Switching preamps in and out

I have only practical experience and my comment applies only to amps with gain knobs (like the OP's amp) where no...and I mean zero...signal gets anywhere in the amp when the gain is all the way down (rare in home audio, common in pro audio), thereby rendering the amp safe for changing input sources, which I've done thousands of times without issue (I am extremely lucky however)...wear and tear on the jacks are also a non issue unless you're pretty clumsy or really obsessed with constant switching out and in. Grounding issues will raise their ugly head even if you turned the amp off first, so there's that. Also, regarding listening for differences...it's possible to switch things around and hear differences immediately and decide if it's good or not so good...although living with things for subtle in depth consideration is always a good idea unless the auditioned item sucks too badly as life is short...all a matter of degree. Have fun in your frenetic input damaging preamp testing voltage mismatch explosion session!

Mike - I will be very interested in your findings. I currently have a VAC Renaissance 2 (modified by VAC) and had an Allnic L3000 for a while.

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.