Question about headroom with dj mixers

R

Richard Stringer

Sophomore
Jan 13, 2011
124
0
16
Boston, Lincolnshire, UK
Hey ya guys, i've got a question, can the amount of headroom a dj mixer has make a difference to the overall spl you can get out of your sound system? The reason I ask is because my old system seemed louder than my current system. My old system was two Peavey Hi-Sys 4XT mid/highs (power : 700 watts continuous each/sensitivity : 100db), four JBL MP418S (power : 650 watts continuous each/sensitivity : 101db), the mid/highs were powered by a QSC RMX2450, one cab per channel giving each cab 650 watts average power, a QSC PLX3002 powering my subs, two subs per channel giving each sub 750 watts average power. I also had a Behringer CX2310 crossover and finally a Vestax PMC05 dj mixer, specs here :


• Inputs: Phono x 2 (RCA), Line x 2 (RCA), Mic x 2 (1/4 inch)
• Output: Line x 2 (RCA), Phones x 1 (1/4 inch stereo)
• RMS Output: -7dBm
• Freq. Response: 25-25KHZ + 1.5dB THD: 0.02%
• Cross talk: more than 70dB @ 1KHz
• S/N Ratio: more than 75dB
• Input Impedance: Phono/56K, Line/10K, Mic/3.3K (ORMS)
• Output Impedance: Line/2K, Phono/8K

Now i'm using two Peavey UL215H mid/highs (Power : 1,000 watts continuous/sensitivity : 100db) which are the newer versions of the old Hi-Sys 4XT cabs but other than that are nearly identical and i'm powering them with a JBL MPX1200 amp, one cab per channel giving each cab 1,200 watts average power. I'm using two JBL SRX728S subs (power : 1,600 watts continuous/ sensitivity : 98db) powered by two bridged JBL MPA750 (re-badged QSC EX2500) amps, one amp bridged per sub, giving each sub 2,000 watts average power. I'm using a Dbx Driverack PA and finally a Stanton SA.3 dj mixer, specs here :

Line inputs: 2 (RCA) x 2 channels, -10 dBV /10 kOhm
Phono inputs: 2 (RCA) x 2 channels, -50 dBV / 47 kOhm
Master output: 2 (TRS Balanced/RCA unbalanced),
+4 dBu balanced / -10 dBV unbalanced
Headphone output: 1 (1/4 inch), 1 (1/8 inch), greater than 32 Ohm load
Frequency Response: 20 Hz - 20 kHz, +/- 1.5 dB
Tone Control : Hi +9/-18 dB
Mid +9/-25 dB
Low +9/-25 dB
S/N Ratio: Better than 94 dB (ref: max input level)
Noise: -80 dBV (Line input to any output)
T.H.D Less than 0.04% (1 kHz)

The Stanton mixer seems to clip a lot quicker than the old Vestax mixer I was using. The Vestax mixer's red lights were at +6dbv on the mixer's vu meters and the Stanton's red lights are at only +2dbv on it's vu meters. Does this mean that the Vestax has (roughly) 4dbv more headroom? The sound, well I mean spl, doesn't seem to be as loud on my current system using the Stanton dj mixer as it did on my old system when using the Vestax, but i'm using more power now with my current sound system than I was using with my older system. Gain structure was set back then with my old system and i'm setting it now with my newer system.
 
Re: Question about headroom with dj mixers

We have another dB scale for DJ mixers. We call it dBP. As in "dB Pioneer." Best we can tell in real world applications, the Pioneer mixer's meters do not correspond with any of the mixing consoles meters that we can figure out. +10 on a Pioneer is approx +22dBu give or take a few dB. The newer Pioneer DJM900 seems a lot more unpredictable that the old 800s which is kind of funny because the old 800s had the attenuation pot on the back, the 900s do not have that feature.

I have no experience with the Vestax or Stanton mixers but it may be that they are using different dB scales for their meters. Also one mixer may not be able to to output as hot of a signal as the other one. The specs you listed ton't tell the maximum outputs of either unit.
 
Re: Question about headroom with dj mixers

I'm worried that you aren't powering the 728 subs properly. The MPA750 only shows a 4ohm bridge rating at 1khz in the manual, producing the 2000watts as you state. Given the 728 can handle program power at 3200watts, peaks of 6400watts, I'm thinking you're not getting everything out of them. The amps also have thermally based power limiting and muting if the temps get high enough inside.

That said, a 418 sub is +3db more sensitive than a 728. It doesn't go down as low of course, but four of them may actually sound louder than two underpowered 728's.

