View Full Version : Building a PA

14th Nov 2013, 12:14
Band number 4 looks like it's making good progress with a 2.5hr set. We are a 2 piece (could become a 3 but splitting $ 2-ways is better).

Our intention is to only play restaurants so we don't need a super loud PA. I already have bits from previous endeavours - a mixer, 100W speakers, microphones, etc. I am only missing a PA.

There are heaps of schematic diagrams on the nerdynet for building a 100W PA but I came across a page where some bloke built a base amp out of a car amp and car subwoofer.

Thanks to the chavs, car audio gear is powerful and dirt cheap. I can buy a 2 channel 100W RMS car amp for $50 (or $30 direct from China), and a 12V/20A power supply for $30-$40.

We are not after high fidelity sound, the inputs are only acoustic guitar pickups and microphones.

Whaddayareckon, will this gear work as a PA?

14th Nov 2013, 12:23
Get yourselves a couple of these:-


Eminently flexible and expandable when you increase the number of participants.

14th Nov 2013, 12:32
G-CPTN, I'd probably get arrested wearing that coat while doing a gig in the park.

14th Nov 2013, 13:00

Winchester Cathedral
You're bringing me down...

DX Wombat
14th Nov 2013, 13:11
I thought you meant one of this type of Personal Assistant.


14th Nov 2013, 13:18
Self-build? Bugga that. Go out and get yourself a Bose L2. Nice bit of kit for "acoustic" gigs.

14th Nov 2013, 13:19
That'd be a bit of Wierd Science there DX.:p

14th Nov 2013, 13:21
DXW: you can build one of those for me, if you like! :E

14th Nov 2013, 13:31
I'm with DXW, I thought you were building the same. In which case, long legs, adjustable tits for days when you prefer 'man made' to natural, and so on.... :)

14th Nov 2013, 13:32
MagnusP, yes I've seen those and other lightweight variations. Very good sound and very versatile, but also very expensive.

14th Nov 2013, 14:20
I thought you meant one of this type of Personal Assistant.

Whereas I immediately thought of PA28.

I'm weird.

west lakes
14th Nov 2013, 14:41
Thanks to the chavs, car audio gear is powerful and dirt cheap. I can buy a 2 channel 100W RMS car amp for $50 (or $30 direct from China), and a 12V/20A power supply for $30-$40.

We are not after high fidelity sound, the inputs are only acoustic guitar pickups and microphones.

Whaddayareckon, will this gear work as a PA?

Yes it will work well fed from a mixer. I've a 2x50 and two 2x25s that I use for additional power on PA systems

tony draper
14th Nov 2013, 14:51
I can remember when chaps managed with a couple of Vox AC 30s.

14th Nov 2013, 19:30
Weep, my brothers: I have just given a small Fender practice amp away, 10 watt, I think it was a 10" speaker. Surplus to my requirements, now in use by Urban Lights (see YouTube if you are interested).

tony draper
14th Nov 2013, 20:01
Many years ago someone gave me a Vox AC 30,twere the time when transistor ruled the Amplification world, I remember humping it upstairs and tried it, twere hissy and crackly and needed some workand probably re valving so humped it back downstairs bunged it in the cellar and forgot about it.
Many years later when the Vox valve sound began to excite folks again I remembered it, alas Bro Draper had taken it up to the tip when clearing out the cellar,when I screamed at him enraged,he simply said he asked me at the time and I said yes dump it,I do not recall this but it may well be true.

14th Nov 2013, 20:30
Don't build it, buy it. Get something cheap from ebay. It's not worth the effort.

I've done live sound for 25 years. Home made gear isn't worth the bin liner you put it in when you take it to the dump. And it will not sound good !

14th Nov 2013, 22:33
Good advice from kms601.

Plenty of low cost kit around.Get an 'all-in-one' system.2 x portable powered speakers with inbuilt mixer and a speaker output for a foldback wedge.

You'll regret building your own,trust me.I did.


