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Old 18th October 2014, 04:46 PM   #1
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Default B1 clone input impedance

Was wondering if anyone knows the input impedance of the Pass B1 buffer clone? Original 2008 R0 version with signal caps (not the dc mesmerise version). Can't see to find this info anywhere.



Cheers
Colin
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Old 18th October 2014, 05:41 PM   #2
flg is offline flg  United States
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From the http://firstwatt.com/pdf/prod_b1_man.pdf schematic, it looks like a 1M and 1M and a FET gate and a 25K pot in ||. So, slightly less than 25K. But, read the link, I'll bet N.P. tells you...

OK I found this in that paper:
A divided input signal appears on the wiper. This signal goes to the Gates of Q100 and Q200
through a resistor R102, R202 and capacitor C100, C200. As a practical
matter, the input impedance of this preamp is determined by this volume
control potentiometer. A 25 K Ohm pot gives 25 K Ohm input impedance. N.P.

So, I may be more accurate than him being Slightly lower than 25K.
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Last edited by flg; 18th October 2014 at 05:44 PM.
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Old 18th October 2014, 07:09 PM   #3
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Thanks for the info. My clone has no pot so I guess the input impedance is 1 meg. I would rather use a JFET Boz instead but have no time to build one and can't find any built ones for sale!

Cheers,
Colin

Quote:
Originally Posted by flg View Post
From the http://firstwatt.com/pdf/prod_b1_man.pdf schematic, it looks like a 1M and 1M and a FET gate and a 25K pot in ||. So, slightly less than 25K. But, read the link, I'll bet N.P. tells you...

OK I found this in that paper:
A divided input signal appears on the wiper. This signal goes to the Gates of Q100 and Q200
through a resistor R102, R202 and capacitor C100, C200. As a practical
matter, the input impedance of this preamp is determined by this volume
control potentiometer. A 25 K Ohm pot gives 25 K Ohm input impedance. N.P.

So, I may be more accurate than him being Slightly lower than 25K.
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Old Yesterday, 11:01 AM   #4
AndrewT is offline AndrewT  Scotland
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I would be very tempted to get it working as shown and then compare by adding a parallel resistor to the 1M0 to bring down the input impedance, Maybe add a 100k and possibly as low as 22k.

Very high input impedance can be more susceptible to picking up interference.
Test with your mobile phone and wireless land line adjacent.
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