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Old 27th July 2006, 08:30 PM   #1
namcir is offline namcir  Philippines
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Default amp input filter to sacd dvd-audio?

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...
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Old 27th July 2006, 08:43 PM   #2
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I would leave it well alone. I think you have misinterpreted either the values or the component placement as there is no way the input filter begins to roll off at 15kHz.
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Old 28th July 2006, 07:14 AM   #3
namcir is offline namcir  Philippines
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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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Old 28th July 2006, 04:52 PM   #4
namcir is offline namcir  Philippines
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anyone who wants to comment about the circuit
anyone who knows about the input filter of an amp?please?
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Old 28th July 2006, 06:18 PM   #5
AndrewT is offline AndrewT  Scotland
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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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Old 28th July 2006, 06:33 PM   #6
namcir is offline namcir  Philippines
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thanks andrew t.
i tried to substitute c2 with 220pf a little improvement in the highs i can hear an airy top and some crispness.
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Old 29th July 2006, 05:19 PM   #7
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I would just put it back to standard. You could be opening it up to stability problems or causing unnecessary cutoff of the input signal.

You could put the network into a simulator but there may be some other input impedance not accounted for.
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Old 30th July 2006, 04:46 PM   #8
anatech is offline anatech  Canada
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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
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Old 30th July 2006, 06:22 PM   #9
namcir is offline namcir  Philippines
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thanks to all of you for your advice glad to read your opinions...

right i'll just put it back to thier original values don't want to have a blown amp again:
thanks
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Old 30th July 2006, 07:00 PM   #10
AndrewT is offline AndrewT  Scotland
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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.
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