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Did a Jubilee, now it's time for a Jamboree


jwc
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Dana. That was the way it was explained to me. One of the MAC Engineers came to a local MAC dealer for a presentation and I got an invitation to go.

He was explaining the patented autoformers on the MAC amps that would controll the output regardless of the impedance of the speakers.

The MC 252 I have apparently is rated a 250 watts/channel for 2,4,8 ohms.

He basically said that the tap chosen should be the one closest to the impedance of the speaker in order to reduce distortion.

The best curve IMO is the 4 ohm tap in series with the 3 x 13 slot.

It may be several days before I get the chance to do more curves. Will need to gather the stuff for the rear load. I will try the larger slot in series front loaded during that time.

jc

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Yes, I'd go with series, too. Perhaps listening would be the best determiner between 4 or 8 Ohm.

I really thought that there would be a drop in SPL in series at 8 Ohms, but motors are very strange animals compared to plain old resistance.

I wonder what the overall impact is once the mids and tweets/network is also in the loop - could have an influence, too.

Dana

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The network I will build will make the jamboree drop like a rock at 600hz. This will be the parallel setup. There won't be a good way for me to A/B the parallel vs the series....as my network won't work with the series.

Initially it will be the good ole Altec 902/511 combo in two way for initial impressions.

Another interesting thing is that this network will work with the dbb's too. So....will have a good A/B.

jc

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Not a lot of difference between the two woofs, but the cast frame is more sensitive at 97 db compared to 95.3 for the stamped frame due to the more efficient magnet structure of the cast frame version.


Bob

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For what it's worth Dana....a lot of these differences between overlayed measurements can easily be the result of minor changes in mic and speaker placement. You might be surprised what a 1/4" change might look like even with nothing else changing.

If you're up for another measuring challenge JC, I think it would be interesting to show an impulse response of the 'anechoic' response to see what kinds of resonances might be happening. You could then compare the group delay of your bass reflex alignments versus the horn bins, which might explain some of that fatness you like? I dunno.

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For what it's worth Dana....a lot of these differences between overlayed measurements can easily be the result of minor changes in mic and speaker placement. You might be surprised what a 1/4" change might look like even with nothing else changing.

If you're up for another measuring challenge JC, I think it would be interesting to show an impulse response of the 'anechoic' response to see what kinds of resonances might be happening. You could then compare the group delay of your bass reflex alignments versus the horn bins, which might explain some of that fatness you like? I dunno.

Mike, I am not so sure the mic placement is as critical as you are suggesting at these frequencies. The wavelengths are realtively large and an error of a couple of inches will not change things that much. It would if it was a highly reverberant space however.

The idea of an impulse-like response is a good one. The problem with short duration signals is that it requires a good deal of repetition and averaging since the energy is spread across the spectrum and there is relatively little energy in any narrow band region. This can be a pactical problem when using chirps or MLS etc. Also there can be a surprising anount of ambient energy (noise) in the low frequencies.

I appreciate you folks showing these curves. I am particularly impressed with the Crites woofers. Is there any plan to measure the the speakers harmonic distortion. I realize this can get difficult if you are near the noise floor to begin with.

Good Luck,

-Tom

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Mike. I'm not so sure I am following your suggested technique. I'm not familiar with it.

As far as a difference with 1/4" mic placement....well....I can't garuantee that I am getting the mic in Exactly the same place everytime but I am pretty dern close. I measured the distance each time and midline.

The testing conditions that I am offering are not of a quality for publishing by all means. I do think it is valuable from a comparison sake. I feel confident in the curve itself....not in the total SPL.

Tom, if you are familiar with a simple technique of measuring distortion...I can consider it.

I will say that the crites drivers are solid out to at least 800Hz. I have run curves on them in a simple bass relex setup. I also have done some impedance testing on them and there is stability out there too. The impedance is the same from 400 to over 900Hz.

Distortion...I don't know.

I don't plan on lugging these things back outside until I have the chance to rear load them. It isn't many steps to do this but I have been bogged down at work.

I also need the time to build the networks. Then actually listen to some music for a change.

jc

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JC,

The idea of listening to music is novel to me. Shouldn't we spend all our time with test signals?

Harmonic distortion would be the easiest to measure. Have a look at the Delgado & Klipsch "jubilee" paper. They have a figure devoted to this. Keep in mind that it may be difficult to get the level of distortion above the ambient noise level if you are measuring outdoors.

I will be faced with the same problem in the months ahead. My "1/8 space corner" for measuring is at the front of my house. Although the lots are about an acre each (I live in the middle of nowhere), I am sure that I will have explain myself to the neighbors.

