One of the questions I get fairly frequently is how to merge a two channel sound system with a surround system in a home theater. This type of approach will use the main speakers for stereo and possibly the subwoofer to improve the bass response of the two channel system while also having the ability to do 5.1 or more channels for home theater use. There are several ways to approach this type of system. All of these approaches must address several problems if the stereo and home theater sound are going to perform at their best.
When integrating stereo and home theater the volume setting on the two channel system should not change the main speaker volume level on the surround system. If this is not done the relative channel levels of the main speakers to the other channels in the home theater system will not be maintained along with the relative level of the subwoofer in the home theater configuration to bass extension in the stereo configuration and the relative level of the subwoofer in bass extension in stereo to LFE level in the theater. Another common problem is maintaining proper bass management in theater and stereo. It is important to blend the subwoofer with the other speakers such that the low frequency response is flat. Obtaining the desired ease of use when switching from stereo to home theater use is one reason to keep the volume control on the stereo preamplifier independent of the home theater system.
The easiest solution is to purchase high quality surround sound electronics and avoid a separate two channel preamplifier so that you can get the best sound possible in two channel mode without the complexity of having two preamps. Most modern high quality surround processors also have the benefit of including equalization that can compensate for speaker deficiencies. The surround processor will also include crossovers for bass management to properly integrate the subwoofer with the main speakers. I would consider an Emotiva UMC-1, Marantz AV-7005 processor or Classe SSP-800 for anyone interested in high quality stereo reproduction in their home theater. How these systems detect and process a two channel signal will be important to how the processor will work for stereo reproduction. Being able to force stereo processing with or without the subwoofer is a useful feature to allow maximum flexibility in sound reproduction. The Classe is one of the best available at this.
The next easiest solution is to use an analog preamp that includes bass management along with a home theater bypass for the subwoofer and main channels. This type of system will work best when the main speakers and subwoofer do not need equalization to obtain an accurate frequency response. Products that offer this feature include the Emotiva USP-1 and Parasound Model 2100. An example of how these connections will appear on the back of the preamp are shown below.
The next easiest solution is to use a stereo preamplifier with the surround preamp that has a home theater bypass, but does not include the subwoofer. This type of system will work best when the main speakers are full range and the subwoofer is not needed for low frequency extension. If the subwoofer needs to be used it will be difficult to integrate it properly in this type of system. The subwoofer needs to be high pass and low pass filtered with the main speakers or the frequencies where the two overlap will be exaggerated creating excessive bass response. This can be done correctly by using the low pass and high pass filters found on many subwoofers on the low level signal inputs. To use these the output from the stereo preamp must be routed through the subwoofer and then to the amplifier for the main speakers. The home theater preamplifier should be set to no subwoofer and large main speakers in this type of configuration. When the system has multiple subwoofers and external crossover is going to be required. Preamplifiers that offer this capability are VAC’s MKI, MKIIa, MKIII preamps, Conrad-Johnson PV14LS and Cambridge Audio Azure 840E. An example of how the connections look on the back of this type of unit are below.
The final options are to either switch the main channel outputs from the stereo and home theater preamps physically or through a switch. Goldpoint sells some nice passive switches the SW2X-I for XLR connectors and SW4 for RCA connectors.
What ever option is chosen careful setup of equipment and selection of the equipment will determine the quality of the results.