How do you set the input impedence in audio amplifier (10k, 47k, 100k?) - diyAudio
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Old 15th October 2008, 10:32 AM   #1
ygg-it is offline ygg-it  Italy
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Default How do you set the input impedence in audio amplifier (10k, 47k, 100k?)

Hello I have to match the 300 ohms output impedance of a cd player to the first stage of my amplifier.
The first stage is a bipolar op amp (ad826) in a unity gain non-inverting configuration without volume pot and without dc-block capacitor.
So the input goes directly to the input (+) of the op-amp.

1) Which should be the best resistance between the input (+) and ground?
2) I see circuits with 10k, 47k or 100k. Which is the best: higher or lower? I think that higher would rise too much the dc offset of the amp....
3) since I don't know if the cd output stage has a capacitor or not, I suppose I must put this input parallel resistance...
4) which "sounds" the best: higher or lower?
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Old 15th October 2008, 11:03 AM   #2
SY is offline SY  United States
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Almost any CD player will be configured to run a 10k load. Given the relatively high bias current of the 826, I'd probably stick to 10k.

I'm not sure why you'd choose the 826 in the first place. The input of a chip amp is not exactly a tough load to drive, so the use of a buffer there is questionable; if you really need to buffer it, a FET-input opamp would be a better choice, since it will have negligible input current and can easily accommodate a 100k input resistor.
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Old 15th October 2008, 11:19 AM   #3
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You should have a look in your CD Player manual.
Specifications.
They usually tell minimum Load Impedance they recommend for CD-out.
Sometimes they tell it like:
Output: Maximum 2.0 Volt RMS into 10 kOhm

For example my CD player (a bit older) should have load of 15 kOhm or higher.
So I go for 22 kOhm .. a bit margin.

22 kOhm is a good value. Most anything will drive such load.
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Old 15th October 2008, 01:26 PM   #4
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You don't have to "match" anything. The use of the word match in audio is typically, though not always, in error. The resistor in your schematic provides a ground reference and current drain in the event the pre is capacitively coupled, or happens to be unplugged. Choose the value so the following opamp has minimal DC offset if the input is unplugged. Something in the 20K - 100K region is typical/reasonable.
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Old 15th October 2008, 01:49 PM   #5
AndrewT is online now AndrewT  Scotland
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Default Re: How do you set the input impedence in audio amplifier (10k, 47k, 100k?)

Quote:
Originally posted by ygg-it
Hello I have to match the 300 ohms output impedance of a cd player to the first stage of my amplifier.
Just as Conrad said. You don't have to match.

What you must ensure is that the CD player can drive the input impedance of the next stage combined with the parallel impedance of the connecting cables.
This is usually a current limitation set by the CDplayer manufacturer.
They usually specify it by suggesting a suitable impedance for the next stage.

Most CDplayers will drive 10k with reasonable interconnects in between.

Check what your player spec tells you.
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Old 15th October 2008, 02:13 PM   #6
ygg-it is offline ygg-it  Italy
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Thanks!
Here it is
CD player NAD

What do you suggest between 10k and 22k ?
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Old 15th October 2008, 04:08 PM   #7
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They spec 300 ohms output impedance. That probably just means that they used a 300 ohm resistor in series with the output to protect and stabilize it. That does not mean it could drive 300 ohms, though doesn't rule it out either. Many power amps are 10k input impedance, which should work fine. I prefer it a bit higher, but it's more personal preference than anything. There shouldn't be much difference between 10k and 22k. I'd likely use 22k unless the 10k resistor bin was closer to the bench.
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Old 15th October 2008, 04:36 PM   #8
AndrewT is online now AndrewT  Scotland
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The datasheet confirms that Burr Brown opamps are used for the output and that there is a DC blocking capacitor on the opamp output.

If you open it up you should be able to read off both the BBopamp type number and the capacitor value.

This will allow us to identify a range of near optimum Load Impedances.
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Old 16th October 2008, 07:09 AM   #9
ygg-it is offline ygg-it  Italy
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Default @ AndrewT

> The datasheet confirms that there is a DC blocking capacitor on the opamp output

Please could you let me know where you get this information?
Thank you
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Old 16th October 2008, 02:32 PM   #10
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It's stated near the bottom of the text on the "Closer Look" tab.

"Apart from the single film type output capacitor, no other capacitors are used in the signal path."
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