Before we get to today’s subject I wanted to share a discovery. If you have an Amazon Echo (my favorite) or the poorer sounding Google Home, you can listen to my podcast by just asking. If Echo, “Alexa, play Ohms Law Podcast” and she will. If Mrs. Google, “Hey Google, play Ohms Law Podcast”. So far they will only play the latest show. I am hoping to figure out how you can request a specific day or show.
On to our regularly scheduled blog post.
One of the most challenging loudspeakers of all time was the infamous Apogee from the late Jason Bloom. These were full range ribbon speakers that gave pause to just about any amplifier. If you could find one powerful enough to drive the Apogees, ignoring their silly low impedance of 1Ω, you could get some pretty magnificent sound.
Here’s a picture of the inside workings of an Apogee ribbon.
What you might notice in this photo is the shiny aluminum strip. That’s the ribbon, and as promised, I will attempt to explain how a ribbon speaker works.
Ribbons move air by means of their namesake. The moving ribbon is typically made from very thin and light aluminum foil—a simple strip that is either straight or pleated (a traditional ribbon) or folded (an AMT). The ribbon is held tight on its top and bottom and free to move along its edges. Those edges are surrounded by powerful magnets. Like the voice coil of a conventional driver, power from the amplifier is applied across the metal ribbon, which in turn develops a magnetic field. As we remember, this moving magnetic field opposes and attracts the permanent magnets along the side and thus it moves, pressurizing the air and we hear music.
Here’s a picture of the setup of a ribbon.
The “pole plates” are the magnets. Note the transformer fed by the amplifier. This transformer is needed to eliminate the Apogee problem of low impedance. You see, the ribbon is a very short piece of metal with low resistance. Using a matching transformer raises the impedance to a more acceptable level for power amplifiers.
Years ago one of the most famous ribbon drivers was made by the British company, Decca. Here’s a picture of the actual ribbon used in a Decca tweeter.
The ribbon is the pleated center strip. The top and bottom lugs with holes are where the wires connecting a power amplifier would be soldered to.
The advantage of ribbons is their low mass. Because the ribbon is very thin, typically about the width of a human hair, it is quick to move and thus both transients and higher frequencies are reproduced with speed and alacrity. Ribbons are among our favorites and are likely what we’ll use in the upcoming Arnie Nudell based loudspeakers we will be producing.