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Old 16th August 2012, 02:22 PM   #231
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Quote:
Originally Posted by Terry McGee View Post
I didn't see much point in measuring harmonic distortion at 10kHz or so, as I was not going to hear even gross 2nd harmonic distortion at 20kHz, let alone 3rd at 30k. I'd be interested in comments on that strategy.

Terry
Yes, but if an amplifier exhibit some amplitude distortion at 10 kHz, it exhibit others distortions, such intermodulation, which are audible when you listen a more complex audio signal... You listen music, no sinewaves

Apologies for my bad english !
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Old 16th August 2012, 02:28 PM   #232
davada is offline davada  Canada
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Originally Posted by P.Lacombe View Post
Yes, but if an amplifier exhibit some amplitude distortion at 10 kHz, it exhibit others distortions, such intermodulation, which are audible when you listen a more complex audio signal... You listen music, no sinewaves

Apologies for my bad english !
If you think your English is bad, what till you hear my French.
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Old 16th August 2012, 03:43 PM   #233
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Originally Posted by DDB View Post
After studying the ebay pcb pix, I thought it appeared similar to a JanasCard inverting Wien bridge but with a jfet AGC. The seller said it was similar but used his own AGC jfet design with 1/2 D-S fed to G for second harmonic reduction.

The seller also indicated he was comtemplating posting the schematic here or on his ebay ad.

A couple of these with a good summer and output circuit could make a nice IM generator.
Looks like I snagged the last one on eBay U.S. So, this is the one to get then? I'm impressed. The SMD's look like they were done by hand. I hope he continues to produce them.
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Old 16th August 2012, 03:44 PM   #234
1audio is offline 1audio  United States
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I think a twin t notch specific to the oscillator would be great. I would like one that has a balanced input as well which means critical tuning of two filters. But, if the q is optimized and the notch depth is 40 dB then passing the output to a conventional analyzer will make everything easier to manage.

I started on this and got stymied when the spice sim did not work right. I need to figure it out before committing to a PCB (which would be a simple expresspcb design). I have attached the sim if anyone is interested in showing me my mistake. . .
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Old 16th August 2012, 05:38 PM   #235
dimitri is offline dimitri  United States
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Damian, wrong resistors ratio (pls change 45.2k to 11.3k)
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Old 16th August 2012, 05:44 PM   #236
TheGimp is offline TheGimp  United States
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Also the notch in the top filter is below 100hZ, divide all cap values by 10 and change 45.2K to 11.3K per dimitri.
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Old 16th August 2012, 07:19 PM   #237
davada is offline davada  Canada
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Originally Posted by 1audio View Post
I think a twin t notch specific to the oscillator would be great. I would like one that has a balanced input as well which means critical tuning of two filters. But, if the q is optimized and the notch depth is 40 dB then passing the output to a conventional analyzer will make everything easier to manage.

I started on this and got stymied when the spice sim did not work right. I need to figure it out before committing to a PCB (which would be a simple expresspcb design). I have attached the sim if anyone is interested in showing me my mistake. . .
Hi Demian,

It is difficult to tune Twin T's and keep them tuned. I can't imagine trying to keep two tune together. The notch pivots on the head of a pin. Can the balanced part be done a different way that doesn't require two filters? Take a look at what Dick Moore had to do with his to get the tuning stable.

Is the idea of balance for interface to balanced inputs/outputs or is there another reason that I have missed?

David.
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Old 16th August 2012, 07:51 PM   #238
DDB is offline DDB  United States
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Anyone planning to lay out the twin-tee filter? I'd hate to do that myself only to find out 237 other people have done it before me. And all of them better than mine. Terry
I have built a version of Dick's tee filter using seven discrete frequencies rather than continuously tuneable, 20Hz thru 100Khz. It works very well but has not been optimized as yet, may not be better than you might do.Click the image to open in full size.

An input buffer has been added, holes are provided to use it inverting, non-inverting or not at all. Since mine is constructed to use either batteries or a lab supply switched in, the board has .1 cog smd caps on IC power pins and 10u tant caps on power rails. I also matched all the caps within .1% so I used 10T 200 ohm pots to fine tune.

The zipped Eagle 6 .brd file was homebrewed, the red traces on top are jumper wires not a two sided board.

My calc says if .01u arm caps are used for 997hz, then 15.963K arm resistors are required. I used about 3K arm resistors for all frequencies but 20hz and matched caps accordingly.
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Old 16th August 2012, 11:04 PM   #239
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Originally Posted by Terry McGee View Post
My 1kHz oscillator from Victor came in today. So far, I've only had time to power it up and check it on Visual Analyser via the Sound Blaster card on my workshop desk computer. Came in at 0.002%, but I'd be confident that that is the soundcard talking. So now I need one of Dick's active twin-tee filters to give the soundcard a sporting chance. Or a much better soundcard. Or both. Sigh....

Anyone planning to lay out the twin-tee filter? I'd hate to do that myself only to find out 237 other people have done it before me. And all of them better than mine.
Terry
Just bought the parts yesterday for a twin-t filter. Have not laid out the pcb yet, but that will go fairly easy and quick. Got the aluminum enclosure picked out, too- for optimum shielding. I had purchased an oscillator from Victor this past Sunday, as soon as it came up for sale. Least I could do after he was kind enough to display his schematic on the www. I was going to wait to build my twin-t until I could first measure the actual output frequency and cater the twin-t to match as closely as possible.

You are certainly welcome to one of my pcb's, but I am no DaVinci when it comes to circuit boards (see post 180). If you could just cover the incremental cost...
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Old 17th August 2012, 12:33 AM   #240
1audio is offline 1audio  United States
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Originally Posted by davada View Post
Hi Demian,

It is difficult to tune Twin T's and keep them tuned. I can't imagine trying to keep two tune together. The notch pivots on the head of a pin. Can the balanced part be done a different way that doesn't require two filters? Take a look at what Dick Moore had to do with his to get the tuning stable.

Is the idea of balance for interface to balanced inputs/outputs or is there another reason that I have missed?

David.
Thanks for pointing out my problems with the Twin T. Sometime when you are too close you miss the obvious.

First a little background. I had custom frequencies made for testing ADC's. The 997 Hz (which came out 995 Hz for some reason) was selected to make sure that every bit gets exercised. If the frequency is an exact division of the sample rate it can hide some problems.

The balanced input is to avoid ground problems. Electronic differential amp in front limits you to the distortion performance of the input amp and it will not be as good. Having the filter positioned so the amp cannot degrade in front of the filter is essential. I don't know if a twin H could be made to work, but that's the reason for the simulation. In any case there are a number of parts that need to match very well which may be a problem. My goal was to play with the q and depth to try to get approx 40 dB depth for a 3% or 4% band so its less sensitive to drift and then use a conventional distortion analyzer on the output.

With distortion products at -130 dB or more all of this is more than 30 dB better than most audio products can hope to be.
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