Audio frequency source/Preamp inpedence

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Re: Audio frequency source/Preamp inpedence

Post by Ivan on Thu May 02, 2013 6:38 am

Hi DragonForce,
I expect you can calculate the impedance of a 1 uF capacitor at 1 KHz and properties of a divider formed by this capacitor and 47 Kohm resistor. These are very basic calculations. I expect you do not try to examine me either Wink

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Re: Audio frequency source/Preamp inpedence

Post by DragonForce on Wed May 01, 2013 5:29 pm

LOL thanks Ivan Smile

Main questions....

If I burn a CD of audio tones, I can use my CD player as a tone generator

How far does the reactance of a 1uF capacitor affect the input impedance of a 47K resistor placed after it (to ground).

Assuming LINE level to be 1V pk to pk, at 47K, would an input preamplifier's input be shunted much by the reactance of that 1uF cap? See, I can't test it, plus I have cloth ears, but I would like to do this properly.

In the UK, we have a saying about spoiling a whole ship for a half penny's worth of tar.....

Thanks for your quick reply Smile

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Re: Audio frequency source/Preamp inpedence

Post by Ivan on Wed May 01, 2013 4:42 pm

Hi DragonForce,
yes and yes. Some multimeters can measure AC voltages up to 1 KHz or even more quite exactly.

What was the main question ?

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Audio frequency source/Preamp inpedence

Post by DragonForce on Wed May 01, 2013 3:41 pm

I'm not sure I have the title of this thread correct yet, no doubt Harry or Ivan will suggest a better one.

This idea came about due to Harry mentioning that any decent audio editing software will remove a DC offset from a signal. Since I use Cool Edit Pro, I already understood that - but then I thought of something else (and I totally blame our glorious leader, it's all his fault Razz )...

Cool Edit has a tone generator. OK it's nothing fancy, BUT - I wondered if I could set up an audio track of 50 Hz (and another of 1 KHz), burn both tracks to a CDR and use my CD player as a signal generator. Obviously it is only any use at the frequency of the audio track, and it doesn't have to be exact (although I expect it IS quite accurate). The reason for this, and hence it's in the questions section of the board.....

My digital test meter is almost certainly not a true RMS device, and is likely to only be reasonably accurate at 50 Hz - is this likely to be about right?

If I wanted to test the line level out on my CD player, I could simply play the CD track (50 Hz) and siimly measure AC voltage, yes?

Basically, I'm looking for an easy way to test the LINE level output on my CD player etc - I am assuming 1V peak to peak, 47K impedance.

The preamplifier for the Baxandall tone control circuit will probably use a 1uF poly film (not electrolytic) cap with a 47K resistor to ground at the input. I'm hoping it doesn't filter out too much of the bass frequencies. To be honest, I doubt if my cloth ears will notice any difference, but I'd like to get it right. Would the reactance of that cap interfere with the apparent value of the 47K resistor thus upsetting the input impedence from what I expect - I guess is the basic question I'm trying to ask... And maybe that's why some people recommend 1uF and 22K.

I'd breadboard the circuit and test - but as I said, I doubt I'd notice the difference, and I want to get it right, so experimenting won't help.

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