Before building Jim W4LF’s Hobbydyne crystal set, I put together an impedance matchbox, for matching the detector diode to a variety of different headsets and earphones, so that I could determine the best ones to use. The world of serious crystal set listening was new to me, so I did some reading up. To give you an idea of how serious this gets, many committed crystal set listeners have heard over 100 different stations on their sets, on the AM broadcast band (thanks to nighttime skywave propagation)!

It appeared that there are a few different kinds of headset that crop up often as being the favored types among crystal set enthusiasts. Of these, perhaps the most storied is the Baldwin Type C radio headset, or “Baldies” as their owners affectionately call them –

There are a couple of reasons why crystal set aficionados often have a set of Baldies in their collection. One of these is for their historical significance. Baldwins are considered to be the first mass-produced headset that significantly resembles modern headphones. Developed by Nathaniel Baldwin in Utah in the early part of the 20th century, they were first patented in 1910. He got his big break after sending 4 pairs to the US Navy. They were very impressed with their sensitivity and performance, and put in an initial order for a hundred pairs. Baldwins were a high quality headset. In the early 1920’s a set cost $14-$16 – about $250 in today’s money.

The other main reason for their continued popularity with crystal set folk, is that even by modern standards, they are quite sensitive. The design is different from most other headsets of the era, being that they employ a balanced armature and a mica diaphragm. A small proportion of Baldwin headsets had a phenolic diaphragm, and were slightly less sensitive as a result. It’s easy to tell if your Baldwins have mica diaphragms. If you unscrew the ear caps and see clear diaphragms like this, yours are mica –

Here’s a view of the bottom of the element. Looking at it from this angle requires that the diaphragm side is downwards. Do not set it in this orientation without placing it on top of the ear cap, as you could damage the diaphragm or the drive rod (see how, in the above picture, the very end of the drive rod protrudes very slightly above the level of the diaphragm?) In this following view, if you look carefully, you can how the plate (circled) that supports the drive rod, is centered between the two poles of the magnet. This is how it should be – slap bang in the center, and not touching one or other of the poles –

Looking straight down at the back of the element, you can see the 2 screws that are used to connect the ends of the headphone cords. I wish that modern headsets were as simple to disassemble and reassemble as these vintage models are –

Scott (from oldheadphones.com) is a big collector of vintage headsets. He told me that of the Baldwin headsets he has seen that have weak output, it has rarely been because of weak magnets, especially with the units that have purple painted magnets, as seen above. I believe that the purple painted magnet variety are far more common than the other type, which have black painted magnets. If your Baldwins have weak output, things to look for, Scott advised me, are armature plates that are not centered between the magnet poles, or are actually touching one of the poles (keep an eye out for debris between the plate and the poles). Another thing to look for, is warped or broken diaphragms.

Nathaniel Baldwin’s story is an interesting and dramatic one. Read it here – it’s well worth it. My Baldwins, like most of the vintage parts and gear I acquire, came from eBay and, contrary to what seems to be popular opinion, overpaying is not necessarily the norm. As the buyer, the power is in your hands. I paid US$15 + $12 shipping for these, which I think was a very fair price. I did clean them up, but only a little. This is the condition they arrived in, which wasn’t bad at all –

They were in pretty good shape, the main areas in which some shabbiness could be seen being in the headband, which showed very little fraying, though there was some dirt and stains present, and just a small amount of corrosion on the metal parts –

Scott from oldheadphones.com has a useful page on the restoration of old headphones. On this page, he is specifically referencing Western Electric 509-W’s, but the same advice can be applied to other types. Following this advice, I gave the metal parts of the Baldies a very light polish with Brasso (only very cursory, as I really wasn’t that bothered with making them gleam), and cleaned the bakelite earcups up a little with Bon Ami, which is a very minimally abrasive household cleaner. The construction of these headsets makes it very easy to take them apart for cleaning. The headphone cord unscrews from the elements – no soldering needed, and the elements themselves can be taken out from the bakelite earcups with no unscrewing at all – they simply drop out. It is a pleasingly very modular type of construction.

He also gave me some useful advice on cleaning the cords and headbands via email. Following this advice, I sprayed some prewash onto the headband, left it to soak for a short while, then immersed it for a while in a basin of cold soapy water, made with clothes detergent, while giving it a light scrubbing with an old toothbrush. Be careful when doing this, as the old fabric can fray quite easily. Then I rinsed it with cold water and left it out to dry overnight. I didn’t do anything with the cord, as it was in good shape. Incidentally, if you can buy something from him, it helps to fund his hobby of collecting vintage headsets.

