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Old 17th January 2010, 08:48 PM   #11
Carlp is offline Carlp  United States
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There are probably better threads to talk about this, but my understanding is that high impedance into low impedance results partly in attenuated low end among other distortions. The bigger question for me, since I'm a tube guy, is what resistance value is needed for the tube grid (given it's grid leak resistor). My recollection is that you can't just put any value pot into the unit and expect it to work. Thus component matching is pretty crucial.

Fewer such problems with silicone units, I believe.

Edit: Remember that the pot is another output going into the tube's grid input, etc. So same rule applies.

Last edited by Carlp; 17th January 2010 at 08:52 PM.
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Old 18th January 2010, 12:30 AM   #12
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Power amps and preamps input can be measured with a multimeter to ground, unless they are capacitor coupled, then measure the resistance to ground after the cap.
Pre amps and cdp's output impedance are a little harder, with a steady 1khz sine wave comming out, load it up with different resitance to ground until the 1khz sine wave is halved in level and that resistor is then the same as the output impedance of the pre or cdp you are trying measuring. Simple as 123
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Old 18th January 2010, 03:28 PM   #13
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udailey, the input impedance in an opamp configuration is determined by the values used in the feedback network. Often with an opamp you can scale the values over a very wide range, thereby adjusting the input impedance.

It also matters if it is inverting or non-inverting to some extent.

I dunno what the chipamp wants or needs, I'd suggest asking in a chipamp thread?



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Old 18th January 2010, 03:31 PM   #14
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georgehifi,

I don't want a tube amp with an unterminated cap at the input. If you do have an unterminated (floating) cap, then when you plug in, you discharge that cap across whatever you are plugging in. Not good.

There should be some value of resistance on both sides of an input cap to prevent this from happening.

The input impedance will then be the paralleled values of the two resistors.

For a tube input that can be in the Mohms, so not much to worry about here.

Of course "standard" input Z is between "47kohm" and ~100kohm... other values do exist.


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Old 18th January 2010, 04:31 PM   #15
Pano is offline Pano  United States
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Do you mean like this?

(R1 could be the input pot)
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Old 21st January 2010, 01:01 AM   #16
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well, yes... but I would not use R1 as a pot because pots are known to open and the end toward C1 is going to be the wiper... if the pot has a resistor in parallel to ground with the wiper ("make ur own taper") that might be ok..

call it a "safety" resistor?

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