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Wayne, compensation question please [message #39032] Tue, 12 November 2002 23:58 Go to next message
ToFo is currently offline  ToFo
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Registered: May 2009
Master
Hi Wayne,
When making my Theater 4's I had trouble getting CH-3's and didn't want the smaller horn for aesthetic reasons, so I ended up with a 2370 horn. I recall it came up once that it is fine in this speaker but may be a little "splashy". You were right about that, and that it's subtle, but I would like to tame that a bit. I have since acquired the CH-3, but there is still something about that 2370 that I can't do without. I really like it's sound in the low thousands.

When compared to CH-3 it seems that it needs less compensation and maybe at a little higher frequency. Actually, my suspicion from looking at JBL's data, is that the slot loaded part makes the 2370 look more like a shelved response, not falling. JBL shows response is roughly flat to 5.5K then drops 4dB, then it is flat to 15K. So when you add the sloped reponse of the cap is this what makes it "splashy"? I have noticed that with PSD 2002 (as opposed to the JBL used for their data) it still falls a little over the top two octaves, but not as quick as on a CH-3. So it still needs a sloped compensation.

My first thought is to try a .33mF bypass on the pad. I know this will make the comp happen higher. In relation to the rest of the HF band, if the comp happens "later" is it also reduced in level due to it's "knee" being further out of band? Also, say I used an LF driver that is more sensitive. (delta pro for instance) This forces me to use less HF padding. would this reduce the effect of the cap without having to raise the frequency of compensation? Would this have the overall effect of making the compensation curve seem shallower, or would it just seem to have moved up in frequency? I think I need shallower compensation, but I dont want it to start too late. I see the comp cap as a first order rising response, but I know it behaves differently as part of the system. I do not have a good handle on all this yet. what is the best approach for a 2370?

Thanks for putting up with us can't leave well enough aloner's,
Thomas

Re: Wayne, compensation question please [message #39033 is a reply to message #39032] Wed, 13 November 2002 00:17 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18792
Registered: January 2001
Illuminati (33rd Degree)
You've assessed the situation pretty accurately - The Peavey CH-3 has a little less in the bottom octave than the 2370. But they're really more similar than you might expect, particularly if a person uses them above 1.6kHz. For the octave below that, the slightly larger size of the 2370 gives a bit more energy and prevents it from being as peaky as smaller horns.

One thing that differentiates the 2370 from the horns I like to use is that it's the only one with a secondary acoustic device in addition to the horn flare. The 2370 has a diffraction slot to increase horizontal dispersion at the highest frequencies. It creates wider dispersion at the expense of an added internal reflection, which causes a slight bit of impedance and response ripple. That's the cause of its "splashy" sound.

Re: Wayne, compensation question please [message #39034 is a reply to message #39033] Wed, 13 November 2002 02:26 Go to previous messageGo to next message
ToFo is currently offline  ToFo
Messages: 219
Registered: May 2009
Master
Wayne,
I dug up an old E-Mail from you about this.

You wrote
"The 2370 will sound a bit more "splashy" because of the diffraction slot in its throat. But overall character is very much the same."

Good call! You nailed that one.

I should have remembered that, It took me a few weeks to really listen and see that this is exactly what it is that bugs me about them. I agree that it is not a big deal, and for most people it would be too small an issue to bother with. If I leave it alone this is still the best set of speaks I've ever had by far.

If I just wanted to be obsessive, what might I do to tame it a bit? Can it be done by a change of value in the existing compensation circuit? I think I might get some smaller caps to see what happens. Is that a viable plan for this? Or will it just push my top octave down too far?

I know I am splitting hairs at this point, and I really appreciate all of your help on this. Thanks,
Thomas

Re: Wayne, compensation question please [message #39038 is a reply to message #39032] Wed, 13 November 2002 07:46 Go to previous messageGo to next message
bqc is currently offline  bqc
Messages: 26
Registered: May 2009
Chancellor
The vertical line in the JBL 2426 response graph after the 10KHz mark
is that a 15K or 20K mark? If you look at the 1K mark, the next
marker is 2K. Usually that means that the next marker after the 10K
should be 20K. Wayne how do read the 2426 frequenc response graph?
is it flat to 20K or just 15 KHz ?


