Added Headroom

[iamoldschool]

I need some opinions here. I just picked up a Crown XS500 that was on the MF web site as a blem @ $262.49. It's rated @ 400W per channel @ 8 ohms or 1450W bridged mono @ 8 ohms. It looked like too good a deal to pass up at that price since I have been gassing for a little clean headroom. I will be running it through my Genz Benz XB2 410 behind a Sansamp RBI. The GB cab is rated at 700W and I have been using a Crown XLS202 rated at 600W brigded mono with no trouble and plenty of volume, but lacking headroom. My thinking is that I can use the XS500 and keep the XLS202 as a backup for either my rig or the monitors.

My question is, will I have too much headroom and have to worry about the cab now? I've never felt comfortable with having less amp than cab even though I haven't needed the additional power. I will now have the headroom, but where do I set the power on the 500? On the 202, I could just turn it all the way up(amp, not preamp!) without worrying about damaging the speakers.

Chime in here, rig masters!

 

[Eublet]

My question is, will I have too much headroom and have to worry about the cab now?

Worry, no. Be careful yes. Listen for sounds of speaker stress, but more than that, just don't turn things up too loud. When you get to the point where you notice that the volume doesn't increase much even though you're still turning up the volume, it's time to back off. The cabinet is moving all the air it can. You'll notice a bit more mid/high-mid presence as you turn up even more. Part of this is natural, but it will start to get really mid present and grindy. That's clue number one IME. Keep going, and you'll hear farting noises from the speaker...clue number 2. You should stop at clue one, not at clue two.

On the 202, I could just turn it all the way up(amp, not preamp!) without worrying about damaging the speakers!

Actually, no you can't, and you shouldn't do what you did. You could have damaged something.

Two things are bad for a speaker:

1) Too much clean power.
2) Any amount of dirty power.

"Dirty power" is what happens when an amp is clipping, and the signal becomes more "square". This square signal can cause a speaker to blow, even if the power is MUCH less than what the speaker is rated to handle. This is how people blow speakers more often than not. They keep turning up their head to get more volume, the amp can't keep up, and the signal starts clipping, and they blow a speaker.

You can run a clean signal into a quality speaker and generally be safer than you can with a poor quality signal at lower power. Either one is dangerous though.

But having a cabinet that is rated higher than your amp is not a license to turn things up. Again, that's the number one way people blow up their drivers. And it's what has created the "underpowering" misnomer.

 

[agedhorse]

Be careful. That's a lot of power if you end up really using it rather than keeping as a reserve for (real) headroom.

 

[iamoldschool]

Thanks for the input. To clarify, I don't normally run the power on the amp wide open. I will bump it up when I need additional volume, then back down the gain on the preamp, but I have seldom had the power on the 202 over 75%. When I first put the rig together, I opened the amp up but noticed the signal was clipping. My reaction was to back the amp down to about 25-30% and turn up the gain on the preamp. My feeling is though, that adjusting volume using the gain is not giving me the tone available, thus the need for more headroom. It did stop clipping the amp, though. Am I correct in my assumption that having clean headroom should allow the preamp and cab both to work better? Another question - Will sending a hotter signal from the preamp (increasing gain) cause the amp to clip at lower levels?

 

[iamoldschool]

Be careful. That's a lot of power if you end up really using it rather than keeping as a reserve for (real) headroom.

That is my concern, it's a LOT of power. Should the volume level be controlled on the amplifier or the preamp?

 

[agedhorse]

Note that the input level controls do nothing to reduce the power delivered to the speakers... they adjust for the amount of drive level from the preamp needed to achieve full output from the power amp.

Turn them down 50% and all that means is that you need an additional 6dB of drive level from the preamp to reach the same identical level.

Turning down the power amp to limit the power is a common misconception, leading to a false sense of security and is one reason for unintentional blown speakers.

 

[iamoldschool]

Note that the input level controls do nothing to reduce the power delivered to the speakers... they adjust for the amount of drive level from the preamp needed to achieve full output from the power amp.

Turn them down 50% and all that means is that you need an additional 6dB of drive level from the preamp to reach the same identical level.

