Source selector

chadh · « **on:** 20 Sep 2005, 04:23 am »

Suppose that my pre-amp has a tape output.
Also suppose I have two sources and each has its own volume control.

If I were to attach my power amp to the tape output from my pre-amp, would I simply turn my pre-amp into a passive source selector? Do I bypass the amplification stage and the volume control completely? Or do I only avoid the the volume control?

I envisage using an SB2 (with its own digital volume control) and the pre-outs from a HT receiver as my two sources. These both have their own volume controls, so the volume control on the pre-amp is redundant. I do need switching though. I would also like to keep the pre-amp in the signal path, as it serves as a high quality headphone amp. I just don't want to degrade the signal from the SB2 any more than I need to.

I guess I don't really understand the physical distinction between the tape-output and the standard pre-out on the pre-amp. Does the tape-out give a true line level output, while the pre-out is amplified and then attenuated? Is there something more? or less?

Less importantly, if the tape out is truly a line level output, so that attaching the power amp to the tape out turns the pre-amp into a passive source selector, would I need any power to get the thing to work? Or could the unit be switched off, unless I was using headphones?

Chad

mgalusha · « **Reply #1 on:** 20 Sep 2005, 12:36 pm »

Chad,

The tape outputs on many receivers and preamps have a buffer circuit to prevent the loading from whatever is attached to it from affecting the rest of the preamp circuitry. Often then is sub optimal and uses a low grade op amp. In this case you might be better off just building or buying a little switch box.

That would keep the signal path as clean as possible as no active components would be in place to alter the signal.

Mike

JoshK · « **Reply #2 on:** 20 Sep 2005, 01:05 pm »

Decware makes a inexpensive no frills source selector. Mapletree audio has one too. Course if you have an pension for DIY, this is about as easy a starting project as you can get.

chadh · « **Reply #3 on:** 20 Sep 2005, 01:05 pm »

Thanks.

I was afraid something like that was the case.

Chad

chadh · « **Reply #4 on:** 20 Sep 2005, 01:11 pm »

Quote from: JoshK

Decware makes a inexpensive no frills source selector. Mapletree audio has one too. Course if you have an pension for DIY, this is about as easy a starting project as you can get.

Thanks for the tips, Josh.

Chad

chadh · « **Reply #5 on:** 20 Sep 2005, 07:58 pm »

Let's imagine then that, despite my complete lack of experience with this sort of thing, I decided to DIY a switching box. It seems like it should be simple:

Two inputs each run to different legs of a switch. The other side of the switch is connected to an output. I attach the switch to a knob or something on the outside.

But...

in my case, I'd like to have two outputs available: one to the power amp and one to the headphone amp. Now, the headphone amp would only ever be used with one of the sources (the SB2 - I won't be listening to two chanels of a 5.1 movie soundtrack through headphones). So, I could...

1) wire input 1 to the headphone amp out AND to the switch, while wiring input 2 only to the switch;

2) wire input 1 and input 2 to the switch, and then have the output from the switch leading to both the headphone amp out and the power amp out; or

3) wire input 1 and input 2 to the source selector switch, then take the wires from the source selector switch to a second output selector switch, where I choose either headphone amp or power amp outs.

Option (3) seems most sensible, but adds an extra switch into the signal path. Is there a problem with either options (1) or (2)?

The other thing I was wondering about was grounding in this purely passive unit. There are grounding wires for all the inputs and the outputs. These can obviously be tied together easily. But do they need to "go" anywhere? or am I simply relying on the input or output cables to provide an appropriate path to ground, through a power cord on one of the other components in the system?

Chad

JoshK · « **Reply #6 on:** 20 Sep 2005, 09:45 pm »

In the cases of 1 & 2 when you have one of the inputs going to more than one output (without a switch) the problems becomes you change the load "seen" by the source. This is no different than wiring a source to two amps and the load impedance is now half of either of the two amps (assuming they are equal).

Assuming your sources can handle such a load there really isn't a problem, but you never really know for sure if that will always be the case if you change components or amp. Option #3 is the best way to avoid this but adds another switch in the signal path.

chadh · « **Reply #7 on:** 21 Sep 2005, 02:40 am »

Thanks Josh.

I guess I can afford to experiment a bit here...just as long as nothing sounds like it will be dangerous!

Chad