Impedence matching with my QSC poweramp?

[thudfromafar]

Basically I had to crank the gain on my preamp and poweramp to get a lot of sound. This is with a plx1804.

I'm using mic cables because my DS24 only has that option. I attached this thing, from the DS24 to the poweramp
http://www.parts-express.com/pe/show...number=240-396

Now I barely have to touch the poweramp knob before it gets too loud.

Obviously I had a slight clue that I might need that piece, but I don't really know what's going on here. Anyone care to elaborate?

[jnewmark]

Something to do with balanced/unbalanced inputs. I always used unbalanced ( 1/4 plug ) to my power amp because the XLR input always had to be jacked up a ton. Calling Bob Lee !

[bongomania]

Interesting.

Here's the usual deal with pre and power amps: an unbalanced output into a balanced input often results in 6dB loss. Considering the line out of most bass pre's is unbalanced, and considering the input sensitivity of many power amps is much higher than what the preamp puts out on average, it's really common to have the first problem, of having to crank everything up to get an expected amount of loudness.

In the case of this QSC amp, I'm betting its 1/4" inputs can run either balanced or unbalanced, and are designed to take unbalanced signals with no loss; while its XLR inputs are balanced only, and you got that 6dB loss.

If I'm right in that interpretation, then another solution for you would be to chop one end off a mic cable, tie one of its signal (hot) wires to ground, and put a regular mono 1/4" plug on that end.

I doubt the impedance changing is what made the big difference here.

[thudfromafar]

bump, if anyone can give more clarification

Originally Posted by jnewmark

Calling Bob Lee !

[Zooberwerx]

What's a DS24?

Riis

[bongomania]

Pretty sure he means a digital PA-management unit by BBE.

[Zooberwerx]

Originally Posted by bongomania

Pretty sure he means a digital PA-management unit by BBE.

Hmmm, okay....not unlike my dbx DriveRack.

To the OP: is this "preamp" a BBE DS24? If so, I have a handful of questions and possible suggestions.

Riis

[thudfromafar]

Preamp is a Presonus Studio Channel

That runs into the DS24 speaker management thingy, which then goes to the poweramp

[Zooberwerx]

Originally Posted by thudfromafar

Preamp is a Presonus Studio Channel

That runs into the DS24 speaker management thingy, which then goes to the poweramp

Nice set-up although I'm not quite sure why you're using the DS24 in a bass rig...but, hey...I like my toys, too! I use my SMS (speaker management system) in our PA.

I'll assume you're using the Presonus's balanced 1/4" TRS output to feed signal to the SMS and the balanced XLR to feed a DI signal to the house. If not, I would using a 1/4" TRS > 3 pin XLR cable. You can also use the same cable, bypass the DS24, and run directly into the QSC's input. The Presonus provides a balanced output and both the 1/4" and XLR amp inputs are balanced, as well. For that fact, I would initially remove the DS24 from your signal chain just to get a baseline on the existing gain structure betweenst the pre and power amps. We can always add the DS24 back into the equation as things progress. Try that and get back to us. Oh, you may also want to defeat any compressors, limiters, EQ, etc. which may skew your results. Add them back, one-by-one...you may stumble across the problem.

The DS24 manual recommends you use balanced XLR (mic) cables for all connections with your power amps...I wouldn't deviate.

Riis

[thudfromafar]

I'm using DS24 for the HPF feature, and soon (when I get my custom cabinet) I'm going to use it for biamping.

I will still try things bypassing the DS24. Might not be at the studio for a few days though.

[Zooberwerx]

Originally Posted by thudfromafar

I'm using DS24 for the HPF feature, and soon (when I get my custom cabinet) I'm going to use it for biamping.

I will still try things bypassing the DS24. Might not be at the studio for a few days though.

Interesting? Going the bi-amp route, eh? What are you using for cabs presently?

A HPF is handy for speaker protection but, from what I can tell, not an option on the 1804. The 1804 does offer gobs of power...600 watts @ 8 ohm, 900 @ 4 ohm, right? The DS24 will provide xover and protection capabilities over and above those of a conventional dedicated xover. I didn't read the whole manual but I'm curious: does the DS24 have a procedure for establishing gain structure / headroom amongst the Presonus pre, DS24, and OSC 1804? I used the suggested procedure via dbx and the results were amazing. My PA doesn't clip, feedback, or produce any other nastiness.

Riis

[thudfromafar]

My cabs are 2 avatar SB112's. Waiting on a "fearful" 15/6 cab from LDS.

I know there is no HPF on the 1804, so that's one reason for using the DS24.

There may be a procedure for gain structure but I'm not sure, I'm kinda a noob with this stuff but hoping to figure it out...

Bongomania's post up there, as well as a few others here, went over my head and I've had to read them a few times

[Zooberwerx]

Originally Posted by thudfromafar

My cabs are 2 avatar SB112's. Waiting on a "fearful" 15/6 cab from LDS.

I know there is no HPF on the 1804, so that's one reason for using the DS24.

There may be a procedure for gain structure but I'm not sure, I'm kinda a noob with this stuff but hoping to figure it out...

Bongomania's post up there, as well as a few others here, went over my head and I've had to read them a few times

I try to avoid some of the technical jargon although its helpful when discussing more detailed issues. For the moment, I'd concentrate on the following:

*Run your rig sans BBE DS24.

*Re-read the manual(s) with emphasis on "set-up". There should be a handful of user-friendly references to matching components for optimal performance.

If it makes you feel any better, when I got my dbx DriveRack, I set up the entire PA in my living room (wife was real happy about that), hooked up a CD player, downloaded a CD's worth of pink nosie, and read the manual over and over until my retinas exploded. Took me a couple of days, but I got the system up and going step-by-step and, in the process, actually understood what was happening and why ("oh...now I get it!"). There is nothing more irritating than an individual trying to use a new piece of equipment without benefit of a bench test and/or trial run.

Best of luck and get back to us!

Riis

[bongomania]

Originally Posted by thudfromafar

Bongomania's post up there, as well as a few others here, went over my head and I've had to read them a few times

Actually as I re-read your post a few times, I think mine was not entirely on-track, sorry about that. Let's see if I've got this right:

The first way you had it hooked up was with a mic cable, XLR out of the DS24 into XLR in of the QSC. Levels were too low. You converted one end of the mic cable, with an adapter, to 1/4" unbalanced, and plugged that into a 1/4" input on the QSC. Now the levels are just right.

Yes?

If that's right, then the sensitivity of the power amp is greater (a lower number = greater sensitivity) when using the unbalanced input; and the sensitivity is lower when using the balanced XLR.

The specs on their website don't show that however. They do mention a different input impedance between balanced an unbalanced connections, but that difference is so small that it shouldn't matter or be audible. Also the impedance specs, combined with the high-impedance output of that adapter plug, make the whole thing kind of backwards.

So really we'll have to wait until Bob can clarify exactly what's going on.

PS: exactly how are you connecting the Presonus to the DS24?

[thudfromafar]

thanks bongo

unfortunately the DS24 has only XLR inputs AND outputs...