Here’s the result. You can see the most improvement in the condition of the headband. Not gleaming, but with the aura of respectability that comes with the evidence of having lived an honest working life –

A very usable pair of Baldwins for everyday use, I think!

While on the subject of Nathaniel Baldwin Type C headsets, I want to share with you a very exciting find that I made while looking for the everyday use pair above. This was also on eBay and, unlike with the above pair, I paid quite a lot for these. They are a set of completely unused Baldwins, in the original box, with the instruction leaflet. The box was a little banged and frayed, but the headset was mint, and looked as if it had never even been assembled, let alone used! I am not an experienced collector of these but, on seeing them, something told me that it was not very common to see a pair in this condition. I ummed and aahed over them, told myself I wasn’t going to pay that much for them, and moved on. Shortly after, I came back, and opened up a correspondence with the seller. He had bought them at an estate sale 30 years ago, he told me, and they had been in storage ever since. He wasn’t sure if they worked, and didn’t know how to test them, but he did own a multimeter. I told him how to do a continuity check on the coils, and they tested out fine. This pushed me over the edge and, after a bit of back and forth, we agreed on a price. They were mine!

I hadn’t seen a pair of Baldies with a green headband before. I think it looks very handsome!

This model was first patented in 1910, and the entire Baldwin operation closed down between 1930 and 1932, so I think it’s fair to say that they are about 100 years old. It’s hard to put into words the appeal of seeing a product that has been sitting in it’s original box for 100 years, in the same unused state the original buyer would have seen it. I can’t help but wonder how this came about. Perhaps this headset sat on the shelves of a radio store somewhere that went out of business and then, along with other remaining stock, was sold, and sat in storage for years? Perhaps it was purchased by a well-heeled customer who bought it and forgot about it?

Just look at that shiny bakelite ear cap, that completely clear mica diaphragm, and that shiny metal, without nary a smidgin of corrosion in sight. Magnificent!

I mean, if you could get in a time machine, walk down Radio Row in NYC in the 1920’s, dive into a store and buy a pair of Baldwins, this is exactly what they’d look like –

Oh lawdy. The shiny bakelite. That is original factory shine. The beautiful green headband. It’s almost too much for me to bear!

That leaflet, that came in the box with the headset? Here it is –

That’s enough of Baldies for now. The other headset of this vintage that I was keen to land a working pair of, was the Western Electric 509-W. Patented in 1918, this headset was also very popular with telegraph and radio operators in the late teens and 1920’s, mainly due to it’s ruggedness and sensitivity. Many believe 509-W’s to be equal to Baldwins due to their sensitivity, robust nature, and quality of manufacture. In the opinion of Scott, from oldheadphones.com, they are ideal for crystal set use, and their performance rival that of Baldwins, only being surpassed by Navy sound-powered headphones.

Continuing my quest to land a bargain, I scored a set of 509-W’s for just $10.50 + $14.85 shipping. They arrived in good, though obviously well used, condition –

A bit shabby, to be sure. Allow me to show you a few more pictures, before I reveal how they cleaned up –

The metal cans were dull, but with no major scratching or other damage. The slots on the two screws that hold the sound element to the nickel-plated brass cans are often knurled or otherwise damaged. Not so in this case, which I took as a good sign –

These are not balanced armature units, like the Baldwins. They are the more traditional style, which makes the poles of the magnets very easy to access for the purpose of re-magnetizing, if that becomes necessary. Like the Baldwins, the headset cord is easily attached and attached with 2 screws. The general rule of thumb for determining if the unit needs re-magnetizing, is if the magnet holds the metal diaphragm on, it is strong enough. These headsets are the more traditional type, with both poles exposed, making remagnetizing easier, if necessary. For these traditional types of headsets, of remagnetizing, Scott says, “I just use a strong rare earth magnet and a couple taps on one pole of the weak magnet takes care of it.”

I polished the metal parts with Brasso, washed the bakelite ear caps with Bon Ami, and soaked the headband in cold, soapy water, then lightly scrubbed them and left them to dry overnight. The headphone cord smelled of tobacco and was a bit greasy, so I also soaked it in cold, soapy water (yes, really) and scrubbed it lightly with an old toothbrush. I then patted it down on a towel, gave it an initial drying with a hair dryer, and hung it up to dry thoroughly, on a hot day.

