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
Why aren't you using the line level inputs??
My understanding from REL’s documentation is that they are designed to be integrated with the mains using a high level connection and that is the recommended method for using them.  At any rate, my preamp does not have a line out to supply the REL with a line level signal.
@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.
Thanks @cal3713. That’s good advice, I’ll check in with those guys. 
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.
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.
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
buy a splitter for you pre amp outs. Send one set to the amp and one set to the REL. 
Have you reached out to REL’s support desk?  They are very responsive. I had a similar question and they responded fairly quick. Not sure with the holidays though
definitely stick with the speaker level connection if possible 
Hello,
I own two REL T 9s. If the amp is differential, linear balanced you only connect the positive red, yellow, or both for more power to the amp. It is bad for the amp to use the neg or black wire. The best is to call Pass. If you have one sub( you should have at least two.) just connect the red to rt pos terminal and the yellow to the left pod terminal. Cap the black with a wire nut, tape, or plastic bag. The only place you would connect it would be the screw on the balanced connection. But it won’t work for one sub due to getting info from two differential terminal. So you just use the pos (red, yellow) and leave the black alone. Please call pass before hooking up the black wire. Happy Holidays!
Thanks everyone for their guidance on this. I will definitely be in touch with Pass before proceeding any further with the F2-J. The SIT-3 manual explicitly states that the black terminal is ground and that it is safe to drive the inputs to subwoofer amplifiers, but the F2 manual says nothing about that.

Will report back just to archive the response for anyone in the future who might be in a similar situation searching the forums for an answer on this.

Thanks again and happy holidays!

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.
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.)
Look at the "OutputOhms" of both amp may explains the results, SIT-3 is 0.26 and F2-J is 15 ohm! The REL subs were load down the F2-J due to its high output impedance.

http://www.firstwatt.com/prod.html

@imhififan, indeed, I am wondering myself if the F2-J's high output impedance is one reason this happened.  But the REL's impedance is 100,000 ohms, which should produce no additional load for the F2-J? 

I admit I don't understand circuit design and basic electrical engineering well enough to understand all this.
indeed, I am wondering myself if the F2-J’s high output impedance is one reason this happened. But the REL’s impedance is 100,000 ohms, which should produce no additional load for the F2-J?
You are absolutely right about it and made me to dig deeper and found that ( correct me if I’m wrong ) the F2-J is inverted phase at output?

http://www.firstwatt.com/pdf/prod_sit2_man.pdf
Output Connection - You can hook this amplifier up to any loudspeaker without danger of damage. Note, however that the (+) Red output connection is actually grounded. The (-) Black output terminal is the live connection. This can be important when you are hooking up active subwoofers to the output of the amp –treat the Red output as ground, and the Black as active.
It may applied to your F2-J?

@imhififan I wouldn't generalize from one First Watt amplifier (the SIT2 link you posted) to another (the F2-J).  Aside from using a basic shared power supply schematic they are (often) completely different designs.
Thank you, @cal3713
“The more I learn, the more I realize how much I don't know.”
Albert Einstein


Just to close the loop on this after hearing back from Nelson Pass. @imhififan was right, actually, although I agree that we should be careful not to generalize about First Watt topologies - but like the SIT-2, the F2 is a phase inverting amp, and the red (+) terminals are ground and the black (-) terminals are live.

Nelson advised to hook the subs up reverse polarity, which I did, and everything is working great. It’s a bit unnerving to hook the REL’s ground wire up to a positive terminal!

I wish that Nelson had put the information about the phase inversion (which is in the SIT-2 manual) in the F2 manual! Would have saved me a headache here...

So for the record and any future adventurers who find this thread - if you have a phase inverting amp, take care with how you make a high level connection to a subwoofer amplifier!
Good work @imhififan and glad it was such a simple solution. Congrats.
@zm
Glad it done successfully ... thanks for letting us know.