Switching preamps in and out

Salectric, I would pose to you the question I posed to Wolf:

Have you ever connected the probe of an oscilloscope to a low or zero voltage circuit point on a piece of equipment, without connecting its ground lead, and looked at the voltage waveform that is displayed on the scope under that condition?

I assume that your answer, and his, would be "no." I have done that. Depending on the grounding configuration of the particular equipment, a 60 Hz sine wave measuring well upwards of 60 volts will sometimes be seen in that situation. And depending on the grounding configuration of the components in the audio system, a similarly huge sine wave might end up being applied to the inputs and/or outputs of all of the audio components that are involved in the interconnect swaps (including the source component), during the fraction of a second or so that the RCA plugs are being inserted or removed, due to the absence of a ground connection during that fraction of a second.

Do you really want to risk applying upwards of 60 volts to the inputs or outputs of components like these, at least some of which cost well over $10K, even for a fraction of a second? Inputs and outputs that are designed to generate or receive voltages that are around 2 volts or less? If so, be my guest. I would encourage others not to do so, even if the approach I suggested compromises the resolution of the comparison that will be made.

I am NOT saying that damage WILL occur as a result of the approach that you and Wolf are espousing. Most likely it will not. But I am saying that it creates a risk, especially given that RCA's are involved, and that it is therefore a very poor practice that should not be encouraged.

Regards,
-- Al

Peter,
Check this thread in three or four weeks. I will post which preamp I like the most and why. I have some initial thoughts, but I want to do more listening with a wider variety of music before I post any thoughts.

Salectric,
I hope you are right. I'm going to put an email in to Kevin Hayes at VAC, and see what he thinks.

I'm going to put an email in to Kevin Hayes at VAC, and see what he thinks.

Mention to him that you are using the unbalanced RCA output of a source component that is solid state, for which the output impedance is 11 ohms.

I would expect that in general solid state components will have greater susceptibility to damage or degradation of long term reliability in this kind of situation than tube components.

Regards,
-- Al

Al, I think we are still not communicating. When the power amp's volume control is reduced to zero, its input connection is shorted to ground. Yes, it's true that the input jack is no longer connected to the preamp's ground once the i/c is disconnected. However, you are overlooking that in any modern amplifier, the power amp is still connected to an electrical ground reference due to the ground wire for the AC power connection. The amp is sitll referenced to ground when the input cable is disconnected. I agree with you that if the gentleman was using a vintage amp from the 1950s that used a simple 2-prong AC power connection, the amp would lose its outside ground reference if the input cable was disconnected and there might possibly be a momentary spike even with the inputs shorted. But not with a VAC or any other modern amp.

I agree, Salectric, that the chances of there being a problem in this particular situation appear to be slim.

I would note, however, that my own VAC Renaissance 70/70 Mk III amplifier has a 3-position switch controlling its ground configuration. One of those positions isolates circuit ground from chassis ground (except at RF frequencies), and hence from the AC safety ground that as you indicate would otherwise prevent circuit ground from floating to voltage levels that are significantly different than those of the upstream components, when the IC's are disconnected. I don't know if any sort of ground lift provision is included in any of Mike's components.

I would also note that two-prong power plugs were not just creatures of the 1950’s and earlier. The STAX and Tandberg components in my system, from the mid and early 1980’s, respectively, all have two-prong power plugs. And some people have cheater plugs on some of their components.

I would also re-emphasize that the front end situation has to be carefully considered, as well as the preamp-to-amp interface. Especially given that the source component is solid state, per my comment to Mike just above.

One of my basic motivations in this thread has been to dispel the impression, which threatened to be created early on, that in a situation where no signal can be propagated through the power amp one should necessarily feel free to do anything and everything with interconnects while power is on. As you appear to realize, but others may not, there are a whole lot of variables that can make that assumption risky. Including those I mentioned in my initial response to Wolf:

… what components are being used, what the designs of their input stages are, what effect a brief overvoltage might have on their long-term reliability, what their grounding configuration is, whether or not they provide a ground lift switch, whether or not he's using cheater plugs on any of his components, whether or not any of his components have two-prong power plugs, and whether the connections are RCA or XLR….

Regards,
-- Al