Hi Guys, I do have some thoughts on this one. For all the experts out there please excuse the depth I go into but, guess I got carried away. I've been thinking about this one for a while.
A long one...
Using your two channel gear as the front left and right channels in your home theater (HT) or other type multi-channel system.
What is a preamplifier pass-through?
Customers request a preamplifier feature they call “HT Pass-through” or “A/V Pass-through”. It is believed that this feature allows an easy and seamless insertion of a two channel audio system into a HT system.
IRD simply calls this feature an “HT” input. It’s just another source connected to the two channel preamplifier. The HT position is selected with the source selection switch.
The HT input has one difference from the normal source inputs, the HT input receives no gain (0dB) nor attenuation (0dB). This is where the name pass-through comes from. When the HT input is selected the two channel preamp simply acts as a unity gain buffer.
Using the two channel system as L&R channels in the HT can improve the sound quality of the HT system, save money and minimize the number of HT components.
The problem is that this basic HT input feature may not be ideal for every HT system.
Some two channel systems may do better and mate more seamlessly into a HT system using the two channel preamplifiers independent gain or attenuation capability.
Exactly how does the HT input work?
When you have a preamp with an HT input all that is required, to add your two channel gear to your HT system, is one set of interconnect cables.
The source of the HT input signal comes from the HT receiver’s decoded front L&R analog output RCA jacks. These are line level signals that are ready to be sent directly to a power amplifier.
Simply connect the receivers decoded front L&R analog output RCA jacks to the two channel preamps HT input RCA jacks, like any other source, using a set of standard interconnect cables.
The signal gain and volume control functions are preformed directly in the HT receiver before being sent to the two channel preamplifiers HT input. The HT remote control controls the systems audio volume.
When using the two channel preamplifiers HT input all the preamplifier functions are bypassed and it acts as a unity gain buffer driving the two channel systems amplifiers with the front L&R channel program content supplied by the HT receiver.
The HT receiver normally has built-in amplifiers, or other amplifiers may be used, that drive the center and rear channel loudspeakers which completes the HT multi-channel system.
Sounds simple, what could go wrong?
The ideal situation is when all of the HT loudspeakers have the same efficiency level & sound characteristics and all the amplifiers used have the same signal gain & sonic characteristics.
Small SPL adjustments for component differences and room conditions can be made with the HT receiver normally in the amount of about +/- 6dB. Even if this is all the adjustment required the adjustment system may not be ideal and many times can adversely effect the systems sound quality.
If you can get a HT system up and balanced correctly using a maximum of +/- 3dB of the receivers available adjustment you will have a great or almost perfect setup.
The problem is when the loudspeakers or the amplifiers used in the system are farther apart than +/- 3dB. This may mean you will not get the best sound. This is often the case when integrating a two channel system into an existing HT system.
The largest differences can be in power amplifier gains which can vary from as little as 18dB to as much as 40dB. An ideal HT system would use all the same type of amplifier.
Loudspeakers efficiencies are usually pretty close but can also vary more than 3dB. Placement and room effects also create SPL differences.
Matching the output SPL of all channels trying to use only +/- 3dB of HT receiver adjustment is what may go wrong when mating a two channel system with a HT system or building a HT system from scratch.
Is there another way?
The pass-through was invented because it is virtually impossible to set a gain of 0dB (x1) with a normal preamplifier.
The reason for this is that it is impossible to set a potentiometer with any degree of accuracy or repeatability, even if there is a silk-screen marking on the faceplate. Most preamps use a potentiometer type level control.
If you could easily adjust the two channel preamplifiers gain to 0dB (x1) you could put the HT input into any standard source input and it would be exactly the same thing as using the HT “pass-through” input function mentioned above.
Some argue that they would not want to do this because they may turn the HT system off forgetting to turn the two channel preamplifiers volume control down. They worry that when they turn it on next time, without checking the volume control, they may damage their loudspeakers at power up. This does seem a bit careless and not like the normal audiophile but, it is one more knob to remember at turn on–off time.
So what do you do if your system SPL levels don’t match close enough or you like to swap components often and never know what you will have next?
Well, with a step attenuator the 0dB (x1) gain problem is completely solved. A two channel preamp can easily be set to 0dB or any other gain required.
This also solves the problem of seamlessly matching the two channel loudspeaker SPLs to the HT systems other loudspeaker SPLs for any system and keeping the <= +/- 3dB rule to achieve the optimum sound.
This is a very expensive solution but, if your two channel system has a step attenuator why not use it to your best advantage?
Just remember to turn the volume down when you turn your system off.
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