Preamps with two main-outs

shadorn's comment about input impedance triggered a question I've been meaning to ask: Can the generally published spec for amplifiers' input impedance, e.g., 47K ohms for the Vincent, be simply measured at the input ports, presumably with the amplifier turned on, or there's a lot more involved in taking this measurement? Thanks again for all the education.

Nevertheless an active amplifier works as specified with input impedance as stated when powered up - I am not sure what happens to the input buffering on all amps when switched off - probably most remain high impedance but that is not a given hence my warning that amps are designed to be used when switched on.

Well said, Shadorne. It may or may not be an issue from a sonic standpoint depending on the specific designs that are involved.

Also, generally speaking I'd expect that the uncontrolled and unpredictable variations in input impedance that may occur in some unpowered components would be more likely to be an issue in the case of solid state designs than in the case of tube designs, particularly if bipolar transistors are used rather than FETs. Also, it's very conceivable that in many such designs the unpowered input impedance could even vary significantly as a function of signal level, which would increase the likelihood of adverse sonic effects on the signal path that is being used. But where the active device itself has a very high input impedance even when not powered, an example being the grid of a tube, the input impedance of the unpowered circuit would usually be determined essentially by resistors. And in those cases it would presumably be little different in the powered and unpowered states.

That possibility of uncontrolled and possibly varying input impedance in some unpowered components is the main reason, BTW and IMO, that having an unpowered component connected to the signal path might in some cases have adverse effects on sonics, that wouldn't occur if the component was powered up. Not the alleged phenomenon that was referred to earlier involving "out of phase backflow current."   Simple calculations will show that for cable lengths that are typically used in a home audio system energy reflected back from a destination component to the component supplying the signal will arrive at the component supplying the signal with a phase delay in the vicinity of 0.2 degrees at 20,000 Hz, and with vastly smaller delays than even that at mid-range and bass frequencies. Not to mention that reflection effects will be essentially non-existent at frequencies below RF.

Best regards,
-- Al
   

Pick up awards at the Wolf Garcia center any time. I have had 2 to 3 things hooked up to my preamp outputs (2 RCA and one XLR) with zero apparent issues for a while now, currently using 2 (tube power amp and a SS amp used for deck speakers) as I no longer have anything that utilizes the XLR outs…the third thing was a little headphone amp…may have to get a pair of XLR adapters to get the headphone amp back in the system, although it’s rarely used. I had wondered about a related topic when Morrow noted you can break in their cables by driving them to another component that is off (preamp to unpowered power amp for example). Seems weird as I would think a component that’s off would act like something that’s not there at all and not draw any signal...but hey…maybe Morrow knows things.

Can the generally published spec for amplifiers’ input impedance, e.g., 47K ohms for the Vincent, be simply measured at the input ports, presumably with the amplifier turned on, or there’s a lot more involved in taking this measurement? Thanks again for all the education.

No, in general I don’t think that measurement could or should be performed with a simple multimeter, Kalali, if that is what you are asking. For several reasons. First, the input impedance at the zero Hz (DC) frequency that is put out by the meter might be much different than the input impedance within the audio band. The input impedance might even be essentially infinite at 0 Hz if a coupling transformer or coupling capacitor is present, while being far lower at audio frequencies. Also, depending on the specific design of the amp and the meter the amp might be over-driven. And if a resistance scale is chosen that would result in a suitably low test voltage being applied, I’d imagine that in many cases a meaningful reading would not be obtainable.

A good way to perform such a measurement, which I suspect is how John Atkinson does it for Stereophile, would be to make use of an audio frequency signal generator whose output impedance and output voltage can be varied in a controlled manner. When a suitable voltage is applied and the output impedance of the signal generator is adjusted such that the unit under test loads down the output of the generator to half the amplitude that is present when its output impedance is set close to zero, that setting of the signal generator’s output impedance would be equal to the input impedance of the device under test, at the frequency that is being generated.

Best regards,
-- Al