Building Amp Cab need advice on speaker spacing?

Phill Martin · Post by **Phill Martin** » 6 Mar 2009 1:46 am

Hello all, I'm going to put a amp together and need advice on spacing the speakers. There will be three speakers one 15 inch and two 8 inch speakers. I'd like to put the 15 on the bottom and the two eights up above it kinda like in the eyes position. So my basic questions are how much space below the 15 inch speaker is needed and how much space between the 8's and how much room above the 15 should the 8's go and then how wide between them and how much space above the 8's can I put the amp. The speakers will be in a enclosed part of the amp with a back on the speakers. thanks in advance.

Rick Johnson · Post by **Rick Johnson** » 6 Mar 2009 5:17 am

Phil
If I were building a closed back
cabinet to house a 1/15 & 2/8's
I would leave about a 1" reveal
on the bottom of the 1/15
above the 1/15 between the 2/8's
I would leave about 1/2" reveal
and maybe at least 1" reveal above
the 2/8's.
If this is for steel guitar, I would
try an port the cabinet with 4-2" ports

The overall dimensions of the cab and
the type of material used will influence
the final sound too. Not to mention
how the baffle is attached.

There a formulas available online to
match speakers to cabinets that will
help the yield maximum performance
of the speaker.
I tried building a cabinet for a
certain speaker using the suggested
formula and it was gonna be
twice the size of a cabinet that I
wanted to build. The size of ports and
the depth of the port tube was another
reason I didn't do it.

Its all about trial and error.

Good luck

Rick

www.rickjohnsoncabs.com

David Doggett · Post by **David Doggett** » 6 Mar 2009 7:26 am

Most formulas available for calculating speaker cabinet dimensions are for full range bass. Unless you are going to play bass or use it as part of a full range PA system, you don't need a cabinet that big. Look at guitar speaker cabinets and use dimensions similar to those. Dimensions are not as critical for guitar speaker cabinets.

Bill A. Moore · Post by **Bill A. Moore** » 6 Mar 2009 7:37 am

Phill, you might also keep in mind, that the 15 will overpower the 8's, if they are in a sealed enclosure. I think you should decide the volume required to tune the 15, and figure a small internal box for the 8's, to isolate them. The 8's won't need to be port loaded, as the higher frequencies involved won't move much air. I assume you are using a crossover anyway to keep the low frequencies out of the 8's. Hope you let us know how it turns out, Bill.

Phill Martin · Post by **Phill Martin** » 6 Mar 2009 3:40 pm

Bill so you are suggestion to isolate the eights I can leave the 15 open in back so as not to over power the eights?

Teh amp and speaker cabinet will be made out of mfd board with a wood veneer over that. The reason for the 15 inch is I'm building a 12 string lap with the lower 4 being actual bass string .125 up, then from the the top three strings will be double string like on a 12 string electric guitar. Can a pot be put on the 15 inch to limit signal and dial in a well rounded sound?
thanks

Bill A. Moore · Post by **Bill A. Moore** » 6 Mar 2009 5:02 pm

Phil, I think some of the cabinets that David has posted here look pretty interesting, with the small slit in back like the THD designs, I hope to try something like that with an old SRO 15 I have. (I'm going to use the external dimensions of a Twin cabinet, and see how it works). I don't know the low frequency of your guitar, but I doubt you'll bottom out the 15, even in an open back enclosure, I built a port loaded 3 cu ft bedroom cabinet for my bass player wife years ago, with an EV 15B, and it's flat to 40 hz, the open E on her guitar. I have only used factory designed crossovers, but I've seen a lot of reference material on the web to design what you need. You just need to remember to match the impedence to your amp. Think maybe 8 ohm 15, paralled through crossover with 2 4 ohm 8's in series will end up with about 4 ohm load to amp. Good luck, Bill.

