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Old 15th June 2009, 10:21 PM   #1
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Default buffer and impedance matching

Dr. Pass,

could you explain how your B1 buffer works and what you intended for it to do?

I understand from reading your article that a passive pot suffers from impedance mismatch.

What distortion results from using a passive pot?

What is the ideal impedance match between a cd player and an amplifier?
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Old 15th June 2009, 10:29 PM   #2
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There's so much information on the F1 out on the internet, on PassLabs website and on this forum. You should research and study more before posting such an open ended question.

Regarding distortion - there is no distortion generated by a passive pot. It's just that: a passive resistor.

The problem comes when the input impedance is so low that there is excessive attenuation due to the previous stage output impedance. Worse, if the previous stage has a output capacitor, then the passive pot will change the frequency response as you change volume.

Impedance match is a radio thing. Here we have voltage sources and so you don't want a match. Ideally you have zero output impedance and infinite input impedance. In reality if Zin = 10* Zout it's good enough.

Do a search on ohms law and RC filters.
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Old 15th June 2009, 11:16 PM   #3
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People argue about this issue all the time. For some unknown
reason, many people like the sound of a buffered volume control
over the sound of just the volume control.

There are some possible explanations or permutations of explanations:

1) Some equipment likes a high load impedance for good sound, and
this is in conflict with some equipment which performs better with a
low source impedance. These two requirements tend to conflict when
a pot is inserted between them, since choosing a high impedance pot
favors the source but not the load, and choosing a low impedance pot
favors the load and not the source. And there may not be a happy middle.

2) Bandwidth on cable runs are affected by cable capacitance interacting
with the pot impedance. This is a variation on #1

3) Possibly pots are more accurate when driving high impedances on
the wiper because of the quality of the wiper contact on the resistive
surface.

4) The subtle alterations provided by the buffer circuit may add a
pleasant character which is preferred by the listener.

5) The listener is suffering from inaccurate perception
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Old 16th June 2009, 04:49 AM   #4
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Default Impedance bridging

What you're actually doing with most audio equipment is "Impedance Bridging". Google that subject and you will find some information to help get your question answered.
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Old 16th June 2009, 06:22 PM   #5
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Also, I'm not a doctor.

Although I'll take a look at it...

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Old 16th June 2009, 09:37 PM   #6
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Quote:
Originally posted by Nelson Pass
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and he is , certainly .....

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