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Old 7th August 2008, 07:01 PM   #11
AndrewT is offline AndrewT  Scotland
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Let's take an extreme case of 4m interconnect having 130pF per metre and a power amp fitted with 1nF RF attenuation caps at the input.
total capacitance seen by the output impedance is 1000pF + 520pF =1520pF.
F-3db=1/2/Pi/R/C/~10kHz, F-1dB~5kHz
That's some treble filter. It should be at least 10times higher and preferably about 20times higher.
i.e. Rs<=500ohms.
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Old 7th August 2008, 07:08 PM   #12
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But, that would be 200x greater (input impedance is 100K, not 10K)
in any case, thanks again,
b
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Old 7th August 2008, 07:55 PM   #13
lineup is offline lineup  Sweden
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I have seen commercial CD-players with 600 Ohm output impedance.
An upper limit for line level & small signal audio
is a value of something like 1000 Ohm ( 1 kOhm ).

Such an output impedance would suit most audio amplifiers, pre- or power amplifiers,
with an input impedance of >= 10 kOhm.
Such as 10k, 15k, 22k, 47k or 100k input jacks.

This would satisfy the Rule-Of-The-Thumb (AndrewT mentioned)
that is preferable input impedance is at least 10 times higher than source output impedance.

It wont hurt, of course, if the ratio is x20 or even more ...
But x10 is considered to be acceptable in most cases.
For Line Level Audio.
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Old 7th August 2008, 07:57 PM   #14
CBS240 is offline CBS240  United States
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Quote:
......input impedance is 100K, not 10K........

1/(2*pi*1520pf*20KHz)=5.24K. A bit less than 100K just for this capacitive load.

I always try to over design for the reasonably worst possible scenario.


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Old 8th August 2008, 08:00 AM   #15
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by CBS240



1/(2*pi*1520pf*20KHz)=5.24K. A bit less than 100K just for this capacitive load.

I always try to over design for the reasonably worst possible scenario.


F-3dB =20kHz will sound like a treble filter has been added to everything.
F-3db >=100kHz is acceptable to most listeners and some prefer as high as 500kHz.
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Old 8th August 2008, 12:53 PM   #16
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The amp (k-502) has 100k input impedance, and .22uf input capacitors...(schematic is here: http://www.circuitspecialists.com/products/km12sch.jpg)
So would that mean i should try for
1/(2*pi*.22uf*100kHz) = 7.32Ohm output resistance?
That sounds a little low to me...I thought I'd be okay with up to ~5K output impedance...
In any case, I will be going with this (with adjusted R5 for more gain) , since I have all the parts handy:
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Old 8th August 2008, 12:58 PM   #17
AndrewT is offline AndrewT  Scotland
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the 0.22uF is the DC blocking high pass filter.
The source impedance interacts with the cable and input capacitance to create a low pass filter.

R5 does not adjust the gain. It isolates the opamp from reactive components in the load.
It also sets the output impedance of the amplifier/buffer.
Change R3 or R4 to alter gain.
Reduce R5 to between 50r and 200r to set Rs in that range.
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Old 8th August 2008, 01:33 PM   #18
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Yes! I meant r4(or r3)...thanks again
B
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