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Question about passive preamplifiers

413 views 9 replies 5 participants last post by  commsysman 
#1 ·
Been reading on passive preamplifiers and an issue I'm seeing is impedance mismatch between the source unit and amplifier.
If I'm running a Denon DP 300f turntable into a Music Fidelity VPS-90 phono stage and then into passive preamp will the phono stage effect the impedance I spoke of coming from the source unit?
 
#4 ·
A typical SS unit will have a Zout of 10kR. The passive pre will change the source Zout as seen by the power amp differently with different pot position. It will change it by zero at 100% as the pot will be a shunt resistor then, and most at the minimum position. Exact nymbers will be determined by the MF Zout, power amp Z in and pot resistance. Capacitance is most likely the most audible issue as at less than 100% of the pot (ie fully CW) it will roll off some of the top end. How bad depends on all the values just mentioned as well as the C.

I agree with Russ that level will likely be an issue. Whilst most CDP/DVDP/BRP will put out around 2Vrms, and therefore have enough to drive most power amps to clipping, I doubt most phono stages will.
 
#5 ·
Thanks for the feedback. I've also read, at Tortuga actually, that almost any tube amp will work with a passive preamp due to tube amps having high input impedances.
 
#6 ·
Talmost any tube amp will work with a passive preamp due to tube amps having high input impedances.
There the shunt capacitance will be an even bigger issue.
 
#7 ·
I see. Why would Tortuga make all these claims I'm just curious? Are they outright lying?
What will work with a passive preamp?
 
#8 ·
What will work with a passive preamp?
A source with more than enough level to drive the power amp into clipping (because a passive pre has only attenuation, no gain, a power amp with lower Zin and short low capacitance cabling between source and pre.

Why do you want one of these things anyway?
 
#9 ·
One could be falling in my lap and I was just wondering what I could do with it. It's sounding like more of a pain than I wanna deal with though.
 
#10 · (Edited)
Passive?




The thing is that "passive preamplifier" is a self-contradictory terminology.

If it is passive, it is not actually a preamplifier, but just an attenuator (volume control) and (usually) some input switching.

The input impedance of the Phono preamp is a match for either the MC or or MM cartridge. Its output impedance is probably on the order of 600 ohms.

The potentiometer used to control the volume in the "passive preamp" should be at least 1000 ohms to prevent excessive loading of the phono preamp's output. That is usually the case (the higher the better).

The Schitt SYS, for example uses a 10,000 ohm potentiometer, so that should work fine; very little loading at all. It is only $49. My only reservation about it is that it only provides for 2 inputs.

The input impedance of most amplifiers is 10000 ohms or higher, so there is no problem there either.

The Schitt SAGA, which is a true preamp with a vacuum-tube, has switching for 5 inputs and would be much better for most systems. It goes for $349.
 
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