14th Nov 2013, 23:21
Agree strongly with the above. :=

I built a 100 watt kit/mixer/amp about 25 years ago and notwithstanding careful checks that all was well, the damned device somehow managed to dump the DC positive rail across the test speakers. A loud splat in protest - followed by the unmistakable smell of burning money! I would have driven over it with the tractor if it weren't the risk to the tyres :*

Buy! It's going to cost less, and (dare I suggest it) give better service in the long run. There are 50 watt basic stereo AC powered bricks with bridging capability
for around $200 AU on this side of the equator. A simple mixer to meet your input needs and Hey Presto! - Robert is your mother's brother . :)

Alternatively by the powered speakers suggested above for even less stress and increased versatility.


15th Nov 2013, 00:52
I'm not keen on building it from scratch as my biggest fear is not burning out the speakers but electrocution (final act :}).

The car audio gear is rugged and simple, they even regulate the input voltage, they work on both 4 Ohm and 8 Ohm speakers, and most of them even have speaker protection. The bang for buck is excellent. I can even install it inside one of the speaker boxes along with the power supply.

Yep these bastards are real heavy.


Had an opportunity to buy a classic Fender Twin Reverb at a good price, which was an amp I always wanted, but wasn't gigging at the time so there was no point. It was even heavier.

Solid Rust Twotter
15th Nov 2013, 07:32
Peavey do a pretty good small PA pushing around 60w through six channels, light and compact enough for a small two piece ensemble. Might find a couple on eBay with matched speakers if you sniff around a bit.

15th Nov 2013, 08:58
Any iteration of car speakers for band PA,even if it is just for vocals and acoustic guitars,will sound like sh*t.

Just my opinion.

15th Nov 2013, 09:31
Already have the speakers which are not car speakers, they are quite dcent and were once upon a time very expensive - I got 'em for free. All I just need is the amplification to drive them and car audio amps look like they may be up to the task.

Also found 12V 20A regulated switched power supplies for $30 delivered. They use them to drive LED strips. Got the idea from these folks.
Building a 12V/24V DC bench power supply for component testing | Krank Engineering (http://krankengineering.com/building-a-12v24v-dc-bench-power-supply-for-component-testing/)

G&T ice n slice
15th Nov 2013, 09:41
Whatever you do, make sure the volume control goes up to 11

15th Nov 2013, 09:49
All the best then cattletruck !

Sometimes it is fun to 'run it up the flagpole and see if it flies'

Out of interest,what type of speakers are they ?

15th Nov 2013, 10:24
Sometimes it is fun to 'run it up the flagpole and see if it flies'Thanks TWT, that sums it up perfectly and gave me a laugh.

The speakers are high end 3-way for domestic audio from my guitarist's brother after he upgraded his hifi system with something newer and expensive. They are true 100w RMS with overload protection however I cannot remember the brand. I plan on sticking a broad aluminum mesh over the front to protect the speaker cones from being torn, and run pedestal stands through the bottom to get the height. Not quite the quadbox stack scenario but they will do for the small cozy venues that we want to do.

The key in this whole equation is the mixer, and I have a good Tascam mixer for the job with extra kit for further signal processing on the sends channel e.g. compressor/limiter. This will ensure I get a nice balanced line-level into the amplification to drive the domestic speakers safely.

15th Nov 2013, 11:29
I've read this thread and have a question; what, precisely, is a PA, or rather what's a PA when it's not the walking/talking version that one is familiar with?

15th Nov 2013, 11:41
Public Address (http://en.wikipedia.org/wiki/Public_address_system).

15th Nov 2013, 11:51
Alternatively, you could only accept gigs from venues with a house PA. ;)

15th Nov 2013, 12:00
They be for propheshonals MagnusP. My guitarist has never done a live gig and is struggling to remember all the material. Our first gig will be held at the nursing home where my guitarist's father is residing, this seems like a good first step, and I doubt they can throw tomatos far enough.

