GuitarNutz 2 - Speaker cab for electric and acoustic

« Speaker cab for electric and acoustic »

Welcome Guest. Please Login or Register.
Jan 29, 2012, 1:05am

[Home] [New Topics] [Help] [Search] [Login] [Register]

The Original GuitarNuts site, where this community was born!

GuitarNutz 2 :: Gear Concerns :: Amps (and Speakers) :: Speaker cab for electric and acoustic

Page 1 of 2 » Jump to page Go

Author

Topic: Speaker cab for electric and acoustic (Read 2,097 times)

JohnH
Global Moderator

member is offline

[avatar]

Joined: Apr 2005
Gender: Male

Posts: 3,060
Location: Australia
Karma: 153

Speaker cab for electric and acoustic
« Thread Started on Jan 10, 2009, 4:15pm »

My holiday project has been building a cab, which I wanted to be able to use with electric and acoustic inputs. I needed a full frequency range available for the acoustic/piezo input, but controllable so that this can be curtailed to still sound good with overdriven electric guitars.

It strikes me that this wish may be becoming more common. There are an increasing number of hybrid guitars that create acoustic and electric sounds, either by modelling or just by mixing of piezo pickups, (Line 6 Variax, Fender VG Strat, Gibson Darkfire, Epiphone ultra, and others), plus most acoustic guitars these days come with a piezo pickup. None of these bring out their true zingy strummy acoustic nature through a guitar amp with a frequency range that falls away at 4 or 5kHz, whereas distorted electric guitars sound fairly painful if you have to hear the full range of distortion products up to 20khz .

What I found out, is that it is easy enough to get piezo horn tweeters working in parallel with a guitar speaker. I built two of them into this cab, and they are switchable to have them off, or with two levels of ‘on’ to allow adjustment of presence. These tweeters are very cheap, they take very little current and do not need a cross over. Hence the main speaker and the amp are happy with or without the piezos switched in. The idea could therefore also be made into a separate small tweeter box to plug in parallel with a normal cab, however, I have it built right into the cab.

My cab is 570mm x 460mm x 360mm at the base and 330mm at the top, and has a 100W 12” Celestion main driver taken from my Marshall DSL combo, where it was replaced by a Vintage 30. The cab dimensions match the front face and front slope of the Marshall, so it can sit under it as a stack. The width matches that of my Marshall cab, and the feet sit into cups on top of the old cab, so that gives another stack option for use with a separate amp.

The cab is built from 15mm ply, butt jointed with 19 x 42 battens screwed, with glue on all joints except the back. I rounded off the corners by hand, and it is finished in black chalkboard paint and black metal corners. I haven’t tried to match the Marshall style, since I would fail, except for the brass screws in the corners, which are a small reference to the rivets that Marshall use. The grill cloth is stapled to a wooden frame and fixed by screws from the back. Stretching the cloth would have been the fiddliest job of all, except I had assistance from some crafty female fingers – (and I now need to build her a deck!).

The size of the cab is reasonably generous for a 1 x 12, and the first intention was to make it a closed back cab. But when I first tested it unfinished, I found that it had a lot more life with the back partly open, and so it now has a large oval slot in the back. This gives it a more space-filling, lively sound. Other theories embodied were, the back and front are not parallel to reduce resonances, and the speaker is positioned slightly above the centre to get it further from the ground and also reduce symmetry which leads to resonance. The back has a central stiffening batten, and some carpet attached internally to damp its vibrations. Also, there is a beam blocker across the front to diffuse treble from the main speaker. The piezos are angled slightly upwards and outwards in addition to the front baffle slope, to help spread the sound.

When I came to test the piezos, I tried various wirings in parallel and series. These bullet horn piezos are usually wired in parallel, with a small resistor in series, but I found this was too dominant. I settled on putting the piezos in series, together with a switchable series resistance of 270 Ohms or 270 + 820 Ohms. This is quite a lot of attenuation, and it also allows the natural capacitance of the piezos to trim down the high treble. I picked these values based on listening tests with three amps (the DSL tube amp, a small Marshall solid state amp and a Crate Powerblock Class D amp) and several guitars, plus feeding in some AC/DC (the band, not the voltages!) to see how it sounds as a full range system. I figure that with the 270 Ohm series resistor, I’m getting a roll off from about 10khz upwards. The combined resistances of 1090 Ohms roll off through all the frequencies that the piezo contributes, but they are still there to be heard as a small enhancement to the sound.

At the minimum setting, the piezos just add a bit of presence and sparkle to the high end without sounding harsh, and the higher setting it works with fully acoustic or music input. With high gain, its best switched off. It is sounding great with the little Crate head on it and is also a good bottom cab for the Marshall.

Here's some pictures:

[image]

Cheers

John

« Last Edit: Jan 10, 2009, 5:49pm by JohnH »