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amt 15th October 2010 04:31 PM

Worth building a keyboard amp?
My son is in need of an amp for his Yamaha keyboard now that he is starting to play in a band. Im unfamiliar with guitar/bass/keyboard rigs other than those used for larger ProSound setups.

Most of the smaller practice type amps Ive looked at are either thrashed (used) or the new ones that are <$200 look pretty cheesy. So is it worth building something on the cheap or just buy a Behringer etc.

What I have on hand is an orphan Eminence 12LTA, a LM1875 and LM3886 amp, and plenty of BB ply. My thoughts were to add a piezo, and mixer/tone circuit ala Elliott Sound.

Universal Preamp/ Mixer (Part 2)

Will I wind up with a better amp than the cheap 8"/10" commercial versions or am I wasting my time.


picowallspeaker 15th October 2010 04:39 PM

I used to try the "power of the chords" using Reason from my computer...well ,attached directly to my amp and the amp to two big trapezoidal monitors with a whizzered 12" ,it sounded pretty good ! You just have to try ...;)

indianajo 15th October 2010 05:53 PM

There are lots of posts on diy about the lack of support (schematics) that behringer gives to their equipment. If the guitar player is going to plug in his amp to the keyboard amp and fry the input parts, (a common amateur guitar mistake) then having built it makes it a little easier to recover. (I just repaired a peavy PA amp the apparently was done to - fried input resistors). Keyboards, if they are used for piano synth, need full frequency response, including some sort of a tweeter in addition to your 12" or 8" woofer. That means chip amp plus crossover. I just realized your reference to piezo is not an alarm or a microphone or a gas lighter, but a piezo tweeter.
For a simpler, cheaper input board, look at the mini amplifier board, at $13 including shipping. Add power supply bypass caps, he doesn't understand them. You don't have to use 4558 op amps, I have direct replaced them with 33078's in a disco mixer with much less hiss. Peavey uses JRC4560's in the late 90's. For power supply I am going through the grocery cart of wall-chargers at the charity resale shop, at $1 each they are the cheapest transformers around. No reason you can't mount them inside the box. Use real fuses or a breaker for real gig hardening (no lost fuse caps in the instrument truck).
Another option is an orphan used full frequency speaker, like a Peavey PR15 or SP2, (really nice tweeter) and build the amplifier inside the box. By orphan, buy a pair one of which is blown up. That way the speaker engineering (which requires a lot of testing, in my opinion) is already done.

bob91343 15th October 2010 07:36 PM

I bought a cheap keyboard amplifier and was dissatisfied with the sound. But the amplifier, 30 Watts, seemed okay so I pulled it out and installed it into a bass speaker. I moved the tweeter and crossover as well and now have a more than adequate keyboard amplifier. My keyboard is a Casio PX-100 which is about as good as I could want, given that none of them sounds all that great. Especially since I also own a grand piano.

Enzo 16th October 2010 02:56 AM

If you want a project to build, then fine. But if your main intent is to give the kid an amplifier for his keys, then really, look at the used pro gear market.

Don;t be distracted by the appearance. Just because the tolex is ripped and scuffed or a metal corner is missing or candle wax is dropped over the top, doesn;t mean the amp doesn't work and sound just fine. And for that matter, something that looks like new can be really screwed up inside.

Listen to any potential candidates. take a CD player with an appropriate cord, and play the music through the thing for a listen.

An old Peavey KB100 is a solid basic keyboard amp, and can be found inexpensively.

Um... plugging a guitar into a keyboard amp won;t hurt it in the slightest. keyboards put out a line level signal, far hotter than anything coming from a guitar. A guitar can't burn up a resistor. Your guitar pickups just can;t generate the power for that. If resistors were burnt up in an input circuit, then either someone plugged the output of a power amp in there, or some piece of gear that was not well grounded and AC on the chassis.

Nigel Goodwin 16th October 2010 07:29 PM


Originally Posted by indianajo (
If the guitar player is going to plug in his amp to the keyboard amp and fry the input parts, (a common amateur guitar mistake) then having built it makes it a little easier to recover. (I just repaired a peavy PA amp the apparently was done to - fried input resistors).

