JBL 2445J HF boost with autoformer in reverse mode - Page 2 - diyAudio
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Old 12th October 2013, 09:11 PM   #11
elac310 is offline elac310  France
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Quote:
Originally Posted by moray james View Post
the auto transformer in the CF3&4 are used to stet up the signal to boos HF response. More than that I cannot say. Best regards Moray James.

I'm still unsure we're talking about the same thing.
auto-transformers usually allow to vary the level starting from 0dB and adjusting to minus 2dB, minus 4dB etc. If switching back from minus 4dB to minus 2 dB means increasing, then OK.

Again, I have inverted my autotransformers so that the nominal 0dB is increased by +1,5 dB, +3dB etc. up to +7,5dB (multiple taps allow up to +15dB but it becomes dangerous for the amp), i.e I am boosting the original signal to increase the response in the UHF, where most 2" drivers drop off normally.
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Old 1st June 2014, 06:49 AM   #12
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Quote:
Originally Posted by elac310 View Post
I'm still unsure we're talking about the same thing.
auto-transformers usually allow to vary the level starting from 0dB and adjusting to minus 2dB, minus 4dB etc. If switching back from minus 4dB to minus 2 dB means increasing, then OK.

Again, I have inverted my autotransformers so that the nominal 0dB is increased by +1,5 dB, +3dB etc. up to +7,5dB (multiple taps allow up to +15dB but it becomes dangerous for the amp), i.e I am boosting the original signal to increase the response in the UHF, where most 2" drivers drop off normally.
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Old 1st June 2014, 11:34 AM   #13
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Using an autoformer for step up is fine, as long as you can live with the step down in impeadance. I think the impedance halves for a +6 boost. The real question is whether your top octave efficiency is equal or greater than your woofer efficiency. If it is then you shouldn't need the autoformer.

In the design of the 4430 and 4435 I found that a first order tilt for response correction wasn't really enough and went to mildly resonant topologies.

As to liking the top octave response of an extended horn vs. moving to a 3way, opinions will vary. I know that a crossover between horn and tweeter horn can only be smooth at a very narrow vertical angle, so it is not without its own problems.

David S
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Old 1st June 2014, 04:11 PM   #14
elac310 is offline elac310  France
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Quote:
Originally Posted by speaker dave View Post
Using an autoformer for step up is fine, as long as you can live with the step down in impeadance. I think the impedance halves for a +6 boost. The real question is whether your top octave efficiency is equal or greater than your woofer efficiency. If it is then you shouldn't need the autoformer.

In the design of the 4430 and 4435 I found that a first order tilt for response correction wasn't really enough and went to mildly resonant topologies.

(...)
David S
In a system where the basic sensitivity is given by the woofers at an impedance of 4 ohm (2 8ohmers in parallel), the 16 ohm 2445J has just enough sensitivity.

Indeed, impedance as seen by the amp is normally droping with an auformer in reverse mode, but the 2445J is interesting insofar as due to its relatively high impedance, there is some safety margin. The 2445 with the original Ti diaph (Ti, diamond surround, non ribbed) goes far up but at a much lower level than the reference level of the system. Hence the autoformer in reverse mode. Note that I've tried other diaphs (Ti ribbed, Aluminium etc) but the original one is the one I prefer in that application and with the horns I have at hand. The aquaplas (unribbed, 16ohm) version was ordered and will arrive soonish. I will test it as well.

In the meantime, I'm experimenting with McCauley 9620. This one goes naturally far up (no need for a tweeter either) but I find that it too may benefit from (a moderate) UHF boost.
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Last edited by elac310; 1st June 2014 at 04:13 PM.
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