The No.1 Website for Pro Audio
 All  This Thread  Reviews  Gear Database  Gear for sale     Latest  Trending
Diagnosing a speaker versus amp problem
Old 20th October 2013
  #1
Here for the gear
 

Diagnosing a speaker versus amp problem

Hi all,

I've just assembled a simple system I'm pretty pleased with, but am encountering an issue with one channel in my highs not working properly. It seems that sound coming from my QSC GX7 amp is excellent in one channel, but spotty in the other. When I turn up the volume, it will increase momentarily and back off. It also doesn't sound clear, rather it sounds like there is some distortion. Did I possibly get a dud amp, or am I the dud here?

Here's the list of connections in my setup:
Macbook -> Peavey USB-P (Peavey :: USB-P USB Playback) - I added this per suggestions in this thread -> JBL JRX225 + Pyle Pro Pexa 3000 - OK or disaster?
Peavey USB-P -> 2 x JBL PRX718XLF subs via XLR cables.
JBL subs -> QSC GX7 amp (no filter or crossover, just the THRU signal (+4dBu) from the subs going into the amp)
QSC amp -> 2 x JBL JRX225 full range speakers

Does this configuration sound reasonable? (I really hope there isn't any damage potentially done with this configuration)

Here is some debugging I've done:
- run the subs alone, checking volume levels using balance and volume adjustments - the subs seem fine.
- full setup, but switched the outputs to the JRX 225 speakers. When I switch the output, the problem is not speaker-specific, it stays with output channel 2 of the amp.
- ran my cell phone (on low volume) via 1/8" to XLR adapter to the amp inputs to run just the amp and JRX225 speakers. The problem persists with channel 2 being problematic.
- also made sure the amp is in full range mode

Am I missing something here, or does it seem to be a defective amp?
Old 21st October 2013
  #2
Here for the gear
 

Update:
I've been doing some additional digging, and it seems that QSC recommends that the gain knobs be set at or near their full level. See section titled "AMPLIFIER SOUNDS DISTORTED."

http://qsc.com/files/9713/4625/5907/...l_revD.pdf.pdf

This would mean I have to turn the volume on the Macbook way down (I'm using Mixx as software for now). Is this generally how this is done? Again, I'm also using the Peavey USB-p device from the Mac to the subs (or amp, depending on config with/without subs). I once read that turning down volume controls in software is undesirable for best quality, but it seems that is either not accurate or doesn't apply in this scenario. Any feedback is greatly appreciated.

Thanks in advance.
Old 21st October 2013
  #3
Lives for gear
 
NeoHippy's Avatar
 

hey,

its true. Amps usually work best when they are driven near their max value.
If you turn down the volume on the macbook you will reduce the SNR. Since the signal is still in the digital domain. That means your dynamic is much less. In other words the noisefloor will be much closer to your peak signal. Thats not good. Best thing to do would be to hook up an analog monitor/level controller between soundcard/macbook and amp.
But usually good controller are quite expensive.
I have a passive monitoring system and I drive the soundcard on max outout and turn down the amp. Thats the way it sounds best in my situation.

cheers
Old 21st October 2013
  #4
Lives for gear
 

I can't speak to any specifics of your issue, but if the problem is that the amp isn't gain-staged properly, then - yes - you'll want to turn down the level of the signal that's going into the amp. And, yes, it's true that you don't want to turn down the level digitally since it eats into the resolution of the signal.

This page is the best explanation of proper gain staging for digital sources that I've seen.

Use with digital sources

Again assuming the level of the amp is the issue, you can use the "gain" knob on the subwoofers to adjust the signal level in the analog domain. However, this won't let you adjust the relative levels of the subwoofer signal and the main full-range signal, so you may need to add a passive attenuator between the sub output and the amp input.
Old 21st October 2013
  #5
Here for the gear
 

Quote:
Originally Posted by NeoHippy View Post
hey,

its true. Amps usually work best when they are driven near their max value.
If you turn down the volume on the macbook you will reduce the SNR. Since the signal is still in the digital domain. That means your dynamic is much less. In other words the noisefloor will be much closer to your peak signal. Thats not good. Best thing to do would be to hook up an analog monitor/level controller between soundcard/macbook and amp.
But usually good controller are quite expensive.
I have a passive monitoring system and I drive the soundcard on max outout and turn down the amp. Thats the way it sounds best in my situation.

cheers
Thanks so much for the response.

