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Old 13th March 2012, 06:40 PM   #1
von Ah is offline von Ah  United States
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Question simple high pass theory in a graph?

Please bear with me on this one.
For simplicity sake, what would a 1st order high pass filter at 20K look like on a log graph? I ask because I briefly tried out adding a tweeter (Apex Jr. Audax) to each of my EL70 MTs, up-firing from the back of the cabinet (unseen, ninja-style). I first used a 1uF cap for a few days, and finding that a bit too bright, went with a .47uF cap. I liked the smaller cap better, but as my attachment scheme was crude at best (sticky-tack), I abandoned the practice.

I'm having trouble visualizing the comparative filter slopes (roughly 20K and 40K) relevant to the frequency response of an EL70. The Audax tweeter is ~93 db at 1W/m, correct? So if crossed at 20K, that would be 6db down at 10K, or in the high 80s. Crossed at 40K, it would be in the high 80s @ 20K, and down to around 80 @ 10K. In dunce theory ( ), that would be more fitting to match to the EL70's output.

Getting to the point, can someone please model these two filter slopes on the same graph as the published EL70 curve? Many thanks!

Last edited by von Ah; 13th March 2012 at 07:12 PM. Reason: miscalculated octaves now fixed
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Old 13th March 2012, 07:34 PM   #2
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A few things.


The published EL70 curve won't work for you for a couple reasons. The biggest is that you have 2 drivers firing into different planes. The second reason is baffle effects. The 3rd is phase. They may be out of phase and create a null. They may be in phase and sum nicely.


That little tweeter is 93db/w/m??? Wow. That's pretty sensitive. I'm not sure I'd believe that if I were you.

The tweeter impedance isn't flat. Although Dave once posted an impedance plot and it is pretty flat. So somewhat reliable.

The tweeter response isn't a flat line, so just saying 6db down at this frequency doesn't work. You're firing it up at very high frequencies where this tweeter is likely quite directional. I believe that tweeter has a phase plug thingy and is quite small. My Dad has a pair but I've never measured them. Bottom line, no simulation without measurements is going to tell you anything. In fact it's dangerous. It will lead you down the wrong path. Better to measure, or trust you ears.

But because you asked.

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Old 13th March 2012, 07:37 PM   #3
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Oh, grey line is 0.47uF and black is 1.0uF. And that assumes a dead flat 8ohm impedance.

Edit - Oh shoot, and another thing. The 0d phase line is 85db and the 60d phase line is 90db. I cropped that off, sorry.
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Old 13th March 2012, 07:51 PM   #4
von Ah is offline von Ah  United States
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Thanks! Your concerns are appreciably noted. I admit I've oversimplified darn near everything involved.

I read the 93db rating in a thread on DIYAudio somewhere, so who knows...

Recently I did fire up HOLM for the first time on this system (no tweeters) and got a rather jagged upper end from ~7-20K. Preliminary results, only. I hope to experiment further, but time is like minnows in a pond -- fleeting and slippery. :P

It would be nice to see if anyone has measured the response of the Audax tweeters.
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Old 1st April 2012, 02:30 PM   #5
von Ah is offline von Ah  United States
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Default Cheap and Dirty

Here is my slap-on-a-tweeter-and-see-what-sticks experiment currently in use.
Seems to add just enough air throughout the room to suit my ears. No measurements yet. These tweeter add-on plugs go down vertically in the wide bare cable openings in the speaker terminals, and the cables from the amp go into the banana jacks in the orthogonal. No soldering!
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Old 1st April 2012, 02:44 PM   #6
AndrewT is offline AndrewT  Scotland
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Speaker drivers can vary in sensitivity from 80dB/W @ 1m to 100dB/W @ 1m and some even beyond that.
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Old 1st April 2012, 03:21 PM   #7
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Is the cap bypassed or something? The banana plugs look like they negate the capacitor.
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Old 1st April 2012, 04:42 PM   #8
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Interesting use of pomonas... but from the picture, as Ryan says, you are shorting out the cap.

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Old 2nd April 2012, 11:24 AM   #9
von Ah is offline von Ah  United States
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Heh, you know, that IS what is happening. I guess a V2.0 is in order... I'll undo that middle connection.

Tinkering will continue. I think I need to go back to the 1 microfarad cap.

Last edited by von Ah; 2nd April 2012 at 11:42 AM.
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