adding sub to:pas 3 pre amp' and st70 power amp system

hi i am new here and wanted help to add a jbl 18" sub driven by an adcom bridged to 600 watts mono. this will allow me to pick up the lower octive that my LaScalla arent able to because the horn lenth is not as long as needed to go that low i would like to have control over croosover point and the amount of rolloff.I have heard that with a beringer 2496 music prosseser would enable me to run two st 70 horinzontaly and have one channel two each of the horns [tube] and a yamaha to the woofer and the addcom to sub..any help will be alot of help Thank you guys .Im also using the new 7199 subtitutions in my st-70's with the tube socket addaptors.thanx again this is a great forum.

It sounds like you want to drive an Adcom power amp or a Beringer 3496 from the output of the PAS-3. Verify what the input impedance of those units is. The PAS was designed to drive tube amplifiers of about 470KΩ and cannot properly drive most solid-state components because their input impedances are too low. The result would be a roll-off in the bass in ALL your power amps, which would confound what you want to do.

There is a simple modification in the later PAS manuals that converts the output circuit so it can properly drive 100KΩ, the input of a Dynaco Stereo 120. That's about as low as you can go without
much further digging. Even some Dynaco transistor power amps (ST-410, ST-150, QSA-300) have too low an impedance to be properly driven by a PAS.

If the input impedance is too low for the PAS to drive properly (most Adcoms are, and the Beringer at 22KΩ definitely is), you would have to use some kind of buffer stage with a high input impedance and a low output impedance in between the PAS and the input of the solid-state device.

You can NOT compensate for the bass rolloff by increasing the size of the coupling capacitors; the output circuit of the PAS does not have adequate drive for such low impedances, which did not exist when it was designed.