Problem with Subs

[Ben Lawrance]

Following on from my "Need an amp" thread, I had a few probs with my subs on Saturday.

 

I've got 2x Turbosound TL1800 Subs (300w RMS @ 8Ohms) which I'm powering off a QSC RMX1850HD amp in Mono Bridge mode. So theroretically feeding the subs 1800Watts (into 4Ohms) But I had the amp on 3/4 on the volume control to get about 1200Watts Ish.

 

I would have thought that it would have been plenty of power and bottom end grunt.

 

But no!!!!

 

 

Standing at the back off the room doing sounds checks and start with the kick drum, PFL on the channel, to get the input gain right, bring up the fader, expecting a huge deep end boom, and all I got was a weedy not very deep sound.

 

Went over to look at the amps and the QSC was oh so slightly clipping.

 

What the f*ck!!!

 

The mid tops were absolutly fine (behringer EP2500 powering a pair of Turbosound Impact121's) nice and loud and very clear.

 

Any one shed any light on the subject???

 

This is the first time I've used this combination of amps with a crossover, I used to run it just from the QSC and link the tops to the bottoms. It sounded ok then, so the amp is fine (it's only about a year old)

 

 

Cheers

Ben

[Bad_Rock]

Did you check the subs without the grills just to test them? With your eyes sometimes you can tell if the woofer is working properly. If you see the woofer moving like it should ( but still the spl it's low it might be the cross over ). It happened to me once. We checked that it wasn't out of phase then we changed the cross over and subs came back to life.

 

I hope it can help you.

 

Regards.

[Rob_Beech]

A number of things to look at.

 

Its very easy to make a simple mistake no matter how many times a week you set up your gear,

So the first thing to note is the settings and wiring.

 

Have you got the high pass filter on. With these cabinets at this power I would recommend it be on and set to 50Hz (I know this sounds harsh for subs but the main punch of kick drum is around this anyway) that will give you more power in the useable frequency range.

 

Also you may find you get stronger bass if you disable clip limiters (but you do then of course have to be careful).

On this note tho you have got a farily large amplifier for these speakers so shoudln't need to drive it to this level anyway.

 

 

The final thing which can catch people out (has with me) is the sesnitivity setting (volume control)

When in bridged mode its important to have the "volume control" for channel B right down at the bottom.

 

As bridged mode uses both channels (with one out of phase) turning up the volume on channel B interacts with that of channel A. Some amps bypass the control here to stop this occuring but if I remember correctly the RMX series does not do this.

and for your information the behringer amps do not either.

 

 

 

On a side note, by turning the "volume control" to 3/4 you will not restrict the amp to 1200 watts or so. It is merely an input sensitivity control, full power is still available but you need a higher voltage to drive it to this level (ie a higher level on your desk).

 

 

Hope this helps. Do let us know how you go on.

 

Rob

[Mr.Si]

Just something to note here

 

They are not volume controls (As I once also thought they were), they are input gain controls and therefore control the sensitivity of the Power Amp.

So to prevent you from blowing your amp or drivers, you need to make sure you have the setup the amp's gain structure correctly, so that it's not overloaded with signal.

 

What is the spec of the subs you were using?

Could there have been an underpowered amplifier for the subs? - they can probably handle about 4 times as much power than they state, esp if they're Turbosound stuff. (peak)

 

Edit: ooh looks like someone else has beaten me to it about the amplifier controls.

[Simon Lewis]

small point:

 

Some amps have variable gain on the front panel, but many are fixed gain with an attenuator on the front panel....

 

Also, you may need to scrutinise your crossover settings, check polarity of connections, and check the placement of your bins.

 

Are they sat on the floor? (this gives more sound due to half space loading). Are they spaced apart? this can cause some cancellation when the spcing is half a wavelength. If that equates to a wanted fundamental or its harmonic, lobing can occur.

 

It's possible that the bins were fairly good at reproducing midrance, so when you added the crossover, perceived overall level dropped a bit.

[Mark Payne]

Hi Ben,

 

IMHO bridging amps into subs is never a good idea.

The RMX amp in question states maximum power bridged into 4 BUT

 

... A 4 ohm (nominal) sub setup (your boxes in parrallel) whill have an impedence that will drop way below 4 at certain frequencies... as low as 2Ohms for example.

 

My (educated) guess is that your amp's clip light is the RMX telling you that it thinks your load is a short circuit and it is showing you its protection.

 

The rule of subs..... less amplifier and more boxes. Damping factor is your friend and bridging amps compromises damping factor.

 

Double the amp power and all you get is 3 dB ...

Double the number of boxes and you get 6dB (assuming close coupling and subs together).

 

Hope this helps... feel free to disagree!

 

Cheers

 

Mark

 

Following on from my "Need an amp" thread, I had a few probs with my subs on Saturday.

 

I've got 2x Turbosound TL1800 Subs (300w RMS @ 8Ohms) which I'm powering off a QSC RMX1850HD amp in Mono Bridge mode. So theroretically feeding the subs 1800Watts (into 4Ohms) But I had the amp on 3/4 on the volume control to get about 1200Watts Ish.

 

I would have thought that it would have been plenty of power and bottom end grunt.

 

But no!!!!

 

 

Standing at the back off the room doing sounds checks and start with the kick drum, PFL on the channel, to get the input gain right, bring up the fader, expecting a huge deep end boom, and all I got was a weedy not very deep sound.

 

Went over to look at the amps and the QSC was oh so slightly clipping.

 

What the f*ck!!!

