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Old 26th September 2013, 09:42 PM   #181
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Default Bill of Material for Dick Moore's Active Twin-T

Does anyone have a complete Bill of Material for Dick Moore's Active Twin-T?
I can not read the schematic, my eyes are not what they used to be.
Please post the Bill of Material to this thread. That was we would have
a source for this information.

Dick Moore does not have a BOM on his WEB site, I asked.
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Old 27th September 2013, 12:21 AM   #182
DDB is offline DDB  United States
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Quote:
Originally Posted by mrsavage View Post
Does anyone have a complete Bill of Material for Dick Moore's Active Twin-T?
I can not read the schematic, my eyes are not what they used to be.
Please post the Bill of Material to this thread. That was we would have
a source for this information.

Dick Moore does not have a BOM on his WEB site, I asked.
This not a BOM but I believe it is a readable schematic of Dick'sTee.
Attached Images
File Type: jpg TwinTee.jpg (147.9 KB, 346 views)
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Old 27th September 2013, 03:33 AM   #183
richiem is offline richiem  United States
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Hi all -- Were I to build my Twin-T today, I would build it for two or three fixed frequencies with a small tuning range for best fit to gear under test. This type of construction will result in a much simpler and more economical build than my original, which covers the whole 20Hz to 20kHz range and uses a LOT of parts.

With two or three frequencies, one set of resistors and tuning pots can be used, with frequency switching done by switching capacitor sets -- this hugely simplifies the build.

If building for analyzing audio gear, I would choose 1kHz, 10kHz, and 20kHz for a three-freq build, and 1k and 20k for a two-freq build.

For those of you comfortable with building from scratch and using the simple design equations (the only kind I'm comfortable with), then my web page likely has everything you need to do a simple build. I never intended the Twin-T to be a kit-type project with all decisions made a priori -- you gotta work through some stuff.

Thx DDB for posting the schematic -- I'll look at it to see if I recognize it right away. Looks good to me. Thanks again. Can I use that one for my site?

Best,
Dick
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Last edited by richiem; 27th September 2013 at 03:37 AM.
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Old 27th September 2013, 03:50 AM   #184
DDB is offline DDB  United States
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Quote:
Can I use that one for my site?
Yes you can but check for accuracy as it was just batted it out when I had difficulty reading the other one.
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Old 9th October 2013, 09:13 PM   #185
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Does anyone know how important the distortion of the op-amps used in the filter are? I know Dick measured very little but curious if that varies significantly based on the chosen op-amp.
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Old 10th October 2013, 01:27 AM   #186
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Quote:
Originally Posted by chris719 View Post
Does anyone know how important the distortion of the op-amps used in the filter are? I know Dick measured very little but curious if that varies significantly based on the chosen op-amp.
Absolutely the op amp chosen matters for both distortion and noise. But more importantly the
input Z of the op amp as it loads the network.
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Old 10th October 2013, 06:37 PM   #187
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Looks like OPA827 or OPA1641 will be the best for U1 if it has to be a FET input. Can U2 be bipolar input?
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Old 10th October 2013, 08:08 PM   #188
davada is online now davada  Canada
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Quote:
Originally Posted by chris719 View Post
Looks like OPA827 or OPA1641 will be the best for U1 if it has to be a FET input. Can U2 be bipolar input?

I know Dick has tried a number of op amps. Check with him. I agree with your choices.
Dick favors the OPA134. The OPA1641 has lower noise.
The Positive feedback in the filter pushes the noise floor up so this is an area to be considered.
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Old 11th October 2013, 12:23 AM   #189
richiem is offline richiem  United States
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I like the OPA1641 very much for all amps, but the extra gain-bandwidth may not be a totally good thing -- nevertheless, the 1641 has worked well for me. I didn't see any less noise than with the OPA134, which remains a very good and very stable amp.
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Old 16th November 2013, 02:41 AM   #190
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I built a quick prototype of the Dick's notch filter with parts on hand. I did build it with a fixed frequency of 1K for this prototype and caps from the drawer. I know these are not the preferred caps, but they were the correct values. I plan to now build the real thing with a 1K setting and 10 K setting. I determined that if I use a switch to change the cap values, I can easily go between the two notch frequencies.

Below are some images. I attribute the noisy notch loop response to the bench supplies I am using to supply the + / - 9 Volts. Also, the op amp is an all purpose 1458 I think. It's what was on hand.

Only question, when the notch is in the circuit, I loose about 1.4 dB. Is this normal? My resistors around the op-amp section are close, but not exact to Dick's schematic.

Feedback is welcome.

Thanks! Dave

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