Why do my 288-8Ks sound like cr@p?

[Earl K]

Really now, who wants to look at such tiny pics ?



Fellas, it's time to learn how to create "links" that go from this site over to an interesting image that is hosted on another site . So;

> Right Click on the Image ( of interest ) at the other site ( webpage ) .
> Choose, "Copy Image URL" from the menu .
> Come back to this site & open the text editor.
> Paste the "friggen" URL ( belonging to the image ) into this sites text editor .
> Write [img] in front of URL
> Write [/img] after the URL
> Now look at the original picture in it's proper, glorious size .





PS : Thanks for the link to the other site .

[vuki]

Thanks,

First measurement was done with IMPulse setting. The Impulserespose was measured with a log sweep. In fact to have ARTA determine if there are any excessive distortion components. At the level of my measurements these all appear to fall into the noise floor of the ADC.

The high pass can be acoustical as well. Think of the lower horn cut-off or bass reflex which both have inductive air-impedances in their modelling. Checking all bolts and nuts is also part of the setup inspection!

Coen

What would be the corner freq. of such high-pass - 20khz? Notice that distortion curves are also very uniform along 6db/oct. line.
Hm, on the second look that would be more like 3dB/oct?

[Earl K]

Something like the above is what I would expect to see ( from a "healthy" 288 mounted on a radial type horn ) .

Not this ;

[Coendert2011]

Well, forget those first measurements.

Did them too tired and way too sloppy. Bad location, loose bolts, on second sight weird settings and a wrong connection that I still don't understand (why it worked in the first place).

Now I've got much better graphs. FR seems ok, but distortion is a little high and uneven compared to above (not playing real loud here).

Thanks to Earl here you are with a nice pic:



Pretty nice eh?! However: D2 and D3 are dancing all over the place....

I did another much louder measurement to eliminate the influence of ambient noise, with this result:



Here is a group shot of the Gremlins:



Ammounting to this high performance compressiondriver THD graph:



Yes folks that is almost 10% distortion....


For the fans: here is the smoothed version in low quality jpeg:



New diaphragms and a little compensation for the 600Hz peak will probably make this an outstanding hifi combination!

Coen

[Earl K]

Yeh baby !! , now that's some good work .

- Notice ( on mine ) D3 ( 3rd Harmonic Distortion / ugly stuff ) is 60 to 70db below the fundamental ( 1K to 5K ) .

- Your D3, ( 1K to 5K ) is about only 30 odd db down ( with that big sub 2K peak getting close to being only 24db down ) .

- Anyways, nice work !! .

- IME, distortion specs like these nicely supports all of your earlier mentioned, auditory-impressions .

- ( FWIW, before ordering new diaphragms ), I would play about with those diaphragms a bit ( loosen off the screws holding the guide-pins / Pano knows what I'm talking about ) to see if you can lessen the distortion ( by shifting the centering of the diaphragm in the gap ) .




PS ; Thanks for the big pics !

[Panomaniac]

Yeah, I was going to ask, do you have the centering pegs? I think I saw them in your photo. Getting the 'frams centered without them is near impossible, as I found out.
I see the ~8.3Khz dip in yours, too.

[vuki]

Did I tell you, or did I tell you?
Do the same measurement with crossover. Do the RTA measurement of the complete loudspeaker at listening distance. Post the schematic of the crossover.
I don't think the distortion is the cause of bad listening impression. Also, it seems to me that diaphragms are OK.

[Coendert2011]

Yeah, I was going to ask, do you have the centering pegs? I think I saw them in your photo. Getting the 'frams centered without them is near impossible, as I found out.
I see the ~8.3Khz dip in yours, too.

No no centering pags or 'spider shim' used. I left the factory alignment in place.
One can improve here.

I will perform a higher resulution measurement on the dip to see how deep it goes.

Coen

[Coendert2011]

Did I tell you, or did I tell you?
Do the same measurement with crossover. Do the RTA measurement of the complete loudspeaker at listening distance. Post the schematic of the crossover.
I don't think the distortion is the cause of bad listening impression. Also, it seems to me that diaphragms are OK.

Ok!

Cross over is second order Linkwitz-Reilly on 650Hz that works wonders on my 802 setup. C=12uF paper in oil 600V cornell dubilier, L=3.3mH heavy copperfoil air inductor. This is a fantastic filter for my 802 setup.

Driver with horn sounds equally bad with and without the co and with and without my diy altec 828 cabinets. I don't expect any difference by inserting the filter nor by adding the bass cabinet. Been there, done that.

Fyi I experimented with various caps like different pios and polypropylene and different cables (stranded, solid core, silver, copper etc). NOTHING cures or even ameliorates the harshness. The high distortion makes a whole lot of sense.

To me it is no longer circumstantial that the phragms are not up to spec, probably the whole batch isn't. They might have have been tin thin or had a bad heattreatment, bad coating, bad whatever, but they break up as hell. Factory misalignment may also play a role, but both drivers suffer the same illness, so it is not exemplary. This is supported by the post of Todd W.

For now I'm curious how my other 288 and 802s perform on the graphs. I do like this sound of the latter.

If I find some extra time I will go through the trouble of measuring in situ. But

Thanks anyway for your suggestions!

Coen

[Earl K]

Coen ,

As far as I'm concerned you've nailed it ( a bad batch of diaphragms , ( metal related ) ; too thin, wrong Alu mix, wrong metal prep, etc. etc. ) .

Distortion in that elevated area is particularly noticeable to the human ear ( & I really have no idea what internet based cool-aid Vuki is drinking when denying the audiblilty of distortion / I have my suspicions that it starts and ends with the initials E.G. ) .

I'd love to see some sweeps of your 802s ( Fr & Dist ) on the same 511Es ( that would be a learning experience for all of us ) . When you have time .

Thanks for the educational thread ( now you know & now we know ) ><