# CarSubAmp

## Project stats

• Difficulty: beginner 1/5
• Cost: ~50€ without battery pack
• Time: ~2h

The CarSubAmp was my first prototype for a really portable Bass Combo (Speaker cabinet + amplifier) that doesn’t even need a wall socket.

### Motivation

During Covid19 Pandemic, I enjoyed playing and excercising Bass outside in the Parks or on my balcony. Which is not a problem when wearing headphones and carrying a small preamp. But, there were occasions when I wanted to play along with, say, an Acoustic Guitar, or somebody would bring a Synth-App along with a MIDI Keyboard - and I was forced to use a small bluetooth speaker that neither had the output power nor the real-time feel.

### Problem

My Bass amp is far from mobile. Weight: 18kg, size: huge, plus: It’s useless without a wall socket nearby.

• Instead of a Class-D amp I used an old Car Radio’s aux in, but it seems to have expected “Line” level which my bass couldn’t supply so the output was rarely audible. Additionally, the idle current draw was >0.5A so it would drain the battery all too quick.
• I tried the power supply with a powerbank (Power-Delivery) first, but the integrated step-up converter didn’t seem to handle full-volume current surges, so the supply voltage dropped, leading to audible distortion.

### Solution

After some thinking, I came up with the idea of using a cordless multi tool’s Li-Ion battery that would be able to handle current surges easily, have a low weight, feature a battery level indicator and the typical protection circuits to avoid overload, short circuit, over temperature, exhaustive discharge etc. I wanted a Class-D power amplifier matched to the voltage range of this battery to be as energy efficient and as lightweight as possible.

### The “CarSubAmp”

To check if the concept was viable, I took a used OEM car subwoofer assembly that normally goes into the spare wheel well, bought a 20W amplifier developer board and after soldering some components, adding a fuse and an 6.3mm input jack, my first prototype was ready to try.

## Project summary

• Very lightweight, <5kg
• 4Ohms Subwoofer assembly “ready to use”
• vented design for better speaker efficiency
• Sound is OK, but it’s clear that the assembly is not meant to be used as a full-frequency range speaker. Muffled.
• Playing low notes (i.e. at or below g1), the system would become noticeably quieter
• Developer board comes without preamp, thus maximum volume a bit low already
• Under high load, vented design “Bassreflex” leads to audible hissing noise
• Due to its small diameter, the speaker would hit maximum excursion limits when playing Hammer-ons on low strings
• All components fixed with hot melt or tape, thus not very durable
• Assembly is sealed with glue, no way of permanently adding a case or enclosure for battery, amp, cables, or input jacks

Altough this design had clear disadvantages, I actually enjoyed it a lot because for the first time, I could literally play anywhere and anytime I wanted. I even caught myself on the couch with this little amp although the “real” Amp was just one room away.

After some months carrying this thing around, 3 re-solderings of broken connections and glueing the battery back into place again and again, I decided not to stop with this prototype.

Updated: