Here’s more chitchat about “audio,” because apparently 2025 is the year I just can’t shut up.
Over the winter — a pleasant time to be indoors with a soldering iron — I completed another amplifier that I have been thinking about for a long time. The first thing I ever built was a little amplifier based on the LM1875 chip from National Semiconductor/Texas Instruments. This is a tiny little thing, the size of a dime, which is a fully functioning 20 watt amplifier. Basically, you just attach a power supply, and feed it a signal, and it works. In bulk quantities, it costs $1.58 each. In the 1980s and 1990s, chips like these (including the big brother LM3875 and its successor the LM3886) went into millions of consumer-fi products, and made millions of people happy, without anyone ever being very impressed about their sound quality. In the early 2000s, amplifiers based on chips like these became very popular among DIYers, both because of their simplicity, but also because it was revealed, especially by a Japanese company called 47 Labs, that they could actually sound quite good — better than most commercial gear at any price, at that time.
That first little amplifier (it was one channel) was driven by +/- 12 volts from a stack of non-rechargeable 6v lantern batteries, making about four watts of output. Those batteries actually lasted about six months, as I recall. It was good! I listened to it on a variety of open baffle speakers. The first was a Fostex FE103, in mono, mounted on a 6″ wide board and with the addition of “wings” in the form of cardboard from boxes taped on to expand the baffle size. This was quite promising, and started a long series of open baffle/fullrange driver speakers. My next step was an Altec 755C — a legendary driver — on a big plywood baffle, which was also good. I also had a Diatone P610, another famous driver, although that never got used before I sold it off. The Altec was later supplemented by a 15″ Supravox woofer on the bass and a Fostex bullet tweeter. Then, I stopped using the 755C altogether, and used just the woofer with the Fostex tweeter, crossed around 6000hz. This was quite nice. Anyway, I had fun with it.
Here’s another fellow who got himself some 755Cs — a lot more expensive these days! I think I paid about $125 for one driver, and $70 for some plywood to put it in.
Naturally, I got the ambition to build things out properly, with rechargeable batteries, +/- 24V (for 25 watts), and a variety of other tweaks that were fashionable at the time. This was a mess. It had some nasty high-frequency ugliness, probably HF oscillation. Even this failure did sound promising, however, especially on fullrange drivers where the HF was naturally rolled off. I listened to it at a friend’s house on his Altec 755Cs (again) in sealed boxes, and it was very impressive.
Now, I want to build it out again, and try to realize the full potential of this silly little thing. It might take me several tries. This is not supposed to be super-very-good. It is more of a benchmark: something to compare other amps to, and an experiment in how far you can reasonably take this cheap little chip. Also, I want to use it to multi-amp the Bill Woods-designed Yorkville U215s I have here. That means using it on the 1″ compression driver of those speakers, direct-wired — exactly the kind of difficult high-frequency task that the earlier version failed at so badly, and which most transistor amps fail at badly, in the process giving horns a bad name since 1970.
I built four channels into a single chassis, with four 12v SLA batteries in a separate box, plus another box for a charger. The brass plate is used as a heatsink. At this time, I will only say that it works.
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“Audio,” or the reproduction of music in the home as we know it, mostly started with the creation of the Long Playing record in 1948. Before then, recorded music was on 78 rpm shellacs, which were rather primitive, sounded bad, tended to crack, were expensive, and held only about three minutes per side. The new LPs were combined with a new round of both speaker and electronics design, which advanced dramatically during WW2, and also due to the expansion of cinemas in the 1940s.
In many ways, audio today is not much better than what was around in 1958, as anyone who has listened to a system of that era — let’s say, the JBL Hartsfield — might well understand. The Hartsfield is 46″ high, and people are still making reproductions of them today. The Klipschorn, of 1948, is still in production from Klipsch, and you can buy one (usually one in the past, they go in the corner) today from Crutchfield.
In movie theaters, we had the classic Altec A5, and in big theaters, the enormous Altec A2.
These speakers were awesome. If you saw Lawrence of Arabia in 70mm, in a big theater during the 1960s, you were probably listening to Altec A2s.
But, they didn’t have much bass, and what they did have was basically a wooly mess. Good for music and dialogue, bad for sound effects. It was called the “Voice of the Theater,” not the “Explosions of the Theater.” During the 1970s, theaters moved to more bass-heavy designs to reproduce the explosions in Star Wars. Dolby sound! All those Altec boxes ended up on the sidewalk, and were sold to audiophiles for peanuts. Not many A2s made it into people’s living rooms, but many A5s did. I used to own one, which I listened to in mono. That single A5 speaker cost me $500. I picked it up from a woodshop outside of Boston where they used to listen to it while working.
There are still people who appreciate these 1950s-era systems, such as this guy:
But, what was “high end audio” before 1950?
78 shellacs were pretty meager. Serious music lovers listened to live radio.
Microphones and electronics were quite good in the 1930s. Recording and playback stunk. Nobody was broadcasting 78 shellacs on the radio. But, you could broadcast a live event over radio, direct from the microphone, and it would be as good as the best recordings today.
Big cities had live music theaters that would broadcast live music. The most famous was Radio City Music Hall, at Rockefeller Center (headquarters of NBC, completed 1933-1939) in New York.
