attenuator may be more accurate than what I measured. I didn't add 49.9Ω per leg to the DC power supply to simulate ULDO's build-out resistors.
The +10 dB position is likely to be more accurate than 0.2 dB.
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fredbloggstwo
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Re: Super Low Distortion Ultra Pure Audio Oscillators Revisited

Post by fredbloggstwo » Sat Jul 31, 2021 10:15 am

Nice one Wayne,

your attenuator board looks a lot better than mine :)

I'll add the osc mods to my board when I am next down the workshop. I also added a switch to the notch filter input to put the osc in circuit. Twisted pair seem to work ok.

Cheers

Mike

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Re: Super Low Distortion Ultra Pure Audio Oscillators Revisited

Post by mediatechnology » Sat Jul 31, 2021 10:40 am

fredbloggstwo wrote:
Sat Jul 31, 2021 10:15 am
Nice one Wayne,

your attenuator board looks a lot better than mine :)

I'll add the osc mods to my board when I am next down the workshop. I also added a switch to the notch filter input to put the osc in circuit. Twisted pair seem to work ok.

Cheers

Mike
Thanks Fred.
I'm intending to do the very same thing - add a switch to direct the notch filter input to the front panel connector or the oscillator monitor out.
Now that I have all the pieces I need to work on signal flow.

I'll soon be connecting that attenuator board to a 50+ year-old Grayhill switch. I'm curious to see what distortion 50 year-old contacts have. I probably have 10 or more 8 position 2 pole Grayhills I bought surplus years ago.

The RC network at the VCA input arose from my finding a passage in the THAT2180 datasheet that reads:
Stability

An additional consideration is stability: the internal op amp is intended for operation with source impedances of less than 60 kΩ at high frequencies. http://www.thatcorp.com/datashts/THAT_2 ... asheet.pdf (page 7)
Gary Hebert gave me the values for a 100K input resistor.
I hadn't seen any stability issues but it seemed like a good idea to include them anyway.
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Re: Super Low Distortion Ultra Pure Audio Oscillators Revisited

Post by mediatechnology » Sat Jul 31, 2021 1:44 pm

I just checked Atto's balanced attenuator error by reading directly in DC mV and then converting to dB. This time I had the build-out resistors included. The load was 597.5Ω.

+10 +0.054 dB
0 (calibration point)
-10 +0.03 dB
-20 +0.052 dB
-30 +0.084 dB
-40 +0.12 dB
-50 +0.157 dB
-60 +0.257 dB
-70 +0.37 dB
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Re: Super Low Distortion Ultra Pure Audio Oscillators Revisited

Post by mediatechnology » Tue Aug 10, 2021 9:04 am

Assembly and Test Instructions for ULDO-Nacho PCB 20: https://proaudiodesignforum.com/images/ ... 1021_1.pdf
https://ka-electronics.com

Directing Orson Welles: "What Is It That You Want?": https://waynekirkwood.com/content/Orson ... u_Want.mp3

Americas Frontline Doctors Lawsuit Motion For Preliminary Injunction Of Covid-19 Vaccine Emergency Use Authorization: https://waynekirkwood.com/images/pdf/Am ... zation.pdf
Americas Frontline Doctors Lawsuit Whistle Blower Declaration: https://waynekirkwood.com/images/pdf/Am ... ration.pdf

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Re: Super Low Distortion Ultra Pure Audio Oscillators Revisited

Post by mediatechnology » Fri Aug 13, 2021 4:14 pm

I added ULDO-Nacho to the KA Store for purchase: https://ka-electronics.com/shop/index.p ... ry&path=67

I'll be adding "ATTO" the attenuator soon.
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Directing Orson Welles: "What Is It That You Want?": https://waynekirkwood.com/content/Orson ... u_Want.mp3

Americas Frontline Doctors Lawsuit Motion For Preliminary Injunction Of Covid-19 Vaccine Emergency Use Authorization: https://waynekirkwood.com/images/pdf/Am ... zation.pdf
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