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Old 2nd December 2006, 04:03 PM   #1
mr.duck is offline mr.duck  United Kingdom
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Default Is this possible from a passive pre-amp?

I came across a passive pre-amp that has 15k ohm input impedance and a MAXIMUM output impedance of 3750 ohms. I cannot see how that is possible prom a passive pre amp. If it's just a basically a pot, then when the volume is turned down very low, output impedance is going to be roughly equal to the value of the pot in series with the output impedance of the source.

If that is possible then perhaps passive pre-amps are not so bad after all. But does anyone know how it's done? I feel like I am missing something.
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Old 2nd December 2006, 04:07 PM   #2
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Transformer?
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Old 2nd December 2006, 04:26 PM   #3
mr.duck is offline mr.duck  United Kingdom
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hmmm could be. I can't find any pictures of the insides. It's a rothwell passive preamp. Seriously expensive (800). I can't believe that having a transformer in there would be better than a simple buffer using transistors.

Is there anything else it can be apart from a transformer?
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Old 2nd December 2006, 04:40 PM   #4
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Quote:
s there anything else it can be apart from a transformer?
Magic? After all - the land of hairy pot'er...
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Old 2nd December 2006, 04:59 PM   #5
Geoff is offline Geoff  United Kingdom
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Default Re: Is this possible from a passive pre-amp?

Quote:
Originally posted by mr.duck
I came across a passive pre-amp that has 15k ohm input impedance and a MAXIMUM output impedance of 3750 ohms. I cannot see how that is possible prom a passive pre amp. If it's just a basically a pot, then when the volume is turned down very low, output impedance is going to be roughly equal to the value of the pot in series with the output impedance of the source.

If that is possible then perhaps passive pre-amps are not so bad after all. But does anyone know how it's done? I feel like I am missing something.
Not only is this possible, it is a certainty. The maximum output impedance of a potentiometer occurs when it is mid-travel (electrically not mechanically if it is a log pot). If the source has an output impedance close to zero then effectively you have the two halves of the track in parallel. As each half has a resistance of 7k5, the resulting output impedance will be 3k75. The output impedance will fall towards zero as the potentiometer is rotated towards its maximum or minimum positions.
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Old 3rd December 2006, 04:47 AM   #6
JesseG is offline JesseG  Canada
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Thank you Geoff

I love to learn things and I study audio stuff frequently but I still cannot figure out impedance!

Jess
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Old 3rd December 2006, 08:16 AM   #7
AndrewT is offline AndrewT  Scotland
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Hi Geoff,
you got it just right.
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regards Andrew T.
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Old 3rd December 2006, 11:57 AM   #8
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Stupid me....
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