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Old 31st January 2013, 07:56 AM   #1661
1audio is offline 1audio  United States
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Quote:
Originally Posted by Samuel Groner View Post
1248W--larger versions probably have lower wiper contact resistance.

Samuel
I thought those were too expensive for what they are. I believe they are much more expensive than the larger ones. Many years ago a Vishay sale rep told be those were invented for hearing aid volume controls and had really low contact resistance variation (noise) but I could never get an update on that story.

I have used the 3/4" rectangular versions for many things. They seems to work really well. I could provide a sample or two if you want to try. PM me and let me know.
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Old 31st January 2013, 08:17 AM   #1662
RNMarsh is online now RNMarsh  United States
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If you use the same part type in each circuit, they will be cancelled as well. External to the output... layout et al is another story.

Was still thinking about app for further improving the 339A oscillator.

I personally dont have too much issue finding best passive parts. Worked that one to death already. Thx-RNM

Last edited by RNMarsh; 31st January 2013 at 08:22 AM.
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Old 31st January 2013, 08:39 AM   #1663
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Quote:
If you use the same part type in each circuit, they will be cancelled as well.
The feedback network in the error sensing amp does not have the same voltage swing across it as the one from the main amplifier, so it can impossibly sense, and thus cancel, the distortion.

Samuel
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Old 31st January 2013, 09:18 AM   #1664
RNMarsh is online now RNMarsh  United States
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Well, you need the oscillator thd a LOT lower before worrying about R thd. At least with the R's i would use. Still for the 339A. Victor's are so good already, even with sm parts, that a change in parts seems like the least of the thd issues with oscillators. Trimmer contacts are a biggy for controls and contacts from switches and the like. But just pure R of most thin film R's have thd that is extreamly low. I'm not worried about them just yet as being the limiting factor for the oscillator. Thx-RNMarsh
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Old 31st January 2013, 11:01 AM   #1665
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Just to put some numbers on the trimmer issue I've re-measured 3 random samples of the Vishay metal foil 1248W parts. The ones I have are 5 kOhm, and for the test setup they were wired as rheostat (variable resistor, two terminal) and set to ~1 kOhm. Distortion measurement was carried out with a resistor bridge (an improved version of the one shown in figure 2 here: http://www.linearaudio.net/images/le...lume1ltees.pdf) and +20 dBu across the trimmer. Observed distortion changed from specimen to specimen, with a varying mixture of 2nd and 3rd harmonic. Worst one was at -93 dB, best at -115 dB (individual harmonics, not THD).

Unless I've not considered everything or messed up the math in the hurry, this result suggests that to keep the trimmer distortion contribution negligible at the -140 dB level, the voltage across it would need to be below -30 dBu. That is for the rheostat directly in series with a fixed resistor, +20 dBu across the total resistance, and assuming that the 2nd harmonic falls proportional with fundamental level. This means that the achievable trimming range is less than 1%. If more is needed, lower fundamental levels must be chosen.

Comparison with other trimmers will have to wait...

Quote:
A change in parts seems like the least of the THD issues with oscillators.
Just saying: if you want to dive towards -140 dB I'd be careful with this sort of assumptions. Verifying everything and assuming nothing is the most promising approach in this territory.

Samuel

Last edited by Samuel Groner; 31st January 2013 at 11:05 AM.
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Old 31st January 2013, 11:34 AM   #1666
vicnic is offline vicnic  Latvia
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I am wondered. Silonex optocouplers NSL-32SR3 seems perfect. Received some days before , today measured. LED current 0.84mA, photoresistor resistance 156 ohm, 1kHz 50mV p-p signal across photoresistor:
Click the image to open in full size.
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Old 31st January 2013, 11:54 AM   #1667
Telstar is offline Telstar  Italy
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So they DON'T distort.
I guessed so (after I heard a preamp built with them) but I have seen previous measurements showing a lot of 2nd harmonic.
any (optic) light on this?
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Old 31st January 2013, 12:09 PM   #1668
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Quote:
Silonex optocouplers NSL-32SR3 seems perfect.
Can you repeat with the R set to ~1 kOhm? Under this condition, and similar level across it as you do, I've seen as much as -100 dB for this part.

Samuel
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Old 31st January 2013, 12:17 PM   #1669
vicnic is offline vicnic  Latvia
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Originally Posted by Telstar View Post
So they DON'T distort.
I guessed so (after I heard a preamp built with them) but I have seen previous measurements showing a lot of 2nd harmonic.
any (optic) light on this?
My previous measurement was about other optocoupler - Vactrol VTL5C7. They showed bad results.
You can see Silonex NSL-32SR3 data:
http://www.google.lv/url?sa=t&rct=j&...,d.bGE&cad=rja
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Old 31st January 2013, 01:11 PM   #1670
vicnic is offline vicnic  Latvia
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Originally Posted by Samuel Groner View Post
Can you repeat with the R set to ~1 kOhm? Under this condition, and similar level across it as you do, I've seen as much as -100 dB for this part.

Samuel
OK. Here is NSL-32SR3 photoresistor set to 1k. Results are no perfect as previous, but not bad:
Click the image to open in full size.
For comparison, here is 1k linear resistor in the same place:
Click the image to open in full size.

Victor.
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