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Old 30th January 2013, 07:41 PM   #1641
Telstar is offline Telstar  Italy
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Quote:
Originally Posted by RNMarsh View Post
THD+N = -105db
About average these days. Have others in-house here that are equal. But, Thx- RNM
24bit should do better, theoretically. I've seen some multibit 18bit AD chips that do that figure (thinking to get one actually)...
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Old 30th January 2013, 07:51 PM   #1642
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Borrow some of the techniques used in the ShibaSoku AD725D.
I just had a brief look at the patent, but I'm pretty confident it too relies on an analog notch filter ahead of further processing. Whatever smart things we do later, the notch filter will limit performance.

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I spotted a few small boxes designed for EMI RF shielding which come with BNC connector at each end. The size is quite limited. Do you suppose your notch filter can squeezed onto a small very board? The boxes are expensive but I think well suited for this purpose.
I've designed the PCB for a specific case (Mouser 546-1457C801E). I'm not sure how much smaller you can get--quality caps/resistors simply have their size. You can replace the 3x 10x parallel caps with 3x single ones to save some space, but you should then match them to get best notch depth.

Samuel
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Old 30th January 2013, 07:59 PM   #1643
PChi is offline PChi  United Kingdom
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I have traced part of the ciruit of a ShibaSoku AD725C and it has 3 cascaded Bridged T Notch Filters. They each have 4 frequency ranges and the trim is with precison scaled (x2) resistors switched in using reed relays, lots of them. I haven't traced the frequency hetrodying part.
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Old 30th January 2013, 08:24 PM   #1644
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Quote:
Originally Posted by Samuel Groner View Post
I'm not sure how much smaller you can get--quality caps/resistors simply have their size. You can replace the 3x 10x parallel caps with 3x single ones to save some space, but you should then match them to get best notch depth.
Samuel
With the two trim technique I seem to be able to walk to a >70dB null even with just 2 single and one 2-parallel caps. I think the pots can be made a small enough portion of the resistor to not be an issue.
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Old 30th January 2013, 09:43 PM   #1645
1audio is offline 1audio  United States
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Originally Posted by Samuel Groner View Post
My sincere apologies for repeating myself every 20 pages or so in this thread, but quoting direct analog distortion measurements at the -150 dB level is simply not credible at the current state of the art (unless someone gives a detailed, repeatable proof how he verified measurement integrity to this level, which is pretty unlikely).
I am inclined to agree. While I have confidence in the -165 dB+ numbers I get from the CLT-1 that is a very special case and not broadly useful. I have two and have used them for two port devices (pots) but I would not extend that to amplifying devices at present. Maybe you can shed some light on how Weiss would verify the distortion numbers quoted for this amp module: http://www.weiss-highend.ch/op-amp/d...bipolar_r1.pdf . They seem too good to be true. For $130 ea. they could be a great value. That's close to the internal cost for the modules I designed for Nuforce.
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Old 30th January 2013, 10:57 PM   #1646
RNMarsh is offline RNMarsh  United States
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Originally Posted by scott wurcer View Post
The service manual is still available and it describes the operation of the fundamental notching in detail. Quite clever, they quadrature detect the residual fundamental and tune out both I and Q components. Maybe we are talking about different things, there was no A/D in 1985 that even remotely could do -100dB.

EDIT - I see, THD + N, I agree a useless metric.
Well that settles it... Then I conclude mine is broken in some way (notch not working?). Who knows whats wrong because A-P wont service it. Too old. Thus, I took it with me to Asia -- to have their A-P service people who will still work on old ones. meanwhile -----

Last edited by RNMarsh; 30th January 2013 at 11:00 PM.
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Old 30th January 2013, 11:31 PM   #1647
RNMarsh is offline RNMarsh  United States
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Originally Posted by Telstar View Post
24bit should do better, theoretically. I've seen some multibit 18bit AD chips that do that figure (thinking to get one actually)...
This is exactly what threw me off. I've been sitting on the side lines for years with the impression they are much better than implied. Surprise! Surprise! Surprise! It hasnt taken me too long to find this out and coming up to speed.

Now, I do have a passive Notch filter -- used it on 339A oscillator a few million pages back. It's the variable freq B&Kjaer Type 1607. BUT just like with the oscillators... I want to keep the testing time down and the use of equipment all to a minimum. So a super osc with really low thd and an active notch so that no calculations are needed and less over-all lash-ups to deal with. Trying to simplify now that i know what's going on and what is needed and why.

How good can an active Twin-T be for the project?

Thx-RNMarsh

Last edited by RNMarsh; 30th January 2013 at 11:36 PM.
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Old 30th January 2013, 11:39 PM   #1648
davada is offline davada  Canada
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Quote:
Originally Posted by Samuel Groner View Post
I just had a brief look at the patent, but I'm pretty confident it too relies on an analog notch filter ahead of further processing. Whatever smart things we do later, the notch filter will limit performance.



I've designed the PCB for a specific case (Mouser 546-1457C801E). I'm not sure how much smaller you can get--quality caps/resistors simply have their size. You can replace the 3x 10x parallel caps with 3x single ones to save some space, but you should then match them to get best notch depth.

Samuel
This much smaller.
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Old 30th January 2013, 11:49 PM   #1649
1audio is offline 1audio  United States
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I have several of those on my bench. They are difficult to work inside of. You could graft a filter into one using point to point wiring since there aren't many parts. It would need to be adjustable after assembly since drift is inevitable. I would use these http://www.vishay.com/docs/57014/6.pdf and really good trimmers. The Vishay foil ones do fit.
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Old 30th January 2013, 11:57 PM   #1650
richiem is offline richiem  United States
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My experience with the active twin-t is that 40dB of fundamental attenuation is enough, but I like to use 60dB to be sure. I'm at the point where I don't know if it's the active filter or the oscillators I have that are my limiting factors for spur amplitudes. I really want to know, but I need a 1kHz oscillator that I know has all spurs at least -150dBV in order to verify. If anyone can assure me that Victor's oscillator will do that, then I'm all over it.
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