Filter components:

Resistors:

use wirewound, Metal film, carbon whatever you like or feel will be better.

Not of any importance really in this application as long as the wattage is

ABOVE 0.25W (which most resistors are)

capacitor 1uF:

Multicomp MCRR50105Z5UM00050 = Farnell order code 121-6447

or:

Multicomp MCRR50105X7RK00050 = Farnell order code 121-6443 (used by me)

or:

AVX SR305C105KTR = Farnell order code 110-0398

or:

AVX SA305E10MARTB = Farnell order code 110-0420

Inductor (choke coil):

EPCOS B82144A2474J = Farnell order code 517-070 (used by me)

or:

EPCOS B82145A1474J000 = Farnell order code 164-4331

or any other small coil, inductor, choke with 470uH and a resistance of

equal or lower than 2.5 Ohms.

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Only the resistor values are to be altered.

You can vary the resistor value

of R3 and R4 (now 39 Ohms) between 27 (to little damping of high freq) to

120 Ohms (a bit too much damping of high freq.). You can put a 100 Ohms

potentiometer in it's place and fiddle with the settings for instance.

If the resistor value of R3/R4 is between 25 and 50 Ohms use 56 Ohm resistors for R1/R2

If the resistor value of R3/R4 is between 50 and 75 Ohms use 47 Ohm resistors for R1/R2

If the resistor value of R3/R4 is between 75 and 150 Ohms use 39 Ohm resistors for R1/R2

The capacitor and inductor need not to be changed as these are the frequency

determining parts of the filter.

Resistor R3/R4 determines the Q factor of the

filter AND amount (dB) of filtering at the filter frequency.

The center frequency is around 7.2kHz.

L1,C1,R1,L2,C2,R2 are for compensating

the impedance peak which is caused by the filter network L3,L4,R3,R4,C3,C4.

You can leave L1,C1,R1,L2,C2,R2 out IF you want if you are ONLY going

to use these cans on a low Z output amp (around 10 Ohms).

In all other circumstances (output impedance between say .. 20 Ohms and 600

Ohms or unknown) this part (L1,C1,R1,L2,C2,R2) of the filter network is ESSENTIAL

in obtaining the right amount of damping.

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maximum highs, sibilance gone HD681 character remains (first filter):

R1=R2=56 Ohm

R3=R4=39 Ohm

slightly less treble but still close to original HD681 character:

R1=R2=56 Ohm

R3=R4=47 Ohm

less treble but still brighter then Sennheiser signature in general (second filter):

R1=R2=39 Ohm

R3=R4=56 Ohm

About equal to Sennheiser signature in general bass more prominent:

R1=R2=39 Ohm

R3=R4=82 Ohm

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Capacitors:

Some say polyprop sounds better (in this picture).

Some will say mica or

silver-mica is better, yet not available in these values.

Multitlayer ceramics

(X7R is preferred over Z5U) are small and can be used inside the headphone without

taking up too much space. There are also smaller sized polypropyline avaiable.

The voltage (which determines the size) is not of importance.

For external filters

the sky is the limit (money wise) and can be constructed to your audio religion.

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Q: For the orginal HD681 filter from the very first page, say I want to attenuate

the ~7.25 kHz frequency 3db rather than 6db? I replace the resistor in parallel

part of the filter with one of lower value correct? What value would I use?

A: Parallel filter (the one in series with the driver) between 27 and 33 Ohm

series

filter (on the input of the filter) R = 68 Ohm