Re: Wayne, compensation question please [message #39042 is a reply to message #39034] Wed, 13 November 2002 11:17 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18792
Registered: January 2001
Illuminati (33rd Degree)
Some CD horns of this size are better canidates than others. They all have a pretty similar overall curve, but each has its own particular "signature," and it's small peaks and valleys are in different places. Again, the reason for the "splash" is a ~7kHz peak in the 2370 from the diffraction slot in its throat. It causes an internal reflection that makes ripples in the impedance and response. You'll see this in Manta Ray and BiRadial horns, and it's there to increase dispersion. Pretty much any horn with sharp edges does this. Those edges are there for pattern control but their disadvantage is they cause internal reflections, impedance and response peaks.

The deal with passive compensation is your ability to tailor the curve is somewhat limited. For example, when attenuation requirements are low, that also necessarily means that top octave augmentation ability is also low. You could add notch filters to the RC filter/damper I've employed, but I prefer to keep it simple. There are some limits to this approach, but the horns discussed on this forum are all good canidates for compensation as is used in the π crossover. In fact, I've not seen any CD horns that wouldn't benefit by some amount of this technique.

If you need a lot of augmentation and a little attenuation, your passive network design choices are generally limited. Likewise, if you need attenuation but the augmentation slope deviates from diagonal too much, then your choices are also limited. But really, when used above the frequency where a horn becomes well loaded, it will generally be flat for a while and then enter a couple of octaves of smooth rolloff at a relatively constant rate. This is where the π compensation technique is most beneficial.

Re: Wayne, compensation question please [message #39043 is a reply to message #39038] Wed, 13 November 2002 11:20 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18792
Registered: January 2001
Illuminati (33rd Degree)
The upper cutoff of a horn is determined by the compression driver used. Most modern ones are good to about 15kHz and few actually hit 20kHz. With π compensation, you can get response to 16kHz-18kHz with just about all the current production compression drivers, some a little higher than that.
Thanks! [message #39045 is a reply to message #39042] Wed, 13 November 2002 11:25 Go to previous messageGo to next message
ToFo is currently offline  ToFo
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tweaking to suit [message #39057 is a reply to message #39032] Thu, 14 November 2002 10:04 Go to previous messageGo to next message
Sam P. is currently offline  Sam P.
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Grand Master
Tom,
Not to second guess Wayne, but sometimes "you just have to fly by instinct". I feel the altec 511's also exhibit that shelved response, which subjectively sounds slightly "hot" when combined with the nominal attenuator bypass HF augmentation.
Try that 0.33uF cap first. Maybe even .2uF. Also, you may have noted that the ONLY difference between a "PI" approved 10dB and 12dB pad is simply the value of R1 specified. You could try adding 1, 2, or more ohms of additional series resistance to drop the HF level some, to perhaps better suit your room/system/ears. Sam

oops, you can also "split" the R1 resistance, and only bypass a portion of it, if you really want to tweak it to death:)

now thats what I call timing. [message #39059 is a reply to message #39057] Thu, 14 November 2002 10:49 Go to previous messageGo to next message
ToFo is currently offline  ToFo
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Master
You must have wrote that while I was listening to my new pad value. You have hit on some other stuff I didn't think of as well. How does that partial bypass work? Say I have three 47 ohm mills in a stack, I put one of the resistors in series with the cap so the bypass only happens to two of them? I think I get it. Cool! (and let me know if Im wrong)

Thanks Sam,
Thomas

revisus stupidus [message #39060 is a reply to message #39059] Thu, 14 November 2002 11:21 Go to previous messageGo to previous message
ToFo is currently offline  ToFo
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Master
Upon thinking (I do that sometimes) It occurs to me that 47 ohms in series with my bypass cap is not such a good idea. I am dumb on this one. Could I just undervalue the stack by a small amount and add it in series with the cap, say 4 ohms to start with? hmmm. does this do anything weird to the crossover? I dunno, damping or something?

Thomas

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