OZ
(O for open baffle, Z for high impedance)
Strolling around the flea market at ATF2002 I noticed some scruffy looking speakers. They turned out to be Philips wide-range speakers, type 9710. There was a pair of 800 ohm version amongst them, hmmm.... This might be a nice pair of speakers to make a project out of. A voice coil of 800 ohms is not an every day find. OTL comes to mind.
I've heard the 8 ohm version of the 9710 in an open baffle once and liked them a lot. Okay, bass reproduction is barely existent below 70Hz and the HF will not make your dog quiver, but it sounds really good. Efficiency is pretty good on these units, Philips published them at 97dB , so it will not require a lot of power to drive (great!!!). I am still thinking of what kind of baffle to make, as I don't want them to fill the room like my other system.
As for powering these speakers: due to the high impedance of the speakers, the OTL aproach seems very warranted here. More so than with 8 ohm speakers I think. I could use a single, low Rp, tube instead of paralleling 4 or more as with conventional OTL amps using the 6AS7 or 6C33. Candidates for my speakers are the 6C41 (half a 6C33), 6C19 (Rp=400) or even a single 6AS7/6080 to drive both speakers :) The biggest question is configuration, should I use an output cap, etc. More on that later.
T/S for the 9710M (8 ohm)
| Diameter | 203mm |
| Efficiency | 97dB SPL @ 1m, 2.83V, 8 ohms (1W) |
| Re | 5.2R |
| Z | 7R |
| Fs | 46Hz |
| Qms | 2.28 |
| Qts | 0.63 |
| Vas | 100 liters |
| Pe | 15W |
| Xmax | ± 0.7mm |
Frequency response of the 9710
