Low-distortion Audio-range Oscillator - Page 139 - diyAudio
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Old 30th December 2012, 06:14 PM   #1381
davada is offline davada  Canada
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Quote:
Originally Posted by Samuel Groner View Post
If the controller has proportional action (and it needs this to be fast settling) there is by definition no (significant) low pass filtering. So glitches surely contribute some distortion.

Samuel
Hi Samuel,

Yes of course. I stated that wrong. So what I intend is not by definition proportional.

Cheers,
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Old 30th December 2012, 06:22 PM   #1382
RNMarsh is offline RNMarsh  United States
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We are not bound by others' faster settling times. We arent using this on a production line. Longer settling times than the norm are OK. [Not minutes, though.]
-Thx RNMarsh
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Old 30th December 2012, 06:51 PM   #1383
davada is offline davada  Canada
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Quote:
Originally Posted by RNMarsh View Post
We are not bound by others' faster settling times. We arent using this on a production line. Longer settling times than the norm are OK. [Not minutes, though.]
-Thx RNMarsh
Well I was thinking more like hours.. No just kiding.

I agree this oscillator is not about fast settling just low distortion.
The settling time of the lamp is not annoying anyway.
100KHz is a bit trying but 10 - 20KHz is a couple of seconds and 1KHz down is less than a second. It might take a second or two more for an leveling circuit to bring it to a reference level. A fast AGC is the wrong thing for this. The lamps are simply to slow responding and cause oscillator squegging.

Another lamp might be just the thing. Does anybody know where I can find a low current miniature dual filament lamp?

One filament for heating and the other.....
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Old 30th December 2012, 06:57 PM   #1384
1audio is offline 1audio  United States
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Its an automotive taillight. Quite a few choices.
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Old 30th December 2012, 07:04 PM   #1385
davada is offline davada  Canada
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Quote:
Originally Posted by 1audio View Post
Its an automotive taillight. Quite a few choices.

Yes but the current is quite high. 2-3 Ampere.

I need the high heat to isolate from ambient T.

How about a lamp driven LDR assembly?
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Old 30th December 2012, 07:13 PM   #1386
1audio is offline 1audio  United States
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They aren't made any more but would be easy to fabricate. At low frequencies you will get flicker.
A sample and hold is not really overkill since they can be done with a few integrated parts. The low frequency limits your options. My ac calibrator dumps the s/h for the 100KHz to 1 MHz range but I have not seen an alternative for 20 Hz and below.
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Old 30th December 2012, 07:45 PM   #1387
davada is offline davada  Canada
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There is this.

Coupled with a lamp.

This can be connected to a Jfet input op amp.
The setup is the same as what Scott posted for the PV coupler.
Max current of 50uA and a resistor to set the V scale.

I have a 7220 lamps connected to an oscillator running at 10Hz and I don't see any flicker.
I know there is some there but it's not visible to the eye.
The ripple from a lamp is at fundamental. This will pass through an integrator after being compared to a reference.
The integrator will attenuate the ripple further.
This might work.
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Last edited by davada; 30th December 2012 at 07:53 PM.
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Old 30th December 2012, 08:42 PM   #1388
davada is offline davada  Canada
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I have the 7220 in series with a 100 ohm resistor.

Input to the series is a 600mVpp square wave.

The QA400 is connected to the vertical output of the scope.
The first shot is the output of the function generator and the second is the voltage across
the 100 ohm resistor in series with the lamp.

I've been trying to measure the tau of the lamp but no luck.

Also with a sine.The waveform is clean on the scope.
The ringing is from the QA400.
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Old 30th December 2012, 09:15 PM   #1389
davada is offline davada  Canada
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I received the PV part and assembled a circuit on some veroboard.
The 339a worked fine. No settling problems so the part is plenty fast enough.
Easy to set up and worked as predicted.

However I could not null the distortion to what I get without it.
Best I could do was about 8dB higher in 2nd H.
So I don't see any advantage with this approach.

But don't through it away it was a non ideal insertion for sure and this may have an influence on the performance.

Cheers,
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Old 30th December 2012, 09:56 PM   #1390
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Quote:
Originally Posted by davada View Post
I received the PV part and assembled a circuit on some veroboard.
The 339a worked fine. No settling problems so the part is plenty fast enough.
Easy to set up and worked as predicted.

However I could not null the distortion to what I get without it.
Best I could do was about 8dB higher in 2nd H.
So I don't see any advantage with this approach.

But don't through it away it was a non ideal insertion for sure and this may have an influence on the performance.

Cheers,
Surprised you can't null the seconds, do you have exactly what you did?
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