During this initial cleaning, I did lightly polish the metal cans, but didn’t pay too much attention to the other metal parts. For minimal effort, the 509-W’s came out quite well, I think –

One feature of this set was a little curious. It came with this washer hanging from a piece of string on one side of the headset. To me, it looked as if it belonged there and was an integral part of the unit. I wondered what it was for. I wasn’t able to locate any photos of other instances of this headset with such a washer, and no mentions of a washer anywhere else. After asking a few other 509-W owners, it became apparent that whatever the washer’s purpose was, it was not a stock item, and was not a known 509-W accessory. Eventually, I removed it, but not before taking these pictures. I think the first picture (the very next one) was taken partway through the cleaning process, before all the metal had been cleaned up –

The headband came out a lot cleaner. The cord looked a little better, though the main improvement was that it felt a lot less greasy –

Shiny!

Western Electric 509-W headsets are not hard to find in fairly good, working condition. There are still a lot of them around. They were well made, and cost $12 in the early 1920’s, the equivalent of a week’s wages for the average worker. Other brands of headphones, that were less well-made and not as sensitive, cost less. Brandes were $8, and other brands even less. I wonder how many sets of headphones being used today will still be around 100 years from now and working well?

Cloth headset cords of this era came with strips of material on the ends that attached to tabs on the cans for strain relief –

The headband is looking pretty spiffy and clean from this side!

Next on the agenda was to put together a sound-powered headset. Sound-powered units are used by the military; primarily, I believe, by the Navy. The sound elements are so sensitive, you can connect two together and they will operate without the need for outside power i.e. the tiny current produced by talking into the mouthpiece will produce a sound in the earpiece. This very informative page on Darryl Boyd’s website has information on many of the available sound-powered headsets you’re likely to find. Jim Frederick W4LF’s favorite is the WW2 sound-powered headset known as the RCA “Big Cans”, which I believe are also referred to as the US Navy “Deck Talkers” (see linked page in previous sentence). He says that they are hard to find and expensive, but so far ahead of the others that they are worth it.

Some of the sound-powered units, such as the US Navy Deck Talkers, have a headset, so little physical modification will be needed in order to use them. With others, the sound elements are in a telephone-style handset. In this case, you’ll probably want to remove the elements from the handset and install them in a headset of your choosing. This page on Darryl’s site lists the specs of sound elements in the various units, and also notes the handsets that have identical elements used for both microphone and earpiece. This can be useful, as you will only need one handset to make a complete headset. I set about looking for one of these handsets, and managed to come up with this Canadian RCA MI-2215-E model, which has 2 identical sound elements –

Touching the two leads coming from the handset across the terminals of a AA battery resulted in absolutely no clicking from the headset whatsoever, which was not a very encouraging sign.

Back view of one of the elements, after the leads had been desoldered, with a strip of tape over the holes to prevent ingress of debris –

Front view of the same element –

Removing the metal back cover from the first element, the armature plate was clearly visible. If you look closely (easier if you’re viewing this on a computer, as opposed to a phone), the armature plate, which was connected to the threaded drive rod, appeared to be jammed up against one of the poles of the magnet –

Looking at the threaded drive rod, you may be able to see that there are two small nuts, one above and one below the plate. Adjusting those nuts determines the default positioning of the plate. I adjusted them with a pair of needle-nosed pliers and successfully repositioned the plate halfway between the top and bottom magnet, so it was no longer in contact with the top magnet. At this point, I didn’t have a crystal set to test it out with (I hadn’t yet built the beta kit in my last post). Instead, I connected it to the output of my impedance matchbox, with the output impedance set to somewhere in the region of 300 ohms (the impedance of this element), connected a longwire antenna and ground to the two input terminals of the matchbox, and connected a 1N34A diode across the input terminals of the matchbox. Success, as the sound of a cacophony of MW AM stations sprang forth from the sound element. It was working!

Buoyed by this success, I set about tightening the nuts, and disaster struck. I overtightened one nut, and snapped the very delicate threaded drive rod. I had lost sight of the fact that it is a thin rod. Unfortunately, the break occurred right next to the nuts. If it had been further away from the nuts, I might have been able to solder or epoxy the rod pieces back together. As the break was up against one of the nuts, repairing it would have reduced my ability to adjust the positioning of the nuts on the rod.

Back to the drawing board and, before long, I acquired this H-203/U handset, manufactured by the Dynalec Corporation. This handset has a push-to-talk button –

When this handset is connected to another handset, pushing the PTT button connects the handset to the other handset so that, when the PTT is not pressed, the other station cannot hear what you are saying. If you connect the two handset leads together and talk into the microphone, you should be able to hear your own voice in the earpiece. If you’re unsure whether you’re hearing your own voice via the earpiece or via bone conduction, alternately pressing and releasing the PTT while talking will clarify.

