#1
Ok, I get the whole having to match impedance on heads and cabs, that's not my problem. My problem is more related to PA systems than guitar amps. Two parts:

First part: How come my 500W/ch PA system can barely keep up with my 120W guitar amp? I miked the amp and ran it into the PA. The channel fader was basically all the way up (trim at about 10 o'clock, any more and I'd send it to feedback hell) and the master fader was at about 75%. MY GUITAR AMP WAS ONLY AT HALF VOLUME. Why is this? Do I just suck and not know how to mix it properly, or is it something more? When I hard pan the channel left or right, I'll get a little more volume out of it, but the guitar amp matches it easily.

Second part: If a power amplifier puts out 500W/ch @ 8 ohms, and 700W/ch @ 4 ohms, wouldn't 4 ohm speakers be louder, because 500W < 700W? I know this seems kind of elementary, but I'm considering upgrading the speakers in my PA system, and I want to know if I would get that much more oomph out of them to make it worth getting 4 ohm cabs over 8 ohm cabs.

Sorry if this is a bit lengthy, been trying to work this out for awhile. Any helpful responses are greatly appreciated.
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I don't like BTBAM. Sue Me.

PLUR

My Solo Project
#2
First: Mic or miking problem probably. Did you test the mic on something else? Also how did you mike it? Most stage mics are very directional, use a stand and actually point it at the speaker up close and personal.

Second: Yes aim for an overall impedance of 4 ohm if you can. ie two 8 ohm bins in parallel is 4 ohms. I often run more than one bin at a time off the same output of my PA, you just calculate the result.



However, you can hardly hear the difference between 500 and 700W through the same speakers. What you will get is marginally more clean headroom with 700W.
Gilchrist custom
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Cathbard Amplification
My band
Last edited by Cathbard at Jun 30, 2010,
#3
Mic placement is definitely not the problem, I've got a Sennheiser e609 guitar mic pointed right at the sucker. I'll try running them in parallel like you said. Will the fact it's a stereo power amp be a problem? I have a QSC GX5, and it sacrifices the bridging option for a built-in crossover. I've hooked up my system with just one speaker attached, and only got half a stereo mix. Does this mean that parallel is no longer an option?
Gig Rig:

Schecter Hellraiser V-1
Crate BV120H
B-52 LS 4x12 cabinet
BBE Rackmount Sonic Max
Boss ME-50 Pedalboard
Digital Reference 2505 Wireless

I don't like BTBAM. Sue Me.

PLUR

My Solo Project
Last edited by swordsofplague at Jun 30, 2010,
#4
Shouldn't cause a problem on a SS amp. You can just run them no load. You'd be better off with a pair though, you can fill a hall more evenly. Running those pairs as stereo shares out the load across both amps. Running it bridged would be a better option but I have done it your way once with a big STK amp because I was scared that if I bridged it my speakers would run out the door in terror before I even switched on the power.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#5
So you're saying if I ran a pair of speakers on each channel (4 total. 2L, 2R) it would balance out the load more? I hope so, because I think that means I could run 2 8 ohm subs and not need a second amp for them.
Gig Rig:

Schecter Hellraiser V-1
Crate BV120H
B-52 LS 4x12 cabinet
BBE Rackmount Sonic Max
Boss ME-50 Pedalboard
Digital Reference 2505 Wireless

I don't like BTBAM. Sue Me.

PLUR

My Solo Project
#6
Those impedances are quoted PER CHANNEL. So you need 4 ohms on each channel. You can think of L and R as two amps, because they are. You could comfortably run two 8 ohm loads in parallel off either side but not two 4 ohm loads.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#7
That should work then. Both the speakers and subs are rated at 8 ohms. With the GX5, one channel is used for the speakers and one for the subs. So I was going to have the speakers chained on one channel, and the subs chained on the other.
Gig Rig:

Schecter Hellraiser V-1
Crate BV120H
B-52 LS 4x12 cabinet
BBE Rackmount Sonic Max
Boss ME-50 Pedalboard
Digital Reference 2505 Wireless

I don't like BTBAM. Sue Me.

PLUR

My Solo Project
#8
A common way to do it actually. Mono mixes are better anyway. At least with a mono mix you know everybody in the venue is hearing the same thing. Stereo mixes only benefit a few people right in the middle whilst everybody else suffers. With your arrangement the pan control becomes a tone control.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#9
We use the PA as reinforcement, rather than use it as all our power. For guitars, half our volume comes from the amp, half from the PA. The stage left guitar gets panned stage right, and vice-versa for stage stage right guitar. That way there's a decent balance between them.
Gig Rig:

Schecter Hellraiser V-1
Crate BV120H
B-52 LS 4x12 cabinet
BBE Rackmount Sonic Max
Boss ME-50 Pedalboard
Digital Reference 2505 Wireless

I don't like BTBAM. Sue Me.

PLUR

My Solo Project
#10
Well you won't be doing that now, left will be bass and right will be mids/tops. It will still sound ok, point the amps towards the centre of the hall. It'll balance ok.
You could run pairs though, a bass bin and normal in parallel on each side. That's how I run and it's never let me down.
Running it how you said gives more control over the sub/main blend. I've used desks wired that way a few times. Often because they have only one badass sub so it's mono or nothing so why not optimise each channel?
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#11
Wait, are you saying do something like this?

