passive i/v stage - low pass filter

[lordearl]

I recently became a convert to the "passive i/v stage" and noticed on the Ciuffoli website the following statement:

In parallel to the 200-220ohm I/V resistors will be inserted a little capacitor necessary to create a high frequency cut-off upper.

The resistor I am using is a 1k value (this allows enough volume to run the dac straight into my power amp - which has a passive stepped attenuator on it), so by my calculations the appropriate value for a low pass cap will be as follows:

1/(2 * 3.14 * 1000 * 0.0000000068) = 23.4 KHz

So the value is 6.8nF (0.0068uF).

A few questions:

(1) I cannot find a suitable place on the board (due to space restrictions) to put these capacitors directly soldered to the ends of the resistor leads. Is it possible instead to solder each capacitor from the tip/signal point of the output RCA socket to the sleeve/ground (i.e. will this have the same effect as soldering them to the ends of each i/v resistor)?

(2) Should the implementation of this low pass filter theoretically affect the sound quality either positively or negatively?

(3) I have located some Vishay 1813 6.8nF capacitors, are these a decent starting point in terms of quality?

[marcelnl]

eeeh I/V after what DAC chip...the 1 k Value might just be too much and could ruin your sound quality, the optimal value of the resistir will depend on the amount of current your DAC chip is capable of.
You might just want to try without the capacitor....I have not used one in my TDA 1541 and TDA 1543 based DACs and it works fine and sounds so much better.

[lordearl]

Originally Posted by marcelnl

eeeh I/V after what DAC chip...the 1 k Value might just be too much and could ruin your sound quality

DAC chip is AD1865 - according to the datasheet any value < 1.5k is safe, SQ is unchanged from 220R, 560R and even 1.5k (tried them all with a lot of desoldering/soldering!)

[guytou]

as marcelnl said , you would choose the easy way and taste some sound without filter. personally I never needed filters in 12 years of nofilter oversampled CDplayers (NOS players is a bit different , I think one can feel garbage on tweeters). now I am running an AD1864 with 800 ohms without any filter(the most beautiful treble I have ever heared ; more refined than a PCM56 for instance)

[lordearl]

Thanks for the input.

I will listen for some weeks without the filter, and then try the filter to test the sound difference. I am using the AD1865 in a NOS setting, so there is no oversampling and no upsampling.

To implement the filter - will soldering the capacitors from the tip to the sleeve on the output RCA sockets work the same as putting the capacitors on top of the resistors?

[guytou]

yes ,you can implement the filter on the RCA sockets , as can be seen on a lot of (cheap) players .
it will work the same any place you put it .

[lordearl]

OK I'll put it from the signal point (tip) to ground (sleeve) on the L and R channels and see if there is any noticeable difference ; )

[guytou]

that's it , lordearl

[lordearl]

Update - I put the capacitors in, it actually sounds better!! Seems to have cleaned the top end up a little!