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In Reply to: preamps: passive versus active posted by Bigromano on April 3, 2008 at 16:19:56:
Obviously, not all active preamps and not all passive preamps are the same, and all types will react differently in different situations.
There are, of course, SS and tube actives. Some have more gain, some have higher output impedance. Some offer only balanced inputs and outputs, some only single-ended, some a combination. Some are truly differential, some use additional circuitry or transformers to achieve a balanced output. All of these factors will affect the sound differently depending on how they are connected and what they are connected to. There can be very little or lots and lots of circuitry, lots of features, filters, controls, etc., all of which will somehow alter the signal. They require electricity to operate, generate heat and noise, and parts can deteriorate or fail.
There are different types of passive, and all have different impact on the sound. Basically, all passives do is allow one input signal to pass through a volume control to the output without any gain. A balanced input can be passed in the balanced mode or passively combined to be single-ended, but a single-ended input cannot be made into a balanced output unless transformers are employed. The source is usually selected by means of a mechanical switch. The volume control can be one of three types: a rotary switch that selects one of a series of resistors (ladder)or an equivalent variable resistor (pot) through which the signal passes directly; a variable resistor (pot) that shunts a portion of the signal to ground compared to a single, fixed resistor through which the signal passes; or a transformer with multiple taps which offers different levels of voltage depending on which tap is selected by a rotary switch. The type of volume control has more effect on the signal than any other aspect of the preamp. Each type offers distinct advantages or suffers unique problems, but they all share the trait that they cannot increase the power level coming from the source. Some types of control are sensitive to source or load impedances, some are seriously affected by cable capacitance, some lose power at low frequencies or compress dynamics. There is no power supply and hence, no noise or heat, because no electricity is consumed (except by a motorized volume control), there is no typical lighted display, and there are almost no parts that will ever wear out or change over the life of the component.
I use a passive preamp to drive three separate systems and two powered subs. The systems are spread throughout my house, with a total of 60 to 70 feet of balanced and single-ended cable in each channel, and I never sense any loss of dynamics or power at any frequency. Those who relate the sonic compromises of passive preamps are usually drawing on anecdotal information or recalling former misapplications of bad designs. When properly designed and deployed, a passive preamp should provide a cleaner path than an active device, giving a more transparent and realistic image of a musical event.
Peace,
Tom E
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- RE: preamps: passive versus active - madisonears 23:09:52 04/03/08 (0)
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