Single or parallel input to power amp Log Out | Topics | Search
Moderators | Register | Edit Profile

Alembic Club » Owning an Alembic » Troubleshooting » Single or parallel input to power amp « Previous Next »

Author Message
jazzyvee
Senior Member
Username: jazzyvee

Post Number: 3451
Registered: 6-2002
Posted on Tuesday, April 30, 2013 - 11:13 pm:   Edit Post

I have a QSC PLX2402 I find that if I run it with the input set to parallel and have a cab on the L and R side of the amp, the sound is not as loud or as clear as if I use it in non parallel mode, use one side of the output to the first cab and link the second cab via a cable between speaker cabs.

I wouldn't have expected any difference in sound. Which is the best way for the best sound.
Any suggestions?

Bass signal to the rig is mono only. I'm not looking at stereo at the moment.
Jazzyvee
jimmyj
Senior Member
Username: jimmyj

Post Number: 443
Registered: 8-2008
Posted on Wednesday, May 01, 2013 - 8:56 am:   Edit Post

Hey Jazzyvee,

For the sake of explanation we'll say your cabinets are each 8-ohm boxes. But you should check them to be sure because it effects the answer.

If I am reading the specs correctly your amp's output is 425 watts (per channel) into 8-ohms and 700 watts into 4-ohms. When you use only one channel and connect two 8-ohm cabinets in parallel to the output you are getting 700 watts.

Since you are running mono you should consider running the amp in "bridge mode" which uses both channels to amplify the same signal. In that case your 2402 would make 1500 watts into 8-ohms and 2400 watts into 4 and it will be even cleaner.

The reason I say you should check the cabinets first is because if they are 4 ohm boxes you would not want to connect them in parallel in bridge mode as 2-ohms is too small a load. But you could connect them in series making 8-ohms and still get 1500 watts!

You're gonna like it!
Jimmy J
jcdlc72
Advanced Member
Username: jcdlc72

Post Number: 244
Registered: 11-2009
Posted on Wednesday, May 01, 2013 - 9:20 am:   Edit Post

I'm not sure if JV meant putting the cabs in parallel, or using the "parallel" mode on the power amp. I use (well, sometimes) a QSC GX5 on one of my rigs, and it has a switch on the back which lets me "feed" both channels with the signal from only one of them. It doesn't puts the amp in bridge mode, tho, and would (at least in theory) be the same as putting a jumper cable between the two channels, to feed them both the same signal. I haven't made such a test, tho, since when I use the QSC it's either on Biamp, feeding it from separate outputs from my Yamaha PB1 pre's crossover outs, or in bridge mode mono. May it be there's something in the switching system that affects tone? May it be dirt? Next time I get to take the QSC from where it currently is, I'd make the test too...
jimmyj
Senior Member
Username: jimmyj

Post Number: 444
Registered: 8-2008
Posted on Wednesday, May 01, 2013 - 10:10 am:   Edit Post

Hmmm... I see what you mean. Why would using two 425 watt amps sharing the same input be any different than using one 700 watt amp? Good question. Might have to do with the "headroom" available in the higher output configuration...?

And with that in mind I'd still suggest trying bridge mode. Louder, cleaner, funner. Ha!

Jimmy J
jcdlc72
Advanced Member
Username: jcdlc72

Post Number: 245
Registered: 11-2009
Posted on Wednesday, May 01, 2013 - 10:56 am:   Edit Post

Totally agreed. For a mono non-biamped signal, this is what has yielded the best results for me. ;)
jazzyvee
Senior Member
Username: jazzyvee

Post Number: 3452
Registered: 6-2002
Posted on Wednesday, May 01, 2013 - 11:16 am:   Edit Post

Ok I need to clarify, i set the qsc dip switches at the back of the amp to parallel mode so one input feeds both channels of the amplifier with the same signal. I then had one cab on the Ch1 and one on the Ch2 output.
Then tried turning the parallel mode off ( stereo mode) and put both cabs linked together in parallel on to the Ch1 side of the amp and it sounded much better.

Both cabs are 8 ohms Mesa boogie 4x10 rated 600w and mesa boogie 1x15 rated 400w.

Jimmy Just thinking about bridge mode, I have to connect the speakers across both the Ch1 and Ch2 so not sure how I would do that with speakon connectors. Probably have to make up a cable to do that.

http://media.qscaudio.com/images/hi_res/PLX3402_back_hi.jpg

Thanks for all the input guys. I'm sure one day I will just be able to plug and play without experimenting... :-)

Jazzyvee
jimmyj
Senior Member
Username: jimmyj

Post Number: 445
Registered: 8-2008
Posted on Wednesday, May 01, 2013 - 12:53 pm:   Edit Post

I know, so many options! But experimenting is half the fun.

