Shouldn't be an issue. I use the same hook up protocol with my ss amp. Both the speaker cables and the high level sub cable are connected to the same posts. You could check with REL, as their customer service is excellent, but i see no problem.
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Is it any kind of issue if the spades from the main speakers are touching the bare wire for the REL at the speaker terminal they are both connected to?
Not a problem. And if you can spare a minute to learn a little electronics you will understand why its not a problem for any amp, tube or solid state, or any sub, or anything else for that matter.
First, it can't be a problem because all you're doing is connecting another load in parallel. In terms of volts and amps this is no different than connecting another speaker, or changing from a 2-way to 3-way speaker.
Second, its not a problem in terms of load. Your sub amp is designed to run on line level voltage not speaker level. Line level is around 2-3 volts. Speaker level, well its a lot more. So what happens, the first thing that happens inside the plate amp in the sub there's a resistor, probably about 20k Ohms but could be 50kOhms. That's a lot of resistance! This does two things. One, it drops the voltage the sub sees way down to line level. And two, it means the load the amp sees is essentially nil. There's essentially no current draw on this circuit. The amp never even knows its there.
I know this is exactly how this works, by the way, because I used this to modify my tube integrated to provide a pre-out to drive my Dayton sub amps. There's even a picture right here on my System page. https://systems.audiogon.com/systems/8367 Those two resistors are all the difference between high level speaker out and line level pre- out.
Third and finally, electrically, as far as the amp is concerned, this is the same whether the resistors are in the amp or in the sub. In the same way it makes no difference whether your wires touch on the speaker terminal or anywhere else. Its all the same.
Right. When I was researching this for my mod there were sub amp circuit diagrams and any time there was high level inputs then always the first thing was a 50k or whatever resistor. The safe thing from a manufacturers point of view is higher value. Because if they use a 20k and the customer hooks it up to some 500 watt amp it might overload the input, but with 100k nobody makes enough power for that to happen.
You can definitely go down a rabbit hole with electricity, but there's a lot of really basic stuff like this that is not at all hard to understand and goes a long ways when you do. Just this one basic thing for example allowed me to mod my amp for $1.99 vs who knows how much if I had taken it to a shop or even done like one dude here said, sell it and buy one with a pre-out!