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Old 11th October 2011, 02:43 PM   #61
klewis is offline klewis  United States
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Quote:
Originally Posted by Frex View Post
Hello Klewis,

Do you use symetric or asymetric inputs of the Juli@ ?


FRex.
Hi Frex,

I used the balanced inputs of the Juli@. The setup was HP8903A output (unbalanced), to Pete Millett's sound card adapter > balanced output from adapter > balanced input of the Juli@.

Some of the frequency roll off might be from the sound card adapater asymetric to balanced line drivers. The THAT corp data sheet only rates them to 20kHz.

Ken
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Old 13th December 2011, 06:15 AM   #62
richiem is offline richiem  United States
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Default Active Twin-T input levels

Hi folks -- just a quick update on the issue of signal levels into the Twin-T. I've needed a better sound card than the one that is built into my PC, which is bandwidth limited by its maximum 96kHz sampling and by its somewhat higher than good noise floor at -120dB referred to near full-scale input.

I have recently acquired an E-MU 0204 USB "sound card" and find its ADC to be a very good device -- 1V, 1kHz THD is well below 0.005% and the noise floor is around -130dB -- much beter than the on-board sound chip in my system. I'll have more accurate data soon. In any case, when used with the Active Twin-T, good harmonic product measurements can be made on fundamentals of 10kHz or more to well below -130dB using the 20dB post-filter gain of the Twin-T.

I'm still in the early stages of evaluation, but I can definitely say that limiting the input to the Twin-T to something around 1VRMS gives the best overall results. At 3V the amps in the Twin-T seem to be sticking their noses into the results. I hope to be able to say something more definitive about this soon and will post here and on my website.

I have also found some rather strange things with a borrowed Juli@ card in terms of audio bandwidth -- it appears that there is some kind of input filtering going on so that using a higher sampling frequency doesn't result in wider measurement bandwidth. In this regard, the EMU 0204 is clearly superior and gives better results if you want to see distortion products out beyond the 22kHz area. The EMU gives good response to about 90kHz with 192kHz sampling, consistent with Nyquist limit filtering.
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Old 3rd July 2012, 12:16 AM   #63
richiem is offline richiem  United States
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Update on the Active Twin-T and input levels -- four 9V batteries for power keep things clean with inputs up to 8VRMS or so, and with the nominal 1VRMS I usually use, the Active Twin-T appears not to add any measurable distortion. With my build of Bob Cordell's state-variable oscillator IG-18 #2, the BIG-18, configured for moderate output buffer gain, I'm resolving THDs of under 0.00004% at 1kHz, and can't prove that any of that comes from the Twin-T -- but can't disprove it either....
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Old 15th January 2013, 04:09 PM   #64
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Default My Implementation of Dick Moore's Active Twin-T

I have been trying to outfit my bench with useful test equipment on a budget. My latest quest is distortion measurement. After a lot of research I have decided to go with a PC based system and Dick's Active Twin-T Notch Filter.

This was a fun project. For the box I re-purposed an HP/Agilent 37203A HP-IB Extender. I found this HP device at a salvage store for cheap and is a common box used by HP to house many of their test equipments including my HP239 Low Distortion Oscillator. This box is a perfect match because I intend to the Twin-T filter with this HP oscillator. The box should also provide excellent shielding.

Because of the small front face plate I had to move the 23 position switches further back into the box using DIY extender rods.

I made no changes to Dick's circuit design. I did add test points to the front to allow me to verify battery voltage without opening the box. I am still looking for front panel knobs that are better suited for this application. But in the interim, these work.

The filter works great and I highly recommend it to anyone on the decision fence. Here are the pictures of my build:

.
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File Type: jpg DSCN0560.jpg (712.7 KB, 116 views)
File Type: jpg DSCN0563.jpg (857.1 KB, 125 views)
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Old 15th January 2013, 04:09 PM   #65
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Default My Implementation of Dick Moore's Active Twin-T

How does it work? I am still testing and getting acquainted with the Twin-T filter and my new PC based analyzer as a system. My first impression is very promising and I am very excited with my results.

Here are some measurements using my HP239 Oscillator as a source.

The first screen shot is a response curve of the notch at 1KHz using ARTA/STEPS and my E-MU204 sound card.

Note: I was able to achieve an amazingly low -90dB notch (as measured on other equipment). The STEPS software is not capable of showing the true depth of the notch, however, I feel confident in the accuracy of the notch from 0 to -10dB. You can see very little attenuation at 2Khz where the first harmonic exist. H2 attenuation would only be about -.01dB.

