Originally Posted by Jim Cate
Could someone direct me to an explanation of the circuitry involved in a Home Theater bypass. - Perhaps a simplified circuit diagram might help. - Is this a feature that is available only with recently introduced, expensive preamps? And if it involves switching the input from the preamp to the HT processor, why couldn't that function be done with inexpensive switches available from electronic supply sources. AFAIK, we are talking about relatively moderate frequency ranges and relatively low level currents. Correctly if I'm wrong, but I can't understand why I would need a $2,500 preamp (rather than three good used legacy preamps, along with some switches) to get the desired result.
It is not a new feature; it's been around for a while. You can certainly find used models with this feature. All you do is feed the signals from a couple of 2 ch sources to a preamp, along with the L+R out from your HT pre-pro or receiver to the preamp into the HT passthrough, which is just another high level input. The preamp line outs get fed to a dedicated amp (there are also integrated amps with HT passtrough - in which case the signal goes to the built in amp). When you are in HT passthrough your HT out bypasses the preamp gain stages (or maybe gets fed through the gain stages at a fixed gain of 1). You certainly don't need 3 preamps or anything wacky like that. If you are listening to a 2 ch source the HT receiver is not in the signal chain, only the preamp.
Another way of describing it is that it is the same thing as a stand-alone 2 ch music system - typical sources like a CD player or turntable, preamp, amp and two main speakers. The two main speakers are also the L+R channels of a HT system. The HT L+R signals are fed into the HT bypass channel (which is a fixed unity gain channel) of the preamp from the HT receiver, this channel, when selected feeds the 2 channel amp and the two speakers connected to it.
The reason that you see premium prices for these is that they are low volume specialty items and there is no point in using a cheap preamp that is no better than the 2 ch capabilities of the receiver you already have. It only starts making sense if you are trying to get very high quality 2 ch playback. And no, you can't get the same effect with a set of switches - you need the high quality analog processing that the dedicated preamp provides.
Theoretically you could just feed your HT out into a conventional preamp and calibrate the preamp volume control so that it matches the other channels of your HT. How much that would compromise the HT sound would have to be tested and would be dependent on the preamp model.