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Old 25th February 2011, 01:11 AM   #1
taeedy is offline taeedy  China
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Default Impedance matching

Dear friends,
A while back, I built some gainclone amps that all seem to be working well.
Recently, however, I came across impendance matching. I read that the output of the source i.e. pre-amp should be connected to the input of an amp where the input impendance should be 10x higher. e.g. should the pre-amp output impendance be at 47k ohms, the amp input impendance should be 470k ohms.
Am I reading that right?
How and where is input impendance on gainclones measured?
On my gainclones I am using for the input to ground resistor a 22k ohms and for the Rf resistor a 22k ohms.

Please excuse my ignorance due to my being a novice.
Any and all assistance will be appreciated.
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Old 25th February 2011, 01:34 AM   #2
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Quote:
Originally Posted by taeedy View Post
I read that the output of the source i.e. pre-amp should be connected to the input of an amp where the input impendance should be 10x higher. e.g. should the pre-amp output impendance be at 47k ohms, the amp input impendance should be 470k ohms.
Am I reading that right?
Impedance not impendance.

Yes you are reading that right, but it is not critical. Input impedance=output impedance is OK. Input impedance < output impedance is to be avoided generally.

Quote:
Originally Posted by taeedy View Post
How and where is input impendance on gainclones measured? On my gainclones I am using for the input to ground resistor a 22k ohms and for the Rf resistor a 22k ohms.
The input impedance is probably 22k, but this depends on the exact circuit. Learning to calculate the input impedance is not simple. Have you got a volume control? Post a drawing.

Quote:
Originally Posted by taeedy View Post
Please excuse my ignorance due to my being a novice.
Everyone was a novice once, no-one minds honest questions.

w
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Old 25th February 2011, 02:32 AM   #3
krivium is offline krivium  France
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Quote:
read that the output of the source i.e. pre-amp should be connected to the input of an amp where the input impendance should be 10x higher.
Speaking line connection we try to send a VOLTAGE along the line.

When you connect gear together you're creating some voltage divider which formulae is R1 / (R1+R2) (edit: in fact Z1/(Z1+Z2) because we are speaking of impedance (Z) but to symplify we consider it as a resistor... ).

If source output Z (preamp : R2) is way lower than load input Z (amp: R1) voltage transmitted along the line stay near unity gain eg: 1v x 1~ 0,9 =1v~0,9v.

If your output Z is equal with your load Z you get HALF the voltage sended (you lost 6db along the line) at input of the amp (for 1v at preamp output you get 0,5V at load).

If your output Z is way higher than your load input Z you divide the signal many times more resulting in voltage at load very small (highly attenuated eg: 1volt preout and 0,01V at amp input).

You could think this is not a problem since next stage (in our example preamp/amp) will amplify signal, but modding the impedance ratio you modify frequency response too (Impedance Z is the sum of all L, C and R of a circuit) and so you modify signal frequency reproduction ( eg you may loose some bass and high end).

The ratio 1/10 is a general recommendation allowing you to be in a 'safe spot'.

In fact in pro gear for line you could find 10r out Z and In Z up to 50K or more: ratio is bigger but voltage divider is much more unity gain (voltage multiplyed by 0,998). As long as you stay in ratio 1/1 to 1/10 it's ok.

Last edited by krivium; 25th February 2011 at 02:54 AM.
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Old 25th February 2011, 07:51 AM   #4
taeedy is offline taeedy  China
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Join Date: Oct 2008
Default My appreciation

To wakibaki,

Thanks for your clarification on the matter. As well thanks for your correction of my spelling.

To krivium,
Thanks for a formulating explanation to the issue.

There are no volume controls on any of the gainclones directly. The signal travels from an Onkyo receiver's pre-amp outputs to a pair of Behringer CX 3400 3way crossovers - one per side - and down the chain to the gainclone amps.
As for schematics, I didn't do any drawings as I followed those available on the Internet.
having used them for over a year, approximately 12 hours a day each day, there have been no issues whatsoever. Clean sound. No hissing or hums.

Thanks again.
Taeedy.
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Old 25th February 2011, 11:22 AM   #5
AndrewT is offline AndrewT  Scotland
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you quote output impedance of 47k.
I think you have mis read this specification.

I think 47k is the recommended load impedance the source should be connected to.
I further suspect the actual output impedance of your source is less than 2k and probably less than 500r.

I prefer to use higher than 1:20 but as said earlier that ratio is not critical. In professional audio it can approach 1:100000, i.e. output impedance 10r, input impedance >1M (balanced transformer).
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regards Andrew T.
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Old 25th February 2011, 08:30 PM   #6
taeedy is offline taeedy  China
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AndrewT,
Your proposal appears to require some further investigation.
Thanks for your suggestion.

Taeedy.
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