[chadh]

How does HT passthrough work?

I understand that the idea is to take the analog signal from the HT processor, bypass the volume control in your pre-amp, and send the signal directly to the amp and speakers.  And if you have only two sources (HT and something else), it seems pretty easy to do this.  All you need is a selector switch that connects the pre-out either to (1) the HT input; or (2) the pot.  The source that isn't HT is connected to the pot, so that the switch now selects between the sources.

But if you have three sources (say HT, CD and Phono), how do you accomplish source selection with a single switch?  Selecting between CD and Phono must occur before the pot, and choosing whether or not to bypass the volume control seems like it should happen after the CD and phono inputs have had the chance to go through the volume control.

I'm wondering about this because I'm thinking of getting a new pre-amp.  It will be customised, and tubed.  It will almost certainly have to play double duty in the HT set-up too.  But this creates a problem:  I don't think I could trust my family to turn off the pre-amp after using the HT.  So one option would be to have the HT input providing a completely passive path through the pre-amp, avoiding any amplification and the volume control, so the pre-amp wouldn't even need to be switched on.  

It's easy to see how that's done with a single selector switch if I have only two inputs.  But if I want three inputs, would I need to have two switches?  or is there a more clever way of doing this?

And is this really much of a problem anyway?  For HT purposes, I don't really care whether I need to balance two volume controls for the front speakers (in the HT receiver and the pre-amp).  My concern is really only with the idea of HT use leading to the pre-amp being left on for inordinate amounts of time.  Should this be a concern?

Chad

[flintstone]

Preamps with pass-through do not need to be turned on, also, their volume control is not active.

Your thinking of the unity gain method of pass-through which is what you need to use if your preamp does not have true pass-through feature.


Dave

[ted_b]

HT bypass or passthru is usually implemented as a bypass switch, unaffected by the selector knob.  Say the HT bypass input is input no. 5.  Well, when the bypass switch is active the signal from input 5 goes through ungained and unprocessed.  Input 5 is still an input, selectable from the selector knob, but in that case (switch in down position) it would be processed and have a variable output.  This is a nice way to allow your front l/r amp and speakers to stay out of the game, and still allow you to listen to 2 channel from the pre-pro if you want.  The Modwright SWL 9.0 SE is implemented this way, as is the Adcom GFP-750.

As far as power up methodology, I always turn on my preamp first and turn it off last.  That way you don't risk that the power amp hears the pre turning on...can be dangerous pulses there.  If my college son (the only one in the family who plays stuff in the HT anyway) was in there I always check on power status anyway.  Luckily, the SWL 9.0 SE has a muted power up feature that returns the volume pot to zero, etc to guard against kid issues (or senior moments).  

[chadh]

HT bypass or passthru is usually implemented as a bypass switch, unaffected by the selector knob.  Say the HT bypass input is input no. 5.  Well, when the bypass switch is active the signal from input 5 goes through ungained and unprocessed.  Input 5 is still an input, selectable from the selector knob, but in that case (switch in down position) it would be processed and have a variable output.  ...

Thanks.  So this really means that to have multiple sources and HT passthrough you need two switches:  one that selects which of multiple sources delivers a signal through the pot and a second switch to chooses whether the outputs receive a signal directly from the HT input or from the pot.

The question left is the advice question:

If you have a tubed pre-amp and wish to integrate your HT and audio systems, do you think the HT passthough is important?  The benefit is that I wouldn't have to have the tubed pre-amp powered up while the HT system was on.  The disadvantages are two-fold:  it places another switch in the signal path of all of the audio inputs, plus it costs money!

I know that nobody can tell me what the right choice is for me, but (as someone who has never owned any tubed equipment) I was wondering if there was any insight about the consequences of potentially leaving the tubed pre-amp on for extended periods, and any feeling about the degredation to the audio signal associated with placing another switch in the path.

[Note:  I'm not really concerned about the degredation to the HT signal when passing through the extra switch.  But the signal from my audio sources would also pass through the extra switch, as that switch seems like it must choose whether the pre-out takes its signal from the pot or from the HT input directly]

Thanks,

Chad

[mcgsxr]

When I had a custom tubed preamp built for me around a year ago, I requested that it have an HT bypass, so that I could combine my HT and 2 channel systems exactly as you describe.

The unit was based around the Hagerman Clarinet, and was left OFF when I was watching movies, and engaging the HT Bypass, so there was zero concern about tube life for HT use.

I cannot honestly say that the switch did not make a difference, since I never heard it without it, but that was a great pre (built by an AC member and sold off to another AC member this year), and I loved it.

Sold it for two reasons.

