Switching preamps in and out
Thanks everyone for input. Here are components. Everything purchased here on Audiogon. I agree with long term listening but I want to enlist some other ears too, so we will be doing switching in and out on that night. Amp is -VAC Phi 300.1a Source is- Playback Designs MPS5 Preamps are: -VAC Sig MKIIa -Messenger -Allnic L-3000 Cables are all single ended |
Mike, the output impedance of the MPS-5 is extremely low, as indicated in the third paragraph of JA's measurements in Stereophile, so I don't think that it would have any problem driving all three preamps at once, via splitters or y-adapters, even though the input impedance of the Allnic is only 10K. The input impedances of the other two preamps being much higher, 250K for the Messenger, and probably something like 67.5K for the VAC, which is what JA measured for the Mk II (non-A). The parallel combination of those three input impedances is 8.4K, which is hundreds of times higher than the output impedance of the MPS-5. So IMO a good approach, assuming that the cost is worth it to you, would be to drive all of the preamps simultaneously, via splitters or y-adapters, and purchase the DB Systems switchbox to select which of their outputs is routed to the power amp. FWIW, I used that switchbox extensively some years ago and I did not perceive it as having any sonic effects of its own when it was inserted into the system. Although my system then was not quite as resolving as it is now. Regarding Wolfie's post, upon re-reading what preceded it I'm not sure that he was understanding that what I was talking about, given that the level controls on the amp would be turned down, was NOT the possibility of damage to the speakers (although I can envision ways in which even that might be possible). What I have been talking about is the possibility of either damage or degradation of long-term reliability of the input and/or output stages of the electronic components. If that wasn't clear, I hope he will re-read everything I said from that perspective. Regards, -- Al |
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! |
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. |