Double check your gain structure.

Check your Driverack settings and amp settings. The 728 subs have a -3db cutoff that's 7hz lower than the 418 subs, 33hz, which could be sucking the amps dry, and if you're running a high pass filter in the Driverack above that point, you're losing out on their performance advantages as well as underpowering them.

Best regards,

John
 
Re: Question about headroom with dj mixers

The reason i'm using the MPA750 is because they're excellent old amplifiers and bridged on subs they can make subs sound amazing. I've tried other amplifiers like Peavey CS4080HZ in stereo and Yamaha P7000S bridged and the JBL MPA750 is the best one so far. Also I know it's only 2,000 watts average power into 4 ohms and the SRX728S are rated at 3,200 watts program but I play extremely compressed dance music so I thought best to stick with an amplifier that's around the SRX728S's continuous power rating. I've set the Driverack's crossover point at 100hz because at JBL's reccomended 80hz frequency it's right at the centre'ish if the kick drum frequency. I've set the high pass filter on the Driverack for the subs at 31hz.
 
Re: Question about headroom with dj mixers

Head room is a poorly understood concept in mixer design, and the simple answer is no... If a DJ mixer even remotely affected how loud it could make the system, what loudness crazed DJ would ever buy it?

Pretty much all mixers use similar PS rail voltage, so are capable of similar output voltages. All power amps are designed so they can be driven past full rated power with typical mixer feeds.

Dj mixers often have only a loose connection between front panel meter level reading and actual output voltage (check for a rear panel output trim).

JR
 
Re: Question about headroom with dj mixers

Richard Stringer said:
The reason i'm using the MPA750 is because they're excellent old amplifiers and bridged on subs they can make subs sound amazing. I've tried other amplifiers like Peavey CS4080HZ in stereo and Yamaha P7000S bridged and the JBL MPA750 is the best one so far. Also I know it's only 2,000 watts average power into 4 ohms and the SRX728S are rated at 3,200 watts program but I play extremely compressed dance music so I thought best to stick with an amplifier that's around the SRX728S's continuous power rating. I've set the Driverack's crossover point at 100hz because at JBL's reccomended 80hz frequency it's right at the centre'ish if the kick drum frequency. I've set the high pass filter on the Driverack for the subs at 31hz.
Click to expand...

I inferred something, but I'll be explicit: the amp is not rated for full range(and low freq. especially)use in bridge mode at 4ohms. The power you're getting from it is more than likely not near the 2000watts it says in the manual, as that is a spec given for a 1khz tone. The amp may be quite good with an appropriate subwoofer, say one that will handle 2000watts continous at an 8ohm load. Even then, the manual states that the amp is limiting hf to 15khz(I know, not precisely related to low freq. use, but more an indication that it doesn't even really like 8ohm bridge mode 100%).

Narrow the bandwidth on the Driverack to 33hz and 80hz, if the subs are crossing over to your tops properly, the kick drum shouldn't be suffering.
Raising the high pass some will tax the amp less, hopefully making for better performance in this situation. I'd experiment and raise it 1-2hz at a time, and listen with some favorite track and stop just below the frequency that you can hear a difference.

Many are using a Crown iTech 8000 on a pair of 728s subs, about 4000watts/ch/40hms in stereo. Besides more power, the amp is being used in stereo mode, so overall performance is typically better/safer.

Best regards,

John
 
Re: Question about headroom with dj mixers

QSC told me a while back that at full bandwidth, bridged into 4 ohms the amplifier will be delivering about 1,700 watts average power, and so that's good enough. I know a lot of people are using Crown I-Tech amps but i've not chance in hell of ever being able to afford one, I was lucky to be able to afford my SRX728S subs, I paid them off slowly, one by one over a year. I got each MPA750 amps for £250, wheras a used Crown I-Tech8000 costs about £2,000, even the I-Tech6000 costs about £1,500 used. With regards to the high pass filter, the music I play is oldskool hardcore rave which has a lot of 30-40hz content and the low frequency makes up a lot of headroom of the tracks being played so i'm gonna leave the high pass filter at 31hz I think, although i'll do what you say and at least have a listen, see if I can tell any difference in setting it at either 31hz upto 35hz.
 
