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Push the levels - desk or PA?

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Push the levels - desk or PA?

Postby Dave B » Tue Jan 29, 2019 3:06 pm

So, I've been thinking about this for a while and thought I'd put it to the collective wisdom here. I'm probably missing something obvious, but would be interested in thoughts.

When we play live (or I do sound for other bands), it's not uncommon to want a little more sound available. I'm struggling to understand which I should drive a little harder - the desk outputs or the PA inputs.

(Yes, I know that the actual correct answer is 'neither - get a more powerful PA', but a) I want to check I'm not missing something and b) it's not always feasible)

I use a QSC Touchmix16 with Yamaha DXR10 tops and let's say that we are in a suitable sized venue. There's an almost nice balance between backline and PA (little bit of kick and snare, keys but mainly vox in the tops), but it could do with going a little louder. The desk is correctly gained with everything peaking nice and close to 0dB with the master bus showing that the overal sound is averaging quite nicely about the 0 mark as well. The tops are on unity so it's line level out to line in.

At this point, I'm struggling slightly because I can't seem to match some of the figures up or even find them. I know that the QSC has a nominal +4dBu output but can handle a peak signal of +22dBu so it has headroom on the outputs. I'm guessing that the 32float internal bit depth means that it's possible to have a valid output up to that level without the signal distorting.

The Yamahas also have a nominal +4dBu input at line level, so 0dBfs should be the same on both systems.

Commonly, when it looks like a healthy / appropriate signal is hitting the tops, but it's a little quiet, people increase the gain on the tops to compensate. Given that I have a bunch of headroom on the output bus, is it better to drive the desk outs a little harder, or drive the amp on the DXRs a little more?

Does it make a difference?

(I'd have thought that working an amp harder would be less preferable - hence me asking this - but I'm completely in the dark here)

I suppose I could strap a limiter at some level on the desk output (around +6dBFS he guessed wildly) to catch the nastier peaks and drive it a few dBs harder so that it is peaking over 0 without taking the proverbial. But this is assuming that the desk bus is happy to be driven and I'm not sure how I check that...

I need to do some more tests as I'm sure that the EVs I picked up last year don't seem to need to be driven quite so hard - but that could be me getting better at gaining and watching levels. Another mate uses a Berry XR18 and he noticed that he has to give the DXR12s he uses a fair amount of gain. Looking at the specs, that has +4/+16 for nominal and peak outputs so I'm guessing that even if I can push the desk, he would still have to be a little more careful.

(I'm going to have to do some more googling and maths to turn 22 and 16 dBu into fs equivalents)

Any thoughts?
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Re: Push the levels - desk or PA?

Postby CS70 » Tue Jan 29, 2019 3:26 pm

The (admittedly few) times I set up a desk, I follow the same gain staging rule as in studio - every channel is set up so that the gain knob is in the range where there's a good signal/noise ratio but still enough headroom to cope with peaks. Then I use the faders from 0 to balance, as a trim. Everything should not move that far from 0. The main out and the PA power amp or volume do the rest.

So basically on the desk my gain+ (post balance) faders are always at the max I can do while leaving comfortable headroom and I then the actual FOH is determined by the mains fader and the PA level.

So in your question terms - max the desk gain and use the PA to regulate.
But definitely waiting for the live music experts to chime in..
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Re: Push the levels - desk or PA?

Postby Hugh Robjohns » Tue Jan 29, 2019 4:00 pm

I think part of the difficulty here is a confusion of incomplete specifications and unreferenced metering, combined with undefined analogue-digital conversion alignments.

In a perfect world, you'd probably want the active speaker amps to enter their protective limiting mode at the same level as the mixer's digital output reaches 0dBFS.

The specs for the DXR10 are confusing, with three different levels quoted:

DXR10 wrote:Input Sensitivity (LEVEL: Maximum) INPUT1 LINE: +1dBu
Input Sensitivity (LEVEL: Center) INPUT1 LINE: +11dBu
Maximum Input Level INPUT1 LINE: +24dBu

So although the back panel says the nominal line input level is +4dBu, the specs suggest it's +11dBu... with the sensitivity increasing by a further 10 dB (nominal input level falling to +1dBu) with the control at maximum.

Perhaps more importantly, the maximum input level is given as +24dBu.

