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Old 31st January 2013, 02:59 PM   #1671
Telstar is offline Telstar  Italy
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Quote:
Originally Posted by vicnic View Post
My previous measurement was about other optocoupler...
Like Samuel, I had seen worse measurements of the Silonex optocouplers.
-120dB is the noisefloor for my system, so a bit of 3rd above that is not a big issue.
OFC for an analyzer that aims for -140 it is, unless lower "values" can be used.
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Old 31st January 2013, 03:46 PM   #1672
vicnic is offline vicnic  Latvia
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Quote:
Originally Posted by Telstar View Post
Like Samuel, I had seen worse measurements of the Silonex optocouplers.
Maybe in little different conditions... My condition is only 17,7mV RMS (50mV p-p) on photoresistor.
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Old 31st January 2013, 05:36 PM   #1673
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by Samuel Groner View Post
Just to put some numbers on the trimmer issue I've re-measured 3 random samples of the Vishay metal foil 1248W parts.
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
Dont put words in my mouth. I clearly stated I was refering to R's (fixed resistors). And clearly said contacts on controls and switches were a biggy -- meaning a big issue for thd.

If you followed closely the 339A upgrades, pots and switches had to be changed and cleaned to get H2 and H3 down to the levels we now have. Also capacitors. Which BTW thd at the same level as Victor's.

Yes, we are and have been aware of the pot contacts for decades now. Demian has seen this also for as many years. These things show up in emplimenting amplifier circuits -- preamp and amps. Stuff not shown in SIM of circuit topologies.

This is an area John Curl has explored a lot and has used the best parts and controls that dont cause added harmonics. Its been a High-End issue for a long time. For myself, I said before, i can add to it my noise analyzer to find R's/pot etc that not only have the lowest thd but lowest noise - both.

The real goal: Once we get a system that can both measure and produce frequencies at -140, then we can develope audio circuits and finished product that measure what the SIM say they are capable of doing.

Thx-RNMarsh

Last edited by RNMarsh; 31st January 2013 at 05:44 PM. Reason: The reason why --
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Old 31st January 2013, 06:29 PM   #1674
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Quote:
Don't put words in my mouth. I clearly stated I was refering to Rs (fixed resistors).
I'm not putting anything in your mouth, I was referring to fixed Rs too. I have my reasons when I'm saying that you shouldn't trust resistors at the -140 dB level unless you've measured it yourself...

Samuel

Last edited by Samuel Groner; 31st January 2013 at 06:33 PM.
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Old 31st January 2013, 06:55 PM   #1675
davada is offline davada  Canada
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Quote:
Originally Posted by Samuel Groner View Post
I'm not putting anything in your mouth, I was referring to fixed Rs too. I have my reasons when I'm saying that you shouldn't trust resistors at the -140 dB level unless you've measured it yourself...

Samuel

Hi Samuel,

Do you have a list of preferred passive components on your web pages?
I've seen the one for cap types.
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Old 31st January 2013, 07:20 PM   #1676
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Quote:
Do you have a list of preferred passive components on your web pages?
Resistors are on the same page as capacitors, but the text is pretty dated. I'm working on a new revision, but progress is slow...

IMHO the key trick to get very low distortion from a resistor (or passives in general) is not so much about picking a special part, but to keep the AC voltage and AC power dissipation sufficiently low by using series (and possibly parallel) connections. With this approach, ordinary metal or thin film parts (I'm often using MiniMELF ones) seem to work well, and even in the sum they tend to cost less than many precision parts (bulk metal, wirewound). The so far open question is how many series/parallel combinations are necessary for a given total resistance value, AC voltage and distortion goal. I'm currently researching this.

Samuel
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Old 1st February 2013, 09:33 PM   #1677
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I am especially curious how this would work with capacitors for an integrator.
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Old 1st February 2013, 09:42 PM   #1678
davada is offline davada  Canada
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Originally Posted by myhrrhleine View Post
I am especially curious how this would work with capacitors for an integrator.
Do you mean in the way of charge distribution?
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Old 2nd February 2013, 08:29 AM   #1679
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I am especially curious how this would work with capacitors for an integrator.
Unfortunately series connections reduce the effectively available capacitance, so there are quickly practical limits at low frequencies.

Samuel
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Old 2nd February 2013, 07:19 PM   #1680
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Quote:
Originally Posted by Samuel Groner View Post
Unfortunately series connections reduce the effectively available capacitance, so there are quickly practical limits at low frequencies.

Samuel
Hi all,

I was just wondering if an SVF oscillator might have lower distortion with series / parallel capacitors?
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