Turning down the power amp to limit the power is a common misconception, leading to a false sense of security and is one reason for unintentional blown speakers.

So, are you saying here that the power amp input level control(assuming bridge mode) should be run at 100%, using the signal output on the preamp to control output level?

I think damaging the cabinet is out of the question for the volume levels I use, but hypothetically, the only way to keep from damaging the drivers is by listening for sounds of speaker stress and reducing the output level on the preamp, right? If so, wouldn't this be the rule for any size amp, whether above or below the cabinet's stated maximum power?

 

[agedhorse]

Either way will work. Depending on the output stage design of your preamp, you may improve your signal to noise ratio juat a little by dropping the amp sensitivity down a little, but this does nothing for maximum power output.

With all our pro audio PA amps, we run them at 100% and balance levels in the DSP speaker processing since noise is not an issue for us.

 

[iamoldschool]

For anyone who looked at this thread and had some of the same questions I had about headroom - I did a little research. The attachment below is from the Crown web site and shows "safe" setup procedure for their amplifiers.

So, it appears that the level control on the amplifier itself should be used to attain maximum anticipated volume level, then adjust the preamp back from there. Set it and forget it. Maybe I should already have known this, and it's the way I have been operating the XLS202, but with over 1400 watts on tap, I didn't want to risk damaging my GB cab. BTW agedhorse, kudos to GB on the XB2 series, mine is awesome!

Thanks,
Richard

 

[agedhorse]

The Crown information is simplified and with a goal of optimizing signal to noise ration. This is only a problem with a noisy preamp line driver and has little to do with overall performance.

(rant on) I wish manufacturers would just settle on a standard nominal gain (say +4 or +8dBu) equals full rated power and quit scrambling everything up between models and lines. It would simplify the drive electronics balancing between different amp products. (rant off)

 

[iamoldschool]

The XS500 came in yesterday and I swapped out the amps before rehearsal last night. BIG difference! The added headroom gave me clarity that I did not know was missing. Even our guitard remarked that he could hear the bass better. Together, we marveled that even though I was running lower volume levels, the bass was more present in the mix. My conclusion: the added headroom will protect my beloved XB2 by allowing me not to push it as hard, yet remain substantial in the mix. Very important in a 3-piece.

Agedhorse - I'm running @ 100% signal sensitivity as you indicated the test amps are run. I have no noise, so I don't perceive the sensitivity setting to be a problem. XLR out on the Sansamp is run @ about 1/4 to 1/3 with some volume left on the Pbass volume pot for solos or bass heavy numbers.

 

[tolson36]

There is a reason you might want to adjust overall volume on the amp (during a gig) instead of with the preamp---which is different than what one might do with the PA amps.

In PA most often sound guys leave amp trims wide open and adjust volume from mixer for max headroom. In noisy bar applications SNR issues are not usually the issue but headroom is more important. (although there are different opinions)--this tends to make it easier to avoid headroom issues during the gig.

Bass amps are a different situation. When we play live bass players often feed the main mixer (using the XLR-out), sound guys IME would prefer that you set your preamp level for a good signal during soundcheck and leave the preamp-level alone after the gig starts. Any adjustments during the gig are done on the master volume of the bass head.

That way when you--the bassplayer--decide to adjust your volume for stage-sound purposes--you are not also upping the level being fed into the main board. If he already had your input at an optimal level--you could clip the input on the mixer by adjusting stage volume too much.

You might ask--well guitar players who are miked can increase their FOH volume regardless of whether they use the gain control or the volume control. That's true but there are reasons why it's more important for the bass player. The biggest one being that bass--being a low-end signal--it's more likely to clip something in the first place and the clipping can be more harmful. Plus, the soundguy already knows he's gotta watch the guitards like a hawk since they have all sorts of effects that could change levels at a moments notice. Anything they can set and forget (kind of) is a bonus.

Anyway--as long as you have set the preamp to a reasonable level and left it alone--there is no big problem to then use the head master volume control (or the volume trim if using a power amp) to make adjustments of your rig level during the gig.

Not only that--but the level of preamp gain established the nature of your tone. I personally like to set that and forget it too.