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Old 6th February 2011, 08:44 AM   #1
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Default RC Oscillator

Dear All,

I am trying to tune an old oscillator I have for minimum distortion.

The model I have is this:

panasonic VP-7201A

Do some of you have a schematic of this thing ?

There are many trimmers (I am tempted to replace them with multi turn models). Some are obvious, like offset and output adj, duty adj and 15V adj.

Other are less obvious, like cds adj (That is the one that has the most dramatic effect) 500kHz adj (almost no effect) and a bunch of them in the multiplier selector.

I managed to get about 0.002 % THD. Do you think it's possible to get any better ? I saw that there are some opamp TL082. Do you think that replacing them with better one would help ?

In general I have a problem with signal generators, as my MAC has an output distortion around 0.01 %, so you cannot really make any measurement on SS amps. Good for tubes though.


Thanks,

Davide
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Old 7th February 2011, 10:32 PM   #2
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That's a pretty good distortion figure for that class of oscillator; improvements should be possible but it will probably be marginal. What figures do you get at 20 Hz and 20kHz?

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Old 8th February 2011, 02:25 AM   #3
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If you try to lower the distortion further it may quit oscillating.

If you need lower distortion (I wonder why), you can make a tracking filter or buy a better instrument.
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Old 8th February 2011, 08:38 AM   #4
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I am playing with the Firstwatt F5. You can tune this amp for minimal distortion to achieve 0.001 % or so. My assumption (Maybe faulty) is that I cannot go lower than the original signal distortion. If my generator hashiger distortion of the amp, what I see in output is the distortion of the generator. Am I correct ?

Thanks,

Davide
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Old 8th February 2011, 08:42 AM   #5
EUVL is offline EUVL  Europe
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Not if you use the distortion magnifier.

Patrick
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Old 9th February 2011, 12:18 PM   #6
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Quote:
Originally Posted by EUVL View Post
Not if you use the distortion magnifier.

Patrick
I'm getting there, I ordered linearaudio and it is on the way. I did not want to buy the kit without understanding what I was doing.

Thanks,

D.
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Old 10th February 2011, 12:55 PM   #7
cliffyk is offline cliffyk  United States
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That instrument is remarkably similar in outward and internal (circuitry) appearance to my Kikusui ORC-11.

Click the image to open in full size.

The Kikusui is rated for 0.002% "typical @ 1kHz", 0.005% from 10Hz to 20kHz, 0.01% from 20kHz to 50kHz, and 0.1% (or better) from 5Hz to 500kHz.

I have the service manual from this instrument and the best I have ever been able to calibrate it to at 1kHz (as measured via a Tek DA4084 analyzer) is 0.0023% unfiltered; adding a 80kHz low pass filter to eliminate noise brings it down to 0.0019%.

So, I would say you have calibrated it down to it's floor--good work!
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Old 10th February 2011, 03:53 PM   #8
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Can you possibly post the schematic?

Art
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Old 10th February 2011, 05:12 PM   #9
cliffyk is offline cliffyk  United States
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Quote:
Originally Posted by audiohead View Post
Can you possibly post the schematic?

Art
Please send me a PM...

-cliff-
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Old 11th February 2011, 10:48 PM   #10
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Thanks, this is really what I wanted to know.

D.
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