"Dumb" Power Amp Question

hyden · #1 07-07-2010, 09:39 AM

I need an amp to get a little more volume than my Mesa Walkabout but I'm broke. I have a Tech21 Sansamp Bass DI and a QSC RMX1450 amp just laying around. I also have a 15" 4ohm cabinet with an Eminence Kappa Pro rated at 500w. My question is -If I bridge the QSC I get 1450w into a 4ohm load - Is it too much? If so - can you run just one side of a power amp without damaging it. I believe the QSC is 450 w at 4ohm per side. Keep in mind - I do realize that if I added another 4ohm cab that would solve my issue. But can't afford it right now.

BassmanPaul · #2 07-07-2010, 09:58 AM

You do the math: 500W driver being fed with up to 1450W or 500W driver being fed up to 450W.

What makes more sense to you??

Paul

Bob Lee (QSC) · #3 07-07-2010, 10:11 AM

Can you tell us more about the loudspeaker's "500W" rating--is that continuous (i.e., "RMS") or program power?

The RMX1450 amp has two channels, and they don't have to be made to do the same thing.

hyden · #4 07-07-2010, 10:16 AM

Quote:

Originally Posted by Bob Lee (QSC)

Can you tell us more about the loudspeaker's "500W" rating--is that continuous (i.e., "RMS") or program power?

The RMX1450 amp has two channels, and they don't have to be made to do the same thing.

Thanks - It is a 500w rms/ 1000w max speaker

sawzalot · #5 07-07-2010, 10:19 AM

So here's the thing--there's no reason you can't bridge the QSC and plug it into the 4 ohm 15" Eminence. It's all about how loud you drive it. You'll be a lot safer running only one channel of the QSC into that speaker because you will only be capable of putting 450 watts into it, though, and realistically you'll probably only put 400 watts into it because it's FTC 20-20 Khz power is only 400 watts. But, I don't know if the Tech 21 has enough output of its own to drive that amp to full power, so you may want to run the QSC bridged to make up the difference. Also, that Eminence speaker is probably capable of taking peaks of 1000 watts, so you can push it a little harder than it's RMS power rating would lead you to believe.

What I'd do is this: bridge the QSC but back the Channel 1 gain knob down to about 50% or 60% of maximum and see how that works. You might have to fool with the gain knob a bit to get to the right level--but by watching the speaker at high volumes you'll find the right balance of volume vs. speaker protection.

hyden · #6 07-07-2010, 10:22 AM

Thanks sawzalot - exactly the info I needed

Bob Lee (QSC) · #7 07-07-2010, 10:24 AM

First, bassmanpaul didn't "insult your intelligence."

I'd suggest running one channel of the amp to start. Set the low-frequency filter on channel 1 to 50 Hz to protect against over excursion. See how that works for you. See if you can get the sound levels you want without any clipping in the amp.

hyden · #8 07-07-2010, 10:28 AM

Thanks for the advice guys - much appreciated.

sawzalot · #9 07-07-2010, 10:29 AM

Just be careful--you'll be connecting an amp in a configuration that can easily destroy that speaker. That gain knob is your safety. 1000w into a single 15 is probably much less effective than 500w into two 15's. More speakers with the same power is usually better than more power with the same # of speakers, IME. More speakers with more power is always better, of course

.

Read the manual for the qsc--you might have to turn down both gain knobs to the same level. I'm not sure how the QSC's are wired up when bridged. Most power amps these days ignore the ch. 2 input when bridged, though.

Tom

A440Hz · #10 07-07-2010, 10:33 AM

And, just to be clear, there's no harm in idling one channel of the power amp. It's like letting a diesel engine idle--no harm at all.

sawzalot · #11 07-07-2010, 10:36 AM

LOL--I can see Bob cringing now. Obviously he's the authority on these amps. I tend toward the Tim Allen School of Sound when it comes to the question of whether more power is better!