15th Nov 2013, 12:58
You're doing a good thing, cattletruck. Best of luck with the gig. I used to do some Multiple Sclerosis concerts, and they were always appreciated.

cockney steve
15th Nov 2013, 13:17
I plan on sticking a broad aluminum mesh over the front to protect the speaker cones from being torn, and run pedestal stands through the bottom to get the height.

I hope you don't poke holes through the bottom! Most HiFi enclosures are airtight boxes...... the speaker is simply an air-pump and the enclosure forms an infinite baffle to stop the "push" from the front of the cone being "sucked" round the back by the "pull " from the back thus cancelling a lot of the acoustic power.....the down-side is that all the equal and identical (but 180* out of phase) acoustic output from the back, has to be contained and absorbed in the "box" (enclosure)...modern roll-surround ,long-travel cones rely on this enclosed air for damping the cone excursions.

The alternative method of construction ,is to use a "tuned-port"/ folded horn construction of the enclosure, which brings selected frequencies round to the front in phase with the front output, thus augmenting the output.....difficult to do it with the full audio spectrum, with any sort of linearity, so usually confined to the power-hungry bass frequencies.

Aluminium grilles have the potential to screw up the higher frequencies and muddy the sound...approach with caution!

The amp idea sounds good.....any fool can make a very good audio-amp nowadays, for buttons money, good transducers are a different story microphones,pickups and speakers are the costly bits...even a preamp/mixer is relatively cheap.

Delighted to see you are not obsessed by sheer power.....IMHO. good amplification means you should not, as a listener, be aware of it.

crystal -clear sound with no distortion,clipping. pops and hisses, loud enough to be clearly heard, but not to overpower the audience's ability to converse with each-other, should be the aiming -point.......unless the performers are that bad that they need the sheer volume to hide their shortcomings!

16th Nov 2013, 12:04
Thanks for the tips Mr C Steve. :ok:

Looks like I won't be drilling a big hole through the bottom of the speaker boxes to easily slot them onto the pedestal stand :\.

17th Nov 2013, 00:35
Just spent the last 10 mins reminiscing my youth after seeing that beautiful picture of the VOX amp! It was amazing how much volume actually came out with only 30W.
I tried to build a clone of the Selmer (anybody remember those) 50W amp. Never did get it to work although I did get a beautiful firework display out of the rectifier tube!

17th Nov 2013, 07:46
Dear Mr C Truck - no need to drill holes. You can get very reasonably priced speaker stands complete with speaker base plates for under $100 a pair here in Aus. Should be able to get similar in your area, I guess.

Hopefully this link works:
Speaker stands in DJ Equipment | eBay (http://www.ebay.com.au/sch/DJ-Equipment-/48458/i.html?_from=R40&_nkw=Speaker+stands)

17th Nov 2013, 13:48
.....the designer of the Klipsch folded horn speakers once opined "what the world needs now is a REALLY good 5 watt amplifier".

In an audio store, comparing lots of speakers before shelling out my hard earned cash, it was outstandingly obvious when one selected the Klipsch ones that it seemed as if the volume had been turned up several notches.
One didn't need 100 watts per channel.

They were out of my income bracket, boo hoo!

TheAncient Mariner

17th Nov 2013, 18:01
Yep - I'll go with that. I did pub and bar entertaining for thirty years and used a Peavey 60 watt six channel with reverb and built my own speaker cabinets. Worked a treat but in order to get a fuller sound as a one-man band, I used the Peavey solely for vocal and had a Fender twin reverb for guitar and drum machine. I took the amp out of the Fender carcass, which I kept, and built a separate speaker cabinet with a 15" speaker. Sounded fantastic and worked a treat. I did need a Volvo estate to cart it about though! Go for the Peavey - highly recommended. When I retired and sold the gear, I put the twin reverb back in the carcass and sold it for 50. Someone had a bargain!

tony draper
17th Nov 2013, 18:26
As a one man band I found the hardest thing to mic up were the cymbals on me knees.

18th Nov 2013, 01:02
I assume you're joking, Tony, but it is actually possible to mike these up using a contact mic. I made my life easier by cheating and using an electronic drum kit.

