Keyboard speaker project

[SimontY]

Greetings to all!

My brother plays the keyboard in a band and needs amplification, so I suggested we make something.

After thinking a bit and looking around on Ebay, I decided it wasn't worth the effort to make any amps... not when one can buy 2 x 400w into 4ohms for £160 new. Plus my DIY electronics and ruggedness don't go hand in hand... So there's the power sorted. The amp arrived today - I've not opened it yet but it's nice and heavy.

For speakers we have decided on a stereo setup. I have plenty of 6.5" drivers handy so I was thinking 4 per side may be as many as we can get away with in something vaguely portable.

I'm thinking a sealed box design, with the drivers wired in series-parallel to obtain a 4ohm nominal load - perfect to use any amount of the amp's potential power reserves.

Now, the problems:

* will we have enough displacement for any kind of real bass notes?

* I don't know the specs of the drivers I intend to use and am not sure how to measure them! The process looks like a nightmare.

 

[SimontY]

The 6.5" drivers are mini 'subwoofer' drivers so can take a beating, and they naturally roll off surprisingly gracefully, so I don't think I'll bother with a crossover on them.

I intend to use probably one normal hi-fi tweeter - I have some cheap 1" soft domes I could use, which are rated at 50w. Perhaps we could use two per speaker for extra power handling.

 

[SimontY]

Apart from the speakers, the only tricky job would be adding some bass EQ - perhaps a Linkwitz-Transform circuit, to reach 45 or so hertz. I've made a few simple things electronics-wise, so I think I can manage this. Plus it would provide a buffer, so the keyboard wouldn't be driving the power amp directly.

This little box could also provide some extra ins and outs should they be desired in the future as part of this setup.

So - what do you reckon? Are there any obvious shortfalls in my proposed design? Has anyone here built a speaker system for bolstering a keyboard (he calls it a digital piano!) before?

 

[Brion55]

Is this for stage use, or more as a studio monitor? I don't think a little 1" tweeter would be much use for a live sound application.

 

[SimontY]

Is this for stage use, or more as a studio monitor? I don't think a little 1" tweeter would be much use for a live sound application.

It's for both, but I suppose practice use is the more important thing. And how about if two tweeters per channel were used? Crossed over at 2nd order fairly high.

 

[Enzo]

I agree, use more tweter than that. Look into a small compression driver and horn. 4x10" horns with driver can be had for under $20 US. A keyboard amp is essentially a small PA system. Look at smaller PA speakers as models. A single 12 or 15 and a horn is a common arrangement. Works well.

Of course with 200 watts a side, you might want more than a few watts worth of horn. A 50 watt horn driver is not all that expensive. A crossover is a crossover, but make it sturdy. DOn't run 100v through a 100v cap, don't expect 20 watts out of a 20 watt resistor.

A keyboard covers the entire audio spectrum, from snorting bass to sparkling highs, so a convincing bottom can be the difference between sounding good and not.

 

[Brion55]

I would agree a horn would be the way to go. You can get a decent 50W compression driver & lens for about $60. I wouldn't waste my money on a piezo horn. IMHO I haven't heard one yet that sounds very good.

Several years ago, I built a very nice keyboard speaker using a 12" (Eminence as I recall) and a 15 X 4 horn (Eminence also). I may even have some photos of it .... someplace.

If the horns are more than you want to invest, you could look into sound reinforcement titanium "bullet" tweeters. You can pick them up for about $30.

If you use the horns, I would put in L-pads so they can be adjusted.

If there's anything else I can help with, ask.

 

[SimontY]

As you can see, the 6.5" drivers are already bought, so I really, really am not going to consider using anything else there... Do you have any thoughts on the loading of these, and the bass extension we should aim for?

But the tweeters, fair enough, you think I need more power handling... and you're probably right! I'll have a look at what's available over here - but remember most things usually cost about 100% more than in the US.

The budget, my brother says, is £300UKP. With £160 gone on the amp we have £140 (max..) to spend on drivers, x-over parts, cabinetry, and any electronics. I'll probably give him the bass drivers for free of course - how kind of me...

I'd guess the cabinets will cost £40 to make, and a buffer stage with bits maybe £40, leaving about £60 for tweeters.

Thanks for the advice thus far...

 

[Brion55]

If you know the free air resonance of the speakers and are willing to do the math, you can design a bass reflex cabinet to get the most out of them. If you don't want to mess with that, an infinite baffle (closed box) with lots of damping material will do just fine. That's the way many bass/keyboard amps are built.

Buy the best horns and drivers you can afford, it will be well worth the expense in results.

 

[Nigel Goodwin]

Hi SimontY,

Your speakers don't really sound upto it, I'm presuming they are either home HiFi speakers?, or car speakers?, and NOT speakers designed for PA use.

6.5" drivers aren't going to provide much bass, nor much volume, and (as already suggested) keyboards output high powers over a very wide frequency range.

I don't live far away from you, and have recently bought a pair of complete PA speakers (each 1x15 and 3 piezo tweeters), these only cost £99 from Mansfield Woodhouse, not very far away from you either!. They will wipe the floor with your 6.5" drivers, which will probably die when you apply any real power to them.

I'll be using them for complete PA, including a keyboard directly into the mixer/amp.

If you want any details, feel free to email me!.

 

[Brion55]

No, the 6.5" drivers wouldn't be up to the task if he were using only one, but as I understand, he is using 4 per cabinet. That is slightly more displacement than a 12" speaker.

 

[SimontY]

Hi,

Yes, that's right - FOUR per side, so a total of EIGHT.

I did actually write a pretty long reply last night but my home Internet connection stopped working before I could post it.

I did some rough calculations, and 4 x 6.5" drivers provide a surface area approaching one 15" driver. So EIGHT would be outgunning a 15" on surface area by a good margin. Furthermore, when four voice coils disperse the power rather than one our thermal power handling should be pretty good. These drivers were used in mini-subwoofers, so they should be capable of taking a beating. And what's more, if they broke they can easily be replaced.. I have loads. Also, if you'll compare to a 15" PA driver, these will likely have a smoother midrange (I've heard it and it's pretty smooth) whereas your 15" will have a ragged midrange and short xmax.

I did say in my attempted reply that this project is becoming more and more likely to be used in practice and less likely to be used on stage, so maximum output may not be quite such a huge issue. I certainly feel an accurate sounding setup will be most useful to him in practice.

I heard my brother Mark's band for the first time last night, and they weren't too bad It's given me motivation to get going with this project

 

[Nigel Goodwin]

I was assuming multiple small drivers, but the bass response and power handling is likely to be relatively small - as is the actual volume of sound they produce.

I don't know if you've ever seen the old Bose speakers?, they used to use 8 drivers of about 6.5 inches - they were commonly used for vocal PA. But the small size meant the bass response was pretty pathetic, even though they were PA drivers.

You didn't mention what sort of drivers they are?, are they for home audio?, or car audio? - 6.5 inch doesn't sound much of a sub-woofer?.