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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
 
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!

imhififan's avatar
imhififan
2,199 posts
 
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

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
2,199 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.
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