desperate help needed for p.a design

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luke691

Member
2007-07-02 2:29 am
I recently bought and built two amp kits (they were an absolute bargain) and was thinking of using them as a small p.a.

I have no idea what speakers i should be using, or, what range the system should have.

from the magazine the project was posted in:
" Features-
Output Power: 36w RMS into 4 ohm
Music Power: 70w into 4 ohm
Frequency Response: -1db down at 28hz and 55khz"


(money wise im fairly limited (im 15) and was hoping to get this project done soon so i could start using it for band rehearsals.

Thanks
 

Dan2

Member
2005-09-17 1:24 pm
If you are only using it for band rehearsals, would you just be using the amp to power on-stage monitors?? ('cause you wouldn't need front of house speakers)

and what instruments are you going to run through the system?? (mics, guitars etc)

Im no expert, but a 10" full range driver with a tweeter in a wedge enclosure should work well. build 2.
 

luke691

Member
2007-07-02 2:29 am
the amp will most likely be for mics, and an upright bass, but occasionally it will be used for guitars.

i have two of theses modules, i've already built one and using a 12v SLA Battery connected to a 50w Rms, 100w max power speaker with a midrange and two tweeters used for radio and cd's the amplifier was to powerful for these speakers.

what sort of power should the tweeter/speaker be able to handle each?

would i be able to use a woofer such as the one on this site

http://www.radioparts.com.au/ProdView.aspx?popup=1&Category=SXBB6015&Product=39155017

and a horn or tweeter and if so would i need a crossover

thanks for all the help
 
luke691 said:
i found this car speaker kit including crossover bass/midrange speaker and tweeter, the kit includes two of each and seems like a decent price

http://www.altronics.com.au/index.asp?area=item&id=C9302

any suggestions should i buy?

is this setup appropriate?


No, don't use car speakers for PA - they are generally vastly over rated for power, and are intended for fairly low sound volumes in a small space.

For PA you want a speaker that fills a room, and withstands plenty of power through it.

Feeding most '1000W' car speakers from a 100W PA amp would probably blow the speaker.
 

luke691

Member
2007-07-02 2:29 am
thanks for all the help,

im most likely to go with a peizo horn/woofer combo
one thing that i still dont understand however is;
why, when using the amplifier that supposedly put out 36w rms, when connected to a set of speakers that could handle 50w rms was the amplifier to powerful. is the label; wrong, or is there another reason how do i avoid this when picking speakers
 
luke691 said:
thanks for all the help,

im most likely to go with a peizo horn/woofer combo
one thing that i still dont understand however is;
why, when using the amplifier that supposedly put out 36w rms, when connected to a set of speakers that could handle 50w rms was the amplifier to powerful. is the label; wrong, or is there another reason how do i avoid this when picking speakers


As long as it's a PA speaker, correctly rated at 50W RMS then you would be fine, but a HiFi speaker probably wouldn't take the level of abuse (and wouldn't be loud enough either).
 

Dan2

Member
2005-09-17 1:24 pm
when you say "too powerfull", im assuming the speaker distorts when you play bass through it. you will probably find that if you had a high pass filter on the hi fi speaker you would be able to drive it at 50W.

for PA, and a small amp you must try to get a big driver (10", maybe 12") with a good sensitivity.
 

Zero Cool

Member
Paid Member
2004-09-20 6:10 am
MN
36 watts at 4 ohms is NOT a lot of power for PA use. and if the speaker you are using is an 8 ohm speaker, your amp will likely only put out 18-20 watts.

I wouldn't use anything less then 100 watts @ 8 ohms for monitor speakers and you will quickly run that out of power too if you are trying to compete with a guitar, bass, drums etc.

For my "Small" PA, i use 4 monitors on stage up front. each is a 15" 2-way box. a woofer and a compression horn. I use a 1800 watt amplifier. 900 watts per channel and each channel runs 2 monitors.

For the FOH, Front of House. I use 2 of those amps. One amp for Subs and one amp for the mains. 5.4 kilowatts total system power.

