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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
zm OP
121 posts
 
@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.
imhififan's avatar
imhififan
2,201 posts
 
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?

cal3713's avatar
cal3713
1,421 posts
 
@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.
Post removed 
zm OP
121 posts
 
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!
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