So the end-to-end system would look like this?
Macbook -> Peavey USB-p DI -> *analog monitor/controller* -> prx subs -> amp -> full range jrx PAs

I intended to punt on a mixer for now (I've already spent a lot...), but would that accomplish the same goal as an analog monitor/controller for now? I'm still learning what the differences and overlap of all these technologies is, and admittedly don't completely understand just yet where I can save some money but have a system that won't incur damage due to incorrect config.

Thanks again.
Mike
Old 21st October 2013
  #6
Here for the gear
 

Quote:
Originally Posted by Brainchild View Post
I can't speak to any specifics of your issue, but if the problem is that the amp isn't gain-staged properly, then - yes - you'll want to turn down the level of the signal that's going into the amp. And, yes, it's true that you don't want to turn down the level digitally since it eats into the resolution of the signal.

This page is the best explanation of proper gain staging for digital sources that I've seen.

Use with digital sources

Again assuming the level of the amp is the issue, you can use the "gain" knob on the subwoofers to adjust the signal level in the analog domain. However, this won't let you adjust the relative levels of the subwoofer signal and the main full-range signal, so you may need to add a passive attenuator between the sub output and the amp input.
Thanks for the feedback. So you're saying the attenuator would allow me to basically adjust signal going into the amp+PAs independent of the subs?

Would a proper mixer with outputs for the PAs and subs accomplish this? i.e. run a set of outputs from the mixer to the QSC amp and another set of outputs to the subs. I've been trying to hold off on a mixer, but it may give me a necessary level of control now that I've added subs to my setup.
Old 21st October 2013
  #7
Lives for gear
 

Quote:
Thanks for the feedback. So you're saying the attenuator would allow me to basically adjust signal going into the amp+PAs independent of the subs?
Yup. In terms of your previous reply to NeoHippy, it would look like this:

Macbook -> Peavey USB-p DI -> prx subs -> *analog monitor/controller* -> amp -> full range jrx PAs

Quote:
Would a proper mixer with outputs for the PAs and subs accomplish this? i.e. run a set of outputs from the mixer to the QSC amp and another set of outputs to the subs. I've been trying to hold off on a mixer, but it may give me a necessary level of control now that I've added subs to my setup.
That would work, just because you'd be able to control those two mixer output levels separately. A standalone volume controller would do the trick too. Even something like the SM Pro Audio Nano Patch could work, although snobs like to take a dump on it. If you want to get extreme, there are compensated attenuators with discrete volume steps that are carefully matched and calibrated...probably cost more than a small mixer that would suit your purposes, though.
Old 22nd October 2013
  #8
Lives for gear
 
NeoHippy's Avatar
 

Quote:
Originally Posted by mikemhobbyist View Post
Thanks so much for the response.

So the end-to-end system would look like this?
Macbook -> Peavey USB-p DI -> *analog monitor/controller* -> prx subs -> amp -> full range jrx PAs

I intended to punt on a mixer for now (I've already spent a lot...), but would that accomplish the same goal as an analog monitor/controller for now? I'm still learning what the differences and overlap of all these technologies is, and admittedly don't completely understand just yet where I can save some money but have a system that won't incur damage due to incorrect config.

Thanks again.
Mike
Hey,

yes a little mixer would work too.
Probably it will be even cheaper than a proper controller.

cheers
Old 25th October 2013
  #9
Here for the gear
 

Just a follow-up on this thread. I tested out my amp, independent from my setup, and came to find that in fact there was something wrong with channel 2. Thanks everyone for the suggestions!
Old 24th September 2016
  #10
Here for the gear
 

I also have a problem with the KSUB, we had someone spill a beer over the top and some had split into the electrical parts. The sub had completed turned off, I had inspected it cleaned the whole circuit board and changed the fuse, The only issue I have is the sub works but when you start to turn up the bass the fuse just blows out.
Topic:
Post Reply

Welcome to the Gearslutz Pro Audio Community!

Registration benefits include:
  • The ability to reply to and create new discussions
  • Access to members-only giveaways & competitions
  • Interact with VIP industry experts in our guest Q&As
  • Access to members-only sub forum discussions
  • Access to members-only Chat Room
  • Get INSTANT ACCESS to the world's best private pro audio Classifieds for only USD $20/year
  • Promote your eBay auctions and Reverb.com listings for free
  • Remove this message!
You need an account to post a reply. Create a username and password below and an account will be created and your post entered.


 
 
Slide to join now Processing…
Thread Tools
Search this Thread
Search this Thread:

Advanced Search
Forum Jump
Forum Jump