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Subwoofers with current source First Watt?

All, I’ve got a rather esoteric question that I hope those of you more technically minded than me might be able to help answer.

Today, I tried hooking up a pair of REL subs in stereo configuration to my First Watt F2-J via the high level Speakon connection. I won’t bore you with all the details of my daylong adventure, but both the subs and the amp behaved strangely connected this way; the sub seemed to significantly reduce the amp’s volume in one channel, while the subs themselves appeared to be receiving very little signal. This was consistent even when swapping the subs to different channels, changing the cords out, etc. Strangely, plugging in and unplugging the sub would change the amp’s volume, even if the sub was turned off. Unsettling stuff.

The F2-J is a current source amp - would that explain these strange results? Is it just not possible to run subs from this amp? Is there any danger to amp or subs here?

I also tried the subs with my SIT-3, which seemed to work appropriately. (Results were still unsatisfying, on the whole, but that’s an entirely different topic.)
I know there are some issues with running subs from Pass Labs amps, so I just want to make sure I’m not trying anything stupid with my First Watts. I can email Nelson about this as well but thought others here might have some thoughts to offer.

Thanks!
zm
cal3713's avatar
cal3713
1,421 posts
 
@zm Unfortunately it looks like you already have your answer through experimentation. Clearly something isn't working appropriately.  Still worth finding out why though.

Pass labs is always helpful with such questions, but if you want alternative perspectives consider posting over at: https://www.diyaudio.com/forums/pass-labs/

Folks over there are very knowledgeable about the first watt designs, like being helpful, and may even help you engineer a solution. A great corner of the internet.
zm OP
121 posts
 
Thanks @cal3713. That’s good advice, I’ll check in with those guys. 
twoleftears's avatar
twoleftears
5,318 posts
 
There are certain amps (because of their topologies) that you can't hook up subs to in the usual way.  I know that Ayre is one of them.  Some of Aesthetix's designs.  Perhaps this one is too.
three_easy_payments's avatar
three_easy_payments
2,097 posts
 
I had looked into this when I used to run a Pass XA30.8 and was contemplating adding a sub.

From the Pass Labs Owner’s Manual regarding subwoofer connection: “Some powered subwoofers require an audio signal ground reference, and their makers may suggest that a black speaker terminal is where this voltage reference might be found. On complementary designs, which would be typical of all power amps produced by Pass Laborato­ries™, this is an incorrect and unsafe assumption. On all Pass Labo­ratories™ power amplifiers a reference of 0 volts will be found only at pin 1 of the XLR input or at the shell of the RCA input. Reference connections for these devices require special consideration when used with our product. If you have such a device, and wish to use it with our product, please contact our Foresthill, California fac­tory for specific instructions.... you have been warned.”

Personally, I'd consider using the line-level connection from the preamp output as this eliminates all issues of negative speaker terminal connections and potential conflicts.
jl35
3,999 posts
 
REL subs are hooked up differently to SE and Balanced amps, but on some SE amps they need to be hooked up as if balanced...Pass should easily answer this, as they are often asked...my friend has 2 REL subs hooked up to an X250.8 and it works great
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