Comparing that with the TouchMix specs, the desk's maximum output level is +22dBu, which presumably corresponds to 0dBFS on the meters... except that the meters don't appear to be calibrated in dBFS, but rather in a quasi-analogue way with some ambiguous reference level below an unknown amount of headroom.

It's no wonder you're confused!

Given the maximum output/input level specs above, it should be the case that, with the speaker's level knob in the centre, the desk meters will hit the top before the speakers overload (with a 2dB safety margin), which is probably a good place to be. But if you wanted the speaker's protective limiters to kick in before the desk clips, you could probably turn the speaker input sensitivity control up by 3dB or so. Might be worth a quick experiment to see which configuration sounds best.

As to whether to 'push' the desk output or the speaker input, there is no right or wrong here. The speaker sensitivity control is an input attenuator, so shouldn't affect the sound character of the amp/speaker itself at all.

On the other hand, sending more or less level from the desk's analogue outputs may well change the sound character to some extent. It's not unusual for budget analogue electronics to start sounding a bit harder or strained as the level exceeds +20dBu or so.

So you may find there's a sweet spot for the average output level on the console... and one might presume QSC know that too and have set the meters' nominal zero level accordingly.

The other thing to consider, of course, is that in a live sound situation, it's always a good idea to maintain some safety headroom margin in the console, so while I wouldn't be averse to pushing up the average level 6dB above the zero-reference level, I probably wouldn't want to go much further.

if you then felt that you needed more from the speaker, you have the option to increase their sensitivity with the controls on the speakers, but you then risk overloading the speaker (or activating the protection) on transient peaks.

Sorry... bit of a mind dump...Not sure if that helps...

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Re: Push the levels - desk or PA?

Postby Dave B » Tue Jan 29, 2019 4:36 pm

It certainly does help if only because I feel better that you are finding it hard to find / read the specs as well! It's not just me...

I feel a quick email to QSC to check their calibration coming on. And maybe another to Yamaha. But mainly, I feel a quick trip to a handy rehearsal room to do some tests.....
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Re: Push the levels - desk or PA?

Postby Wonks » Tue Jan 29, 2019 5:21 pm

I though I understood it, then realised I didn't. Those quoted maximum input levels (especially the mic one at +20dBu when the line is +24dBu) really mess things up.

This may just mean these are the maximum voltage that can be applied without damage to the circuitry (though the clipping light will come on). The input sensitivity values give for the full output without clipping.

referring to the block diagram, there is a common pre-amp for mic and line inputs on input 1, so in line mode, this appears to give 10dB of gain. The level control is still an attenuator but as the line signal is always boosted by+10dB before the attenuator, there's no extra noise penalty in setting the level pot to +10.

So to me, it would seem that with the level control in the centre (0dB gain), a +11dBu input signal will give full output, whilst with the level control in the +10dB position, a +1dBu signal will give full output (at least those figures make sense).

Repeating this in a slightly different way, with the level control in the centre 0dB position, you don't want more than +11dBu from the mixer, and with the level control in the +10dB position, you don't want more than +1dBu from the mixer (so practically you look at the meters for either +10dBu or 0dBu peaks).

When I was running these speakers from my Soundcraft desk, I'm pretty sure I turned up the level control to about +6 and the desk main outputs meters were normally a bit below 0dBu, with the occasional louder peak.
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Re: Push the levels - desk or PA?

Postby Mike Stranks » Tue Jan 29, 2019 6:51 pm

Generic reply rather than specific to the kit you've specified - although I was using DXRs.

When I was 'live-sounding' for a guitarist of our acquaintance everything was tidy and with micro-adjustments on the channel faders until the encore... Then I always turned the wick up quite a bit - it was always the same piece which needed LOUD.

For that I always pushed up the desk main faders, but never enough to go 'meter red'. Had the desired effect without any nasties becoming evident.

Of course, as has already been mentioned this was with careful gain-staging and having plenty of headroom for the main concert stuff.

That may be useful... or not! :D

PS. This reminds me somewhat of the very polite debates that used to go on between two of our (sadly no longer contributing) forumites about whether you should run amps with maximum gain and adjust levels accordingly at the mixer or whether you should gain-stage to the mixer meters and then raise amp gain controls for required room volume. (I was an am in the latter camp.)
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Re: Push the levels - desk or PA?

Postby Hugh Robjohns » Tue Jan 29, 2019 7:50 pm

Wonks wrote:...referring to the block diagram, there is a common pre-amp for mic and line inputs on input 1, so in line mode, this appears to give 10dB of gain.

