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In Reply to: Re: any problems using a passive pre amp with a valve amp? posted by JRGTR on September 30, 2001 at 10:43:33:
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.
Follow Ups:
Thanks for your input. I was talking just about conventional passive units (working with resistors, not transformers). With "conventional" passive preamps I am positivetely sure all the the requirements I said are true. BTW I´ve heard transformer based passive preamps are not so transparent or neutral sounding like "conventional" passive units due to the existence of a transformer in the signal path. Ultra high quality passive units uses the "less is more" approach in the signal path with ferocity. Of course the compatibility of the transformer based type units is the strong point.
No ! A good transformer coupling still can get transparent or neutral sounding as traditional Resistor type passive Pre-amps. It only some manufacturer made it too narrow in band width due to some technical reason. I had try many difference way to winding it . I finally has a very good result. You may be can found a Model Passive T1 in AA Review or in UHF Issue 62. I think it will change your mind in Transformers coupling. I will open it detail later Due to We are applicated patend in China to avoid copy. We have many completitor in China like to copy our products design now. e.g A transformers coupling Headphone amp.
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