Passive Linestage answers
Robin said:
Mitt,
Thank you for your positive response, ideas and suggestions as well...
Please see my new post above (to Spike)....
What is a Passive Linstage? How does it work? Is it an amp or pre-amp or a better switching devise? What would be a good brand to consider for a Passive Linestage?
Robin,
First off, I'm an amature, not an expert, so others may want to weigh in here. I do have some understanding and will elaborate as succinctly as I can, but a bit of history regarding how Pre-amps in general and volume controls specifically work is necessary. Sorry if this is too rudimentary, but I don't know how else to explain it.
All of our systems consist of source components (cd players, turntables, tuners, tape decks), amplification components (phono stages, Pre-amps, power amps, integrated amps, receivers - this last includes a source component in the tuner section), and transducers (speakers or headphones).
Source components typically output very low level signals called line level signals. Phono cartridges - especially moving coil cartridges - produce such a low level signal that an additional amplification stage (a phono stage) is required to boost this very low level signal up to a line level. Line level signals are the inputs to your typical receiver/processor/pre-amp. The minimum function of the input stage of your receiver/processor/pre-amp is to provide switching functions between sources and volume control. Many receivers/processors/pre-amps also have controls for things such as balance, phase, tone controls, tape loops and in the case of many receivers and processors, multi room controls, Dolby Digital and DTS processing etc. Some, but not all, of these devices also add an additional level of amplification to provide even more gain to the signal that is then fed to either an internal or external set of power amplifiers -2 channel, 5.1 channel, 7.1 channel etc.
The amount of gain contributed by a pre-amplifier stage to the audio signal is the core issue that I am trying to expound upon here. The differences between passive line stages, good pre-amps, and lower level pre-amps and receivers centers around how gain is employed (if it is employed), and how the volume control on these components operates. In a passive line stage there is no additional gain stage in the signal path. Line levels signals are passed through the device by means of a very high quality switching network and volume is controlled by means of an attenuator (usually a series of very high quality, closely tolerenced resistors or resitor networks). Run at maximum volume, the attenuator adds no resistance to the signal path. Volume is controlled by adding or subtracting resistance.
High Quality pre-amps actually function in much the same way to be honest with you. Full function pre-amps (those that include a phono stage) do have gain associated with the phono stage, and most have some amount of gain associated with their specific circuit topography, but many, depending on their design, function essentially as a passive device through at least a portion of their overal gain capability. That is to say that, depending on the circuit designers intent and execution, gain may not be added until the volume control (or attenuation) reaches a certain point, at which time changes in the volume control deviate from gain attenuation to active amplification. I know that BAT, ARC, Conrad Johnson, McCormack, Manley, Cary and others employ such hybrid (passive/active) type circuits to one degree or another.
Lastly, lower level components and receivers function as more fully active devices, often employing single resistor networks or even single solid state digital circuits (chips) to control volume functions that are handled by the many closely specified discreet devices that make up higher quality attenuators and volume controls. A lot of the money that we spend on more expensive equipment of this type is tied up in these high quality discreet components that literally cost pennys to manufacture as mass market solid state devices.
If you are interested in a passive linestage many would say that the best one available is the Audio Synthesis Passion shown below. It has 3 line level inputs as well as a "Direct" input that bypasses the input selector completely. This may be just the sort of thing you are looking for, although the Manley Steelhead that aliveatfive and Spike mentioned is a supperb unit as well.
Keep in mind what I mentioned though about the circuit topography of many GOOD modern pre-amps, and of course, above all, trust your own ears.