David Doggett · Post by **David Doggett** » 6 Mar 2009 5:08 pm

Why do you want 8" speakers. They are likely to make a steel guitar sound harsh. Most lap steelers prefer 12" guitar speakers. Doubling up on the high strings shouldn't change that. Years ago bass strings on one neck of a quad Stringmaster were standard. And people played them through open-back 12" or 15" speakers. If you are going to use those bass strings at high volume, you may need a closed-back enclosure, whether you use a 12" or 15" speaker. Either of those regularly handle the high strings of guitars, lap and pedal steels with good tone. Just because you are using double courses on the high strings and adding some bass strings does not mean you have to reinvent a whole new speaker system. Any good 12" or 15" guitar or steel speaker cabinet should work fine, and to be on the safe side with the bass strings, you might want to make it closed-back.

Phill Martin · Post by **Phill Martin** » 6 Mar 2009 5:43 pm

The two eights are coming out of a Crate 20 amp the amp has a great presence and projection with the two eights. They are Jensen’s 8ohm Mods. The 15 is a blue Marvel out of a Peavey blues amp it is 16ohm I will be wiring up the speakers to make 16ohm over all. Bill thank you for your input I'll take a look at thoses cabinets. this amp will be run moderately loud it will be my out in the shop amp so it won't bother the wife to much she has chronic migraines. The main reason of combining all this together is that the Crate chasse in the original amp cabinet has a problem of over heating and you have to tear the whole thing down to change a tube. This will be much more user friendly so to say.
thanks

Michael Maddex · Post by **Michael Maddex** » 6 Mar 2009 8:14 pm

Photos of similar project ( 1 - 15", 1 - 6" ):

http://fuzzcraft.com/boulder.html

HTH.

Phill Martin · Post by **Phill Martin** » 6 Mar 2009 8:43 pm

That really looks nice your pohos state that you were going to go in to biz building speaker cabs for bass. The inpedance of my three speakers will be 2 8ohm and one 16 ohm. I was planing on wiring the two 8 up in a series to give me 16ohms how would or can I hook the 15 inch 16ohm up to maintain 16ohms?
thanks

Michael Maddex · Post by **Michael Maddex** » 7 Mar 2009 9:10 am

Phill Martin wrote:That really looks nice your pohos state that you were going to go in to biz building speaker cabs for bass. The inpedance of my three speakers will be 2 8ohm and one 16 ohm. I was planing on wiring the two 8 up in a series to give me 16ohms how would or can I hook the 15 inch 16ohm up to maintain 16ohms?
thanks

Phill, nice as it is, that's not my work. That's a site I found searching the web for speaker cabs.

I don't think that you'll find a way to get 16 ohms total out of one 16 and two 8s without some sort of dummy load, which IMHO would be a waste of power. You might try this: Parallel the 8s to get 4 ohms and then series that with the 16 for a total of 20 ohms. Then try it out and see if that isn't close enough for your needs.

HTH.

Phill Martin · Post by **Phill Martin** » 7 Mar 2009 12:20 pm

The amp wants 16ohm load the two eight wired in a series would give me 16 then if it would be possible to wire the 15 inch 16ohms in a parallel would that give me 8 or 16 ohms?

David Doggett · Post by **David Doggett** » 7 Mar 2009 4:05 pm

That would give you 8 ohms. That probably would work fine with the amp, especially if you don't run it wide open for extended periods. If you wire everything in series, that would be 32 ohms. That would also be okay, but you would lose a little volume.

Phill Martin · Post by **Phill Martin** » 7 Mar 2009 9:54 pm

David what would be best for the amp running on 8 (1/2) power or 32 twice the amount of the 16ohms on the spec sheet?
thanks

David Doggett · Post by **David Doggett** » 8 Mar 2009 11:04 am

Most amps can take a load between 1/2x and 2x their suggested rating. Is it a solid-state amp or tube? Seems like tube amps are safer with a speaker impedance higher than their rating rather than lower. And it seems like I remember hearing that solid-state amps are just the opposite. I know how to calculate the impedance of multiple speakers, but I'm not an electronics expert. You should as Ken Fox or Jim Sliff or somebody who knows more about this.

Phill Martin · Post by **Phill Martin** » 8 Mar 2009 3:47 pm

David it is a tube amp I hooked it up last night all in a series and it was a little weak not the punch of the two 8ohm 8inch speakers that it has now. I'll hook it up as a series and a parallel tonight and see how it likes just 8ohms. thanks for all your advice and I;ll post the results.