The monitors are slightly over powered and the FOH is slightly underpowered. But thats what i have for amps at the moment.

Speaker sensitivity is the most important factor if you are going to try and use these amps. you have to find a very very sensitive speaker to get any sort of volume out of these little amps.

Do the math.

If you tried to use car speakers. that say have a sensitivity of 84db for a 1 watt of input. Every time you double the power, you gain 3db. SO

1w = 84db
2w = 87db
4w= 90db
8w = 93db
16w = 96db
32w = 99db max

now lets compare that with a speaker (like some Yamaha SW115V speakers) that have a sensitivity of 99db for a 1 watt input

1w =99db
2w = 102db
4w = 105db
8w = 108db
16w = 111db
32w = 114db max

So you see you have 15db more volume just by using a more sensitive speaker!

Those calculations are for an ideal world and do not take into account power compression and other real world things that happen.


A small amplifier is more likely to distort and cause clipping. Clipping creates heat inside the speaker. heat kills speakers. you can burn out a 100 watt speaker running it on a 10 watt amp with heavy distortion. but you can run a 10 watt speaker on a 100 watt amp all day long as long as you are not physically over driving the speaker.

It is better to have too much power then not enough and 32 watts is NOT enough as you will see it will distort very very quickly.

Is it possible you can bridge the 2 amps together for more power? Might have to build an inverter op-amp board to do it. you might gain a couple more watts.

Most small PA Mixer/Amps have at least 150 watts per amp built in.

Find yourself an old Peavey CS-800. BIG heavy tanks that people will practically give away. 200 watts per channel @ 8 ohms and you can beat the crap out of them day and night. and when not being used for the band, they make great door stops. wheel chocks, jack stands etc.

Very cool you built some amp kits at age 15! keep that up!
 
I think you need to put this into perspective? :D

Yes, 36W isn't much for a PA system, but for an acoustic gig it's probably plenty?.

As for the massive monitor powers, the limiting factor is the drums, these haven't changed over the last 50 years - an acoustic drum then was as loud as an acoustic drum now, you can't buy a 1000W drum kit now, while a 1950's drum kit was only 10W.

They managed perfectly well back then, and gigs were plenty loud enough, but there just seems a never ending push for more power?.
 

Zero Cool

Member
Paid Member
2004-09-20 6:10 am
MN
And i forgot to mention the Inverse Square Law. The numbers i listed above are for measurements 1 meter from the speaker. everytime you double the distance, you lose 6db. So if you are in a small room, those numbers might be real. but if you are in an average size room. you start dropping off quickly.

For example. lets take our car speaker with 32 watts of input = 99db at 1 meter from the speaker. lets say our room is 40' long.

99db max at 1 meter
93db at 2 meters
87db at 4 meters. this is not very loud at the back of the room.


Using a higher sensitivity speaker lets look at those numbers

114db max at 1 meter
108db at 2 meters
102db at 4 meters at clipping. this is closer to a realistic level. but you have NO headroom at all. and once you calculate losses due to power compression and other factors. you lose at least another 6db from those numbers.

Now if you could build a bunch of those amplifiers and build yourself an active multi way system....you might be getting closer. But i would bet that the bass will still require more power then 32 watts.

Maybe you could build a box with 2x 15" woofers & a horn per box and use 3 amp modules per box? that might be closer to something that is usable.


Zc
 

Dan2

Member
2005-09-17 1:24 pm
for our band (drums, bass, acoustic guitar, vocals) i made up a set of "temperary" monitors about 2 years back - we are still using them, but they do sound REALLY bad.

each monitor has 2 20cm paper cone full range drivers - i made 3 of them. unfortunately they cant handle the abuse that we put them through - but they are loud enough (only just).

we are upgrading to 12' 400W monitors - i bought the speakers i just haven't had time to make the boxes. BTW any ideas on how to build those wedge enclosures??

Our bass guitarist has his own bass amp - so we don't run that through the monitors.
 
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