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avid 20th August 2006 08:37 AM

newbie questions for DIY audio - line-in attenuation
I'm currently putting together a LM3875 chipamp using a PCB set from Peter Daniel. I only have a basic understanding of electronics and have some questions regarding line levels and attenuation:

1) What is the line out voltage and impedance of a consumer CD player?

2) Is the above device considered a high-impedance or a low-impedance device?

3) How do I determine the optimum resistance of a potentiometer to be used as a variable attenuator? (I've seen a lot of posts where a 50k pot is being used, but I don't really understand how that number was determined.)

4) What would the value of the resistor be to make a linear pot a logarithmic attenuator? (I have read Rod Elliot's Better Volume Control project page and don't quite understand how he determined the values of the pot and resistor.)

While I would love some straight-forward values as answers to my questions, I'm really more interested in figuring out how and why a value is determined. Some minor coaching would be helpful.

Let me know what you think.

Thanks for your help...


sreten 20th August 2006 12:08 PM


The standard output of a CD player is 2V maximum.

Source impedance should be low, though generally line level
signals are treated as high impedance and should be loaded
with ~ 10Kohm to ~ 100kohm.

The lower impedance has minor noise advantages, and can
be too low for valve output line stages, but should be OK
with normal tuner and tape line outputs.

For the fake log pot use a 100k linear + ~ 15K (13K or 16K),
or a 47K pot + ~ 7.5K (6.8K or 8.2K), I'd go for the latter.

For dual log pots anything above 10K to 100K can be used.


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