In decades of repairing various guitar and PA amps etc. I've never seen one damaged in that way?, or heard of one before. Although depending on the input design I suspose it is possible?.

In many respects a keyboard amp is pretty easy, it's just a linear high quality amp, a PA is ideal for it. A decent little HiFi amp would do it as well, as long as you don't push it too hard. It's certainly essential though to use a tweeter as well as a bass unit to get a decent sound.

I happen to have a nice little 60W Fender keyboard amp, which I got for free :D - it's been used for guitar and all kinds of things. It's only rarely been used for keyboards though, we usually just DI the keyboard directly to the PA - when we happen to use a keyboard.

wakibaki 16th October 2010 08:31 PM

Take him to the shop, let him try a number of amps, buy him the one he wants. The one he wants, even if you have to buy a secondhand one elsewhere for budgetary reasons. If there's a financial constraint he'll understand that. An amp that's a bit battered is not the end of the world. Don't rush him into making a choice. He won't understand that. Don't just buy the most expensive one because you're flush.

You should preferably buy one with a 12" speaker. Don't build or adapt an existing amp, these can be fine or even preferable for a mature player, but a young person needs something with street cred.


indianajo 16th October 2010 08:51 PM

Like the street credit thought. Kids are vicious.
My points are 1. Piano one of the hardest sounds to do right. He may never use it, but a good sounding speaker on piano is good for most everything else. I've had 4 generations of speakers over the years, the latest are the best yet on piano. Give your kid the opportunity to hear the difference in fidelity on a good piano CD for test. The speakers I mentioned are about $200 used, the KB100 is about the same. Right on about at least 12" woofer, but I like a 15" woofer better on deep piano bass. Besides, the other kids may respect a 15" woofer more. Building a good amp into a speaker is doable, building a good sounding speaker for piano is hard to do.
2. Amateur bands have a lot of 1/4" phone plugs, for various purposes from guitar pickup, to mikes with phantom power, to keyboard output, to PA amp output. Getting them crossed is part of band learning, and can fry things. Pro bands use speakon connectors for the high power outputs, but I've never seen a used amp I could afford in this county with speakons. Pro bands also use XLR connectors for the phantom power things. Spending more money for more reliability is something kids have to learn.
3. If you can fix things, behringer doens't give out schematics. Lots of other brands do. A toasted 1/8 watt resistor looks just like a toasted ceramic capacitor these days, schematics are nice to have.

chris661 16th October 2010 09:01 PM


Originally Posted by wakibaki (
..........but a young person needs something with street cred.


I know exactly what you mean, being 16 myself. I have a heavily modified guitar amplifier from the late '70s. I still find myself looking at Marshall gear, amongst others, for the street cred. Mine sounds nicer (than the one I'd buy), but no-one's heard of it (that said, plenty have heard the amp itself :D).

Apply that here, and you're absolutely right, get one that has a decent balance between sound and street cred.

Apart from that, make sure it holds the sound together when turned up. Nothing worse than liking the sound at low levels, then it all falls apart when you try to keep up with the drummer. I wonder if 1x12" is really enough to hit the lower notes on a keyboard when the drummer's going for it... Perhaps 2x12 is in order?


amt 16th October 2010 11:38 PM

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Guys, heres the deal. My son just turned 10. He doesnt need street cred since hes not allowed on the street:D What he needs/I want is a small lightweight amp he can actually carry and that will produce enough sound to equal the other kids' amps. They all use small Fenders, Rolands or Peaveys. Ive got plenty of pro stuff available to me for gigs (someday) but right now compact starter is whats up.

He is also quite happy building something since he gets to participate and has been schooled in the zen of diy audio.

My only concern is the quality of the amps. Ive seen lots of larger amps used but they are just too big. Lots of little guitar amps though.

Ive got the Eminence 12lta mounted in a sealed/ported box and will fire it up with a chipamp and see how we are doing. My thoughts now are build it as a powered speaker and later, add a mixer like this Mackie.

Buy Mackie 402-VLZ3 Compact Audio Mixer | Unpowered Mixers | Musician's Friend

Boils down to this: Is a nice chipamp, the 12lta and the Mackie a better collection of parts than a $150 8" commercial amp?

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