Live PA help

[BAG]

Hi All,

I am hoping that with all the knowledge on this site someone might be able to help me out.

I've recently purchased a second hand PA. The FOH consists of a Mackie PPM608m and two passive RCF ART 312 speakers (they are actually a bit older than these exact ones but are basically the same) . It has an excellent sound for vocals, acoustic guitars and also the electric guitar inputting with a DI from the effects pedal. It sounds great with plenty of volume for small gigs to the point that i'm only running it at around 60% before getting volume complaints.

I have tried plugging in my acoustic bass via a DI and found that there is not enough volume particularly on the lower register (E string) and when I boost things up (either more volume or bringing the bass EQ pot up high for that channel) the bottom end falls apart a bit with some distortion of the bottom end. I get a similar problem with my home made stomp box making the bottem end fart at higher volumes.

I am thinking that I need a sub bin to handle these frequencies.

My question is this though........ if I get a powered sub and take a line feed out of the mixer, the sub will handle the lower register through it's built in crossover however assuming that the main speakers are still at the same volume, and without a crossover for these speakers as they come out of the powered side of the mixer, what is to stop them still distorting at the lower frequencies? Will the sub simply be putting out enough bottom end that i don't need to drive the mixer as hard on the bottom end thus not driving the poor little 12" speakers so hard in the lower frequencies.

I don't want to spend the money on a sub if i'll still have the problem.

[launched]

Unfortunately, bass frequencies are power thefts. If you DI a bass directly into your PA, it will be difficult to mix due to the large demand for 100-200hz frequencies. A powered sub will cure the problem for an entirely speaker-powered PA because most have a "high pass" for the smaller powered speakers - Easy as pie to configure. However, your potential powered/passive setup will require you to isolate the higher frequencies for the passive speakers. If your mixer has a Pre-EQ output or subgroups, use that option for a powered subwoofer so you can still manage the EQ for the 12" speakers and get the low end out to the sub. If not, you will need a crossover.

Based on your music posts, I'm not sure you need a subwoofer though. A subwoofer is a badass animal designed for room shaking thump of kick drums and plucked low E bass strings. You may be better off using a bass amp separately which you probably already have lying around. For an acoustic gig, a bass amp should cut through fine and you won't have to worry about the lower frequencies farting everything out. Problem solved and you won't have to spend any extra money.

Anyway, my two and hope I helped some.

Mark

[BAG]

Thanks for your help there launched.

The Mackie powered mixer outputs from it's internal amps to the speakers post-EQ and has no way of inserting a crossover and unfortunately also outputs it mixer signal post-EQ. It's sounding like the bass amp may be the way to go.

Unfortunately i don't have a bass amp and was hoping to kill two birds with one stone so to speak, that is the bass and the stomp box. I might have to try the stomper with a bass amp as well. Mind you, a reasonably priced bass amp is similar to the sub so that not it's neither here nor there as far as cost goes.

[Flash Harry]

The schematics of this mixer show that there is a separate sub out line. It seems to suggest that this is limited to frequencies below 100hz which will not help you in this situation.

It's more likely that you need a bass bin, which will work below 400hz and should be OK down to 40Hz. This will cover your bass/guitar frequencies.

I would suggest that you drive it from an amp of at least 500W RMS and you will need an crossover which will allow you to run it by splitting the main out of the mixer.   

[BAG]

Thanks for your input Harry. That is what i was planning on doing however the problem remains in that the main speakers, horn and 12" (high-mid effectively, hence the inability to handle the very low frequencies) come straight out of the powered mixer. Therefore there is no way that i can see to insert a crossover (which i believe needs to go between the mixer and amp) into this line to reduce/cut the sub frequencies to them which cause the farting/distortion from the bass and stompbox.

The only way i can think to do it would be to take the mixer out feed to a crossover and then from the mid-highs to an amp for the top boxes and the lows to an amp for a sub, but this sort of defeats the purpose of having a powered mixer and would get me in trouble with the household cost controller.

The budget is pretty tight and any gigs we do are small/medium rooms so i'm thinking of trying out one of these or this. They get very mixed reviews but they seem good for the price and it will never be pushed too hard.

Mind you, if anyone can come up with a concept for a sub bin that would work i'm all ears.

[launched]

The one thing I didn't think of is possibly putting a crossover in your speaker line between the speakers and the mixer. Those are pretty cheap I think, but have no idea how well it would work. You could then hook a bass bin (Subwoofer) to the separate sub out line, and manage the 12's with the in-line crossover.

I still like the idea of a bass amp, though. I really think it will suit you better if you can swing it and it will sound a helluva lot better. A subwoofer won't reproduce the "sweet" frequencies that a bass amp can offer. We have two 18" Mackies and all they give you is thump thump and floor noise - I wouldn't use them for an acoustic gig (If I ever did one ). And we sometimes are asked to turn them down...

[BAG]

The one thing I didn't think of is possibly putting a crossover in your speaker line between the speakers and the mixer.

Can a crossover work post amp level? I thought they had to go in between the mixer and the amp. With it being a powered mixer there's no way that i know of (although that doesn't mean much ) without pulling it open and changing the internal wiring. Something i'm not technical enough to attempt.

I must admit, i am leaning towards the bass amp. As you say, it will sound better. I can still possibly put the stomper through that although the stomper does sound decent enough through the PA currently, even if it is a little "woody".

[launched]

It's called a High Power Crossover, or possibly Passive Crossover. They will work at a post amp level. However, I am not an expert regarding these devices and have no idea which one you should use - I did some Google searches and there are all kinds of wattage ratings and frequencies. But they look to be very inexpensive.

Hopefully somebody here can tell you whether this is a realizable option and what high-pass frequency would give you the best results.

And like you said, there's always the 'ol reliable bass amp

[BAG]

It's called a High Power Crossover, or possibly Passive Crossover. They will work at a post amp level. However, I am not an expert regarding these devices and have no idea which one you should use - I did some Google searches and there are all kinds of wattage ratings and frequencies. But they look to be very inexpensive.

Hopefully somebody here can tell you whether this is a realizable option and what high-pass frequency would give you the best results.

And like you said, there's always the 'ol reliable bass amp

Thanks for that launched. I didn't know they existed and i'll have a look into them. But as you say, the bass amp is probably the best sounding option, and if i can get better at bass i can also use it with other soloists/duo's etc. Mind you, i also like the idea of everything being DI'd and all of it coming through a PA

[Flash Harry]

High power passive crossovers are quite lossy, you're better off doing it at low level and amplifying the individual outputs separately.

You can make a passive low level crossover simply and cheaply, with a handfull of components, some bread board and a soldering iron. If you're interested, let me know.

The mixer/amp that you have would lend itself to running a separate bass channel post mixer output.