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Old 30th March 2012, 08:05 PM   #1
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Default 2 SECTION AIR VARIABLE CAPACITOR

anyone know of a reason i couldn't use a 2 SECTION AIR VARIABLE CAPACITOR for cartage loading?

i want to be able to change the loading from the front panel.
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Old 30th March 2012, 08:26 PM   #2
DF96 is offline DF96  England
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Worth a try, but microphony might be an issue unless it is a high quality cap with stiff vanes.
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Old 30th March 2012, 08:28 PM   #3
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Should be possible, but I would be concerned about introducing noise. Either line noise, or RF. It would need to be in its own shielded enclosure. Is the loading cap in parallel with the lead from the TT? You would also be "tuning" the end of the transmission line and I suspect that the compensation needs to be as close to the cartrage as possible.
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Old 30th March 2012, 08:28 PM   #4
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microphony from music making the fins vibrate?

or some other reason?
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Old 30th March 2012, 08:32 PM   #5
DF96 is offline DF96  England
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Yes, the vanes will act a bit like a capacitor microphone. As firechief says you will need to screen it from hum fields. The 'transmission line' is far too short for it to matter which end you place the extra capacitance - put it wherever convenient.
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Old 30th March 2012, 08:37 PM   #6
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Quote:
Originally Posted by firechief View Post
Should be possible, but I would be concerned about introducing noise. Either line noise, or RF. It would need to be in its own shielded enclosure. Is the loading cap in parallel with the lead from the TT? You would also be "tuning" the end of the transmission line and I suspect that the compensation needs to be as close to the cartrage as possible.
i thought of the rf sheilding wouldnt be a problem......was planing on it for dust too.

yes it is in parallel, im thinking alephOno.

"You would also be "tuning" the end of the transmission line"
can you explane more on this?
would it be any diffrent than say.... loading plugs or dipswitch to change values?
thanks to all
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Old 30th March 2012, 08:46 PM   #7
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"As firechief says you will need to screen it from hum fields."

side question/confirmation

extra noise in this situation cause of the huge fins acting like lots of antennas right?

and the loading cap types doesn't matter (for sound quality)cause the audio that goes through it goes to ground/- and not to the amp stage right?
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Old 30th March 2012, 08:48 PM   #8
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You could try the tuning capacitors used in the cheap little transistor radios. They have plastic dielectric between the capacitor plates and they are completely enclosed in a plastic case. They don't look very susceptible to vibration or microphonics. They used the plastic dielectric so they didn't have to space the plates apart like they had to do with air dielectric tuning capacitors so they can make them very compact to fit in the transistor radio case.
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Old 30th March 2012, 09:00 PM   #9
DF96 is offline DF96  England
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If the vanes vibrate then the capacitance will vary. Fortunately it probably won't have a DC bias but it is at a relatively high impedance point so it could modulate the signal. Nothing to do with RF or antennas. The screening is for 50/60Hz fields, as a variable capacitor can be quite large so could pick up hum.

The loading cap quality could, in principle, affect the audio because it is imposing a load. In practice the signal levels are low so any non-linearity will be very small even for a poor quality cap.
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Old 30th March 2012, 09:06 PM   #10
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Quote:
Originally Posted by DF96 View Post
If the vanes vibrate then the capacitance will vary. Fortunately it probably won't have a DC bias but it is at a relatively high impedance point so it could modulate the signal. Nothing to do with RF or antennas. The screening is for 50/60Hz fields, as a variable capacitor can be quite large so could pick up hum.

The loading cap quality could, in principle, affect the audio because it is imposing a load. In practice the signal levels are low so any non-linearity will be very small even for a poor quality cap.


ok cool, i think i will give it a try then.
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