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General Asylum General audio topics that don't fit into specific categories. |
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General Asylum General audio topics that don't fit into specific categories. |
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In Reply to: There is no such thing as a "passive" in an electronic system. posted by Chip647 on March 8, 2025 at 13:02:16:
Just like the old 2 stage vs. 3 stage single ended amp debate.
A 2 stage amp with a one stage preamp is a 3 stage amp. It doesn't matter which box that third stage is in.
All preamps that I know of use a passive pot somewhere. It boils down to how much total capacitance the pot has to drive. In an active preamp there is only a short wire run to the amplifying device. In a passive preamp that cable run needs to reach the power amp.
A active preamp can be made to have a very low output impedance that keeps the cable capacitance (and stray and Miller) from being a problem. With a passive preamp all of these things need to be thought of and solved.
As long as that is successfully done, there should be no loss in dynamics (or highs).
A 10k pot has a worse case output impedance of 2500 ohms. That is at the -6db position, all other positions yield a lower output impedance.
2500 ohms against 320pF gives a -3db point of the low pass filter of about 200kHz. That will make 20kHz completely untouched by the filter. If the shunt capacitance is higher than that, then the phase at 20kHz will start to shift.
2500 ohms driving 1500pF will cause a -3db point of about 40kHz and 20kHz will be 1db down. I'm not sure how many of us old guys would hear that but I don't know for sure. To be safe I keep all the filters, that I have control over, at or above 200kHz.
I use a old school 25k Daven stepped pot so its worse case output impedance is 6.25K but my cables are only 3 feet long and very very low capacitance (about 45pF total) and the input tube I use in my amp has the lowest Miller capacitance I have ever seen in a triode. About 40pF. 6.25k ohms driving 85pF gives a -3db point of the filter about 300kHZ. So that filter has no effect at all on 20kHz.
In some cases a passive might very well "rob dynamics" but if the system as a whole is designed right that will not be the case. Unless there is another mechanism causing it but so far no one has come up with one.
Maybe active preamp stages act as "dynamic expanders" and a passive preamp doesn't? :-)
Tre'
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- You have a good point. - Tre' 17:53:26 03/08/25 (0)
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