MobFobAmp

A second prototype of my mobile Instrument Amplifier/Cabinet combo with foldable design. 28l cabinet with 17cm woofer. 15W amplifier power. 7.3kg incl. battery. Especially suitable for Bass Guitar, Acoustic guitar, Organs.

Project stats

• Difficulty: challenging 4/5
• Cost: ~140€ without battery pack
• Time: ~15h

Impressions

Impressions of MobFobAmp. Click to enlarge.

MobFobAmp Sound sample

A/B/A comparison Sealed vs. Vented

MobFobAmp and Piano, Sealed, Preamped, at maximum volume

All of these tracks have been recorded with my Smartphone, so no high-end gear used. Still, I think they represent the actual sound of MobFobAmp quite well.¹

“MobFoBamp”: Motivation

To overcome many of the constraints of my previous prototype, I decided I’d need a bigger speaker, and a more voluminous enclosure to yield a better response on lower frequencies. Unfortunately, a bigger enclosure would considerably reduce mobility, so maybe I could get rid of all the air inside the box for carrying…?

The Idea: Folding box

A folding box ^{[ 1 ]}

Following the concept of a folding box like shown, I decided my combo should be collapsible for easy carrying. I chose a sealed design for starters so I wouldn’t have to care about calculating vents, resonance frequencies and group delays.

I had to think hard how to integrate speakers and electronics while keeping the ability to fully fold the cabinet.

Dimensions

I selected the cabinet’s width in a way that it would fit snugly into my bass gig-bag if needed. All other dimensions fell into place by itself - height had to be at least twice the width so top and bottom lids could be accomodated when folded (but not too high to keep stiffness as high as possible. Depth had to be less than (0.5* width) - (materialStrength + hingeThickness), otherwise folding wouldn’t work as the sides would get in their way.

Sound Hardware

For the power Amplifier I chose to stick with the module I already bought earlier. I chose a 6.5inch Hi-Fi woofer (50W, 4Ohm) with low resonance frequency to match my sealed cabinet, which I figured would have a volume of ~28l.

Simulation

Then I ran some simulations to see whether cabinet design and speaker would be a good match.

Estimated Qtc would be at 0.63 instead of the recommended 0.71 (optimal step response), so the cabinet would be somewhat overdamped and sound “tighter” but loose volume at the low end compared to an optimal setting. As you can see, building a cabinet always is a compromise between sound, power, size, weight, frequency response, and volume level that can be obtained from that setting.

Summary

When I finished building that cabinet after some 15h, I was quite happy to see the cabinet is airtight and not flimsy at all. Unfolding would take me below 60 seconds, folding it back to its original state even less than 30 seconds. As maximum level still was a bit low, I added a compressor stomp pedal to the setup which had a gain knob I could use as a preamp.

The sound of this thing is amazing. Tight Bass, warm mids and subtle, but brilliant highs fitting the characterisitc sound of the bass guitar very well. Maximum sound level is limited, but it’s always enough to keep up with an acoustic guitar. I’m using it as a practising amp all the time.

Sound quality

For sure, the most important aspect of an instrument combo is how it sounds like. Due to the foldable design, it can be operated in two different setups.

• Sealed: Tight and well-balanced sound, good low-end punch. Without preamp, maximum level OK to play along with an Acoustic Guitar
• Vented: Bottom Lid opened ~3cm, latch suspending cabinet against bottom panel. Much louder sound with pronounced low-mids, less balanced and a bit boomy, maximum level on par with Piano played f. All Audio resources can be found in the Impressions section.

I’m really happy with the sound quality MobFobAmp provides. The option to play “open” adds flexibility and makes the sound really stand out (with the drawbacks mentioned above).

I also have performed some measurements to see how close the design gets to an ideal frequency response, sending some White Noise to MobFobAmp and then recording the sound output with a microphone. For comparison, I displayed a spectrum analysis in the frequency domain. A perfectly neutral sound output would hit the orange line at 0dB for the whole spectrum. You can see, that the -3dB limits or useable frequency range is from 30Hz to ~1.5kHz.

Add-Ons

9V power supply for Pedal Board

I’m using some effects pedals when practising, and I also wanted to be able to use them when I’m taking MobFobAmp with me. That’s why I decided that it might be a good idea to have a +9V supply rail with barrel connector (barrel = +) on board. So I bought a very well-known positive linear voltage regulator from the 78xx series called 7809 in a TO-220 design that can supply up to 1.5A of current at 9V, and accepts an input voltage range of 11-35V. The component can be used with minimal additional parts as shown in the image on the right. This way, I can now drive a pedal board from the battery of MobFobAmp!

Piezo Tweeter

When I first used a drum computer with mobfobamp, I felt that the highs just were not enough. While perfect for an electric Bass, the drum machine’s hi-hat, cymbals, and even the snare sounds dull and mumbly. So I ordered a simple piezo driver which I could connect in parallel to the main speaker - this works perfectly well. I added a 2W/320Ohm resistor in series with the driver to get the levels just right to my ears. The result is great TODO: and can be heard here.

MobFobAmp Pros:

• Foldable to very compact dimensions
• Low Weight: 7.3kg incl. Battery, 6.9kg without
• Runs >20h on a battery charge as a practising amplifier and moderate volume
• Runs >3h on a battery charge at maximum volume
• Sounds really good for Bass, Organ or Acoustic Guitar
• Playing low notes down to B0 works fine and sounds punchy
• 9V DC out for preamps or effects boards
• Can be operated in “Bass Reflex Mode” for more Bass

Imperfections:

• Without Piezo tweeter, highs are muffled
• Low power amp headroom: When turning up the external preamp too much, the power amp will distort notes, especially low ones, first with flutter, then with the typical sound of transistor saturation
• Prototype’s rat’s nest wiring is still prone to mechanical damage, so an additional enclosure for the electronics would be nice
• Of course, not enough volume/ power to match a drumset or plugged band setup.
• Protective grill for speaker would be nice

MobFobAmp - DIY Replica

If you like this idea, feel free to build your own copy! You’ll learn what it takes below. Mobfobamp DIY instructions

1. Piano by Feliks Weber ↩