The Box
I made this busking amp from an old guitar practice amp. All I needed was the box and the speaker. Any old amp would have done, although the speaker had to be able to handle over 7.5 Watts. I had a Vox DA5.
I removed the electronics leaving on the speaker and thye enclosure. I screwed a reinforcing block on the bottom with a hole so I could mount the box on a mic stand. Mic stands are bad enough to carry about but speaker stands are out of the question. In practice a chair or a wall, or even simply the ground is OK. The rear panel can be opened and the amp tilted up. I added a separator board as cables and mics can be stuffed inside the box and I did not want them hitting the electronics.
The Amp
To get the power, I bought a Class D Amp. from Amazon. It cost me £7.74 (in 2019) The amp is capable of providing 30 Watts (15+15) into 8 ohms with a 24V supply. It is sold as PAM8620 although that is actually the ID of the chip. It is sold by lots of Chinese suppliers Although it is a stereo amp I only use one channel, and a 12V supply. The amp is specified to drive to within a volt of the rails or which gives about 7.5 Watts into an 8 ohm speaker from a 12Volt supply. It has a volume control and I found it could reach max power with only 0.7Vp-p at the input with the volume turned up full giving it a max gain of 40 (or 32dB). The potentiometer is used to mount the amp to the replacement top panel.
The Power
I have tried a few different power supply solutions, including C cells, Alkaline AA cells and rechargeable AA cells. I currently use a Lucas 2.3AH 12V lead acid Battery which cost me £14.95 from ebay. My logic is:-
A lead acid battery can be charged while still in the amp with either my Ctek car battery charger or in the car via the cigar lighter socket. So it can be charged easily in between gigs. The battery is 2.3Ah and if I discharge it to 50% that is 1.15Ah or 13.8Wh. The amp can give 7.5W peak, but in normal operation it will only be taking around 3 watts. So four hours continuous playing is no problem.
In an emergency, the lead acid could go further if I allow it to discharge to under 50%. This is not good for a lead acid battery but it should only happen very occasionally, if at all.
The Final Article
I usually drive the amp from my balance orientated mixer or from my modified Behringer MX400
Afterword
It works well.
Is 7.5 watts enough? Maybe you’d like it to be twice as loud?
Sound is measured in Decibels The old value was bels (where a decibel = 1/10th of a bel). You needed an extra bel to sound twice as loud. That requires 10x the power or 75 watts. This assumes a similar sensitivity speaker.
Anything between 3 watts and 10 watts is easily loud enough for busking or a small venue.