Solid State Thoughts/Suggestions for High Efficiency OBs?


Junior Member
Hi all ... regular reader, but I haven't posted a lot. Looking for some thoughts on tube vs. SS amplification with my folded OBs...

I am running a pair of hand built by me "folded" (not sure that's the right term) OBs, which are a big-ass cabinet with a partially open back, using a 15" P. Audio BM15CXA woofer (or an Eminence, which I have as well). The top end is a SEOS horn with a 1" Radian 475Pb driver. The woofer is 101dB and the horn driver is 111dB, so pretty damn sensitive. The crossover is a simple Linkwitz-Riley first order design, with a capacitor, an inductor and 2 resistors. I pad the horn down about 14dB, which seems to work for me. It's a simple design without a lot of "stuff" in the signal path. Very detailed, and a lot more bass than other, more traditional, OB designs I've personally built and tried (Fruglehorns, Decware Zen Open Baffles).

I'm currently powering them with a pair of Welborne 45 Star Chiefs, which are my first foray into the 45 world (but ceratinly not the tube world). Preamp is a tastefully modded and hand built TS Grounded Grid. I'm thinking about pushing more power to the OBs, and possibly moving to a SS amp. At one time I was running a First Watt J2 with my Cornwalls, and it was a fantastic pairing. Sadly, I no longer own the J2 to try with the OBs. Anyway, since I have a lot of tubes in the front end of the chain (the GG pre and my PH16 tube phono pre), I'm intrigued by trying a SS amp, and am looking for thoughts and suggestions. Or do I just stick with what I have (which sounds great btw). My curiousity is being driven by what more power may do for me, even though the speakers are plenty effiecient enough for the 1.5W Welbornes.

Any and all discussion and thoughts is welcome ... looking to learn and absorb before I pull the trigger and make some dumb decisions! This is a typical case of "can't leave well enough alone"...



Senior Nobody
Great looking speakers.
Class D amps have come a long way in the last few years.
You could also try biamp with SS on the woofer.


Junior Member
Nelson Pass anything.
That's where my head is at. THe J2 I owned was a keeper, except I didn't keep it. May be time to look for another, although I'm not sure which model is the go to for OB speakers. Time to do some reading up ...
I agree, to date, to my knowledge, nothing much beats the best DHT's, or PP Pentodes with top notch iron. Tubes handle impedance swings more gracefully and voltage amplification (tube) vs current (solid state) has advantages. Class D amps have been a godsend for some jobs, and anything Nelson Pass, Class A, SE solid state. darn tooting.

Solid state has that wonderful LF control.

** However... There's serious unexplored country in solid state. By the mid 1950s large power transistors were common place, but never saw much action in HiFi (Ultralinear watt wars had begun, and there was little interest in the petty wattage SE solid state produced at the time). The early half dollar sized Germanium transistors can be easily used Class A, SE -- nice wattage (3 watts give or take) configuration and as experiments we did years ago yielded -- they can sound amazing. Real Germanium stuff went out in audio by the early 1960s... that's why it's so seldom seen. The earliest SE Germanium amps were hybrid, tube front end, with a single, huge transistor output, running pure Class A. The recording industry (Neve, EMI, etc) has paid top dollar for early Ge SS stuff for years -- germanium transistors have a warmer and sometimes more detailed sound than the silicon counterpart -- there's a clue there.

Despite the cool surplus parts available.... as well as many adaptable, HiFi capable circuits, no low volume manufacturer to my knowledge ever tried the "2N" early 2 or 3 digit power outputs as intended (many mfr's of the 50s, including Delco, Bendix, RCA -- even Western Electric (amazing quality) and Tung Sol (their final days) !

Even the late 1960s Nobsound mentioned above, is "too young" in design to make use of the Class A germanium's. The first "official" production solid state HiFi amp was the late 50s/early 60s Transi-Tronics S-15 TEC amp--infamously poor reliability --- I think it made daring use of the then "new" Silicon transistors (but may have been Germanium) that took more heat and pumped out 20Wpc of that the infamous SS power (it's part Ge, part Si in circuitry) ! The TEC's size (10" long) and weight shocked the Audio scene back then... sort of like the Sputnik in tube-land!

I wanna say Walt may have tried to talk Nelson into exploring Germanium power outputs in SE .... but there is natural hesitation, they don't make them any longer, so you have to test the old ones like tubes --- and supply concerns for any mfr...
Much like DHT tubes, these early, these huge SS output's with their massive glob of germanium (or GE Alloy in the case of WECo), may fail to impress on paper vs newer devices.... but can have a very organic sound. Same could be said about DHT's and 300B's. One day, one day --- somebody will do this!
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