I am the guitarist and sole chordist for a 15 member Jazz band. I also sometimes play melody and solo. The couple of guitar amps that I own are not loud enough to play over the band while still sounding clean. My music professor suggested some Jazz guitar combo amps that were in the $700-$1200 range but he also said that the lower end suggestions may not be loud enough for what I’m looking for.
Not being happy with the reviews of the low end amps and not wanting to pay $1200 for the high end ones I decided to look into building my own combo amp. For my situation (playing a hollow body for jazz) I was interested in the cleanest sounding amp I could get. I have played directly through PA systems in the past and like that kind of clean sound. I considered just using a powered PA speaker on stage but after some research decided to put together my own combo amp.
The components I ended up selecting are professional PA system components designed to put out loud clean sounds for dance clubs and movie theaters. I selected a rack mount 1500 Watt power amp from Crown in combination with a 15 inch PA replacement speaker with a flat response from 54 Hz to 4.2 kHz from Eminence.
Step 1: Combo Guitar Amp: the Amp
The Crown XLS 1500 is a stereo PA amplifier capable of 1500 Watts in bridged mode into 4 ohm speakers. This amp is a class D amplifier which is a digital amplifier that uses pulse width modulation (PWM) in the power amplification stage. PWM allows the power transistors to switch between the states of fully on and fully off to power the speakers. Traditional class A and A/B amplifiers rely on linear transitions of transistor states between partially on and partially off. The class D amplifier is much more efficient requiring smaller heatsinks and power supplies and cost less to make. I purchased the Crown XLS 1500 for $199 on Amazon. The amp weighs less than 12 pounds. In my configuration of bridged mode into an 8 ohm speaker the amp is rated 1050 Watts.
One thing I learned for this build is that professional audio equipment has a different line level specification than consumer devices. The Crown amp expects +4db input levels where consumer products expect -10db. Even though the Crown amp has ¼ inch plugs I was not able to plug my guitar directly into the amp and play. I needed a preamp between my guitar and the amp. Even with my DigiTech multi-effects pedal plugged into the Crown amp the volume was not enough even at full volume. Luckily I have a small Behringer 4 channel mixer that has has the proper preamp and +4db output required for the amp.
Step 2: Combo Guitar Amp: the Speaker
A general guitar speaker produces a unique colored sound. They have names like “Big Ben” and “Cannabis Rex”. Solid body guitars that do not have significant instrument body resonance and therefore benefit from speakers with coloration. Electric guitar players select speakers based on style of music and the sound they like. Most acoustic guitar amplifiers have a clean sound and often include a tweeter to play the high frequency harmonics acoustic guitars produce. I am playing a hollow body jazz guitar so I want a clean sound without the high frequency harmonics of an acoustic guitar. Instead of using a speaker designed for guitar I have chosen a speaker designed for a PA system.
The Eminence Delta Pro-15 speaker was selected because of it clean spectrum from 54 Hz to 4.2 kHz and high power rating. The frequencies of the notes on a guitar range from 82 Hz for low E to just above 1000 Hz for the upper notes. There are a number of harmonics above these frequencies required to reproduce a good sound. This frequency range matches most other jazz guitar amps that I was looking at.
The 15 inch diameter is larger than most guitar speakers. The speaker is rated for 800 Watts of music program and has a sensitivity of 102 dB at 1W/1m. This speaker outputs plenty of clean sound allowing me to keep up with the jazz horns and drums. I purchased this speaker for $135 from Amazon.
Step 3: Combo Guitar Amp: the Cabinet
There are a few type of speaker cabinets used for instrument speakers. For guitars combo amps the most common is open back. Other types are sealed and ported. A sealed cabinet encloses a specific volume of air to produce a proper back force on the moving speaker cone. A ported cabinet includes tuned port that extends the bass response.
I am fortunate in that my grandfather has a wood workshop and the patience to help construct my vision. I wanted this combo amp to look different from amps on the market utilizing hardwood as part of the design. I choose a sealed cabinet for the ease of building and not needing to extend the bass response. Eminence recommends a 1.0 to 1.5 cu. ft. sealed enclosure for this speaker. The face and 1.2 cu. ft. box behind the face are constructed from ¾ inch MDF board. MDF is a very dense particle board that works well for speaker cabinets because it is stiff and does not resonate much. To reduce sound on the inside we stapled old cloth to most of the inner surface. We added oak sides (stained and finished) for aesthetics and to mount the 19 inch rack mount amp to. The overall amp weighs about 65 pounds. I added handles and wheels to help move it around.
Step 4: Combo Guitar Amp: End Product
The amp has a very clean sound at all volumes. I have been using it for jazz band rehearsal and the amp works effortlessly keeping up with the horns and drums, I only have to turn it up to about 25% to be loud enough. The finished combo amp looks and sounds great. It cost about $400 in parts and a full day to construct. I learned a lot about amps, speakers, and woodworking by building versus buying. It’s definitely a conversation piece.
There are some good questions and more info in the comments, if you're further interested. Or feel free to message me any questions!