18th Nov 2013, 08:46
Rossian, my fav hangout has a professional PA system installed with many low powered speakers installed in the roof and just one big subwoofer on stage. Seems like that's the ideal way to share the sound energy without having it bounce off walls or be absorbed by the first two rows of tables.

100W is more than enough power for our needs and we probably won't be using all of it, but it gives us that extra bit of headroom to keep the sound clear.

Ahh yes, we tried the drum machine however my vintage Yamaha RX8 doesn't have enough memory, so I made up some MIDI's but then we began sounding like karaoke music. We sound better in pure acoustic form and decided to work it that way - only problem is we tend to chase each other in the timing department - which isn't an issue if your going to play the Zorba :E.

19th Nov 2013, 15:37

Back in the early disco era, one had some extra capacity at a small electronics fab facility & thus was rather easily persuaded by a distributor of auto-sound products to make him some 100w-100w good-quality Mobile sound amps that would substitute for a Japanese product he was selling, even as his margins were drying up due to an epochal exchange rate trend. First deal was for 1000 pieces, with more K's to follow.

Because of the need for fitting the new design into very similar spaces and places, we designed the new unit product to have very similar external shape as the competitor (about the weight and volume of a thick textbook), and took advantage of some very new release audio IC's to make it perform a bit better. Nearly half the space inside was switching power supply, necessary to get audio output voltages (from 12vdc in) high enough to do the rated output into 8-ohm speakers. (something soon learned was that many competing high power audio amps for cars were often quoted on basis of 2-ohm net speaker loads -- saving much cost in the power converter. A cheap trick that, no doubt still common.

We found it necessary to incorporate SERIOUS size power wiring to satisfy audio-output changes of 200 W across 20hz - 40khz (all specs are from dimming memory). The 12v supply cables were similar to dainty jumper-start cables, because no room was available for the big capacitors commonly used. Point there is that you'll likely want a righteous and well-behaved power supply to the mobile amps if running your sound system from ac mains power. Modern switcher power supplies do that well, but some will be much better than others for audio purity.

And then there's the heat. Your typical mobile amp (afaik) has large heat-sink surfaces and small or nonexistent internal fans. There may well be assumptions about airflow over the unit that are not well documented -- resulting in potential for distortion from self-protection circuits as temps escalate during crescendos leading to the gran finale.

Mobile audio amps are inexpensive and often great value for the $$$, but maybe is wise to reserve some extra time to work through the learning curve for boundary condition deployment cases.

I never could understand how people could bear to have one or more high-power sound systems inside a relatively small road vehicle. Had to wear industrial earmuffs mself, to bear listening to one amp set with good speakers at full power in a rather large room.


20th Nov 2013, 10:22
Thanks for the informative story arcniz :ok:

A cheap trick that, no doubt still common

Oh-oh, notions of self doubt now entering my head.

you'll likely want a righteous and well-behaved power supply

Got my eyes set on something from here:
12V 20A Power Supply | eBay (http://www.ebay.com.au/bhp/12v-20a-power-supply)
They may possibly produce noisy DC but car audio amplifiers regulate the input voltage anyway.

20th Nov 2013, 12:13
Cattletruck says,

Oh-oh, notions of self doubt now entering my head.

There, there. Is mostly just a matter of either a) trial and error, with allowances made for caution, and/or b) trying to better characterize what you likely really need. Fortunately you are not doing this on some other planet or even in an airyplane where the numbers really have to be done with sincere and uncompromising righteousness.

Am just heading off to an uncertain horizon. Will try to have a simple, thoughtful response for you in 36hrs or so. Might best put your ccard back in the holster for that duration, tho. Sight unseen is tough, but specs unspec'd nearly alays'l getcha in the end.

Hint: have a quick read on "DC Impedance" -- which is really quite similar to resistance, but takes into account the effect of sudden changes in power demand. Aside from heat, that's the gotcha.

One can have giant dam full of water handy, but too small a hose to really get what is needed at the desired time and place.