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Bill F. 31st August 2004 03:09 PM

Tube possibilities for 2.9k-ohm speaker?
 
I will likely be buying a couple cases of inexpensive industrial 3" wideband speaker drivers to tinker with. The interesting thing about these speakers is their dcr of 45 ohms . . . which got me thinking . . .

If I put together an 8-foot biplolar line array of 64 drivers connected in series, I'd have a loudspeaker with a dcr of about 2.88 kohms. This would seem to open up very interesting amplification possibilities.

I am not an amplifier expert, so please help me brainstorm here. With a load like this, what type of tube-based amplification do you think would be cool? Lower turns-ratio OPTs? OTLs with less compromise, fewer tubes, and more interesting tube types, etc?

I'd appreciate your ideas.

Fuling 31st August 2004 06:11 PM

What about a parafeed-like circuit but without output transformer?
YouŽd only need a plate choke and a high quality capacitor (well, double for stereo of course)

2,88k is a fine load for several good output tubes like 2A3, 6B4G, 300B, KT88, EL34, you name it.

Sch3mat1c 31st August 2004 10:31 PM

Mind you actual impedance is a bit more than that, how much depends on frequency etc... I'd guess 64 ohm each? Not like it's predictable enough to matter.

Hmm... 3"... maybe 1W? That's a 64W array, sweep tubes ran from maybe +500V (say 100Vs, for 6CD6/6LQ6/...) would do that easily. :)

Er, well no, not class A, not alone. You'll need like an 845/211 for that kind of power output. 620V peak required, so maybe +800 to +1000V supply... run class A2 in the former case... maybe 12dB NFB... should be good... :D

Tim

rcavictim 1st September 2004 12:27 AM

Be sure to install a bleeder resistor across the speaker output terminals in the amp to help charge the output coupling cap. No room here either for second class speaker wires. Check electrical code, you might need to bend conduit for this installation. ;)

Bill F. 1st September 2004 01:29 PM

Thanks for your input, fellas.

It's kind of a crazy idea, and I may or may not do it, but I was just wondering if anyone thought it would be worth doing.

There are obviously some important considerations involved with such high-voltage direct drive. Speaker wire has already been mentioned. Then there's the small matter of keeping fingers away from the driver frames, especially the ones early in the series. I imagine there's a possibility of voltage arcing from voice coils, or of a misaligned or damaged coil touching a top plate--could give you quite a zap. :hot:

I'll be receiving the drivers soon: 2 cases of 120ea for a total of $80 shipped. I couldn't pass them up.

rcavictim 1st September 2004 04:53 PM

Quote:

Originally posted by Bill F.
Thanks for your input, fellas.

It's kind of a crazy idea, and I may or may not do it, but I was just wondering if anyone thought it would be worth doing.

There are obviously some important considerations involved with such high-voltage direct drive. Speaker wire has already been mentioned. Then there's the small matter of keeping fingers away from the driver frames, especially the ones early in the series. I imagine there's a possibility of voltage arcing from voice coils, or of a misaligned or damaged coil touching a top plate--could give you quite a zap. :hot:

I'll be receiving the drivers soon: 2 cases of 120ea for a total of $80 shipped. I couldn't pass them up.



Bill if the driver frames are on an insulating material you can solve the issue of voltage buildup between driver frames and voice coils with a resistor divider tapping all the driver frames along a long voltage divider. Connect the resistor junction to the voice coil at every driver. 100K 1W carbon resistors ought to work fine. The value is not at all that critical. I just pulled this number out of my head. 250K, 500K? Probably OK. Since there will only be the AC music signal impressed on the array, parasitic capacitance will play a factor, sort of like tube miller capacitance at the highest end of the range and may cause uneven distribution of HF drive or problems with time lag in the array phase (distribution). This is probably the factor that would lean towards using as low a divider resistor value as practical.

Regarding the purchase price of those drivers....you dog! :smash:


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