Switching preamps in and out

Wolf's responses are right on the money. Since your amps have volume controls on their inputs, you can turn them down to zero and then safely disconnect and connect i/c cables. This will leave the power amps powered up continuously and avoid stressing them through repeated off/on cycles. It also means that you avoid any changes in sound due to the power amps being turned on and off.

As far as the preamps are concerned, you can certainly leave them all powered up so each one is ready to go when you want to change from one to the other. This will leave each one in a good-to-go status without worrying about changes during warmup. You will be switching the input connections for each preamp as well as the outputs, so that only one preamp is actually connected to the source at a time. The idea of running all 3 preamps in parallel and using a switching box between the preamps and the power amps makes no sense at all. You not only have the sonic colorations of the switch itself but also the unnecessary i/c cables. It is far better to take advantage of the amps' volume controls and then move all of the input and output connections from one preamp to the next.

Two final comments. First, leaving the unused 3 preamps powered up while you are listening will change the sound compared to just having the one being tested powered up. There is nothing wrong with leaving all 3 on for some initial comparisons, but eventually you need to listen while just one is powered up. The true sound of the preamp will only be demonstrated when the others are turned off and the power cords unplugged from the outlets.

Second, the point about needing to do longterm listening is valid. There are aspects of the sound that become apparent only through longer listening. Sometimes differences are masked when you change quickly from one component to another. That doesn't mean that you shouldn't ever change quickly from one to another, only that you shouldn't use this as the sole means for evaluating components.

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.