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Old 31st December 2012, 06:15 PM   #1401
davada is offline davada  Canada
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Quote:
Originally Posted by vicnic View Post
Hi David,
I don't see other reasons for distortions increasing, as circuit at FET's gate. PV also has very big and nonlinear output resistance. I don't know, how big effect it makes - needs calculating or measuring. PV output resistance in series with FET's gate capacitance (also nonlinear).
Maybe you can shunt PV with capacitor (about 1000pF) for experiment...
Hi Victor,

Scott is going to investigate this. So I think I will leave it him.
I tried this out of curiosity.

Cheers,
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Old 31st December 2012, 09:21 PM   #1402
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Quote:
Originally Posted by davada View Post
Hi Victor,

Scott is going to investigate this. So I think I will leave it him.
I tried this out of curiosity.

Cheers,
Vivnic is right when I first proposed this there was a cap, mainly for non-linear C. Since the DC gate current is negligible the R does not figure in. I have interest in this so I will mock it up and make a measurement.
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Old 1st January 2013, 12:51 AM   #1403
davada is offline davada  Canada
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This one?
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Old 1st January 2013, 03:03 AM   #1404
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by davada View Post
I've done a few more mods to the 339a oscillator and to the balanced modulator board.

A3U1 was replaced with a 1468 in a earlier mod and R1 is set at 200 ohms.

Cheers,
A3U4 should be A4U4 and A3U2 should be A4U2 and A3U1 should be A4U1.
R1 refers to A4R1.

Last edited by RNMarsh; 1st January 2013 at 03:20 AM.
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Old 1st January 2013, 03:28 AM   #1405
davada is offline davada  Canada
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Originally Posted by RNMarsh View Post
A3U4 should be A4U4 and A3U2 should be A4U2 and A3U1 should be A4U1.
R1 refers to A4R1.
Quite so.

Thanks for pointing that out.

Cheers,
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Old 1st January 2013, 03:52 AM   #1406
davada is offline davada  Canada
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I have pare of those dual jfets Scott recommended.

I might give those a try.

They are very tiny don't know if my eye are good enough to solder them.
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Old 1st January 2013, 11:24 AM   #1407
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Originally Posted by davada View Post
This one?
Yes, the dual FET thing looked good but does not seem to help. Series/paralleling the FET's you have to get just the right Vp/Idss relationship looks better.
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Old 1st January 2013, 11:33 AM   #1408
Bonsai is offline Bonsai  Taiwan
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I see Glen Kleinschmidt has completed his audio THD analyzer with some remarkable resilts.

An Audio T.H.D. Analyser
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Old 1st January 2013, 05:03 PM   #1409
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I see Glen Kleinschmidt has completed his audio THD analyzer with some remarkable resilts.

An Audio T.H.D. Analyser

Pretty ambitious, I'm not sure I could finish this if I started.
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Old 1st January 2013, 07:05 PM   #1410
1audio is offline 1audio  United States
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Originally Posted by davada View Post
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.
I think you need to set up a bridge circuit to measure them. With a little tweaking you could see the time constant from any two resistance points on the lamp pretty easily. The other secondary effect of the lamps would be the reactances at high frequencies. I don't know how inductive a filament would be. A quick check of a 6S6 lamp (12V 6W) I got 100 nH at 100 KHz. However different manufacturers will be very different.
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