... I picked up three 288-8k drivers about a month ago to play around with on 805B horns. Two had working diaphragms the third had no diaphragm.

The two I had measured pretty close to each other but neither seemed to be anywhere near what was claimed for these drivers. They were extremely rolled off on the top end and really didn't even has as much bass as the 902-8B I'm running on 511B. I talked to a few different people and most thought this was a function of the horns. As such I built boxes to hold the 2404H tweeters I have and was planning on crossing to them around 5kHz.

Since I needed a new diaphragm anyway for the third unit I ordered one from Great Plains Audio.

This is one of the 288-8K with the used diaphragm in it.....


Shawn

Measurement made with True Audio with the mic right up to the mouth of the horn with a 96kHz sampling rate.

This is what happens when you put a new diaphragm in it. Note... the old diaphragm was the exact same model Altec diaphragm. Obviously after seeing what the new one could do the old one is tired and has seen better days!

Not sure what the glitch is above 20kHz. Could be driver ringing, might be the noise source or it might be the diaphragm rubbing slightly. I'll have to use ETF to look at some time domain measurements to see if it is the diaphragm ringing or not and will also measure my noise source with the FFT.

Needless to say this new diaphragm sounds *much* better. Now I can understand how Altec was able to use these in a lot of two way systems. It is interesting to comare to the 902/511 in that regard. The 902/511 is more extended on access but because of the very different dispersion patterns of the horns it almost sounds like the 288/805 has more top end.... esp. if you aren't on axis

Shawn
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Very interesting. How much the diaphragms cost?

If you loosened the dowel pins, installed the diaphragn with the paper shim, tightened the dowel pins, removed the diaphragm, removed the shim, and re-installed the diaphragh; it shouldn't be rubbing.

The graph looks too good for a rubbing driver anyway.

Don't know, could be cr(A)p in the g(A)p.

Tom,

The diaphragms are $100 each from Great Plains.

DJK,

"The graph looks too good for a rubbing driver anyway."

I tend to doubt that too. When I was fooling around with the old diaphragms I made one rub and it was really obvious sounding from the scratching noise it made on material.

I doubt it is stuff in the VC. I cleaned the VC in both drivers. Both drivers are showing the spike so I tend to think it is either the noise source or just the resonance point of the diaphragm itself. I'll plug the noise source into the FFT tonight to see if that is the cause.

Shawn

Interesting Shawn. Since you are crossing them off at 5K with a steep slope, that resonance will be irrelevant. What would concern me is that 5Db spike at 1KHz.

To what do you attribute that? Horn ring, mike ?

Rick

Rick,

"Since you are crossing them off at 5K with a steep slope, that resonance will be irrelevant."

That was where I was going to cross them based on the old diaphragms. With the new ones I'm planning on crossing higher... probably around 8 to 10kHz. From Al's plots of the 2404 it looks its smoothest above about 8kHz.

I may also high pass the 288s lower since they appear to have no problems getting down deeper.

"What would concern me is that 5Db spike at 1KHz. "

Yeah, I need to look at that at a finer resolution to see what that is all about. It could be one really narrow spike pulling up the 1/3 octave average there. Should be easy enough to notch out with a parametric filter in the crossovers though. I also want to remeasure that at a distance to see if it is a problem from further away.

Shawn

Checked the noise source on the FFT, it doesn't have that peak in it. So it looks like that peak is in the driver somewhere.

The attached is the same measurement of the new diaphragm scaled to 1/24 octave.

Shawn

And to show how different 'tricks' will change how a drivers response will look to the casual eye... this is the same measurement as above but with the vertical scale changed.

Shawn

looks like those old diaphrams ...saw a lotta Watt's

I received the other two diaphragms yesterday so I was able to get the other 288-8ks up and running properly. I have them on the 805Bs in the LaScala's now. These are seriously large horns and drivers, they dwarf the 511B/902 combo.

Right now they are still running as two ways with Al's ES600T networks (set at -10ish dB of attenuation) and sound very good. I think I have about 4dB of boost in the treble on the Lexicon to help out the high end a little. I could easily live with them like this though I think.   But.... since I have the JBL's.... and the crossovers... and the amps... next up is converting the center speaker to a three way tri-amped setup. After I get that all dialed in (crossover points, levels, time alignment... possibly EQ) I will duplicate it for the L/Rs.


Shawn

The planned crossover points:

From the JBL Sub1500s to the LaScala Bass bins 80hz fourth order
From the LaScala bass bins to the 805/288s 400hz elliptic at around 210dB/octave (still need to program that in the crossover)
From the 805/288s to the JBL 2404Hs probably 8kHz. Not sure on the slope yet as I need to work that out in the crossover

System is dual duty music/HT. For the dedicated LFE channel (movies) there is a ContraBass behind the screen above the left side speaker.

Shawn

Shawn,

You are an animal!

Terry

Hey Shawn,

Where did you buy the diaphragms for your 288s?

-Jon

From the first post:

"Since I needed a new diaphragm anyway for the third unit I ordered one from Great Plains Audio."

www.greatplainsaudio.com