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It is not alway true !

No ! It is not alway true. It only problem on Resistor attenuator or Normal V.R Pot. It don't have problem on Transformers tapped V.R.

1. No need a High impedance input in Power amplifiers due to the turn ratio will higher the Input impedance from Passive Pre-amps. e.g turns Ratio 3:1 (When work at 1/3 Volumn) , Power amps input impedance at 10K. Input resistance seen by CD or other source will be . 10K x (3 x 3) = 90K ohm. So Use step down tapped transformers will increase to high impedance especially in low level. (Sometime down to 100:1 or more . So Even 10K Input impedance power amps will made it become 100M seen by your source.
2. No need a Low output impedance due to same reason as point 1.
3. Yes ! It really need a Higher output to drive your power amplifers to full power. It nay be at least 1V or Over is suitable but most CD/SACD have good enough voltage to drive it very good.
Some Mistake on test output Impedance of Transformers coupling Pre-amps : It can not test by Ohm meter.
Output Impedance is not Fix but domain by it turns ratio. MOre lower the Volumn. More higher input impedance on transformers passive pre-amps.
The about theory only suitable for step down type transformers Pot. Step up type will be reverse it.



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