Re: Question about headroom with dj mixers

Richard Stringer said:
QSC told me a while back that at full bandwidth, bridged into 4 ohms the amplifier will be delivering about 1,700 watts average power, and so that's good enough. I know a lot of people are using Crown I-Tech amps but i've not chance in hell of ever being able to afford one, I was lucky to be able to afford my SRX728S subs, I paid them off slowly, one by one over a year. I got each MPA750 amps for £250, wheras a used Crown I-Tech8000 costs about £2,000, even the I-Tech6000 costs about £1,500 used. With regards to the high pass filter, the music I play is oldskool hardcore rave which has a lot of 30-40hz content and the low frequency makes up a lot of headroom of the tracks being played so i'm gonna leave the high pass filter at 31hz I think, although i'll do what you say and at least have a listen, see if I can tell any difference in setting it at either 31hz upto 35hz.
Click to expand...

First, my apologies, the spec where filtering bandwidth >15khz is on the MPA1100.

Ah, 1700watts average power into a subwoofer cabinet that can literally suck that amplifier dry, especially with the use you're giving it. As an experiment, perhaps you could take each of your 750 amps in bridge mode, and run one 728S(one amp per driver). The amp will do 1500watts/8ohm/20-20khz at that setting. Leave your DSP settings for the high pass at 31hz, but I'd still lower the crossover point from 100hz down closer to 80hz. Also, have you time aligned the subs to the tops, and double checked polarities?

There are other options besides an iTech 8000. Just adding two more MPA750's and dedicating one in bridge mode to each driver of the pair of 728S cabinets would probably do the trick.

Best regards,

John
 
Re: Question about headroom with dj mixers

For one thing, finding another two MPA750 amps so I got one powering each driver would be virtualy impossible, it's taken me a year to find the two I have, and another thing, the venue is always the same, a little rock pub/venue with only a 150-200 person absolutely maximum capacity, they've barely got enough power circuits to run my system already so having four MPA750 bridged wouldn't be much good because there simply wouldn't be enough power to feed them. There's only 3 double 230v/13A wall sockets to run the whole pa and lighting system. I'm not clued up on time allighing subs and mid/highs so I wouldn't know what settings to use or what the ideal sound would sound like.
 
Re: Question about headroom with dj mixers

Richard Stringer said:
For one thing, finding another two MPA750 amps so I got one powering each driver would be virtualy impossible, it's taken me a year to find the two I have, and another thing, the venue is always the same, a little rock pub/venue with only a 150-200 person absolutely maximum capacity, they've barely got enough power circuits to run my system already so having four MPA750 bridged wouldn't be much good because there simply wouldn't be enough power to feed them. There's only 3 double 230v/13A wall sockets to run the whole pa and lighting system. I'm not clued up on time allighing subs and mid/highs so I wouldn't know what settings to use or what the ideal sound would sound like.
Click to expand...

Sorry, I meant to say try the pair of amps on one sub as an experiment.

You may already have power supply issues with the three circuits you have on the new rig. The old subs on one PLX amp, were more efficient, using less AC. One of the trade offs was a higher low freq. corner on the old subs. Going lower uses up a lot more power.

At this point, I'd double check all my cables for correct polarity, check the subs to make sure they're all hooked up with the same, find someone to tweak the settings on the rig with the Driverack(measured with Smaart say), get the gain structure sorted out, and experiment with the high pass on the subs.

With less efficient subs and not enough power for them, even going back to the four original MP418S subs on the 750 amps would be something I'd consider to get back some of the lost SPL in this club.

Best regards,

John
 
Re: Question about headroom with dj mixers

It's ok John. :)

I prefer older heavyweigh transformer amplifiers because at the budget end of live sound they're better for subs than the lightweight switch mode amps. I'm talking about amps in the price range of say, the QSC RMX2450, amps that price, or around that price, used older heavyweight amps bought for the same price in my opinion give better low frequency output. I've tried various amps and found that switch mode amps on subs weren't very impressive. There'd be no point in using higher power amps than the MPA750 bridged anyway because of the fact i'd lose a lot of the increase in power due to power compression. So basically, instead of using two bridged MPA750 amps, one per subs, if I used an amp or amps that could output 3,000 watts into 4 ohms (the impedance of my subs) i'd maybe only get about 1db of extra spl because of power compression. I don't clip my system ever anyway, so for the sake of about 1db more spl, I might as well just stick with amps or an amp that can give my subs at least 1,600 watts average power each.
 
Re: Question about headroom with dj mixers

While this seems to be a pretty common perception, I am not aware of any justification or basis for this belief based on the technology of switching PS. Transformers do not store** power supply energy, capacitors do. Capacitors can be sized marginally for either design approach

Funny the 2450 does not seem like an old iron design to me... more like the newer 2U stuff.

