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Old 29th May 2012, 11:10 PM   #1
Tino is offline Tino  Canada
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Default a couple questions to help me improve my home built speakers

Hi guys, a few months ago I built a pair of speakers using Vifa D27TG45 tweeters and a pair of vintage Camber 8" drivers. For the most part I'm happy but I would like to do a few things and would like some suggestions.

Right now its crossed over at 12db/oct at 2.7Khz for the tweeter and 2Khz at 6db/oct for the woofer. I've got the tweeter padded down by about 3db to match the woofer. I dont have any circuits to keep the impedence constant or anything like that (zobel?)

At the moment the speakers sound good but occasionally I detect a bit of harshness from the tweeter and or it sounds a tad honky. I think its because the Vifa maybe doesnt like to be going down at that low or maybe being crossed over that shallow. I also find a bit of heaviness somewhere in the lower mid.

Using an old Audiocontrol SA3050 RTA, I see that I have a slight rise from about 400hz to about 1Khz by about 3db with the highest or center being maybe 650Hz. I also notice the woofers by themselves roll off without any drama starting at 2.5Khz if I let them run full range.

So first question. Is the rise caused by the baffle step? I hear a lot of people talking about baffle step compensation so could this be it?

Second question - if I let the woofers run full range (with the baffle step or whatever it needs to correct for that slight rise in the middle area), and just cross the tweeters higher, would that be fine? Is there any reason not to let the woofer go up to 2.5Khz and roll off smoothly on its own? I'd imagine the sensitivy would go up a bit and I might also need less padding on the tweeter then. I know I've heard that large drivers dont sound so good playing higher frequencies but at least playing pinknoise withthe woofers alone, i see the roll off to be free from any major peaks.



Here are the speakers. If I can make them sound more awesome, I'd be extra happy! Thanks in advance for any help or suggestions.

Click the image to open in full size.
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Old 29th May 2012, 11:24 PM   #2
ra7 is offline ra7  United States
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Nice setup... is that an old Threshold Statis amp? Cooool!

You are probably right in that the woofer rise is coming from the baffle step. Are you sure there are no resonances before the woofer rolls off at the top end? If these resonances are not notched out, they can sound harsh - it may not be the tweeter.

Try Passive Crossover Designer. It's free. You really need to combine your target acoustic slopes with the natural roll off of the drivers.
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Old 29th May 2012, 11:36 PM   #3
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the dead give away is your tweeters Fs is 650Hz where your response spike is. At resonance the tweeters impedance is maximum and therefore it will have a greater voltage across it than if it was at its nominal impedance and be louder than it should. You can fix by introducing a network in parallel with the tweeter that has an impedance minimum that compensates for the impedance rise at resonance (RCL network). A network may also be needed to compensate for the voice coil inductance causing a rise in impedance at high frequencies (parallel RC network).

I second the recommendation to use software with optimisers to design the crossover as I have ended up with very different to expected topologies to obtain target curves before. Speaker workshop is worth considering. I use SPL copy to trace curves. Here is a guide on such things: FRD Consortium tools guide
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Old 30th May 2012, 03:51 AM   #4
Tino is offline Tino  Canada
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It is indeed an old Stasis amp. Its the SA3. I like it

Ok so at least I'm on the right track with the baffle step correction. As for resonance. Hmm well I have not examined too carefully other than the result of the audiocontrol meter which I believe is only 1/3 octave resolution, but nothing really jumps out as being peaky. I do have trueRTA on a laptop but I have not bothered to run through the calibration and all that but maybe I should. At least then I'd get more detailed results

I will also definitely check out that passive crossover design. Will do it now actually. Thanks

Quote:
Originally Posted by ra7 View Post
Nice setup... is that an old Threshold Statis amp? Cooool!

You are probably right in that the woofer rise is coming from the baffle step. Are you sure there are no resonances before the woofer rolls off at the top end? If these resonances are not notched out, they can sound harsh - it may not be the tweeter.

Try Passive Crossover Designer. It's free. You really need to combine your target acoustic slopes with the natural roll off of the drivers.
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Old 30th May 2012, 03:54 AM   #5
Tino is offline Tino  Canada
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Oh wow I did not even think of that. Before even doing a baffle step compensation, I should then put that circuit on the tweeter and see if the broad bump in the response is gone. If it goes away, I'd definitely owe you a few beers

...going now to load some new software on my test laptop including getting trueRTA finally set up properly

Quote:
Originally Posted by kipman725 View Post
the dead give away is your tweeters Fs is 650Hz where your response spike is. At resonance the tweeters impedance is maximum and therefore it will have a greater voltage across it than if it was at its nominal impedance and be louder than it should. You can fix by introducing a network in parallel with the tweeter that has an impedance minimum that compensates for the impedance rise at resonance (RCL network). A network may also be needed to compensate for the voice coil inductance causing a rise in impedance at high frequencies (parallel RC network).

I second the recommendation to use software with optimisers to design the crossover as I have ended up with very different to expected topologies to obtain target curves before. Speaker workshop is worth considering. I use SPL copy to trace curves. Here is a guide on such things: FRD Consortium tools guide
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Old 30th May 2012, 02:41 PM   #6
AllenB is offline AllenB  Australia
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Quote:
Originally Posted by Tino View Post
Is there any reason not to let the woofer go up to 2.5Khz and roll off smoothly on its own?
Your upper limit to cross a woofer depends on the tweeter. To match a dome tweeter you will want to cross an 8" driver somewhere below 2.5kHz, and if possible, an octave or more lower than that.

Quote:
I'd imagine the sensitivy would go up a bit and I might also need less padding on the tweeter then.
if the mids and highs don't pad down to match with the bass, you'll be missing out on the natural weight of a performance. For this reason the sensitivity will be mostly fixed for a given configuration.

Quote:
I know I've heard that large drivers dont sound so good playing higher frequencies but at least playing pinknoise withthe woofers alone, i see the roll off to be free from any major peaks.
The on-axis response is not indicative of the overall response. If you listen to your speakers a little off axis you will see that you can overlook a peak that occurs only on-axis if it corresponds to a dip at other response angles.
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Old 31st May 2012, 02:31 PM   #7
system7 is offline system7  United Kingdom
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I don't know if this helps, but Paul Carmody described the troubles he was having getting some non-ferrofluid Vifa tweeters to avoid distortion due to low crossover point. His solution was to pad the tweeter so it saw a low impedance of 4 ohms at even the Fs resonance point:

Click the image to open in full size.

The details of the crossover don't matter much, it's that shunt R2 of 4R that is fixing it. Always a good thing to do if you've got gain to spare on the tweeter. A Fs notch across the tweeter is quite a complicated thing that changes the order of the filter. I found it was little different from just using a higher order filter in practise.

Natural rolloff on paper cones with just a coil is just not very good. Phase and breakup is terrible:
Click the image to open in full size.

I went for second order here, which sounds nicer.
Click the image to open in full size.

As it goes, I'm not very happy having no shunt resistor across the tweeter right now, but it's a work in progress. Similar drivers to yours though.
Restoring Monitor Audio R300 bookshelf speakers.
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