 

The mid tops were absolutly fine (behringer EP2500 powering a pair of Turbosound Impact121's) nice and loud and very clear.

 

Any one shed any light on the subject???

 

This is the first time I've used this combination of amps with a crossover, I used to run it just from the QSC and link the tops to the bottoms. It sounded ok then, so the amp is fine (it's only about a year old)

 

 

Cheers

Ben

<{POST_SNAPBACK}>

[gazman]

u should definitely check your wiring, I think if you're using speakon outputs the bridge mode wiring is 1+/2+, and if you're cables aren't right...

 

I agree about the high pass filter, also I think ur driving the subs hard with 900 watts each, thats 3x the rms level and if u use or plan to use compressors or simply drive the amp to hard the average power is going to melt those coils...turbosound or not

 

room placement is a big issue too, could just be the venue you were in is an awkward venue for sub placement. have you used any of the same gear there before?

 

cheers,

gaz

[Ben Lawrance]

Never used this combo of gear before, thus why I got a new amp (see "need an amp before friday" thread)

 

I'm not using the speakons on the back off the amp, I'm wiring into the terminal posts, to enable me to run both subs straight off the amp.

 

I'm using the right connectors. The 2 red ones (and yes they are the right way round for pos and neg)

 

Would the size of my speaker cable be an affecting issue. I'm using 2x 4mm2 somme medium stuff from studio spares. Both cables are 10mtrs long.

 

Cheers

Ben

[Bad_Rock]

You should definitely check wiring. It wont be the first time that a cheap amp comes with the negative pole on red. I agree about the high pass filter and sesitivity setting.

[dbuckley]

... and all I got was a weedy not very deep sound.  ...  QSC was oh so slightly clipping.

<{POST_SNAPBACK}>

 

Assuming this isnt a room issue, this reeks of the subs being out of phase with each other.

 

Or... theres something wrong with the mono summing in the crossover area. Which is the same thing but at line level rather than speaker level. But thats less likely as you're flirting with amp clip.

 

For the speakers situation, the easiest way to confim is to put the subs physically together, run them up (doesnt need to be loud, the garage will do), and unplug one. if you get a significant increase in bass then phase error is confirmed.

 

For crossover phase problems, pan the kick mic (or a CD, doesnt really matter) hard to one side, does the bass increase? if so, its a phase integrity thing somewhere around the desk to crossover.

 

You could also try running unbridged, ie straight stereo, which would eliminate any under impedence protection concerns.

 

4mm cable is "adequately generous" :-)

[Bad_Rock]

Hey Ben!

 

Did you find what was the problem? I'm still curious about what would it be.

 

Regards.

[Ben Lawrance]

Hi Guys

 

Thanks for your help.

 

unfortunatly, I have not had time to find out what the problem was. Which sucks.

 

I have been told that it could be to do with the current the amp is drawing.

 

If it can't draw enough then it just put's up with what it's got and sends the signal out.

 

I'm not too sure of this because I would have thought it would try and pull the current and trip the breaker if it tries to hard and there is not enough there.

 

Any ideas.

 

Cheers

Ben

[mackerr]

You didn't say how this is all wired up. What are the crossover settings you are using with the subs? What kind of crossover? Have you tried running them in normal stereo mode? Is there any difference? As mentioned before, you should have the subsonic filter IN. You can set it at 30Hz, the important part is to get rid of energy below what the speaker is capable of reproducing. The TL-1800 has a rated -6dB point of 44Hz, so the 50Hz amp filter is a little aggressive. Try to avoid boosting extreme low frequencies in any eq's, you may be trying to get more out of the speakers than they can deliver, and driving the amp into clipping without having any useful audio.

 

The first step should be testing the speakers one at a time on the amp, then in normal stereo mode, then in bridged mono mode.

 

Mac

[Ben Lawrance]

Setup wired as such

 

Folio SX out L&R down a 30mtr multi.

Out of the stage box with 2x 10mtr XLR-XLR cables into back of rack

That goes into the L&R input on the crossover (behringer 2way stereo)

Mono sum out into qsc RMX1850HD in Bridge mode using a 30cm XLR-XLR cable.

Using the binding posts, 4mm speaker cable soldered onto speakon chassis on the back of the rack (y-split sort of thing - 2 subs into one output) createsa 4ohm load.

4mm speaker cable with speakons on the end into the subs.

 

I am going to go through all the cables with a meter and check the polarity of the leads as I made them all. Some of them were made at midnight so that might be the first place to look.

 

 

Cheers

Ben

[mackerr]

That goes into the L&R input on the crossover (behringer 2way stereo)

Mono sum out into qsc RMX1850HD in Bridge mode using a 30cm XLR-XLR cable.

Using the binding posts, 4mm speaker cable soldered onto speakon chassis on the back of the rack (y-split sort of thing - 2 subs into one output) createsa 4ohm load.

4mm speaker cable with speakons on the end into the subs.

<{POST_SNAPBACK}>

What is the bandpass of the mono sum out of the crossover. You want it to be something like 30Hz to 100Hz. The HPF at 30 will keep you from trying to amplify stuff the speakers just can't reproduce, the 100Hz LPF will keep interference with the mains to a minimum.

 

Check each speaker individually on one channel of the amp to make sure the speakers are fine. If they both work fine by themselves on one channel try them both together on one channel to check for correct polarity. When you have a speaker sounding good, use that setup to check all your cables. When you are sure all the cables and speakers are fine check the mono bridged mode and the speaker "two-fer".

 

Mac