What it is

Not everything that ends up on the bench is broken. Sometimes you build something because you're tired of making a mistake that hasn't happened yet, but will.

At DriftTone, a lot of the work involves plugging unknown gear in to check it's working. An amp that came in with a fault. A pedal that may or may not be doing what it should. Something someone's built, or modified, or had a go at before bringing it here. Most of the time that's fine. But you only have to connect the wrong thing to a decent amplifier once to turn a routine test into an expensive afternoon. I'd been using my main guitar amp as a bench monitor. That needed to stop.

The solution was a disposable amplifier. Something that could take a hit, cost almost nothing to replace, and still do the job.

The DriftTone £4 Amp, finished enclosure on the bench with tools
DriftTone £4 Amp. The marker pen was the right call.

What went into it

The heart of the build is a Bitsbox LM386 audio amplifier kit, a small PCB based on the classic LM386 chip, a component that's been turning up in low-power audio projects since the 1970s. The design itself is a descendant of the Ruby amplifier from Run Off Groove, a well-regarded low-power amp circuit that's been a favourite of DIY builders for years. The kit costs around £4. It's designed to run on 9V, handles line-level input, and drives a small speaker directly. Gain is adjustable: a jumper on the board sets it to either 20 or 200 times, and you can drop a resistor in there instead if you want something in between.

The board went into a scrap diecast aluminium enclosure from the parts bin. An 8 ohm, 0.5W speaker, the kind that turns up in old clock radios and cheap intercoms, was sourced and fitted. Everything else was leftover stock: a volume pot, a quarter-inch jack for the input, banana plugs for the power supply connection, a line-level output jack for routing to other test equipment.

The biggest job was physical: drilling out the speaker aperture and cutting holes for the hardware. Cosmetics were not the priority. The enclosure has "DriftTone £4 Amp." written on it in marker pen. That felt right.

Inside the £4 Amp enclosure, LM386 PCB, speaker, and wiring
LM386 PCB, speaker, and volume pot inside the enclosure
The £4 Amp in use on the bench with line input and banana plug power connected
In use on the bench: line input and 9V banana plug supply

What's next

The current version accepts line-level signal only. A future revision will add a small JFET preamp stage (also from Bitsbox, also a few pounds), which will bring instrument-level signals up to a usable level. At that point it becomes a proper bench amp, able to test guitar pickups, pedal outputs, and amplifier stages directly without any additional equipment in the chain.

Why it matters

There's a version of this story where someone plugs a faulty unit into a vintage amp, something misbehaves on the output, and the repair bill doubles. The £4 amp exists so that story doesn't happen here.

If the worst occurs (a DC fault on the output of something under test, a wiring mistake, a component that fails in an interesting way), the collateral damage is a four-pound kit and an afternoon. That's a trade worth making.

It also just makes the bench more practical. Quick signal checks, listening tests during a repair, confirming a stage is working before reassembly, all of it now happens through something that doesn't matter. The main amp stays off the bench.

What was spent

Sometimes the most useful thing on the bench isn't a piece of test equipment. It's a thing you don't mind breaking.