1 - new house, new space for dedicated 2 channel in basement
2 - SB3 entered my life, and actually allowed me to sell of a ton of gear, since it acts as the transport/DAC/pre for my system today.

I would not flinch at ALL to include a good HT bypass in a tubed pre again, should I desire to return to that type of setup, it was very useful to be able to use the same amps/speakers etc for my music and my wife's movies!

[PhilNYC]

Thanks.  So this really means that to have multiple sources and HT passthrough you need two switches:  one that selects which of multiple sources delivers a signal through the pot and a second switch to chooses whether the outputs receive a signal directly from the HT input or from the pot..

Isn't this just one switch?  For example, the input selector switch has 3 positions...CD, Phono, and Passthru.  The first two positions are wired to go thru the volume control and the Passthru is wired directly to the preamp's output.

[ted_b]

My tubed Modwright SWL 9.0 SE is the most musical pre I've ever heard, period.  It has an HT bypass.  Have I heard what it might sound like without an HT bypass?  No.  However, it is not an issue, it's a simple clean relay (not a switch) in the path.....and I'm an analog elitist!!      Pre must be on, though, cuz Dan has the mute circuit closed when its off.

Dan might want to comment.

[chadh]

Thanks.  So this really means that to have multiple sources and HT passthrough you need two switches:  one that selects which of multiple sources delivers a signal through the pot and a second switch to chooses whether the outputs receive a signal directly from the HT input or from the pot..

Isn't this just one switch?  For example, the input selector switch has 3 positions...CD, Phono, and Passthru.  The first two positions are wired to go thru the volume control and the Passthru is wired directly to the preamp's output.

Okay - that's possible.  But I just don't see how such an arrangement works internally.

Suppose I have three inputs: 1, 2 and HT.  And suppose these are wired to a single input selector switch.  To what does the other side of the switch connect?  presumably it connects to the active circuitry and the pot.  But this means that the HT doesn't bypass the preamp active circuitry or volume control.

What seems possible is that there is a switch that controls whether the HT signal or the signal from the gain stage and volume control will go to the pre-out.  But such a switch cannot help with selecting which of multiple sources will go to the gain stage and volume control.

So...if source selection and passthrough can be accomplished with one switch, how do you do it?

Chad

[PhilNYC]

Chad...ah, I see what you're saying.  My bad, I misunderstood, and I think you are right (unless a manufacturer made a custom switch that had a separate out for the 3rd input position)...

[pugs]

I just switch to AUX on my Odyssey Tempest for HT bypass

[hmen]

I'm sure there are many ways of implmenting HT passthrough. This is how I do it:
 My 2 channel preamp is a BAT 31i. There are five inputs and you are able to program an input to have extra, less or fixed gain.
  My Reciever is a Marantz 7200. It has pre outs.
 I run the center and rear speakers directly from the receiver and run  the the pre outs for the front speakers into one of the inputs of the BAT. I  fixed the gain on the input to a level where the front speakers blend in when I'm using the reciever. It only took a little playing around to get the right level.
 I guess you could do this with any preamp if you find the right level and just change it to that level whenever you use the HT. However, if you do this make sure that your receiver and and 2 ch pre don't use the same remote control command language. Marantz and BAT both use the same command language so every time I adjust the volume one machine it changes the volume on both (except for the fixed gain input.) All you need is a receiver with pre outs or a HT preamp.
 If you're getting a customized pre then being able to set a fixed gain
on one input or having the gain on one input set by the factory is all you really need. You can make adjustments on your receiver as well so you do have flexiblity on how the gain could be set by the factory.

[Levi]

I have an HT and 2ch setup.  It is probably similar to hmen's implementation.  I use the VK-30 pre and VK-500 for 2ch (front L-R) and Bryston SP1.7 pre and Bryston 9B-SST for the center and rear.  I connect the Bryston SP1.7 balanced out to the VK-30 balanced input.  Whenever I am ready to play 5.1 channel for Movie watching.  I simply switch the VK-30 into unity gain.  The SP1.7 takes care of the levels for all 5.1 speakers.  Volume will then be adjusted via my Bryston Pre.  Since the VK-30 is being used as the source, their is a slight tubey sound when there is music being played in movies.  I like it!

If any of my family member wants to watch regular shows (Oprah, Olympics, Sports etc), I just turn the Bryston on.  99.999% I can get away with it.  It is not suprising that the center channel is the most important part of any audio video setup by anchoring the sound to the screen anywhere in the circle.  Well that's another topic.

[Doug_B]

I just switch to AUX on my Odyssey Tempest for HT bypass

And the Tempest passes the HT Bypass signal through when the unit's power switch is off.

Doug

[chadh]

Thanks everyone for the info.

Chad