Question : Problem: Increasing headphone impedance to reduce background noise (using iPAQ)

Hi - Our engineer is unavailable and I thought someone might be able to answer this quickly.  My electrical engineering skills are rusty!
To reduce background noise, we have a need to increase the impedance of a set of professional headphones used by our hearing testing software that runs on an iPAQ.  We purchase the transducers and build a custom cable with a jack capable of fitting into the iPAQ, and we want to insert some circuitry to increase the impedance so we reduce the background noise.  We're using 50 ohm transducers, and we want to bump up the impedance to something over 100 ohm --- the exact value to be determined as we test different impedances and output levels.  Here's a rough sketch of the circuit we are using in each side of the headphone cable.  We get a total impedance of 100 ohms (Z) when R1=120 ohm, R2= 390 ohm, R3=100 ohm, and C1=100,000pf.  Question: What would the R and C values need to be for Z=110,120,130,140,150,200,250 ohms?  The frequencies we use range from 250Hz to 8kHz.  Thanks in advance!
Brett  
PS Any recommendations on companies or individuals who could create the cables and/or printed circuitry for us in small make to order quantities? (under 50 units at a time)
---------
\_____/ speaker 50 ohm
 +| -|
   |  |
   |  +--------------+
   |                      |
   +-----VVVV------+
   |      R1             |
   |                       |
   +----------+        |
   |             |         |
   |             |         |
   |            ---        |
   >           /-\ C1   |    
   >            |         |
   > R2       |         |
   >            |         |
   |             |        |
   +----------+       |
   |                      |
   |                      |
   +-----VVVV------+
   |      R3            |        
   |                      |
   |  +--------------+
 +| -|
   |  | Z=total impedance
   \  /
    ||
    || 3.5mm jack
    ||
    \/  

Answer : Problem: Increasing headphone impedance to reduce background noise (using iPAQ)

Brett,

If you eliminanted all but one reistor you would have a total impedance of 50ohms (for the speaker) + R (the single resistor value). It's a whole lot simpler. You may find that it sounds better with the capacitor as it will be acting as a high pass filter (http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/experiment/highpass/hpf.html). My electronics is a little rusty (and I've just woken up) so I can't work this out exactly at the moment.

The reason the capacitor dominates the resistance at the moment is that in the calculation you'll find it impossible to get a value higher than the smallest parallel resistor, it's just the nature of things.

The reflections will be reduced when the impdance of your cable is matched to (i.e. the same as) the imput impedance and the impedance of the load.