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[4 way sound system] issues with the sub stage

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hurukan View Drop Down
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    Posted: 17 August 2021 at 7:00pm
Greetings !!

I got a 4 way sound system driven by a preamp (hand made) we usually play reggae/dub and
the bass/subs are very "deep" on some particular tracks.

Usually we play full sound with 4 "mogales" loaded with 18 inches RCF speakers alimented by two QSC3000A
amps bridged. The "pressure" from the sub stage is always nice with the whole four "scoops".
We sometimes use the "half of the sound system" and one time on two we got issues with the sound.

In this configuration we use only one qsc3000A in bridged mode instead of two and we connect the output "speakon" to the input of one of the "mogale" and then relay to the other one using another "speakon" the connectics mounted in parallel. So for me we got 8 ohms in the circuit (I suppose).
BUT: when playing with the preamp at 12 o'clock (half of the input gain) on some tracks the QSC3000A starts
to clip when one year ago in the same configuration it didn't :{
We have a crossover (thomann t-rack) that is trying to make things to prevent the amp to clip.
It has been configured to compress on 8:1 with a +4db threshold --  the amp still clipping even if the threshold is set to 0db. The issue is bigger when playing with other source than "phono" (mixing table controlling a computer for example or a wav player).

The whole preamp is mono configured (all the inputs and outputs are mono).
The "mixing table" is connected to the preamp with a stereo to mono (palmer PLI-06 line level convertor) but the cable used was a XLR to stereo instead of mono -- we lost all the bass/sub frequencies just like it was a phase inversion.
There wasn't a amp clipping issue here but we did not have bass/subs at all.

I don't know which element I have to check first.

1. The scoops seems to work (no sounds indicating they are blowed up) and on most of tracks they are delivering
more than a "correct" response.
2. The amps are working ok unless we play "heavy dubs" but in some configuration they are clipping.
3. The crossover -- it seems to do its job, the compression led is lighted when we "sweep" the frequencies from 40Hz to 90Hz (the HPF and LPF frequencies)

I didn't yet tested the speakons cables AND connectors, nore verified if the connections at the speaker level were correctly seated. Still not recovered from the three dayz of sound system session ^^

I use line isolators at the output of the four outputs of the preamp they are mono to mono (palmer PLI-05) --  the outputs from the preamp are not balanced -- there is no particular difference (more hum/noise) when they are off the circuit between the preamp and the crossover.



 




 

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Elliot Thompson View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Elliot Thompson Quote  Post ReplyReply Direct Link To This Post Posted: 17 August 2021 at 7:22pm
I am uncertain what you are expecting. You've downsized from four boxes using two QSC MX 3000a amplifiers bridged to two boxes using one QSC MX 3000a bridged. So you are getting half the volume. If you are pushing the amplifier beyond it's limitation (due to not having enough equipment to cover the venue properly) the amplifier is going to clip. 

Best Regards, 
Elliot Thompson
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Post Options Post Options   Thanks (0) Thanks(0)   Quote JonB67 Quote  Post ReplyReply Direct Link To This Post Posted: 17 August 2021 at 9:05pm
What are your subs? 8 ohm? Have you checked if you didnt load them?

2 x 8 ohm drivers in parallel is 4 ohm load. 

Bridge the amp and thats 2 ohm a side. Can it do that reliably at the level you're pushing it?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote fat_brstd Quote  Post ReplyReply Direct Link To This Post Posted: 18 August 2021 at 2:02am
First question is have you tried both amps and do they both display the same behaviour? If so then its unlikely it is the amp, if one is worse than the other/goes into clip earlier then its probably time to get a better bass amp.

TBH those old MX3000a's are great amps but they are old now, like 25-30+ years old which is quite an age for something that you are pushing to its absolute limits by running in 4ohm bridge. Also you shouldn't really be able to push that amp into clip on 2 drivers because it puts out so much power I would expect the drivers to go pop if you are lighting up the clip lights.

You should note that because of the way that horns couple means that 1 stack of 4 will sound more powerful than 2 stacks of 2 so maybe you are just expecting too much from the pair of scoops.
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hurukan View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote hurukan Quote  Post ReplyReply Direct Link To This Post Posted: 18 August 2021 at 3:29pm
Yes... I'm not clear enough... I'm not really efficient in electronics :{

. The two mogale are each 8 ohms (one connected to the amp and the second relayed by the first mogale)
. The two amps tested were set to bridged (mono). Tested one and the second with no noticeable improvement concerning the clipping when the crossover was compressing (threshold at compressor).
The QSC 3000A could catch 4 ohms (regarding the specifications)

I will first check the impedance at the "mogale" level. Checking the connections.
At the speakons I suppose I would get 8 ohms (or 4 ohms as the circuit is parallel mounted) ?

I think I didn't understand when I put the bridged to the two LF18X401 I was stating the 3000W would be shared on the two speakers (ie: 1500W each) was I correct ?

Thanks for your guidance ^^




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Elliot Thompson View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Elliot Thompson Quote  Post ReplyReply Direct Link To This Post Posted: 18 August 2021 at 6:45pm
One QSC MX 3000a Bridged = 3200 watts @ 4 ohms = 1600 watts per 8 ohm Speaker.

To Bridge the QSC MX 3000a you must have both gain pots set at full. It does not follow the same principal as other amplifiers that use only channel one gain pot at full.

Best Regards,
Elliot Thompson
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imageoven View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote imageoven Quote  Post ReplyReply Direct Link To This Post Posted: 18 August 2021 at 9:45pm
Originally posted by hurukan hurukan wrote:


The whole preamp is mono configured (all the inputs and outputs are mono).
The "mixing table" is connected to the preamp with a stereo to mono (palmer PLI-06 line level convertor) but the cable used was a XLR to stereo instead of mono -- we lost all the bass/sub frequencies just like it was a phase inversion.
There wasn't a amp clipping issue here but we did not have bass/subs at all

Sounds like you are combining the left and right channels somehow and this is causing cancellation 
Keep pushing on, things are gonna get better.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote imageoven Quote  Post ReplyReply Direct Link To This Post Posted: 18 August 2021 at 11:52pm
Yes the palmer PLI-06 has a balanced mono output, so putting this into an unbalanced mono input you will get cancellation. 

Replace this with a mono sum cable (Google 'rane why not y') from the mixer to the preamp 

Edit: this should solve the loss of bass, not the perceived early clipping of the amp.


Edited by imageoven - 18 August 2021 at 11:57pm
Keep pushing on, things are gonna get better.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jo bg Quote  Post ReplyReply Direct Link To This Post Posted: 19 August 2021 at 10:04am
Post was not clear, but if this was going from mixer to pre and channel were inverted, wouldn't the cancellation be broadband? Can't see why only the bass has gone...

About clipping, you state that you have the pre input at 12 o'clock like last year... This means nothing, it is not an absolute level, it is always relative to the incoming signal. If source is louder amplifier and pre will clip earlier. Did you use the same mixer at the same level last year?
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