Tom

dpbass613 · #12 07-07-2010, 10:39 AM

bridge it and run the amps gain at 80-85% which is the suggested way to run the amp, So this is by no way shape or form exact science but a good explanation...

Brigded running gain at 85% 1450 X .85 = 1232.50 wts
Pending where you run your preamp gain, lets say somewhere around 50% 1232.50 X .50 = 616.25

So of that 1450 wts you are only really using about 616.25. Its actually better to overpower a speaker than underpower for this reason, (technically you aren't overpowering it)

You want the driver to move its full distance, when you underpower a speaker the driver is moving a shorter distance but alot faster...and more frequently...which actually will burn the speaker out, versus blowing it when you drastically over power it.

And idling an amp isn't a great thing to do, you can do it but, i personally would avoid it, they get used to their normal usage, and when you change that its not going to perform at its best until it gets used to that new usage. QSC's are great durable amps, though so you really shouldn't have to many issues.

Speaker quality matters to in this case...if its a quality eminence you should be good to go, its a beater speaker than watch out.

Bob Lee (QSC) · #13 07-07-2010, 10:40 AM

Quote:

Originally Posted by dpbass613

Brigded running gain at 85% 1450 X .85 = 1232.50 wts
Pending where you run your preamp gain, lets say somewhere around 50% 1232.50 X .50 = 616.25

No.

The gain control does not limit how much power the amp can put out.

Bob Lee (QSC) · #14 07-07-2010, 10:41 AM

Quote:

Originally Posted by dpbass613

when you underpower a speaker the driver is moving a shorter distance but alot faster...and more frequently...which actually will burn the speaker out, versus blowing it when you drastically over power it.

Uh, no. This is not correct.

sawzalot · #15 07-07-2010, 11:01 AM

I should have mentioned the highpass filters on these amps--I'd definitely engage it at 50 hertz (or 30 hertz if it sounds like you're losing your super-low end) to protect against over-excursion. It's amazing how much subsonic energy gets transmitted from a bass guitar to the amp, and one of these power amps can easily damage a speaker with a big low-frequency transient.

mulchor · #16 07-07-2010, 11:03 AM

Wow, some really original creative thinking there.

bassplayer7770 · #17 07-07-2010, 11:11 AM

Quote:

Originally Posted by sawzalot

I should have mentioned the highpass filters on these amps--I'd definitely engage it at 50 hertz (or 30 hertz if it sounds like you're losing your super-low end) to protect against over-excursion. It's amazing how much subsonic energy gets transmitted from a bass guitar to the amp, and one of these power amps can easily damage a speaker with a big low-frequency transient.

I noticed this with my Crown XLS 1500 power amp. Before I setup the HPF at 50Hz, I noticed the 15 in my cab working rather hard. When I engaged the HPF at 50Hz, it cleaned up my low end, and the 15 doesn't appear to be working nearly as hard. I'm digging it.

dpbass613 · #18 07-07-2010, 11:21 AM

Bob,
You need to speak with your QSC reps in CT i think, I used to work in a music store and this is how they trained us. So i apoligize for any incorrect information on what I posted above, but that was passed along to me from a QSC rep.

Is it not correct when you decrease the gain on a power amp you limit the output? therefore less movement in the speaker?

Bob Lee (QSC) · #19 07-07-2010, 11:32 AM

I'll speak with anyone. Do you know who they were?

Alex1984 · #20 07-07-2010, 12:19 PM

I believe the gain is more or less like an input sensitivity thing, it adjusts the level of input required for the power amp to output at full power. So with a large enough input, even with the gain turned way down, you should still get the full output.

I believe the SansAmp, being a preamp pedal has enough output to feed the power amp.

Idling one channel should be safe and harmless to the amp (at least for SS amps). Also, IIRC, I think feeding a speaker less power only limits how far it moves, not the frequency, otherwise we'd have ourselves a pitch shifter. :P