10dB attenuation, surely?

So to me, it would seem that with the level control in the centre (0dB gain), a +11dBu input signal will give full output, whilst with the level control in the +10dB position, a +1dBu signal will give full output (at least those figures make sense).

Hmmm. I think those are more likely to be the norminal input levels. Otherwise where does the maximum input level of +24dBu fit in? ...or maybe that level can only be supported with the input attenuator turned down further?

Confusion reigns! :thumbdown: some careful practical system experimentation is required I think.

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Re: Push the levels - desk or PA?

Postby Bob Bickerton » Tue Jan 29, 2019 7:52 pm

Don’t have time to check this now, but I seem to remember reading about how the levels on the TouchMix when run a usually sensible levels were a little lower than expected. I certainly have run mine ‘hotter’ than I would usually be comfortable with, but email QSC or do more reeasearch. It is a ‘foible’ as I remember.

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Re: Push the levels - desk or PA?

Postby Wonks » Tue Jan 29, 2019 8:36 pm

Hugh Robjohns wrote:Hmmm. I think those are more likely to be the norminal input levels. Otherwise where does the maximum input level of +24dBu fit in? ...or maybe that level can only be supported with the input attenuator turned down further?

The maximum quoted mic input level quoted is as +20dBu. Given that in mic mode the pre-amp will be giving another 30-40dB of gain, and the quoted line level maximum input is +24dB, I really can't see how that can be for maximum volume output. I really think that that is a physical voltage maximum.

A quick web search for 'nominal input voltage' for an amplifier on the web keeps coming up that it's the voltage required for maximum output before clipping, so I based my setting assumptions on that (and which makes sense to me).

Unfortunately the Yamaha site seems to be down at the moment (I've obviously broken it) so I can't access the manuals to check anything else.

Yes, the mic/line pre-amp gain may be anything (not thought through fully). It's feeding into an A/D converter as the signal is digitally processed, so the pre-amp will raise or lower the signal level as appropriate for the converter. But whatever it does, (if the block diagram is correct) the full signal level is passed through from the volume pot with the pot on +10dB, and is attenuated by 10dB when the pot is in the 0dB position.
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Re: Push the levels - desk or PA?

Postby Dave B » Tue Jan 29, 2019 9:43 pm

So if I read this right, the power amp runs at full power at all times, and the knob is merely controlling how much is being fed in?

Which is going to upset Mike as he is in the second camp and that describes the first! :D

Bob - cheers for that and I think I will try and wangle more detail out of QSC. I'm just trying to mentally compose a polite email describing what I'd like to know (without coming across as a total numpty. That might take some time! :) Will post if/when I get a response.

Muchos grassy ass - all good stuff chaps. I know at least one other chap who will be interested this, so all knowledge welcome!
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Re: Push the levels - desk or PA?

Postby Wonks » Tue Jan 29, 2019 10:17 pm

Well, the pre-amp appears to run flat out (so it might be a 'class A' design), rather than the pre-amp gain being set by the volume knob. The signal goes through A/D, DSP then from the two D/As off to the two power amps. Almost every power amp runs at a fixed gain, with the level of the input signal determining the output.

Both HF and LF and are class D, and with no input signal to speak of, they really don't use much power at all. Actual power consumption of the DXR10 at idle is only 35W, which includes the power for all the DSP etc.
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Re: Push the levels - desk or PA?

Postby Tim Gillett » Tue Jan 29, 2019 11:26 pm

With a powered mixer all this is moot. The master gain is all we have. Ideally we don't need to readjust the powered speaker's input gain mid gig. Adjusting the mixer's master fader is all we need. For me the master output meter tells me system headroom. That's it.
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Re: Push the levels - desk or PA?

Postby Mike Stranks » Wed Jan 30, 2019 11:37 am

Dave B wrote:So if I read this right, the power amp runs at full power at all times, and the knob is merely controlling how much is being fed in?


Well that's my understanding and that's what the debate between the two very experienced and knowledgeable guys was all about.... ie where is it best to apply the overall gain reduction?