Unlike the previous handset, the identical microphone and earpiece are not soldered in, so it is easy to remove them. They just pull out, revealing the neatly-wired supporting circuitry –

Even the capacitor leads across the earpiece have been neatly preformed. I very much approve!

These are the identical sound elements from this handset –

The terminals are easily soldered to, though you have to be careful to apply the iron for the absolute minimum of time, so as not to melt the plastic. I used a little flux from a flux pen, to help things along –

Next on the agenda was to find a suitable headset or old pair of headphones to install these elements in. I found this new unused pair of earmuffs on Facebook Marketplace for $5 –

This set of earmuffs turned out to be well suited for these sound elements. The aperture in one end of each side of the earmuffs is very slightly wider, and there is a good thickness of foam lining the back of the earmuff –

It was possible to push each sound element in at an angle, such that it ended up being held firmly in place by being pressed up against the ledge/flange around the opening, by the foam in the back –

The headset lead enters one side, at the bottom of the can. Heat-shrink tubing over the lead and tie-wraps on the inside and outside of the can hold the lead in place –

A lead runs in between the cans to connect the elements in series (in phase). It runs between the tops of the cans, underneath the cushioned piece that surrounds the headband –

These ProCase brand earmuffs are available on Amazon at the time of writing for $16.99, although the FB Marketplace price of $5 from a local private seller was a no-brainer, of course. The only possible drawback to them is that the fit is very tight, though as the purpose of earmuffs is to keep ambient noise out, this is an intentional part of the design. One could argue that a sound-powered headset will only be worn for the very weak DX stations, and the absence of outside ambient noise is helpful when trying to copy them. I used an old headphone lead from a pair of AKG 240’s with the molded plug at each end chopped off, but you can use anything that works for you.

Also included in the assortment of headsets/headphones/earbuds/earpieces that I tested with my newly built crystal set and impedance matchbox, was a classic piezo earpiece, often known as a crystal earphone. The styling hasn’t changed over the years, and many of the cheaper crystal radios marketed to youngsters in the past, such as the rocket ship crystal radio, came with one of these. The metal diaphragm is connected to one of the leads, and used to be soldered to it. In recent years, many of the units sold had a foil diaphragm with the lead held to it partially by glue and partially by the pressure of the plastic case against the lead. As a result, there was a high rate of failure. Both the piezo earpieces of this type that I bought failed soon after I acquired them. However, the old-style with a soldered connection are still available. Mike’s Electronics sells them. There is also a seller on eBay called protechtrader who sells them (they recently increased the price significantly, from $10.70 to $14.99, both prices including shipping). The earpieces from the eBay seller have a characteristic black lead and plug. You can see the brass color of the diaphragm too, which I assume is also the case with the one from Mike’s –

These piezo earpieces have a 3.5mm mono plug. The 3.5mm jack on my impedance matchbox is wired to a stereo plug, so a 3.5mm stereo male to mono female adapter was pressed into service.

Also tested were an MDR-W14 yellow headset from an old Sports Sony Walkman cassette player/ AM/FM radio combination, and a pair of C Crane earbuds, both low impedance –

I forgot. I also tested my beloved AKG 240’s, of which I own 2 pairs. I used these for years, when working as a DJ/announcer, and voiceover guy in Los Angeles. I also use them with my Elecraft K2 on CW, as they are very comfortable to wear for long periods. I wasn’t expecting a lot from them for crystal set use though, as they are known for not being as sensitive as the more consumer type low impedance models. They are intended mainly to be driven by the headphone amplifiers present on mixing boards and similar professional equipment, which is capable of providing a greater drive level than the amplifiers in small consumer products such as Walkmans/radios/MP3 players etc –

OK, the big test. How do all these different headsets/headphones/earpieces/earbuds stack up against each other? Firstly, allow me to say that I haven’t yet performed extensive testing with many very weak stations, so take these preliminary results with a pinch of salt. Also, at this point, I still have some significant improvements to make to my antenna, by lengthening the outside portion of it from 45 feet to about 75 feet. This will very possibly yield a far more noticeable improvement than which headset I choose to use will.

That said, here are my initial impressions.