Serious audio fans had serious radios. Among the most serious was the Zenith Stratosphere. It was 50 inches high.
It cost $750 in 1935, which was about $40,000 in today’s money.
https://www.youtube.com/watch?v=igpyP5dbNDw
The radio portion is on top, the amplifier is on the bottom, and in the middle was the speaker, which was a combination of two cones (I think 12″ wideband drivers, common in those days), field coil of course, and a horn tweeter.
And here, audio fans, is the schematic of the power amplifier:
It looks to me like a pair of 76s, parallel single ended, into an interstage transformer; a pair of 42 pentodes, triode-wired, in push-pull again with an interstage transformer, and for power, eight 45 triodes in push-pull, and no feedback of course.
Yes!
Fed by a live broadcast from Radio City Music Hall, I can imagine that it sounded very, very good. Today, you wouldn’t use eight 45s, but rather a pair of 300Bs, which would produce about the same amount of power. But, the 300B wasn’t developed until 1938, and was intended for commercial use, mostly in movie theaters.
Let’s continue our tour of audiophilia in the pre-1950 era with this:
It is an RCA Photophone theater system from 1935. It won an Academy Award. Yes, the speaker won an Academy Award. Now it is in someone’s living room, with his dog and wife.
The very first speaker used in theaters, beginning in 1929, was the Western Electric 555 driver and 15A horn. Here is a set of eight of them. They are about five feet wide.
These are still loved by audiophiles today. People are still making them new.
I also used to listen to speakers somewhat like these. I used Western 720A drivers, which were the 1940s permanent-magnet version of the original 555. My horns were from 1936, and went down to about 120hz, while the 15A horns went to about 80hz. Not so much different. They were very nice.
The 720A is famous because it was part of the Western Electric Beachmaster System, literally a speaker for use during amphibious invasion.
These Beachmaster systems actually saw good use after the war, in places like football stadiums.
Original Western Electric big horns like the 15A, or even reproductions, cost a ton. But, both Western Electric and RCA made big straight horns, mostly for public address use. You might find one somewhere. They can be very good. The Western 720A drivers themselves have gone way up in price. But, RCA also made a similar driver in the 1940s and early 1950s, which are hardly known today but are probably just as good.
Today, there are still great deals on theater gear, actually the stuff from the 1970s and 1980s that replaced the Altec gear:
Getting four JBL (=quality) 15″ woofers in a box for $1200 is not bad at all. Plus some nice horns. If you can find one with the famous 2360 “butt cheeks” horn, even better. But, alas, prices have been rising for those too.
If you are wondering why anyone would make something so ridiculous, this is the reason:
Round horns are very “beamy.” The high frequencies come out like a firehose in a very narrow beam. This can be OK — although a little weird — for home use, where there is typically one main listener. But, they were made for commercial use, in theaters. You had to have an even dispersion of high frequencies across the entire audience. This gave rise to the “multicell” approach, or a lot of smaller horns, in the 1930s and 1940s. In the 1950s, manufacturers experimented with “radial” horns, which had a wide horizontal dispersion but narrow in the vertical axis.
By the 1980s, manufacturers like JBL were making diffraction horns, which used a narrow slot to spread a wave about 90 degrees in the horizontal direction, and forty degrees in the vertical, on the 2360A horn:
Today, after a hundred years of horn technology, one of the most common formats for “constant directivity” (not-beamy) horns in prosound gear is the simple straight-sided conical horn. This is often combined with a little transition section in the throat to produce the “quadratic throat waveguide.” But, some of the most sophisticated horn designers don’t bother with this. One opinion seems to be that the additional length created by the transition area (it roughly triples the “narrow” section before the conical section starts) re-introduces all the problems with beaminess and curved sides, at higher frequencies, from the original exponential or tractrix designs. The only reason that there needs to be a transition at all is because driver designs include a little throat section between the output and the phase plug, of about 1″ or the thickness of the magnet. This little throat section is often of an exponential curve, which then inspires a transition to the conical curve. But, you can either eliminate this throat area entirely, by moving the phase plug to the output, or also make it a conical expansion, as was the case for some drivers from ElectroVoice.
In this driver, the little integrated throat section seems to be completely superfluous, a holdover from a time when magnets were bigger. Here, a compact Neodymium magnet is used.
That is probably more detail than most people would like.
I would look in Facebook Marketplace and Craigslist for local deals on JBL or Klipsch gear.
Continuing our tour of great gear from Klipsch, here is the KLF series of the late 1990s and 2000s. The KLF20 and KLF30 are the ones you want. The larger KLF30 was the best of the series, but tends to be considerably more expensive on the used market, around $1200. The KLF20 was almost as good — the horns are basically the same, just the woofers are a little smaller — and can be had for around $800. Here’s one guy who is selling both:
Hook them up to something like this Chinese 300B amplifier. It would be a good way to start.
Lower on the price scale, we have this:
This is definitely a Cheapy-McCheapo amplifier, but it still might be pretty good, like a 1957 Volkswagen Beetle. Go poking around to find some of those JBL systems with the 2360 horns, and you would have something pretty nice that even a high schooler could afford.
