Low-distortion Audio-range Oscillator - Page 134 - diyAudio
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Old 29th December 2012, 06:06 PM   #1331
davada is offline davada  Canada
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Quote:
Originally Posted by jackinnj View Post
I think he was referring to the now obsolete LT1088 == Linear's "thermopile on a chip". Jim Williams did an ap-note on True RMS and mentioned the device. Here's a link to the datasheet: http://cds.linear.com/docs/Datasheet/lt1088.pdf

Yes this is the one. I really disappointed this is out of production.
This thing can do 0 to infinity Hz.
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Old 29th December 2012, 06:29 PM   #1332
davada is offline davada  Canada
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Quote:
Originally Posted by scott wurcer View Post
True RMS detectors are plentiful? I tried a Tellabs precision wirewound thermistor designed to compensate KT/q in log amps in a chamber. Settling was hopeless.


Hi Scott,

What doesn't work with one oscillator type might work with another.

I have a multiplier along the lines of what Demian is suggesting. It doesn't work with a Bridged T but it works perfectly with a SVO. It doesn't work with a fast ALC because the tau of the lamp is too long. It doesn't respond fast enough. It is capable of stabilizing the SVO on it's own. All it really needs is an auto set level sort of thing. Some thing to just bring the level to a dc reference slowly but not so slow it is annoying.

Can you share some part numbers for these plentiful thermal RMS detectors?

Cheers,
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Old 29th December 2012, 06:48 PM   #1333
1audio is offline 1audio  United States
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For precision I would not recommend a thermal converter, unless you want to do down the vacuum thermocouple route Thermal Elements for Ballantine TVC. And that is way expensive. If the sine wave is clean enough (which it is in this case) peak detection will get you the most resolution easiest. Thermals are of course thermal. . . A good peak detector and a good voltage reference should get really good results. I would sense at the output terminals before the source Z setting resistor. Its more of a "leveling loop" like a microwave generator would have.
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Old 29th December 2012, 06:53 PM   #1334
davada is offline davada  Canada
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Quote:
Originally Posted by 1audio View Post
There is a positive temperature coefficient thermistor in an evacuated bulb that is very easily had in many values. Its called an incandescent light. The thermal inertia is essential to not add even order distortion as it changes with the waveform.

There are still issues of overall amplitude stability which my idea was to add a dc current to tweak the overall resistance to the target value that is controlled by a precision level detector.
Hi Demian,

It's taking me so long to get this project together I may as well share the work I've done on my incandescent lamp multiplier. I keep getting distracted with other things like modding the 339a and such. I can't etch PCBs indoors because of the corrosive fumes attacking my furnace ducting. It was -32C, -25.6F outside yesterday so you can see etching outside is out.

I'll write up a short summery of the multiplier and follow up with a more detailed paper with graphs. All I ask is for others to let me publish the first working oscillator using this proven multiplier design but everyone is welcome to experiment with this.

This is a working physical model.

More to come.

Cheers,
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Old 29th December 2012, 07:16 PM   #1335
RNMarsh is offline RNMarsh  United States
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Can you use an outside service to make the pcb? Like ExpressPCB or similar? -RNM

Last edited by RNMarsh; 29th December 2012 at 07:19 PM.
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Old 29th December 2012, 07:26 PM   #1336
davada is offline davada  Canada
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Originally Posted by RNMarsh View Post
Can you use an outside service to make the pcb? Like ExpressPCB or similar? -RNM

Hi Rick,

Yes suppose I can but I don't like doing this for development boards.
It seems like such a waist of money if I make a mistake.
There is an outfit in Taiwan that will do cheap boards but I've never tried them
and they take forever to deliver parts. I waited two months for some LT parts.
Half the cost though.

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Old 29th December 2012, 07:39 PM   #1337
dimitri is offline dimitri  United States
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Interestingly enough, but I prefer to do PCB for every project. 33each.com or miniboard service from ExpressPCB ($17 each)
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Old 29th December 2012, 07:55 PM   #1338
davada is offline davada  Canada
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Originally Posted by dimitri View Post
Interestingly enough, but I prefer to do PCB for every project. 33each.com or miniboard service from ExpressPCB ($17 each)
Hi Dimitri,

That is certainly approaching my cost of producing PCB. At that price it doesn't seem worth it to make my own.

Do you have to use their software or can you send the gerbers from your own preferred CAD?
Or can they process from the output files of most popular packages?
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Old 29th December 2012, 08:06 PM   #1339
dimitri is offline dimitri  United States
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David,
ExpressPCB will accept proprietary output format from their free soft, the board size 3.8 x 2.5 inches only,
33each - normal gerber files from your CAD, e.g. FreePCB; with 33each you are limited with max size 60 sq. inches - but this is pretty large PCB.
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Old 29th December 2012, 08:28 PM   #1340
1audio is offline 1audio  United States
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I like expressPCB a lot. Its not real cheap but the boards are good quality and its easy to share the source files. And plenty quick. They arrive just as you thought of the other changes you want to make. I would not use them for leading edge stuff (you can't anyway. . .).

However Canada shipping may make it lots less attractive.
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