Sean Casper's Single Ended AD1

Now here is an amplifier that makes music sound “real” in my room! Introducing the single ended AD1 by Sean Casper. This amplifier is certainly a work of art and passion.

The AD1 is a super archaic triode from Germany, circa 1936. Called the “Queen of the European Triodes”, the AD1 seems to be a sort of 4 volt version of the 2A3. Made by Telefunken, the AD1 represents the last and perhaps greatest of the European triodes. Coming to market literally right before WW2, the tube was a German masterpiece and was intended for use in large radios and theater amplifiers. Think Klangfilm here, folks. Unfortunately, it seems that the War cut short production on the luxurious AD1, and it was not made in large quantities. After the War, the focus immediately went to smaller, cheaper and more powerful pentode tubes, and the AD1 was just about extinct already. A few industrial variations were made with higher plate tolerances (350v instead of 250v); the 4683 pictured here for example, as well as the beautiful EBiii.

Sean Casper’s AD1 creation is one of the best-built boutique amplifiers that I have owned; a nearly perfect build with quality parts and real attention to detail everywhere you look. Its well in line with the build quality of the Jeff Korneff and Jeremy Fix amps that I have owned in the past, very robustly built, but Sean's amp is more refined in feel. At 4 or so watts per channel it is powerful enough to pair with a pretty wide variety of speakers.

The sound of this amp is something really special. The AD1 seems to be capable of opening yet another screen door on the sound stage. The AD1 is driven here by the very interesting C3M tube, a lesser known industrial tube made exclusively for the German Post (more info on that driver here). The result is that on my Zenith baffles, instruments and voices in material that I am very familiar with sound subtly more open, and with more precise placement than ever before. Tonality is also excellent. The twang of a banjo or the rattle of maracas is somewhat more realistic sounding with this amplifier. As I write this I’m listening to Johnny Cash speak into the mic after a short version of “You Are my Sunshine”, and its like he is standing just behind me in the studio; there is an easy to discern sense of depth, the laughter from the others in the studio come from all directions but all are distinct voices. That's something this amplifier really does well. If there are multiple singers in a recording, its quite easy to pick them apart from eachother, something not all amps do well. It’s very pleasing.

I hope to one day be able to try the original Telefunken AD1 tube, as well as Emission Lab’s recent re-issue, but finances will need to catch up to desire. At any rate, this amplifier is flat out superb in every way. Both the AD1 and Sean Casper deserve my highest recommendations.

The rectifier used here is the AZ4. Of the same era as the AD1, the AZ4 is readily avaiable in NOS quality as well as a modern mesh plate version by Emission Labs. I'd love to own all EML tubes for this one!

The C3M tube was originally housed in a black metal sleeve, which can be easily removed to reveal a beautiful mesh plate pentode that, to me, looks very much like a coin base 13EM7 compactron tube. The C3M was originally made for use in repeaters for the trans-oceanic phone cables. As such, the tube is the result of a money-no-object investment, and has superb linearity and reliability, costing hundreds of dollars each when they were first made in the 1960s.

Above is an example of an early AD1 tube by Valvo, probably made to the Telefunken patent specs. Sadly, this one is a dud, but if it was in NOS condition it would be worth quite a lot of money. The AD1 is a very rare tube and prices certainly reflect that.