As expected, the AKG 240’s are the least sensitive, though not by as large a margin as I expected. They was, at a rough guess, about a 6 dB difference between them and most of the other headsets which, surprisingly, all seemed to be roughly the same in sensitivity. However, the AKG’s have a wonderfully flat audio response, and AM radio sounds great on them. They’re really good for listening to stations that are moderately strong, or greater.

The piezo earpiece was sensitive, though the sound was very restricted and tinny and, because it’s just one earpiece as opposed to two, doesn’t sound as loud as the other units as it’s only in one ear. Wiring two in series or parallel might help with volume, though the frequency response will still be very tinny. Plus, it kept falling out of my ear, which was annoying.

The Baldwin Type C, Western Electric 509-W, homemade sound-powered headset, Sony Walkman MDR-W14 headset, and C Crane earbuds all seemed to be quite sensitive, about as sensitive as each other, and about the same volume on weak to moderate strength signals. On strong signals, the Baldwins and sound-powered headset weren’t quite as loud as the others due, presumably, to the physical limitations imposed by the balanced armatures.

My initial impression was that, if anything, the Sony Walkman MDR-W14 headset was very slightly more sensitive than any of the others tested here. However, I’m not sure if that’s really the case, or has more to do with the fact that sound is transferred more efficiently to the ear because of the way that the headset earpieces sit in the ear canal. Being earbuds, the C Cranes also sit in the ear canal, but the Sony headset has a slight edge over them. However, I believe this is mainly because the Sony set has more response over the whole audible frequency range, while the C Crane earbuds purposely have their frequency response shaped to favor voice, with a sharp drop-off above about 7-8KHz.

The Baldwins and Western Electric 509-W’s both have somewhat restricted frequency response; the WE’s have slightly more bottom end.

Both the C Crane earbuds and Sony MDR-W14 headset sound a lot louder on strong signals, yet are also sensitive on weak signals. Of the two, the Sony has the widest frequency response. For most types of listening, my favorite headset, of all of them, is the Sony. My sense is that it would also be good for listening for weak DX stations. It’s possible that a headset with less bottom end might increase intelligibility and copy on the very weakest of stations. It’s for this reason that I’m thinking it’s worth keeping several headsets to hand, just as many serious listeners keep a selection of diode detectors on hand, for the most challenging of DX catches. Interestingly, my initial take on these headsets aligns with what Al Klase has said, namely that in his experience, modern earbuds, even the cheap ones, are about as sensitive as sound-powered headsets.

I find all of this a bit frustrating, because I went to quite a lot of expense, time, and trouble, only to discover that my favorite headset to listen to my new crystal set on, was a cheap Sony Walkman model that I already owned! I am not too surprised by this finding, as I had already read Al’s remarks, but needed to find out for myself. I could have saved myself quite a lot of money. On the other hand, I do enjoy owning a few pairs of vintage and antique headphones. The Baldwins and WE 509-W’s both occupy significant places in radio headset history, and my mint Baldies are museum grade. There’s a definite pride of ownership at play here.

Bear in mind that with vintage headsets, there can be variation in their performance, especially if they’ve been treated poorly throughout their long life. Sound-powered headsets often received rough treatment while in service. If you have a set that are in poor physical shape, they may have received a lot of knocks during their life that degraded their performance. In other words, my very brief test drive of these different sound-producing devices was preliminary at best. Nevertheless, things are looking good for the combination of matching transformer and a modern headset/earbuds using neodymium disc magnets and lightweight components.

It turns out, after all those vintage headsets, and a homebuilt one, that my favorite way to listen to this crackin’ little crystal set is on a pair of Sony Walkman MDR-W14 headphones. They have great fidelity, are the loudest on strong stations, and appear to be sensitive as well. I do wonder if a pair of US Navy Decktalkers (the famed RCA “Big Cans”) would beat them on very weak signals, but Al Klase appears to have a pair of those, and still said that modern earbuds are about as sensitive as anything else he has used. Who’d have thought! Crystal set enthusiasts – what are your experiences?

Growing up as the youngest of 4 boys, I was well positioned to receive all the hand-me-downs. Although that might sound as if I just ended up with second-rate stuff, that was not the case at all. I inherited a lot of great things from my older brothers. I couldn’t have cared less that they’d had them before me. The stash consisted of all sorts of board games, Dinky toys, and books, as well as something that would fuel my imagination for many years to come – a crystal set. My very own crystal set! Manufactured by Ivalek, this little beauty sat in it’s white plastic case by my bedside, delivering quality programming from the BBC 24/7 –

In truth, in the big world of crystal sets, this little mass-manufactured set wasn’t a very good performer at all. In fact, I’d go as far as to say it was pretty awful, having the following schematic, which I don’t think should ever be used for anything other than a teaching tool, but not a practical build –

The above is almost the simplest crystal set you can build. The Ivalek also has a switch that switches in extra inductance for the long wave band, but we’ll ignore that. The main problems with the above schematic are –

1. The antenna is not impedance-matched to the coil, so it will load it down, reducing Q and therefore, selectivity.
2. The diode is not impedance-matched to the tank, loading down the tank, and also affecting Q, and selectivity.
3. The headphones may not be matched to the diode affecting – yes, you guessed it, circuit Q and, therefore, selectivity.

