Open Back Combo Amps – Reduce Phase Cancellation
This article is for all you players that use an open back combo amp or open back extension cabinet. This tip comes from a personal experience I encountered on a gig. During the soundcheck at a recent gig I noticed that my 5F6-A 4×10 Bassman clone I built sounded a little thin and harsh in the midrange. It also sounded very two dimensional and lacked dynamics. This amp usually sounds nice and thick with plenty of girth and has that juicy tweed tone that goes perfectly with my Tele’s. My rig sounded glorious the night before and after a quick check I knew that there wasn’t anything wrong or broken. It still sounded bad however and I was not going to spend the entire gig with my ears hurting from the harsh non-musical tone.
The only thing I could think of that was different from the previous night’s gig was the placement of the amp onstage. The night before my amp was about 10-12 feet from the back wall of the stage and on this particular night my amp was about 2-1/2 to 3 feet from a wall. The wall was also smooth and painted with gloss paint so it was a reflective surface. What was happening was the sound coming from the back of the amp was bouncing off the wall and combining with the sound coming from the front of the amp. Now we all know (or maybe not) that the sound from the back of an open back combo is out of phase with the sound coming from the front of the amp. This results in a phase cancellation similar in sound to hooking up one speaker with opposite polarity to another in parallel. That’s why I was getting the thin, harsh, very unmusical sound. Had the wall been covered with fabric or carpet or if I had a closed back cabinet this would have never happened.
So now that I figured out what was going on I needed to fix it quickly because we were about 10 minutes before the start of the gig. I needed to deaden the back of the wall directly behind my amp. I took the padded bag for pedalboard and opened it up. My pedalboard bag is about 30 inches by 16 inches. Completely open it then is about 60 inches by 32 inches and the inside is like a sort of velvet material. This is very sound absorbent material and it has 1 inch or so padding. I leaned the open pedalboard bag against the wall behind my amp and instantly my rig was about 85% better! Most of the reflections were eliminated and the phase cancellation was minimized.
Next time you are a gig with an open back combo and it is close to a reflective wall deaden it with whatever you can get a hold of. Your ears and soul will thank you for it! �