The next screen shot is using my new QuantAsylum QA400 USB Audio Analyzer, HP239 Oscillator and my Twin-T filter. The QA400 reports 0.0125% THD and taking into account the Twin-T -40dB notch, the actual THD of the oscillator is 0.000125%.

In the final screen shot I further verified the accuracy by calculating the sum of the first 9 harmonics at 0.00013%.

This is a learning process for me. If any of my analysis is incorrect, please set me straight.

Thanks for looking. I also want to thank Dick for all the personal help he provided me, and for sharing his work with everyone. His web site at moorepage.net is an excellent resource for this project as well as his many other useful projects.

.
Attached Images
File Type: jpg Notch Response.jpg (229.0 KB, 131 views)
File Type: jpg HP239 TO TWIN-T(40DB NULL) TO QA400 2.jpg (384.0 KB, 115 views)
File Type: jpg Untitled.jpg (148.0 KB, 106 views)

Last edited by dennismiller55; 15th January 2013 at 04:34 PM.
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Old 15th January 2013, 06:03 PM   #66
davada is offline davada  Canada
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Quote:
Originally Posted by dennismiller55 View Post
How does it work? I am still testing and getting acquainted with the Twin-T filter and my new PC based analyzer as a system. My first impression is very promising and I am very excited with my results.

Here are some measurements using my HP239 Oscillator as a source.

The first screen shot is a response curve of the notch at 1KHz using ARTA/STEPS and my E-MU204 sound card.

Note: I was able to achieve an amazingly low -90dB notch (as measured on other equipment). The STEPS software is not capable of showing the true depth of the notch, however, I feel confident in the accuracy of the notch from 0 to -10dB. You can see very little attenuation at 2Khz where the first harmonic exist. H2 attenuation would only be about -.01dB.

The next screen shot is using my new QuantAsylum QA400 USB Audio Analyzer, HP239 Oscillator and my Twin-T filter. The QA400 reports 0.0125% THD and taking into account the Twin-T -40dB notch, the actual THD of the oscillator is 0.000125%.

In the final screen shot I further verified the accuracy by calculating the sum of the first 9 harmonics at 0.00013%.

This is a learning process for me. If any of my analysis is incorrect, please set me straight.

Thanks for looking. I also want to thank Dick for all the personal help he provided me, and for sharing his work with everyone. His web site at moorepage.net is an excellent resource for this project as well as his many other useful projects.

.

Nice job Dennis.

I wish I had access to such junk stores as you but I live in the great white north.
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Old 16th January 2013, 05:14 AM   #67
RNMarsh is offline RNMarsh  United States
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Davada -- I was up there for awhile (B.C.) in May.... beautiful..on a trek to Alaska... but I am glad to be back where it is warmer most of the time. :-) -RNMarsh
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Old 16th January 2013, 01:11 PM   #68
davada is offline davada  Canada
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Originally Posted by RNMarsh View Post
Davada -- I was up there for awhile (B.C.) in May.... beautiful..on a trek to Alaska... but I am glad to be back where it is warmer most of the time. :-) -RNMarsh
If you traveled Alaska Hyw you might have gone through Fort St John.
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Old 16th January 2013, 06:09 PM   #69
RNMarsh is offline RNMarsh  United States
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Just bouncing off the coast all the way up to Alaska... by boat. My Apache lady friend with me... had driven that route a long time ago and it wasnt a good experience. Very hard on vehicles. She is hard core outdoors person and it was still hard. Pretty... but WAY too cold. -RNM

Last edited by RNMarsh; 16th January 2013 at 06:14 PM.
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Old 17th January 2013, 10:22 AM   #70
JPV is offline JPV  Belgium
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Quote:
Originally Posted by richiem View Post
Update on the Active Twin-T and input levels -- four 9V batteries for power keep things clean with inputs up to 8VRMS or so, and with the nominal 1VRMS I usually use, the Active Twin-T appears not to add any measurable distortion. With my build of Bob Cordell's state-variable oscillator IG-18 #2, the BIG-18, configured for moderate output buffer gain, I'm resolving THDs of under 0.00004% at 1kHz, and can't prove that any of that comes from the Twin-T -- but can't disprove it either....
Did you try to use Arta with a deep FFt buffer 132000. This will decrease the noise level in the bins and with the EMU card it gives huge headroom. No need for a twin T filter anymore in normal measurements.
See file with spectrum of tektronix 505 oscillator
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