How to make passive speakers active? Simply stick an amp on the back. Here is the how and why of mounting power amplifier modules on a pair of passive studio monitors.
Some decades ago I spent an entire school vacation working at a local pro-audio company for a pair of studio monitors. The resulting speakers were Alesis Monitor Ones. Over the years several plastic bits have cracked and fallen off, but they're still sounding fine and are still my main monitors today.
Recently my sub-optimal wiring and amp started to pick up Morse code from a local radio amateur and I decided to deal with this once and for all. Despite using passive monitors, I actually prefer active speakers, where amplifier and speaker are in a single enclosure. Amplifier and speaker can be perfectly matched, it simplifies wiring and saves the space that would otherwise be taken up by an external amplifier. But most of all I like that professional equipment uses balanced wiring which is a lot less susceptible to interference. To get in on those advantages I decided to turn my passive monitors into active ones.
There are purpose-built power amplifiers to mount on the back of speaker cabinets. Those typically only have an input jack and sensitivity control. Everything you want done to the signal, such as volume control, needs to be done before it reaches the amplifier. In my case there already is a volume dial on the audio interface, so I'll just use that. Obviously everything will be connected using balanced connections.
A very affordable power amplifier module I came across is the Thomann the t.amp PM40C. It's a compact, 50 W class AB amp, which is a good match for the speakers. The rest of the specs also looked promising, so I ordered two of these.
More advanced amplifier modules exist, with multiple channels and adjustable crossover. With these you can bi-amp instead of using a passive crossover network. This is technically superior, but it comes with a matching price tag. Since I'm not entirely sure I can properly configure the crossover, and was trying to do this project on a budget, I decided to go for the simpler and cheaper solution.
When I received the modules I noticed a few usability flaws, but nothing major. The speaker wires are very short. I had to extend them to reach the speaker terminals even though the amplifier is literally mounted on top of them. Secondly, sensitivity control is a potmeter, which makes it difficult to set identical levels on both amplifiers. And last, the module has connectors on both sides. This means plugs and cabling will stick out on both sides of your speaker enclosure, unless you mount the unit sideways. Still, the amplifier is hard to fault at the price tag of 66 Euro each, including VAT and shipping.
Inside the amplifier module are a big toroidal transformer and a small PCB. Overall build quality seems to be quite good, but it's obviously built down to a price. The main amplifier chip has the markings rubbed off, but seems to be a Texas Instruments LM2876. On the internet I found detailled performance measurements by Timo Kirschke.
The new amplifiers and balanced wiring do not sound any different from what I had before. However, the hiss, mains hum and radio interference have all gone. Without playing audio I can now barely hear if the speakers are powered on. And since the amplifiers are mounted out of sight on the back of the speakers, when seen form the listening position, it does not look any different from before.
With the speaker upgrade finished I will now add balanced inputs on the subwoofers.
Relevant data sheets can be found in the downloads directory of this article.