# Power Amp Bridging



## Peter Stoffregen (Oct 27, 2012)

Hello Everyone;
I have just finished my new (sub)woofer cabinets, (two for stereo because I need to cross over at 100 or 120 Hz), basically copying the JBL 4645c.
I am using RCF LF18X401 drivers. They have a 50 mm max cone excursion but are about 3 db less efficient than the JBL woofers.
I have 3 Phase Linear Amps (I must be a masochist) Two 700B's and an old 700. I am running out of power at 350 W/chan, driving the subs.
QUESTION; Can I use the balanced (+ & -) output of my Rane Crossover to drive two series resistors, centre grounded, to produce opposite polarity signals for L and R inputs of the 700B's. One Woofer would then be connected across the Amp's hot output terminals.
I have some pro audio back-ground but would like some reassurance before I start experimenting. 
Thanks for any help, I really do not like the idea of using these new monster amp class H or multi-tier power supplies etc. Does not sound very Hi-Fi to me, although that could be condemnation before investigation,

Thanks again, from Peter in Hillsdale, Ontario, Canada


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## gazoink (Apr 17, 2013)

Peter Stoffregen said:


> Hello Everyone;
> I have just finished my new (sub)woofer cabinets, (two for stereo because I need to cross over at 100 or 120 Hz), basically copying the JBL 4645c.
> I am using RCF LF18X401 drivers. They have a 50 mm max cone excursion but are about 3 db less efficient than the JBL woofers.
> I have 3 Phase Linear Amps (I must be a masochist) Two 700B's and an old 700. I am running out of power at 350 W/chan, driving the subs.
> ...


I'm sending you a PM with contact for someone who will not only have your answer but be able to rebuild and upgrade the Phase amps. He's an amp designer, and I've had him do some amp repair for me, he's reasonable and the only guy I know who works on old Phase stuff.


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## gazoink (Apr 17, 2013)

Looks like you have private messages off. So PM me first, and I'll forward the contact.


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## Peter Stoffregen (Oct 27, 2012)

Hi Gazoink;
I haven't quite managed the private message thing yet, (I have less than 25 posts). Are you referring to White Oaks? I have rebuilt one of my 700B's with Joe's new PCB loaded with his components. I have e-mailed him with my latest query, but not heard back yet.


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## lcaillo (May 2, 2006)

There were a couple of discussions at AudioKarma about bridging these.


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## Peter Stoffregen (Oct 27, 2012)

Thanks Leonard;
I joined Audiokarma, and was given a jpg of an official PL service bulletin suggesting a transformer to with centre tapped secondary (for phase inverting the signals). Also, mentioning minimum load of 8 ohms. At least this tells me it's been done.
Someone said the trade-offs with bridging are reduced performance specs (except for power). I would guess that damping factor may suffer. This could be an issue with those heavy RCF cones. What is your opinion re; bridging. I never liked the idea, but I really would like to keep the 700B's in my system.
Thanks again, Peter
PS; Is there a newer amp out there that is "HiFi" and around 1000 W/ch. I was looking at a QSC RMX 5050.
I would appreciate any expert oppinions.


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## lcaillo (May 2, 2006)

I was never a fan of the 700 to start with. You do have to keep the impedance up and you are correct about damping factor. I would not mess with bridging. Just asking for problems, IME.


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## Peter Stoffregen (Oct 27, 2012)

Still trying to bridge my PL 700B's, so the subs can keep up with my line arrays. Here's another photo. 
Can any one suggest a reasonably priced mic/(level meter) to use with the REW program? I am using my old Radio Shack 33-2050. I have been warned that they are not accurate above 3 or 4 kHz, but I had to learn the hard way.


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## Peter Stoffregen (Oct 27, 2012)

Trying again, line array images;


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## Glen B (Jun 11, 2013)

Peter Stoffregen said:


> PS: Is there a newer amp out there that is "HiFi" and around 1000 W/ch. I was looking at a QSC RMX 5050.
> I would appreciate any expert opinions.


How hi-fi sounding does an amp need to be to drive subs? A good pro amp should serve your needs.


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## gazoink (Apr 17, 2013)

Crowns have been on sale lately... I think Sweetwater...


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## Peter Stoffregen (Oct 27, 2012)

I am concerned about hum and noise with an amp this size sitting with gains wide open. Also, Cone mass is 215 grams, so I suspect an amp with high damping factor would be important.


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## Douglas_Doherty (Apr 26, 2013)

I've used the 5050s on a number of occasions. They'll give you nearly 1600W into 4 Ohms and around 1000W into 8 ohms. They sound pretty good and are forgiving. Don't forget the ear can tolerate huge bass distortion (relative to .1%!). They are really really heavy! You might look at some of the Crowns too.


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## Peter Stoffregen (Oct 27, 2012)

Hi Doug; 
Almost bought a 5050. I have a RMX 2450, seems very well laid out inside compared to my Phase Linears, but I still would not use it for mids or highs in my home theatre system. Next experiment is to use the 2450 on my new subs and watch for the clip lights. The used equipment market seems more limited in Canada, I would like a Crown Macro 5000 or 5002 or even a Crown Studio Reference I (too pricey,and maybe a service future issue?) Crest CA 18 seems like much better specs than the RMX and maybe made in USA?
I know it's just for subs, but I am fanatical. The subs are bass reflex (front loaded) so S/n ratio is somewhat more important, I think. Also the 'subs' are crossed over at 120Hz. With peaks to 120 db at 12 feet, the line array 5-1/4" mids can't go any lower, see photo in this thread. Any experience with Crest,
Thanks Peter
PS; The old Phase 700 I am using for subs at the moment, I bought from "Triumph" and was rumoured to be used previously by "The Who". The owner's manual says it is normal for the VU meters to 'peg' on loud continuous passages, I bought the amp with the needles broken off! Found new meters but still not completely sure if the amp is clipping, but the inefficient new subs are pushing it too hard IMHO.


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## Douglas_Doherty (Apr 26, 2013)

I've used the 2450s too. They're all pretty reliable and pretty good, but as you say, not for super high performance systems. 

Good luck

D


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