Power amp questions

[bc_dup1]

I'm in the process of replacing my Ashdown rig (325 watt 1 x 10" combo, 4 x 8" cab) because I'm finding it underpowered in some situations. Previously I had a Hartke 350 head and 4 x 10 that was always loud enough in any situation but I needed something more portable.

 

I'm now awaiting delivery of a used SWR Grand Prix preamp. For a power amp I had more or less settled on a QSC PLX 1602 but then noticed a thread at Talkbass where someone had asked if he should buy the 1602 or the 2402, and the consensus seemed to be, definitely go for the 2402.

 

This gives me a dilemma because I'm already feeling guilty about my level of bass gear expenditure and the 2402 is more expensive. I don't want to pay extra for power I don't need. But on the other hand I don't want to repeat the mistake I just made, ie pay for gear that will need replaced soon because it's underpowered!

 

Looking at the specs of the 1602:

 

2-ohm 800W

 

4-ohm 500W

 

8-ohm 300W

 

I'd have thought that it should be plenty loud enough given I'd no problems with the Hartke. Cab wise I'm thinking of keeping the Ashdown 4 x 8 and possibly adding a 1 x 12 or a 1 x 15.

 

What I'm not sure about, though (sorry, I am extremely non-technical and some - sorry if some of the following sounds laughably ignorant):

 

1 I'll probably be using 2 8 ohm cabs. So presumably I get 300w per cab. How would this compare with, say, the Hartke head going into the 4 x 10 or the Ashdown going into 2 x 8 ohm cabs? I understand that when I put the Ashdown into two cabs in parallel meant that the ohmage was effectively halved to 4 ohms. Would this be the case with the QSC as well, or the stereo set up mean that the cabs were driven separately, so that the resistance was still 8 ohms a side?

 

2 I also understand that I can increase the power using "bridged" mode but that this will be mono. Does that mean I could only have a single output or can I still drive two cabs.

 

My gut feeling is that the 1602 should still be much louder than the Hartke and therefore more than adequate for my needs, but please tell me if I've got this wrong!!

[alexclaber]

Difficult...

 

It seems an awful waste to use the QSC in parallel delivering only 300W into a cab that will handle 600W (and would thus be best of with a 1200W amp). However, if you want to add a different cab like a 15" or 18" you may need to run in parallel to balance the volume, and also to let you have your high pass filters set a different frequencies.

 

If you bridge the QSC into your 4x8" and whatever other cab you choose then you'll get 1600W and that will be a lot louder. Running in parallel I don't think the difference will be anywhere near as large as you hope.

 

Alex

 

P.S. You have a PM from me

[_Sweet Willie_]

Wow, a lot of questions there.

 

First of all, 300W per cab for two cabs is still going to be loud. A 4x8 + a 1x15 w/ 300W into each will make a lot of noise. That's 600W of power. This would be the scenario if you ran the 1602 in stereo.

 

You can still use two cabs if you run bridged mono. I'm not looking at the outputs of the PLX amps right this second, but you would either daisy-chain the cabs (amp-to-cab-to-cab) or there might be a way to use two outputs off the amp. You would have more power, but you would lose the ability to control the volume of each cab separately.

 

Going from the 1602 to the 2402 there is a change in the power structure/design of the amp -- not just the total output in watts. I've heard from one source that the design for the 1602 is more reliable than that of the 2402. I'm not competent enough to explain the technical issues behind this, but it has something to do with the way the switching power design of the 2402 works. Even though the 2402 has more power, the 1602 may be a better designed or functioning amp. This is contrary to what you've heard over at talkbass, and my info comes from one individual, albeit someone who is very knowledgeable and reliable about this kind of stuff.

 

I hope this helps.

[rumpelstiltskin.]

Originally posted by bc:

For a power amp I had more or less settled on a QSC PLX 1602 but then noticed a thread at Talkbass where someone had asked if he should buy the 1602 or the 2402, and the consensus seemed to be, definitely go for the 2402.

unless you really need the headroom badly, i recommend sticking with the 1602 instead of the 2402. the reason for this is for amps larger than the 1602, QSC utilizes sort of a dual power supply -- one for most of the output, and a higher voltage one for the peaks. every time the power supplies switch, there is a lot of distortion. this can be very bad if you're not turned all the way up and your peaks are just tickling that range.

 

the 1602 should be quite nice, and really adequate power wise. and the light weight is really nice. i wouldn't hesitate to use a 1602, though i doubt QSC will ever be my first choice. they're nice amps -- decent sound, very light weight, and inexpensive for what you get. but unless you really need the extra power, you may as well get the 1602.

 

I don't want to pay extra for power I don't need. But on the other hand I don't want to repeat the mistake I just made, ie pay for gear that will need replaced soon because it's underpowered!

yes, that's something of which you should be aware. but you're at least doubling your wattage and increasing the cone surface area of the loudspeakers you'll be using. that translates to noticeably more loudness. your rig won't be earthshakingly loud, but you'll be surprised

 

1 I'll probably be using 2 8 ohm cabs. So presumably I get 300w per cab.

that is correct. you have two amplifier channels both capable of creating 300W into 8 Ohms. this is compared to your current amplifier, which would onlybe able to make 325 total which is divided equally between your two cabs.

 

which brings up what happens when you use a 1602 bridged-mono. this is a very popular setup. for those who don't know, bridged-mono refers to using both amplifier channels to act as one in order to double the amount of power available from either of the individual channels. it usually involves throwing a switch and connecting your speakers to the two red binding posts (or the 1+ and 2+ of a speakon connector). since both channels are being used by bridged-mono mode, you don't have another channel you could use.

