Why does the I/C from the source to passive pre have to be as short as possible while from the passive pre to power amp can be longer?
Who is using passive preamps and why?
Seldom has there been any discussions on passive preamps in the forums and although my experience with them has been limited I have found them so far to be very enjoyable and refreshingly different. They seem to fall into their own category, somewhere between solid state and tube. Finding a preamp that is satisfing has been difficult. Some active solid state preamps can be very good but they seem to inject grain to some degree in the upper registers and some tube preamps are not too far behind. So far I think they should at least be matched up with an amp that has sufficient gain which is often overlooked. Which passives are you using and with what amp? Why do you like them?
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Cdc: Why does the I/C from the source to passive pre have to be as short as possible while from the passive pre to power amp can be longer?Actually, the reverse is true. The high output impedance of a passive preamp will form an RC low pass filter in conjunction with the capacitance of the cable that is connected to its output. The longer that cable is, and the higher the capacitance per unit length of the cable, the more likely it is that significant upper treble rolloff will occur. The relatively low input impedance of a passive preamp will produce a minor increase in the amount of current flowing through the cable between source and preamp, thereby perhaps increasing some subtle cable effects if the cable is long, but those effects will be minor in comparison to the effects of excessive capacitance in the cable connecting passive to power amp. Clio09:Question: Jack tells me the transformers are wound for 100 ohm impedance, meaning the source must have an output impedance of 100 ohms or less. Any of this make sense?Transformer impedance ratings, as distinguished from the reflected impedances corresponding to the connected source and load impedances and the turns ratio, have always been something of a mystery to me. Perhaps someone else will comment more knowledgeably. But I believe that the rated impedance of the transformer itself corresponds to how lossy the tranformer is in terms of flux leakage and dc resistance, and represents the amount of current limiting that would occur in the primary with the secondary short circuited (and a voltage source with near-zero output impedance connected to the primary). The higher the output impedance of the source component, the greater the losses that would result from that transformer impedance. Best regards, -- Al |
Almarg: The higher the output impedance of the source component, the greater the losses that would result from that transformer impedance. While I only quoted part of your response Al, Jack did mention DC resistance and sound degradation, particularly in the bass the higher the output impedance of the source. The PVA is really designed for opamp driven sources and when I asked if I could use my 600 ohm Zout Otari tape deck with the PVA I was told that I would get some bass distortion and potentially other losses due to the mismatch. With my Kenwood KT-8300 tuner and digital set-up the sound is fine. |
It has been a long time since I read Stereophile, but Sam's Corner in the February issue has a write up of the old Halcyon PVA and the Lightspeed Attenuator that I own. The article prompted me to put the Lightspeed back in my system for a whirl. I'm not regretting it. This opto coupler designed passive is truly remarkable. As usual careful system matching applies. I believe minimum amp Zin is 50k ohms and high sensitivity is best. Here is some additional information for anyone interested: Lightspeed Attenuator Single input/single output only. I have a version with dual attenuators. One of the designers comments is that this type of design removes the sonic effect of the switch from the equation. |
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