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(This is the 14th of many promised articles which explain an idea in isolation. It is hoped that ideas may be adapted, linked together and implemented.)

I had difficulty getting an operational amplifier to work. In the case of audio, signals are often positive and negative relative to a ground wire. It isn't obvious how to get this working with a battery powered audio amplifier. Most of the explanations and circuit diagrams omit details about virtual ground and expect it to be known already.

The correct method for wiring audio input to a battery powered audio amplifier involves powering the operational amplifier from battery and also connecting two equal, large-value resistors in series across this power supply. The mid point between the resistors is a halfway Voltage. This can be used as an input to an operational amplifier. It should also be tied to the ground wire of the input.

This arrangement allows an operational amplifier to work with "positive" and "negative" Voltages without incurring any Voltage clipping. Shortly after learning this, I ensured that a friend at my makerspace avoided the fruitless avenues that I encountered. This has spurred development which continues to present. This includes irregular use of a TDA7379.

After some fun experiments with mains fluorescent lights and sine waves at 101Hz or 121Hz, my friend made a laser-cut box for a speaker. The first attempt had a horrible resonance. It was really glaringly bad. With hindsight, it was a poor choice to make a cuboid box with two identical lengths. After some modification to the design, a much better result was obtained.

When someone at the makerspace subsequently said they wanted to make cube speaker enclosures, with JBL speakers and a US$12 Chinese amplifier, I thought it would be truly awful. The concept was good. The speakers were supposed to fit into Ikea pigeon-hole shelving. Official accessories for this shelving include wicker baskets and wine racks. It is also possible to purchase computer cases which are the same size. Speakers would be an obvious addition. That's the general opinion when the concept is explained.

However, I was concerned about the resonance of a cube box, a speaker brand favored by Pimp My Ride rice-racers and an amplifier fresh off of EBay. I was expecting it to be dire. However, after being invited to a sound test, I was astounded. I don't have much sound test experience but I've never heard anything so good. The amplifier was excellent and JBL make really good 4 Ohm speakers. The secret is to make a cube box with really thick panels. Don't skimp. Make it from particle board but make sure it is heavy, 3/4" particle board.

Let's run through some known content. AC/DC? No complaints. Foo Fighters? No complaints. Bob Marley? It is possible to hear the limitations of the analog recording *and* mastering processes. Boston? Oh, that was special. I've never heard such distinct sound separation. It was possible to hear the phasing effect when one enclosure was twice the distance of the other.

I hoped these speakers would be commercialized. There was even an outside possibility of them getting branded as BBC monitor speakers. Unfortunately, this was another former Apple developer from the lean period in the 1990s who subsequently endured health and financial problems. Even worse, the cube speaker designer died in Jan 2017. However, some of the work continues.

There is no substitute for a speaker cluster with an 18 inch (or larger) woofer but a car speaker in a thick box is an acceptable compromise for a small apartment. A one foot cube speaker should be considered as the mid-range option but there is plenty of scope for cheaper alternatives. I wonder if it is desirable to put a two inch or three inch speaker inside a six inch box? This would be a miniture version of a cube speaker and they would probably be sold in packs of six or packs of eight to reduce shipping cost. (Actually, packs of eight may allow re-use of packaging.)

The really cheap option would be US$2 stereo speakers which plug into a headphone socket. I attempted to repair one variant for an ex-housemate. They are impressively cheap to the extent that I laughed over the hair-thin wire and the plastic box which holds the speaker coil against the magnet. Regardless, this is a very economical option to bootstrap a 30 element speaker array. I hope that a theoretical US$20 version would have better, longer cables and would be sufficiently loud and consistent.