L sub--->L mid/top--->AMP<---R mid/top<---R sub

This seems like the simplest way to do it, I just didn't know if that would work, given there's no crossover in use (I'd have to set the amp to full-range). Sorry, still trying to figure all of this out. It seems this amp's have made things a bit complicated lol
Gig Rig:

Schecter Hellraiser V-1
Crate BV120H
B-52 LS 4x12 cabinet
BBE Rackmount Sonic Max
Boss ME-50 Pedalboard
Digital Reference 2505 Wireless

I don't like BTBAM. Sue Me.

PLUR

My Solo Project
#12
In an ideal world you'd have four amps and an active crossover before them. In the real world, it'll work fine with two (L & R). The sub will probably filter out what it doesn't want to accept. Depending on the efficiency of the sub you may have to wind back the bass a bit on the desk. Thats why some people split L & R between bass and main and just run mono. You can adjust the bass balance better.
Try both and decide for yourself.

My normal biggish gig setup is an 18" W bin in parallel with a 15" straight bin (also has a small horn in it) in parallel with a 12" horn. I have the same on both sides. the impedance works out within spec and it sounds fine. I was lucky that the efficiencies balance out nicely.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
Last edited by Cathbard at Jun 30, 2010,
#13
Ok this is beginning to sound like it's wired similarly to the subs in my car stereo (and those bump, believe me). I'll try the parallel thing first, and then go down the list of possibilities if that doesn't work. Thanks so much for your help
Gig Rig:

Schecter Hellraiser V-1
Crate BV120H
B-52 LS 4x12 cabinet
BBE Rackmount Sonic Max
Boss ME-50 Pedalboard
Digital Reference 2505 Wireless

I don't like BTBAM. Sue Me.

PLUR

My Solo Project
#14
correct me if i am wrong cath, but some speaker setups use onboard crossovers and low pass filters. for example a 3 way speaker system might have crossovers so the 15's only get lows and the 6's get mids and the horn or tweeter get the highs, so there may be some channeling inherent in your system.


Quote by swordsofplague
Ok, I get the whole having to match impedance on heads and cabs, that's not my problem. My problem is more related to PA systems than guitar amps. Two parts:


most solid state PA's don't actually use impedance matching principles, they instead using impedance bridging. the basics work so that a small impedance from the amp can be match with a speaker load of a minimum impedance. look it up on wikipedia for more/accurate info


Quote by swordsofplague
First part: How come my 500W/ch PA system can barely keep up with my 120W guitar amp?


there are a number of reasons why, one of the main reasons is the completely different motives in design of the units. the PA is designed for accurate sound reproduction across a full frequency bandwidth and will usually operate within it's linear response range. a guitar amp is designed to source a guitar tone, it give up fidelity for the sake of more power and really specializes in operation within a limited frequency range and can be pushed to outside it's linear operation range.


Quote by swordsofplague
I miked the amp and ran it into the PA. The channel fader was basically all the way up (trim at about 10 o'clock, any more and I'd send it to feedback hell) and the master fader was at about 75%. MY GUITAR AMP WAS ONLY AT HALF VOLUME. Why is this?


did you try to keep the guitar amp's volume quite low and amplifying it that way? a super loud signal through a PA like that will cause some ungodly noise. you say it was only at half volume, but i am unfamiliar with the amp, but i envision many at my amps at half volume and wonder how a PA could keep up at all (any of my music man amps, my sunn, my THD, my epiphone, would all blow a 500 watt PA away).

also, projection is big here. most people like to isolate the source cabinet from the PA speakers to help prevent feedback, and this also isolates stray noise from the cab as well and lets you control it's mix more fully between stage and audience.

i think it is a combo of competition with the amp volume and the loud nature of the input signal that was the main cause.


Quote by swordsofplague
Do I just suck and not know how to mix it properly, or is it something more? When I hard pan the channel left or right, I'll get a little more volume out of it, but the guitar amp matches it easily.


no, i think this is a great learning experience.

Quote by swordsofplague
Second part: If a power amplifier puts out 500W/ch @ 8 ohms, and 700W/ch @ 4 ohms, wouldn't 4 ohm speakers be louder, because 500W < 700W? I know this seems kind of elementary, but I'm considering upgrading the speakers in my PA system, and I want to know if I would get that much more oomph out of them to make it worth getting 4 ohm cabs over 8 ohm cabs.


cathbard already mentioned you gain a bit more headroom and not real perceptible volume, so i don't need to talk about why or nothing (basically it that 700 watts just doesn't put hardly anymore 'sound' out than 500 watts)

but upgrading speakers? may work may not. a few more speakers may allow you to point the speakers in the right place to hear the guitar better through the PA. it will also give you more clarity and headroom.

i am not overly sure what your end goal is, but if you want to have the ability to amplify not just the guitar, but a whole band with a PA then i would recommend a nice mixer and a few power amps and a mess o speakers. i would only recommend this for playing fairly large venues. else it is usually easier to run a guitar amp in the mix using 'stage volume' from the amplifier itself. this will be quite easy for most smaller gigs.

if your goal is just to amplify a small amp with your PA, then it is easier and cheaper to get a bigger guitar amp.
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