A lot of simple “toy” crystal sets that were marketed to kids in the 50’s, 60’s, and 70’s employed this simple schematic and, as a result, we all got the idea that crystal sets were fun, but not very good, and not to be considered as a “serious” receiver for extended periods of listening. In my case, making things worse, was that I thought the telephone earpiece I was using was high impedance. 8 year-old me had no idea the impedance was actually closer to 150 ohms. The end result of all of this was that my crystal set had very broad tuning indeed. On the other hand, it was very loud, because we lived only a few miles from the BBC Droitwich transmitters. The longwave transmission, on 200KHz at the time, was 500KW in power, and covered most of the UK. The medium wave signals, though not quite as powerful, were certainly not QRP either.

The lackluster performance of my Ivalek crystal set didn’t put me off. I just thought it was the neatest thing that I could leave it on all the time, and would never have to change the batteries. Plus, I had plenty of chances to listen surreptitiously under the covers at night, when I was supposed to be sleeping!

I also had this magnificently compelling book –

The Boys Book of Crystal Sets contained construction details for 12 different sets of varying complexity and, presumably, performance. The young me spent hours and hours gazing at all the articles and schematics, and thinking how grand it would be if I had an air-spaced variable capacitor or two, some litz wire to wind a coil with, and an empty tobacco tin or chassis, to build my crystal receiver in. I’d be the king of the hill! But an 8 year-old boy living in the countryside in the early 1970’s didn’t have the means to procure such elite and specific luxuries, so I settled for reading each article a couple of hundred times, and day-dreaming.

About 10 years ago, I came across a web-site belonging to Jim Frederick W4LF. Jim is a big fan of Cushman scooters, which are uniquely American motor scooters that boast an earnest following of enthusiasts. The Cushman Scooter company was formed in Lincoln, Nebraska in 1903, and produced their last scooters in 1965. The majority of Jim’s site is given over to discussion of these machines. As you might guess, it wasn’t these that interested me, but a single page on Jim’s site that is dedicated to another of his interests – little radios and crystal sets. On this page Jim shows pictures, with brief descriptions, of the neat little radio receivers he has built over the years. They include regenerative receivers, crystal receivers, and some amplified crystal receivers. I loved not only the fact that he was using double-tuned circuits with, in many cases, adjustable capacitive coupling between the tank and the detector for greater control over the selectivity of the circuit. I also really appreciated the attention paid to the casing and overall appearance of the final product. These were some really appealing little receivers!

Recently, I had the honor of assembling a pre-production beta build of the 3rd generation of Jim W4LF’s Hobbydyne Crystal Set Receiver kit. I won’t go into details of the build here, but the kit should be available soon, at this site. If it is not yet active, save it in your bookmarks, and check back later. When the site is up and running, it will be the place to get more info on this kit.

I’m very happy with the end result, and I hope you’ll agree that it is a very good-looking little crystal set, with it’s African mahogany base, and garolite front panel –

From top left to top right, the knobs are – a rotary switch to add extra capacitance to the antenna tuning capacitor, for help in tuning different lengths of antenna, the variocoupler, which controls the coupling between the coils, and also the selectivity and, on the far right, the Hobbydyne selectivity enhancement control. The circuit of this set is heavily based on, and very similar to Jim’s original Hobbydyne circuit, which was featured in Dave Ingram’s column in the Nov 2005 issue of CQ Magazine.

The brass binding posts on the left are for antenna and ground connections, and those on the right are for the headphones.

The headphones are a set of Western Electric 509-W’s. They’re about 100 years old and work well, if you’re looking for a set that will plug directly into a crystal receiver without the need for impedance matching.

As soon as I had constructed Jim’s Hobbydyne kit, I started looking for two things in quick succession –

1. A good set of high impedance headphones and
2. An impedance matchbox, to experiment with different types of headsets/headphones.

