http://www.colomar.com/Shavano/lpad.htmlwhy wasn't the the 8 ohm side of the Lpad used to replace the preceeding R1 resistor like in conventional Lpad use?
it isn't often that the leads are brought out to where this would be possible, if I
understand the question. -grumpy
I believe we are talking about the same thing. I guess when Altec originaly built the crossovers the 2 gang potentiometers where not available yet or too expensive.
Jorg, ( I leave in minutes but ,,, )
- Per your overlay of the ( 8 ohm ) LPad on Altecs fixed Tpad .
- If one uses a 16 ohm variable Lpad at that postion & then makes R1 , 4 to 8 ohms ( & then followed by any 8 ohm driver ) / one creates a variable 16 ohm ( more or less ) T-Pad that also has very usable isolation from the HF contour circuit ( some isolation is needed ).
- I use this configuration on the bench exclusively when mocking up circuits for 8 ohm drivers .
- If you used this configuration ( instead of your present 1201-8a padding arrangement ) / you would need to recalculate ( for a 16 ohm load ) the CLC elements preceeding the new padding topography . Alternately, just strap a 20W, 16 ohm conjugate resistor in front of the new padding ( & keep the present CLC elements ). I recommend you try it both ways .
I'll try it once I get an RTA set-up of my own so I can see what I'm doing. I'm curious though. Why do you recommend a 16 ohm lpad? The R1 fixed resistor in the Tpad is/was a 4 ohm and the variable R2 resistor in the M19 is a 0-32 ohm. An 8 ohm lpad seems to be a closer match to the Altec design, just the R1 resistor being variable as well.
I ran sims, and that kinda works. The shapes of the curves change somewhat as the values are adjusted, but not fatally.
With an 8-Ohm L-pad, you begin at 3 dB of compensation, and the midpoint is 6 dB, so the useful range is in the lowest settings.
With a 16-Ohm L-pad, the starting point is 6 dB.
Changing the fixed R to 8 Ohms with that gets 10 dB compensation at midpoint, and 15 dB max (all at 1.2 kHz,) which I consider a useful range.
I'd have to build it up and measure the reflected impedances; too tough for me and the calculator to figure.
Aside from its inherent symmetry, (you appreciate that the left side of the "T", your connection to terminal 3, varies from 8 (or 16) Ohms to zero Ohms (CW) depending upon the control setting,) I consider the original circuit to have the advantage of but one wiper contact when made adjustable, with the parallel leg of an L-pad easily serving as the adjustable R.
I am clueless as to how the reflected impedance of that varies with the setting, as well, and would have to measure. Note that Altec typically used this filter AFTER the main crossover attenuation L-pad, i.e., between that and the driver, providing an element of isolation as Earl recommends above.
And finally, this compensation filter should not be conceived or used as attenuation, which must be otherwise provided for balancing. Presumably, the amount and character of compensation should not vary with level, and once correctly established for a particular driver/horn combination, attenuation should be idependently adjusted for the desired balance with the woofer.
The Model 19 crossover does not operate this way, and worse, attenuation and compensation adjustments are interdependent. Hardly anybody gets how to use it, and it was widely ridiculed by pros who didn't understand it. Markwart apparently abandons M19's attenuation adjustment and uses compensation for level balancing.
For the truly gluttonous, here again is my M19 XO analysis:
With respect to Skywave's project, I expect this is all academic; he's likely out of headroom with the compensation already provided in N800-F....
Just finished cloning n800f with the parts I already had in my hands. Turned on the music. Sounds good. Turned on my laptop and opened up the Lasing Heritage forum. What do I find? More options. I noticed that N800f has attenuation as you said they were 25ohm pot and 20ohm parallel to the LF. My question is does T filter goes before or after the attenuation or does it matter? I am confused.
The 8-Ohm version of that crossver, N800-8K, uses an L-pad for attenuation in lieu of the pot and fixed resistor. Use a 16-Ohm L-pad with N800-F as alternate for a 16-Ohm HF driver, or an 8-Ohm one with an 8-Ohm HF driver....
For Zobel for 16ohm LF, am I doing the same thing as Skywave?
Skywave's not doing Zobel yet, I don't think.
But for dual 16-Ohm 414s in parallel, what Earl said.
18 uF and 7.5 Ohms for starters.
8 Ohms's fine, too....
Ok. I will wait then.
I know you guys are having fun here and I began reading this thread because a buddy has 9844s and we just fixed them.
He was not happy with the HF and felt they needed an additional tweeter!
I found a bunch of info in the "Look at my new babies" thread and let him sit here and read.
Neither of us are up to the level of building XOs and testing the results.
He took a much easier route and bought a set of BMS 4550s.
Even I can hear a remarkable difference in them now and he is completely satisfied with his new 9844s now. So much so that he has me veneering the cabinets for living room duty as his wife wasn't going for the battleship grey.
I would love to see, while you are at it, what the equipment will show if you take them back to the two way XO and just add the BMS's.
Hi, Gary, and thank you for coming forward with your experience.
I believe we've pretty well established that there are better options than slapping a tweeter on vintage Altecs. Many enthusiasts are skeptical regarding those more radical approaches and instead remain committed to getting the best they can out of what they have. For others, the mere consideration of alternatives is insulting: they are perfectly content to enjoy Altecs for what they are; there's nothing to improve. I just got reamed again on another site for suggesting otherwise.
There are certainly performance enhancements to be achieved within this context, and Altec themselves showed how to do it with their own subsequent product improvements. Beyond this lies even more potential for those desiring to continue the process and explore outside the Altec boundaries. It's great good fun, learning all the way, and there is always further satisfaction in knowing that others are benefiting from the experience as well....
I'm transfixed on this thread.
Still waiting for parts. Hope to have them in a few more days.
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