amp input filter to sacd dvd-audio?

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i need help as i want to mod may nad 352 amp input filter to be compatible with sacd or dvd-audio is this possible?
the rc filter (rc shunt to ground) is 22k & 470pf what should be the right value? is changing the rc filter will cause it to oscillate or unstable at high frequency?
i wonder why some amps have a frequency response of 20hz-20khz while others at 3hz-100khz is this by design or just because of the input filter?
any suggestions are gladly appreciated...
 
sorry if i am confused:(
but here is the input schematic:
values are
R1=220K
R2=470
R3=22k
R4=100
C1=10uf
C2=470pf

what i'm planning is to replace C2 with 150pf mica....

i have seen in some designs like the symasym & others that C2 varies from 100-680pf. any enlightment on this i dont know anything about amplifiers i'm only a diyer?
so again what makes those amps capable of handling 100khz compatible to sacd or dvd-a?
 

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Hi,
R3 & C1 set the bass (high pass) roll off frequency.

R2 + R4 & C2 set the treble (low pass) roll off frequency.

Unfortunately due to the complex arrangement of components I cannot calculate it accurately but it approximates to -3db @ 0.7Hz and 600kHz. or -1db @1.4Hz and 300kHz.

These are very wideband values. Your amp will be bandlimited inside the gain circuits to less than these values.

But, remember the values I have given are approximate.

Can someone confirm the real input filter numbers.
 
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Hi RICMAN,
Both AndrewT and richie00boy has given you correct advice. Use the original values. The filter is well above and below the limits of your ability to hear. If you are hearing something going on, my money is on something not good happening. The effects are audible, but the cause is unknown at this point.

-Chris
 
Hi,
since the low pass roll off is set quite high, you may want to increase the HF RC time constant.

Fitting a cap a couple of values higher, 680pF or even 1nF.
NP0 (=C0G) ceramic or polypropylene would be good.

I think the extra inductance of polystyrene would prevent the roll-off going towards RF.
 
Hi,
a standard single pole RC filter made up of one capacitor and one resistor has a well defined pass and attenuation characteristic.

The RC time constant is simply the product of the resistor in ohms and the capacitor in Farads.

So your 100r and 820pF will have a 0.082uS time constant.

The -3db frequency is given by the standard formula
F=1 / 2 Pi R C = 1.94MHz

Unfortunately in your NAD case there are two series resistors and other resistors forming a more complex arrangement.
If we just ignore all the other components and just use R2 + R4 then -3db occurs @ 340kHz.

Do a search both in this Forum and more widely and you will find all the data you need and even the accurate manual method for analysing your NAD network.
 
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