Bi Amp Issue Monitor 70

ceedawwg

I am attempting to bi-amp my 70's with the left and right channel of an Adcom GFA-7500(150wpc x 5) for the highs and an Adcom GFA-565(300wpc mono,dual spkr outputs) for the lows. I have checked my wiring 10 times over. With jumpers in place and only the 7500 running hi's and lows, they sound very good. Remove jumpers and add 565 to the lows.The hi's(drivin by 7500) all but dissappear. Same setup/experiment with my NHT VT-2's works flawlessly both ways. Only major diff being 6ohms nominal on NHT's and 8ohms on polks. Any idea's?

Jed Leland

Hello,
Welcome to CP. I'm not completely clear how you have the mono amp powering the bass/midrange of the Mon 70s. As a test try using four channels of the five channel amp as the bi-amp power sources? In other words, have channels 1 and 2 power the low pass section of the speakers and channels 3 and 4 power the high pass section.
Let us know what happens.
Cheers, Jed

ceedawwg

Low/Mids
- Sub out of receiver to line in on mono amp
- Spkr out(dual) of mono amp to 70's low pass(L and R)

High's
-Left main pre out of receiver to left channel of multi ch amp line in
-Spkr out(left) of multi ch amp to left 70 high pass

-Right main pre out of receiver to right channel of multi ch amp line in
-Spkr out(right) of multi ch amp to right 70 high pass

-As I said, if I unplug the 70's and plug in the NHT's(no other changes)it works perfectly. It just doesn't seem to want work on the 70's

Thanks for the welcome

ben62670

Welcome to Club Polk. Please don't feel insulted, but biamping Monitor 70's with nice equiptment like that is not worth any gain. Nice amps, but the monitors are not really top of the line. Please keep in touch. Let us know what you are running.
Ben

ceedawwg

Point taken and no offense taken as these are not a permanent solution. My NHT's are spiked in place and are not going anywhere soon. I was just curious what these 70's were capable of. I got a brand new set as payment for a debt ($200) and am just confused as to why they will not accept bi-amplification.

Jed Leland

Hello,
I think I see the problem. Most Polk speakers have a crossover point around 2.5kHz. This means the mid range/bass components need to play to around this frequency to match what the tweeter is going to play. When you use the sub-out connection, on the receiver, for the signal that's going to the mono bass amp it is going to only play sounds 80 or 90Hz and below. This is way too low a crossover frequency. The mid range/bass speakers, in the Polk, are only going to play up to 80/90Hz and leave a very large gap between where the tweeters are going to play.
The best arrangement would be as I suggested, true bi-amping. Sending full range signals to four identical amps, then each of these four amps connected to the corresponding sections of the speakers. No frequency shaping required.
Cheers, Jed

ceedawwg

Makes sense to me...I'll give it a try, thanks for the input Jed.