Good Luck,

-Tom

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Mike, I am not so sure the mic placement is as critical as you are suggesting at these frequencies. The wavelengths are realtively large and an error of a couple of inches will not change things that much. It would if it was a highly reverberant space however.

I can't think of a good way to say it, but are you saying this outta speculation or experience? I can provide quite a few measurements that I've taken (and trust) that show this behavior does happen. I agree, it's only going to happen when there are reflections present at the microphone, but that's going to be true of every room [;)] Or heck, here's a link that shows what 4" can do: http://www.ethanwiner.com/believe.html

I don't mean to say that JC's measurements are pointless either, or that any of the comparisons aren't showing any trends. I was just noting that within the framework of the measurement, that I would interpret the performances as being nearly identical. I suppose JC could always take a few measurements of the same configuration with the mic in a few similar but different locations to quantify the magnitude of variation. Who knows, perhaps he's not having any issues at all. I was just commenting on a possible ambiguity. Heck, even in an anechoic chamber I wouldn't trust the measurement to within +-1dB. Of course I have no experience with measuring in anechoic chambers so I'm sure Roy will slap me upside the head if I'm being crazy [;)]

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I think the curves are quite useful. They also seem entirely consistant enough for our purposes.

The reason I keep coming back to the Jubilee as a comparison is that I think that it acts as a reference, overall size, shape, freq. response, and mouth size, etc., are closer than anything else we could compare it to. It serves to give an ability to determine what portion/element(s) in the curve is likely result of the testing methodology. I think there are 3 portions that are consistant between the Jamb and Jub and do not seem to change. I attribute these to being anomolies that are the result of the "particulars" of the testing methodologies.

JC has also done the impedance testing as he went, so we have other information available.

But I'm still trying to decide which configuration is "best" overall, since I have to tell Al K. which one to do. That's what the thought behind the above curves is, anyway. JC knows I have wanted to go with a series config from the start for reasons of power handling ability. However, I'm still not quite sure.

Dana

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Mike, I am not so sure the mic placement is as critical as you are suggesting at these frequencies. The wavelengths are realtively large and an error of a couple of inches will not change things that much. It would if it was a highly reverberant space however.

I can't think of a good way to say it, but are you saying this outta speculation or experience? I can provide quite a few measurements that I've taken (and trust) that show this behavior does happen. I agree, it's only going to happen when there are reflections present at the microphone, but that's going to be true of every room [;)] Or heck, here's a link that shows what 4" can do: http://www.ethanwiner.com/believe.html

I don't mean to say that JC's measurements are pointless either, or that any of the comparisons aren't showing any trends. I was just noting that within the framework of the measurement, that I would interpret the performances as being nearly identical. I suppose JC could always take a few measurements of the same configuration with the mic in a few similar but different locations to quantify the magnitude of variation. Who knows, perhaps he's not having any issues at all. I was just commenting on a possible ambiguity. Heck, even in an anechoic chamber I wouldn't trust the measurement to within +-1dB. Of course I have no experience with measuring in anechoic chambers so I'm sure Roy will slap me upside the head if I'm being crazy [;)]

Mike,

It is not speculation it is in fact from experience. I used to work in anechoic and reverberant chambers, so I know the two extremes.

That Ethan Winer thing is frequently mentioned and it had been puzzling to me at first. At 1000Hz the wavelength is 13 inches so you would be going from a peak to ref level in 3 in. (1/4 period). At a 100 Hz it would now be 10 times that. This assumes a single wave and no multiple reflections. What Winer was measuring was in a fairly reverberant space (hence my caveat noted previously). As such, he got some wild fluctuations; however at some the frequencies (wavelengths) they still seem unreasonably large. According to his data, if that was true then listening to a pure tone would have been "doubling and quadrupling" in the "loudness" (in physical units of SPL this would be 10 & 20 dB which is what he measured). This may happen at the higher frequencies (with the reverberation and standing waves) but it was suspicious at some of the lower frequencies (I have to assume there was even more "room effects" than I had imagined).

But try it yourself, since that will be the most convincing. If you play a 100Hz tone and very slowly walk around your living room you will get those kinds of fluctuations. But will it be over a difference of less than one inch? It may be at the higher frequencies but unlikely at the lower frequencies (again, I do not know how reverberant your living room is).

Anyhow, JC has the double advantage in that he is doing this outdoors and at a low frequency region. There are far fewer reflective surfaces (and if the singnal/measurement is gated then even some of those can be eliminated). Again, the mic is positioned on-axis so that minmizes some other problems.

Please do go ahead and try the "living room expt". See if you think 1/4inch difference at a low frequency is going to wildly fluctuate.

If this was done in a tiled bathroom or a stairwell, then the fluctuations could get wild.

Good Luck,

-Tom

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