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You snooze, you lose, suckers!Since nobody hopped on my deal for NOS TL-404s I decided to put them to use. I only wanted to sell because I hate to see parts sitting unused for more than one decade. 9 years, OK, that's reasonable, but beyond that it is criminal.
So. I'm searching through my tweeter collection trying to find one that mates well with my WE728Bs. Liking the TOA HT-371 but it is way too sensitive without attenuation. A simple $2 resistor attenuator can fix that, but wait...I'm a super-pimped out dude. How about a set of $500 autoformers instead of 4 resistors? Yeah, that's more like it. Too bad they are not cobalts, but oh well...
I hooked the tweet to the 4 ohm tap and the input to the 500 ohm tap. Sweeping the impedance of the combo with my DATS speaker tester, I get the impedance at intended crossover point which points to use of a 0.25uF cap for nominal 8kc crossover.
Having the cap in front of the autoformer save the 3W TL-404 from the full mighty onslaught of power from my 300B amp. It only gets HF power, which will be quite low/ I terminated the input with 10ohms of NI WW resistor so my amp sees a stable load.
Doing these experiments on a jig I worked up so I could swap in different parts. Had to double stick tape down the TL404 since it is a pre-bracket model.
Sounds really really good. Better than a $2 resistor network with a 2uF cap? I don't know and I am afraid to find out.
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Free your mind and your ass will follow -- Parliament/Funkadelic
Edits: 10/01/15Follow Ups:
Interesting to see that the reflected impedance of the combo is around 100 ohms.
Yes, actually a little bit less than 100 ohms but I went with standard cap values that were close at hand. 0.18uF might be better but I need to measure the acoustic response of the woofer and the tweeter with a mic to dial things in. It is reasonably close to my preferred balance already though. Cap type/flavor might matter as much as precise cap value here.
There is a strong drop in the impedance and phase curves vs. frequency from 200 ohms at 4-5k.. I think it wise to measure actual impedance at the input terminals. Seems to be a complex load.
This crossover sounds really fine, although I wouldn't run out and buy TL404s to do it...but maybe it is much better than resistors and a cap?
I am building another version for an Altec 802 using an Atlas Sound HT371 transformer wired 12.5:1 with a bypassed resistive attenuator on the secondary for HF EQ. That one came out to 100 ohms also. I hope to be listening to those tonight.
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Free your mind and your ass will follow -- Parliament/Funkadelic
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