It's good to know there is a market for heavy old iron... :) I wonder if there's a similar market somewhere for my old 286 PC?

JR

** transformers do transiently convert electrical energy into a magnetic flux in the primary, then back to electrical energy in the secondary, but this magnetic flux is not persistent and the conversion process occurs on a time scale far too short to be meaningful to bass notes. Even in a slow old heavy iron transformer.
 
Re: Question about headroom with dj mixers

Yeah I know that capacitors store electrical energy, but I was under the idea that with budget amps, the transformers are able to extract more power from the mains continuously than budget switch mode amps, and that's the reason why older toroidal amps are better for low frequency than newer budget switch mode. I thought budget switch mode amps can only deliver bursts of their rated power and can't deliver it continuous if they needed to. Also from what I know more expensive touring switch mode amps CAN deliver continuous power because they have bigger built in power supplies. When I mentioned the RMX2450 I meant that from my experience and also the experience of a lot of people i've talked to, newer amplifiers like the RMX2450, Crown XTI4000, Crown XLS2500 etc..the older toroidal amps like the QSC EX2500, MX3000A, Crest CA9 etc...are better becauser they're able to pull the power needed from the mains supply plus i've heard people say the older amps had bigger capacitors or more capacitors to store more electrical power for when subs need it, wheras newer budget range switch mode amps seem to quickly run out of stored power.

That's the talk anyway and it's been the talk on a lot of forums and from a lot of sound engineers too.
 
Re: Question about headroom with dj mixers

Richard Stringer said:
Yeah I know that capacitors store electrical energy, but I was under the idea that with budget amps, the transformers are able to extract more power from the mains continuously than budget switch mode amps, and that's the reason why older toroidal amps are better for low frequency than newer budget switch mode.
Click to expand...
There was a recent thread here or somewhere about low vs mid frequency power output. At <<120 Hz, capacitor size and therefore PS ripple voltage can make a few volt difference to peak output. So for bass frequency PS cap size matters.

The typical constraints imposed by transformers are #1 Unloaded PS rail current/voltage (limited by design, ie turns ratio and core size) , and #2 continuous power, limited by heat dissipation and internal insulation resistance in the face of temperature rise due to IxR losses. Some value amps, use circuit breakers to protect undersized transformers from overheating, by shutting down the amp before the internal one-shot thermal fuse opens.

HF switching supply transformers have a huge thermal advantage from passing smaller amounts of current many times per second, instead of large current pulses, at the much slower mains frequency. But there is a cost for this higher technology so generally all things equal the higher technology approach has to cut some corner somewhere to match price.

It is only a very recent trend where the switching and class D technology has matured to the point that the material savings from smaller iron and smaller heat sink can actually translate to lower cost over all, even with the higher inherent complexity.
I thought budget switch mode amps can only deliver bursts of their rated power and can't deliver it continuous if they needed to. Also from what I know more expensive touring switch mode amps CAN deliver continuous power because they have bigger built in power supplies. When I mentioned the RMX2450 I meant that from my experience and also the experience of a lot of people i've talked to, newer amplifiers like the RMX2450, Crown XTI4000, Crown XLS2500 etc..the older toroidal amps like the QSC EX2500, MX3000A, Crest CA9 etc...are better becauser they're able to pull the power needed from the mains supply plus i've heard people say the older amps had bigger capacitors or more capacitors to store more electrical power for when subs need it, wheras newer budget range switch mode amps seem to quickly run out of stored power.

That's the talk anyway and it's been the talk on a lot of forums and from a lot of sound engineers too.
Click to expand...

Talk is cheap... these are all constraints within the purview of competent design engineers. The are high and low performance amps in any technology.

The reality is a modern "value" amp will generally be designed to a lower performance standard than an older not-value amp. So in street dollars you could get more actual amp for less money by buying old, but this is not related to the technology used per se. More influenced by market trends toward cheaper and cheaper power amps. I recall when $1/watt was a cheap amp... today that is expensive. Note my idea and yours about "old" may vary some too.

JR

PS: regarding pulling more power from the wall, only the premium PFC (power factor correction) and better yet PFC+ regulated PS voltage amps can pull demonstrably more power from a given mains distribution. You will not find to many cheap PFC amps.
 
Re: Question about headroom with dj mixers

A lot of my decision to go with older amps is to do with cost, if I could afford a Lab Gruppen FP10000Q or something of that quality and price I would do, but the JBL MPA750, MPX1200 etc...they go for as little as £250 here and for an amp of that quality, even with it being heavy, it's well worth it.
 
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