While planning the construction of an impedance matchbox, to match a variety of crystal set detectors with a range of headphone impedances, I also kept an eye out on eBay for headphones, and the necessary parts to build a sound-powered headset. That is the subject of a whole new post, which will come after this one. The construction of just one crystal set, which you’d think would be a simple affair, while not quite turning into a rabbit hole, was certainly becoming quite involved!

I have a habit of over-estimating how involved I’m going to become in pursuits when still in the beginning stages. For instance, when digital photography was really starting to take hold, in the early-mid 2000’s, I decided to get back into photography, which had first grabbed my interests as a teenager. This time though, I was an adult with more disposable income. I went a bit hog-wild, buying a nice camera and a whole slew of lenses, accessories, and even some studio lighting equipment (not the cheap kind either). Although I had a lot of fun with all that gear, I realized over the next few years that I had overbought, and spent a fair bit of time selling all the photo accessories that were surplus to my needs, eventually distilling them down to only the essentials. All this is a prequel to me saying that if you want a crystal set which performs well but, being realistic, you’re not going to want to eke out the very last drop of high performance from it, you might be happy with connecting a set of high impedance headphones directly to the output of your new crystal set, and leaving it at that. I wanted to be able to test out a variety of different types of headset, headphones, and earbuds, so an option that offered a variety of output impedances was definitely on the cards for me. A selection of different input impedances would also allow experimentation with other crystal sets in the future.

Crystal set builders have used various methods to match the diode detectors on their receivers to headphones over the years, with a variety of transformers being used. Darryl Boyd’s very informative site at crystalradio.net has a section devoted to detector to headphone impedance matching, with a number of approaches detailed. One of the most recent solutions has been the very useful Bogen T725. More recently, an auto-transformer has come onto the market that is wound specifically for the needs of crystal set builders. The KPB-02 auto-transformer has both inputs and outputs (on the same terminals, being an auto-transformer) of 200KΩ, 100KΩ, 40KΩ, 20KΩ, 10KΩ, 5KΩ. 2.5KΩ, 1.5KΩ, 800Ω, 500Ω, 300Ω, 150Ω, 64Ω, 32Ω, 16Ω, 8Ω, and 4Ω. It was custom-made for our needs and, as such, has input and output impedances that satisfy any possible need a crystal set builder could conceivably want. Look for the KPB-02 on eBay, being sold by seller mkmak222. This auto-transformer formed the basis for my all-purpose crystal set impedance matchbox.

On the input of the box is a Benny, consisting of a 0.1µF capacitor and a 500K audio taper potentiometer. The Benny is named after Ben Tongue, who wrote a series of detailed technical articles on the subject of crystal sets, which, taken together, probably represent the most detailed analysis of this type of receiver architecture ever published. Ben is no longer with us, but you can find his articles here. You might want to save them all just in case one day, they are no longer hosted anywhere online. He talks about the Benny in article 01. Very briefly, it helps to reduce audio distortion on strong signals, by equalizing the DC and AC audio loads on the diode detector.

Both rotary switches are 12 position types but on the first one, on the input side, I only used 7 positions. Some switches have an adjustable stop, so that the switch will only rotate to the number of positions set by the user. You’re highly unlikely to encounter a detector with an impedance lower than 2.5K, so there is little point in going any lower. The last two positions, of 5K and 2.5K, are included in case a device such as a MOSFET is used as a detector; with diode detectors, the impedance is going to be somewhere in the 200K to 10K range.

There is more potential for variety when it comes to the output impedances, so you’ll probably find yourself using all of the positions of a 12 position switch. There are 20 position switches available, which would allow a builder to make all of the 17 impedance taps available. However, they are a bit pricey. On top of that, the one I found didn’t have a lug to prevent a loose switch from rotating, and I like to make use of those.

In my matchbox, all of the taps from 100K down to 1.5K were utilized. I found that range covered all of the vintage high impedance ‘phones I tried out, as well as the piezo earpieces in my collection. Kevin Smith, when building his impedance matchbox, divided his ranges a little differently from mine. His ranges were 100K down to 10K for magnetic and piezo ‘phones, 1.5K down to 300 for sound-powered headsets, and 32 to 8 for modern low impedance ‘phones and earbuds. The sound-powered headset that I put together turned out to have an AC impedance at 1KHz of about 3K, so the middle impedance range in the hundreds of ohms wasn’t needed. When describing his impedance matchbox, Kevin talks about the lack of a need for exact impedance matching, due to a listener’s inability to distinguish much of a difference in volume when the mismatch creates a volume difference of 3dB or less. He calls it “the 3dB slop”. If you build your own matchbox you will notice, when stepping through the impedance taps, how for any given set of ‘phones, there are several switch positions that give acceptable and almost equal volume. Looking at the schematic above you can see how, if you did happen to have a headset with an impedance of 800 ohms, for example, the closest tap available at the switch, is 1.5K, which would still give an acceptable match. Likewise, if your headset had an impedance of 500 ohms, the 300 ohm position would be adequate.