"Parallel" mode on your amp is the same as "Stereo" but sends one input to both channels. You would indeed need a special speaker cable to do "Bridge Mode" The upper speakon connector is wired for it but needs to be pin 1+ for positive and pin 2+ for negative. (See arrow).



It may be easier to use the two red binding posts to make a cable - or use a banana to speakon adaptor there.

Either way, if you then link your two cabs together you will have 4-ohms so the load will be fine. 2400 watts though so watch out!

You have many options... In "Parallel" mode you have separate volume control over the 4x10 and 1x15 channels so sort of a pseudo bi-amp situation. You could also try actual bi-amping in "Stereo" mode if you have a pre that does that. Better still, Series stereo outputs in "Stereo" mode - bridge pu to the 4x10 and bass pu to the 1x15. Yay!

But for mono I would still recommend getting into "Bridge" mode! Be sure the amp is powered down before you throw any of those switches.

Have fun.
Jimmy J
edwin
Senior Member
Username: edwin

Post Number: 1524
Registered: 5-2002
Posted on Wednesday, May 01, 2013 - 2:07 pm:   Edit Post

I have a QSC amp as well, and I've run it in bridge mode before. It let all the magic smoke out of my cabinet when I got carried away playing reggae.

2400 watts is a lot of power! Although that rating is at 1khz and 1%THD, so it's not really apples to apples with the stereo specs. Sadly, my QSC is, so it was 2200 watts (into 8 ohms) all the way down to the bottom of my B string. In general, having more power than your cabinet is rated for is a good thing, but you have to make sure that you keep yourself in check. So, I say go for it! It's easy to make up the cable, most Speakons don't even need a soldering iron.
jazzyvee
Senior Member
Username: jazzyvee

Post Number: 3454
Registered: 6-2002
Posted on Wednesday, May 01, 2013 - 3:01 pm:   Edit Post

I hope I don't get loud enough to let the magic smoke out of my cabinet as well. I don't use any kind of compressor so I'm always mindful of the transients in my playing and the need to avoid that smoke.

I'm not after huge volumes as 99% of my gigs have a FOH PA and stage monitors. I just like things to sound clean, thick, powerful and very HIFI.

If I get a chance at the weekend I'm go down to the electronics shop and get some bits.
Jazzyvee
edwin
Senior Member
Username: edwin

Post Number: 1527
Registered: 5-2002
Posted on Wednesday, May 01, 2013 - 3:32 pm:   Edit Post

I think you'll be very happy! Bridging for me is all about headroom.

The scary thing is that my bass was in the house so much I didn't notice when the speaker cab started billowing smoke until I smelled at which point it was long gone. I think the soundguy likes bass!
keith_h
Senior Member
Username: keith_h

Post Number: 1863
Registered: 2-2005
Posted on Wednesday, May 01, 2013 - 4:14 pm:   Edit Post

It sounds like your cabinets are out of phase when you run them in dual mono. This would not necessarily show up when you daisy chain the speakers.

What type of cables are you running from the amp to speakers when you are in dual mono? Are they wired correctly?

Keith
jimmyj
Senior Member
Username: jimmyj

Post Number: 446
Registered: 8-2008
Posted on Thursday, May 02, 2013 - 1:47 pm:   Edit Post

Good thought Keith and worth checking into. Out of phase cabinets will mess up the low end in particular.

If you are using speakon to speakon cables here's a way to check them to see if one of them is incorrectly wired... Run one channel only and daisy chain the speakers. If it sounds good try swapping the cable from amp to cabinet-1 with the cable between cabinet-1 to cabinet-2. If it suddenly sounds weird then the cable between the two cabs is probably wired incorrectly.

Let us know what you discover.
Jimmy J
jcdlc72
Advanced Member
Username: jcdlc72

Post Number: 246
Registered: 11-2009
Posted on Friday, May 03, 2013 - 6:34 am:   Edit Post

...and that is just one more of the hassles avoided by going direct? ;)
terryc
Senior Member
Username: terryc

Post Number: 2082
Registered: 11-2004
Posted on Friday, May 03, 2013 - 7:00 am:   Edit Post

jeeze....whatever happened to amp + speaker, plug in and play?? LOL
jazzyvee
Senior Member
Username: jazzyvee

Post Number: 3458
Registered: 6-2002
Posted on Saturday, May 04, 2013 - 5:24 am:   Edit Post

Same thing that happened to bass and treble only on basses.
There is a time and place for them all. :-)
Most of the time I use my Mesa walkabout and 4x10 powerhouse cab with all eq set flat.
Jazzyvee

(Message edited by Jazzyvee on May 04, 2013)

Add Your Message Here
Post:
Username: Posting Information:
This is a private posting area. Only registered users and moderators may post messages here.
Password:
Options: Enable HTML code in message
Automatically activate URLs in message
Action:

Topics | Last Day | Last Week | Tree View | Search | Help/Instructions | Program Credits Administration