Back in the day when we didn't have powered speakers, a friend to whom I was apprentice for a while always ran the amp at maximum gain and controlled overall volume at the mixer. Although that's what I was 'taught' I've always preferred to set levels and gain-stage at the mixer and then adjust the speaker controls - separate amp or onboard speaker amp - to get the required 'volume' in the room. Usually I'd set that to slightly 'louder' than what I would deem to be gig-appropriate so that I had something in-hand if needed.
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Re: Push the levels - desk or PA?

Postby MarkPAman » Wed Jan 30, 2019 11:50 am

I know the setting where desk max coincides with what the speakers max. I've worked this out by running the desk up to just clipping, then turning the speakers/amps up to the same point. More or less gives me what the maths predicted, maybe just 1 or 2 dB higher at the speaker on my usual setup - because the desk clip light comes on early. So that's my ideal place.

But - if it's going to be a significantly quieter gig than that (and thankfully it often is) I run he speakers/amps lower, so that I'm still mixing with a decent level from the desk.

But then again - at the occasional more lively pub gig, I may decide to run speakers/amps flat out, so that drunken punters, or bands, can't turn the volume up on the speakers without me knowing. Un-muting after the break to find I now have massive feedback from every mic is not fun!
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Re: Push the levels - desk or PA?

Postby Hugh Robjohns » Wed Jan 30, 2019 12:07 pm

Wonks wrote:The maximum quoted mic input level quoted is as +20dBu.

:?: The specs I found definitely claims +24dBu for the line input. The mic input is specc'd at +20dBu max (page 81):
https://uk.yamaha.com/files/download/other_assets/3/325553/dxr-dxs_en_om_c0.pdf

I really think that that is a physical voltage maximum.


Max input signal level in dBu is the same thing as a maximum physical signal voltage...

A quick web search for 'nominal input voltage' for an amplifier on the web keeps coming up that it's the voltage required for maximum output before clipping...

It would be an odd interpretation that equates nominal input level with clipping level... but we are potentially discussing 'Japanlish' here, so who knows. Careful experimentation with the equipment is the only way you'll get to the bottom of the interfacing signal levels and optimum gain structuring.

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Re: Push the levels - desk or PA?

Postby Hugh Robjohns » Wed Jan 30, 2019 12:09 pm

Dave B wrote:So if I read this right, the power amp runs at full power at all times, and the knob is merely controlling how much is being fed in?

This is the normal case. Power amps generally have a fixed voltage gain -- typically 25-30dB. Any required level adjustment is normally courtesy of an input attenuator.

Small signal amplifiers (eg, mic preamps) usually (but not always!) do have adjustable gain, but most other gain stages are typically fixed gain and are either preceded or followed by adjustable attenuators (eg, faders).

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Re: Push the levels - desk or PA?

Postby Wonks » Wed Jan 30, 2019 12:15 pm

I was just trying to point out that +24dBu for the line and +20dBu for the mic input which is then increased by 30 dB or more in the pre-amp in mic mode, to say +50dBu just doesn't make any sense. Therefore those figures logically can't be for maximum signal output.

Nuff said. Dave can check it out at his next rehearsal, tomorrow night. Take a pillow to cover the speaker, Dave!
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Re: Push the levels - desk or PA?

Postby Sam Spoons » Wed Jan 30, 2019 12:16 pm

I have DXR10s and when I asked the tech dept what the power amp specs really meant they were unable or unwilling to tell me, It seems that (semi-) pro PA manufacturers have become infected with the same disease as 'In Car Entertainment' makers. Yamaha quote 'continuous' and 'dynamic' for the power amp outputs.

I do realise the important figure is the max SPL but, being a little "Old School" and, I guess, a bit of an anorak, I like to know these things. I also know that the concept of 'RMS Watts' is itself flawed but it is, at least I believe it is, consistent.
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Re: Push the levels - desk or PA?

Postby Wonks » Wed Jan 30, 2019 12:23 pm

The problem with rating larger class D power amps is that their music (and obviously peak) power rating can be much larger than the available electrical power supply, so whilst their practical music power rating can be a lot higher, their RMS rating remains limited as they can only produce a large RMS value for a few milliseconds before all the stored energy in the capacitors is drained and the voltage levels plummet.
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Re: Push the levels - desk or PA?

Postby manleyelop » Mon Feb 11, 2019 10:56 pm

This op's question really interests me also i usually have my power amps on full each of these are through a dbx crossover would it be best to have my desk fader levels more around unity & back off the power amp level?
i usually find my faders are below unity but still get good sound results but would rather do things different if what im doing is bad practice
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