The KPB-02 auto-transformer doesn’t have a built-in mounting bracket. I cut a strip of flexible plastic from the top of a small storage container and used to it mount the transformer to the lid of an ABS plastic project box –

In the next photo, you can see an earlier version of the matchbox, which utilized chunkier, more modern binding posts. They are the type often marketed for speaker connections. I discovered that it is not as easy to connect the bare wire ends of the metal tips often used on vintage headphones to them, as it is with the more traditional style of brass binding post. I also ended up changing some of the impedance taps from the ones shown in this photo. There are vinyl bumper feet on two sides of the box, so it can be used in two different orientations –

Although a single pair of binding posts are used for the input, more output options are provided, in the form of a 1/4″ jack wired for mono, and a 3.5mm jack wired for stereo headphones, with the velements placed in parallel, in addition to the binding posts for the metal pins on vintage headphones as well as bare wire ends –

The internal wiring in the first version of this matchbox, before the wiring to the impedance taps was changed a little –

On realizing that traditional brass binding posts were going to work better in this application, I took out the speaker posts, made a trip to Ace Hardware, bought the appropriate brass hardware, and fitted the impedance matchbox with 2 handsome sets of brass binding posts. I also changed the wiring to some of the impedance taps on the transformer. Note the new labeling –

Unfortunately, swapping the location of the binding posts necessitated a lengthening of the wiring, and made it a bit more untidy. Nevertheless, I wanted brass binding posts on this matchbox, and am glad I added them –

The brass nuts on these binding posts are called brass flanged knurled-head thumb nuts on the McMaster Carr site, though I got mine from my local Ace. The 3.5mm jack is wired so as to place the 2 elements in parallel. I did this so that they would be fed in phase. If you use the ring and tip connections to feed them in series, you will end up feeding them 180° out of phase, though I’m not sure if that makes a noticeable difference in practice. The 1/4″ jack is wired to the sleeve and the tip only, for mono jacks. Another change I made, was to place the binding posts on the end of the enclosure that faces the operator. In the previous version, they were at the back, causing the metal tips of vintage headphones that were connected to the binding posts, to foul the two jacks. Small ergonomic details like that make quite a difference to the usability of a piece of gear –

One thing I noticed, stepping through the different output impedance positions for a given headset, was that although nearby switch positions to the optimum one produced almost exactly the same volume, the tonal quality changed. If the switch position is set higher than the headset impedance, higher audio frequencies are favored. As you rotate the switch through the optimum position to impedances that are lower than optimum, more bass response is favored. This could possibly be used with very weak signals, as a switch position that favors higher frequencies could, if copy was very marginal, perhaps improve intelligibility enough to be able to ID a station.

An impedance matchbox is a useful piece of gear in an experimental crystal set receiving station. It makes constructing subsequent crystal sets easier, as the builder doesn’t have to keep replicating the audio circuitry after the detector diode.

In the next post, I plan to show you the various vintage as well as modern headphones and headsets that I tested with the Hobbydyne Crystal Set and impedance matchbox combination.

In the meantime, more information on Crystal Sets, and DX’ing with them, can be found in the following, as well as the various individual blogs on the subject (I’ll let you find those!) –

The Crystal Radio DX Group on Facebook – a group founded and run by Steve VE7SL. Intended specifically for discussion of crystal set DX listening events, as well as circuits and techniques specific to DX’ing with crystal sets.

The Crystal Set Radio Group on Facebook – a larger group, for general discussion of crystal sets. If you are a newcomer to the world of crystal sets, this would be a better group for you than the previous one.

The New Radio Board – intended as a new version of the now defunct (and much missed) Radio Board Forum, this board contains discussions of construction of several different types of receivers, including solid state radios, tube radios, and crystal sets. Links to the different topics can be accessed from the lists of hashtags.

A good introduction to the subject of crystal sets can be found in this engaging talk given by Al Klase to the New Jersey Amateur Radio in 2022, titled ,“Understanding and Building Crystal Radio Sets”. The graphics to go along with the talk are here.