Electrodynamic Loudspeakers, Vacuum Tubes, and a Big Iron on His Hip (the real EricLab)

Greetings everyone. This edition of The Audio Link is decidedly different from previous posts. I will warn those who are invested in DEI that photos in this post could offend; after all, my shop includes Snap On and Ridgid tool calendars, and you can (barely) make out the late Jo Raquel Tejada on one calendar. I enjoy working from home, where free thinking and creativity are fostered; i.e. a place where you don't have to walk on egg shells or worry whether you used a gender-specific pronoun when referring to an it-them. Ok, I'll get off this "PC" soapbox and write...

Nope, changed my mind... I'm going to briefly espouse my view on society's emphasis for STEM, AI, and specialization in today's work world. The audio and audio electronics I embrace isn't about meeting specifications (namely for marketing purposes), the latest app or technology, or profit. Instead, I prefer to venture into a world where philosophy, art, music, craftsmanship, and "hearing is believing" have precedence. I'm certainly not anti-technology; in fact, it was a recent Seltech article titled "Electrodynamic Loudspeaker--The Lumped Element Modeling Approach" that sparked my choice of topics for this edition. Additionally, many thanks to Frédéric Fallais for his kind replies to my thoughts on LEMs and other loudspeaker models.

A lot of my (paid) work involves an in-depth understanding of sound, hearing, and psychoacoustics. However, my humble beginnings (as well as humble present) have a lot more to do with electronic design than "acoustic" design. I sort-of became pigeon-holed as an acoustics engineer, but audio emcompasses many disciplines, to include the aforementioned humanities. Reading the Seltech post brought back fond memories of dangling a loudspeaker (raw driver) from a string in the middle of a room to measure its free air resonance (Fs), and adding a known mass of modeling clay to measure additional parameters needed for determining the "ideal" enclosure volume for a specific driver. I'd take this a step further by applying a "golden ratio" to determine an enclosure's L x W x H (the principle here was too maximum the spread of standing waves that could occur--sort of an eigenvalue / wavelength idea). In the end, my earliest homebrew loudspeakers left something to be desired; but then, so do the vast majority of commercially available loudspeakers.

Today I'm working on an invention that, for now, is entirely self-funded. Because of this bold adventure, I'm liquidating a lot of my personal audio-electronics and hearing lab equipment. I might even be throwing in the HATS (head and torso simulator). I'm selling items on a popular auction site (you know the one), but I've recently discovered that hard work doesn't always pay off... at least not for the person doing the work. I take the time to shoot clear and detailed photos for my auctions, and I attempt to write clear and concise descriptions. But, as today's world would have it, the site not only enables, but encourages sellers to use my photos and descriptions (or "write with AI"). Good grief, I suppose plagiarism will be amply rewarded* in schools, while AI generates term papers, dissertations, legal correspondence, etc. (*Rewarded for resourcefulness, not for the lost art of originality.) But hey, the world is changing, and there's still plenty of room for innovation. I truly appreciate the support and feedback I receive from friends, to include my very good friend Ted Leonard (founder of Prescott Pro Audio). Ted is a great person, promoter, and he's well connected with some top names in the music and entertainment industry. For the time-being, he's one of about three people who have seen early versions of my "invention".

Vacuum tubes, as many know, haven't disappeared (same for vinyl LPs). One of the "fun" aspects of building with tubes is the high voltages involved. I will warn novices and old hats alike that tubed circuits don't work on your sissy-boy 5V or 3.3V supplies. I have built amplifiers with plate voltages in excess of 700VDC, and there's plenty enough current to cause a fatality. Instead of protecting an electrical component or PWB from ESD damage, you need to protect yourself from getting zapped. When measuring high voltages, it's prudent to keep one hand in a pocket so it doesn't touch a conductive surface that would allow current to pass through the body. Your DMM's ground or reference lead should be affixed to the chassis or power supply, not held in your hand. Folks, this is serious do-do: The high voltages used in VT circuits can be lethal.

Now let's get to another topic I wanted to touch on: Electrodynamic speakers; specifically, speakers with field coil magnets. As you might imagine, the use of an electromagnet (vs permanent magnet) introduces a new variable in loudspeaker design. Field coil loudspeakers pre-date loudspeakers with permanent magnets, and their heyday was long before Dr. Albert Neville Thiele and Dr. Richard Small presented their ground breaking work (the well established Thiele Small, or T-S parameters). The photo below shows a few vintage electrodynamic loudspeakers that have been refurbished. I use these speakers in custom guitar amplifiers (combo amplifiers). I can dynamically vary the "B" in Bl (a loudspeaker parameter).

When building with vacuum tubes, one tool that is extremely useful is the radio chassis punch. The following two photos provide examples of tools in my shop. I'll discuss specialized hand tools and amplifier construction in future editions of The Audio Link.

Notes about the title of this post, and "A Big Iron on His Hip". Who can forget (no matter how hard I try) the Marty Robbins song and the lyrics "For the stranger there among them had a big iron on his hip." For me, the big iron isn't a revolver, but the iron shown in the photo(s) below. If you look online for videos on how to solder, you'll find close to Avagadro's number of YouTube videos covering this topic. Despite the plethora of videos, I'll be giving soldering "tips" that I haven't seen elsewhere. I started soldering at around age 7, using a Craftsman soldering pistol. When the copper element broke, I replaced it with heater elements I fabricated from coat hanger wires. Really, really NOT recommended (but what do you do as a farm kid?).

As I had stated above, I'm putting "most" of my eggs into one basket, with the conviction that my latest invention will do well. Once I have a prototype finalized, I'll be going the usual route of applying for patents or other means of intellectual property protection. When that time comes, I'll be reaching out to VCs and interested parties for funding. But for now, I can continue to share what I know (and continue to learn) along the way, and without giving away trade secrets. When it comes to secrets of good craftsmanship and design, a lot of this simply takes practice and experience. I'm happy to share the successes as well as foibles (which are a part of learning). For the remainder of this edition of The Audio Post, I'm providing deluxe show-and-tell photos as a sneak preview of what's to come. Cheers! ELC