 

you were also correct about what happens to your cabinets in this configuration. since there is now only "one" amplifier channel, you have to connect your cabinets in parallel, which reduces the load on the amplifier to 4 Ohms. in that case, you'd create a total of 1600W, or 800W per cabinet.

 

My gut feeling is that the 1602 should still be much louder than the Hartke and therefore more than adequate for my needs, but please tell me if I've got this wrong!!

you have it spot on.

 

robb.

[bc_dup1]

Thanks to all who've replied. I'm much easier in my mind that the 1602 is the right choice.

 

Robb, one of the reasons I'm going with QSC is that I'm in the UK. Some of the rival power amps preferred by some players are not readily available here and power amps are generally much more expensive. For example, the best price I can find on a 1602 in the UK is £675 (ie about $1,050). That's after a big discount (Sound Control, generally regarded as one of the cheaper UK music chains, quoted me £899!). I'm not sure I'd make the same choice if we had the same options as you guys have.

[bassjigga]

You might consider the RMX series. They are a bit heavier, but they cost less and the power is clean and plentiful. I currently use an SWR Grand Prix with an RMX 1450 and it sounds great. I run it mono at 4 ohms which delivers 1400 watts. I think they are currently around $400. There's also 850, 1850, 2450, depending on your needs.

 

Dave

[rumpelstiltskin.]

let me rephrase myself a bit, in order to be crystal clear. though the PLX series would not necessarily by my first choice, it is not far from the top. again, you get a lot for what you're paying, even in the UK.

 

though my preferences lie with crown and my current employer, peavey (i wish we'd make more lightweight amps like the DPC1400x), QSC is probably third. i'd much rather have a PLX1202 in 2U than a stewart world 1.2 in 1U. there's just something about actually getting advertised power that makes me happy.

 

robb.

[SteveC]

bc - what a timely topic. I, too, am awaiting the arrival of an SWR Grand Prix Preamp. I've tried other high end preamps, but always seem to come back to the SWR front end. I am thinking about trying a bi-amped rig with low and high cabs.

 

Anyway, I was also debating the QSC PLX 1602 or 2402. I'd like tokeep the weight down as well, which is why I am looking at these.

 

The Peavey digital single space is great, but quite expensive.

[bc_dup1]

Yes, that Peavey looks great but the best price I can find on it is still nearly 50% more than the 1602.

[Jeff Addicott]

If you were running 4 ohm Acmes (very inefficient power hogs) one per channel, apparently it's great to have the extra headroom of the 2402.

 

But if you have a stack of relatively efficient (i.e. non-Acme) speakers, both 8 ohms with similar power handling, you might as well daisy chain them and run bridged into the 4 ohms with a lower wattage amp (like the 1602 or the 1202). You'll save a few bucks and still be killer loud. I like this concept because the bridged amp is just about right for either the single or both. Just make sure the amp is robust enough to bridge into 4 ohms (sounds like the QSCs are cool for that).

 

BTW, my Stewart World 1.2 does pretty good (with the cooling fins). Of course, the 1U concept is a bluff; if anything, it's more like a 3U by the time you give it ample room. I just put a clip-on fan on the back of the rack and now it feels cold to the touch even after bridging into 4 ohms all night! And you gotta love the weight: 11 pounds! The Peavey would be cool to cram it + a 1U preamp into a nifty little 2U rack. Not $1200 cool, though!!

[bc_dup1]

To update I've now taken delivery of my Grand Prix and 1602. Will update on progress once I get the chance to try them out in a proper situation but at bedroom level I'm blown away by the SWR - not that I've heard many high-end preamps to compare, but I just love the sound of this.

[Dave da Dude]

Hey guys,

 

Sorry to butt in here, but ...

 

I've just been surfiin' 'n dreamin' about megawatts of bass power, but I can't seem to find the URL for Stewart power amps.

 

Anybody know what it is?

 

Dave

[_Sweet Willie_]

Click here for Stewart Audio.

[bc_dup1]

Originally posted by Jeff Addicott:

[QB]

But if you have a stack of relatively efficient (i.e. non-Acme) speakers, both 8 ohms with similar power handling, you might as well daisy chain them and run bridged into the 4 ohms with a lower wattage amp (like the 1602 or the 1202). /QB]

Please excuse my technical illiteracy but what's meant by "daisy chain" here? If I'm understanding the 1602's manual correctly here, there's only one output when the amp is run bridged. How do I connect up two 8 ohm speakers?

[SteveC]

I just hooked up my 1602 and it is so great. The "daisey chain" in the PLX manual is referin gto hooking multiple amps together.

 

I think what Jeff is talking about is doing that to your speakers. In this case, it means you run the one output from the amp into the first speaker. Then run from the first speaker to the second. With 2 8 ohm speakers, this makes a 4 ohm load, thus drawing upon all the 1602 power - 1,600 watts into 4 ohms.

 

I'm bridging mine and running my one 8 ohm 700 watt cab with it. It sounds FANTASTIC.

[dohhhhh6]

Could someone please tell me exactly how the ohms work? I used to know, but that knowledge is kinda afloat in my head right now.

[bc_dup1]

Hi Steve,

 

Glad your 1602 is sounding so good - still haven't had the chance to check mine out in anger, partly out of consideration for the neighbours and partly because I still don't have a cab that's really going to show it off to best advantage.

 

I'm still not sure about how you rig up the cabs, can yuo just run a cable from the input socket of one cab to the input socket of another?

[_Sweet Willie_]

Originally posted by Da LadY In Tha Pink Dress:

Could someone please tell me exactly how the ohms work? I used to know, but that knowledge is kinda afloat in my head right now.

Do a search for "impedance." There's a thread that was revived recently called "the truth about impedance." It should even be on the first page of the LDL thread listing. That should answer your question.