[Adamay]

Jason suggested that I ask this question here, although I suspect it's a terribly naive question.   I've got a simply lovely custom tubed pre whose designer says it works best with amps having an input impedance of 100k ohms or greater.  The Nuforce's input impedance is 47k.  If the input impedance could be raised, I would theoretically get better dynamics & bloom.  Is it possible to modify the Nuforces to raise the input impedance?  Are there likely to be detrimental effects? If there are, is it possible to know in advance the nature of the tradeoff? Thanks in advance for any responses from the technically-savvy among you.

[nuforce-casey]

The short answer: yes, we can do that for our customer, raising input impedance to 100K.

The long answer:  Common rule-of-thumb is that the input impedance to be at least 10x that of the output impedance.   Going to the extreme may or may not bring any real benefits.

One aspect of the issue here is that resistor is noisy, more so that the Op-Amp (which rejects CM noise).  For example,  with 20Khz bandwidth, if the input impedance increases from 47K to 100K, the noise will increase from 3.9 uV to 5.7 uV.  If the power bandwidth increases, the noise increases in a 'square' relation.

So finding an optimum value such that maximum voltage is transfered (the 10X rule) yet keeping the input resistor as small as possible can improve performance.

[Adamay]

Thanks for your reply.  The preamp's maker tells me "the higher the better" for the amps' input impedance, up to an ideal 470k ohms.  1) Can I accomplish this myself by simplying putting a high-quality resistor in series on the hot lead of the incoming interconnect at the amp end?  If so, how high in resistance can I go without triggering the amps' incoming RFI-shielding circuit?  2) If resistors on the IC's are not ideal, how high could you raise the amps' input impedance?  200k ohms?  Higher?  Thanks very much, in advance.

[nuforce-casey]

Thanks for your reply.  The preamp's maker tells me "the higher the better" for the amps' input impedance, up to an ideal 470k ohms.  1) Can I accomplish this myself by simplying putting a high-quality resistor in series on the hot lead of the incoming interconnect at the amp end?  If so, how high in resistance can I go without triggering the amps' incoming RFI-shielding circuit?  2) If resistors on the IC's are not ideal, how high could you raise the amps' input impedance?  200k ohms?  Higher?  Thanks very much, in advance.

We can change the input spec., mind you this is not just swapping single resistor.  So please let us know when you are ordering a 100K or 200K unit upon placing the order.