Author Topic: Single or parallel input to power amp (Read 337 times)

jazzyvee · « **on:** April 30, 2013, 11:13:11 PM »

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 · « **Reply #1 on:** May 01, 2013, 08:56:33 AM »

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

JuancarlinBass · « **Reply #2 on:** May 01, 2013, 09:20:10 AM »

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 · « **Reply #3 on:** May 01, 2013, 10:10:48 AM »

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

JuancarlinBass · « **Reply #4 on:** May 01, 2013, 10:56:24 AM »

Totally agreed. For a mono non-biamped signal, this is what has yielded the best results for me.

jazzyvee · « **Reply #5 on:** May 01, 2013, 11:16:58 AM »

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 · « **Reply #6 on:** May 01, 2013, 12:53:21 PM »

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 · « **Reply #7 on:** May 01, 2013, 02:07:22 PM »

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 · « **Reply #8 on:** May 01, 2013, 03:01:03 PM »

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 · « **Reply #9 on:** May 01, 2013, 03:32:31 PM »

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 · « **Reply #10 on:** May 01, 2013, 04:14:02 PM »

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 · « **Reply #11 on:** May 02, 2013, 01:47:46 PM »

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

JuancarlinBass · « **Reply #12 on:** May 03, 2013, 06:34:22 AM »

...and that is just one more of the hassles avoided by going direct?

terryc · « **Reply #13 on:** May 03, 2013, 07:00:06 AM »

jeeze....whatever happened to amp + speaker, plug in and play?? LOL

jazzyvee · « **Reply #14 on:** May 04, 2013, 05:24:48 AM »

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)