Low-distortion Audio-range Oscillator - Page 122 - diyAudio
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Old 18th December 2012, 06:01 PM   #1211
RNMarsh is offline RNMarsh  United States
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Thank you for the good timely info. yes, very old --- so old, i thought maybe something new has come up since way back when. Or at least some new insights garnered over the decades. I suspect that we can make amplifying ciruits better than we can measure them... or soon will be doing so If we cant over come these old issues with new methods/thinking. We'll look into alternative long channel fet parts that may be available now that werent back in the 339A development days. Thx moocho-RNmarsh

Last edited by RNMarsh; 18th December 2012 at 06:04 PM.
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Old 18th December 2012, 06:07 PM   #1212
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Originally Posted by RNMarsh View Post
Are there other jFET choices available today for lower H2?

Control fet --> push-pull compl jFEt's ?? (170/74) for cancelling the H2?

I can live with longer agc time constant/settling time if it helps.
There is a sweet spot for local degenerative feedback where all of the second can be canceled.
I believe it is equal to the internal source resistance.
Most people use a pot to trim the value while watching distortion on a meter.
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Old 18th December 2012, 06:14 PM   #1213
RNMarsh is offline RNMarsh  United States
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AN105 by Siliconix is a basic primer on the subject.

Myhrrhleine -- thanks... good idea -forgot about that one. Approx 1/transconduct. is a good place to start. Or a 50-100 ohm multiturn trimmer might do it. Thats easy to try and adjust other values if need afterwards for the oscillator. (just as SW did in his SW-OPA circuit for the input jFEts) Thx-RNmarsh

Last edited by RNMarsh; 18th December 2012 at 06:31 PM.
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Old 18th December 2012, 08:02 PM   #1214
davada is online now davada  Canada
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Originally Posted by scott wurcer View Post
The down side is that high Vp low Idss FET's are higher resistance and it's harder to make them be just a small portion of the feedback mechanism.

Would parallel Jfets of this type work to bring the R down?
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Old 18th December 2012, 08:58 PM   #1215
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by davada View Post
Rick what are getting for distortion at 1KHz 1Vrms off that oscillator of yours'?

Cheers,
without output opa changed yet and no external notch filter (all internal gains are as stock) -- here is what i get from the THD guru known as the 725; at 1V rms and 1KHz --->

H2 = .00008%
H3 = .00008%
H4 = .00005%
H5 = .00003%

THD+Noise = .00012-15-ish% or approx -117 dB

-RNMarsh

Last edited by RNMarsh; 18th December 2012 at 09:07 PM.
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Old 18th December 2012, 09:06 PM   #1216
RNMarsh is offline RNMarsh  United States
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Originally Posted by davada View Post
Would parallel Jfets of this type work to bring the R down?
parallel jfets and then series R to tune out H2?
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Old 18th December 2012, 09:25 PM   #1217
davada is online now davada  Canada
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Quote:
Originally Posted by RNMarsh View Post
without output opa changed yet and no external notch filter (all internal gains are as stock) -- here is what i get from the THD guru known as the 725; at 1V rms and 1KHz --->

H2 = .00008%
H3 = .00008%
H4 = .00005%
H5 = .00003%

THD+Noise = .00012-15-ish% or approx -117 dB

-RNMarsh
And this is straight off the oscillator I presume?

That's really not bad. See if LME49710 helps with that.

It's about the same is what I get. So what's up with Victor's oscillator?
Maybe it didn't like the power supply. Tis possible. I need a new one.

I found that a big C for oscillator buffer amp feedback cap reduced disto by another 2-3dB.

At the moment 0.1uF. It's big and a smaller one would probably due but it was the only poly
I had at the time.

So let's work on the fet thing.

Cheers,
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Last edited by davada; 18th December 2012 at 09:30 PM.
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Old 18th December 2012, 09:31 PM   #1218
RNMarsh is offline RNMarsh  United States
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Originally Posted by davada View Post
And this is straight off the oscillator I presume?

That's really not bad. See if LME49710 helps with that.

So let's work on the fet thing.

Cheers,
Its from the output binding posts on the front panel of the 339A !

yes, lets focus on that jfet.

Thx - RNMarsh
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Old 18th December 2012, 09:38 PM   #1219
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EXCELLENT, Richard!
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Old 18th December 2012, 09:45 PM   #1220
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Has anyone read the Siliconix app note
AN73-1? It is: 'FET's as Voltage-Controlled Resistors' Lots of examples of FET linearization. Anti-parallel, anti-series, feedback, ...
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