Centering a new fram via tone
OK guys - need some help.
I have a pair of 291-16K drivers (thanks Chris!) for which I have bought new GPA aluminum frams.
Unfortunately the 291s do not have the adjustable locater pins. Just a pair of small screws. So even though the new frams came with the spider shim, they are giving me a headache getting them centered. Without the pins, the shim is no good.
I can play a 1KHz tone and look at it on FFT (arta). As I shift the fram around, the harmonics change. Even tho I can get to a spot with low distortion, or a nice even fall-off, once I start to snug the screws the 3rd harmonic shoots up. I've tried and tried, but every time I tighten even a bit, the distortion shoots up.
Any hints or tips for doing this right. I know some of you guys have practice at this. What worked for you? Thanks!
Thinking on the principle that measuring devices do not compensate as ears do....how about a silly scope program?
Feed different frequencies and adjust for the cleanest sine wave?
Oscilloscope
There's tons of free ones BTW.
Just thinking rub will distort the waveform, cleanest wave should be the least rub?:coffeedrinker:
Next suggestion...call GPA
Your neighbors called. They like your music.
Is there anyway to replace the locator pins.This is on the GPA site that you probably have already seen but here it is anyway I hope it helps some.
http://www.greatplainsaudio.com/LgDrvrDiaRepl.pdf
Tighten the screws so much that the diaphragm cannot move by hand. Than try to center it by lightly tapping it on the sides with something (rubber bat is ideal) and control the process with ARTA. Once you get clean FFT (I would use even lower freq) tighten the screws. If you don't succeed this way something is wrong.
Hey Vuki - thanks. That's just what I did! But no joy. =(
Can't seem to get the screws "just right" so that the fram will move when I tap it, but not change when the screws are tightened. It seems to be a pretty delicate balance. Maybe I just need to keep trying.
It's frustrating because I CAN get a good looking harmonic structure with a little tapping and movement. Each harmonic drops by about 6dB has the orders go up - perfect. But tighten it and that all falls apart. And it's mostly the nasty 3rd that rises, don't want that.
I have tried 700Hz and will try again. Distortion is different at every frequency.
Pano,
- FWIW to your existing plight, my late run 288-8Ks ( they're almost 288-8Ls ) have the proper pin-dowels in place ( you made me look at my worst performing driver ) .
- It's a bit of a mystery to me why your 291s are missing the dowels .
- Some conjecture here; perhaps someone removed the proper OEM parts because the whole "spider-shim" thing was just too much hassle for them to deal with ( ie; some "SR-Rocker Tech" who got frazzled after swapping out one too many blown diaphragms ) .
- I'm guessing that the spider-shim / when forced out against a new voice-coil from the inside of the coil / can help "true-up" a possible out-of-roundness ( if found in a new coil-winding ) with the locating pins then "freezing" the new "best fit" ( maybe this is just a hopeful thought on my part ) .
- It's a possible benefit that you can't explore because you aren't able to implement the recommended replacement procedure .
- Therefore, I'd contact GPA ( again ) and have them snail mail you the parts those 291s are missing ( dowel & perhaps even the proper small headed machine bolt that goes through the dowel ) .
- Of course, this assumes GPA still uses this unique diaphragm centering system and maintains a stock of these small parts .
- I'm curious, how does your 3rd harmonic distortion compare to what I've posted over at the DIY Forum ( in your old thread ) ?
- ( That "STEPS" plot was from my best performing 288 driver ).
- Unfortunately, I have no "spider-shim" to play around with to see what sort of alignment I can achieve .
<> Earl K
Hey Earl. Yeah, maybe best to call GPA to see if I can get the dowels. At least give that a try.
I'd love to get this right by harmonic measurement. Then I'll know it's as good as it can be. Bu the shim and dowel may be the best way.
I have a spare spider shim, if you need one. Just PM me.
Will do ( Thanks !Originally Posted by Panomanic
) after I return from todays gig .
Gotta run <> cheers
ps;
- I'd perfer that the distortion method be made to work well / but hey / I'll take whatever gives the best results .
- I've found my older alnico 802s happen to have dead-on accurate locating pins ( with very tight tolerances ) .
- They're especially easy to align when using either generation of "loading cap" / ( when I think about it ) they work as one continous load-distribution washer .
- Sadly, my 902s are a completely different story .( never alignable )
Hmm, this implies the mounting plate/phase plug isn't properly aligned.
GM
Loud is Beautiful if it's Clean! As always though, the usual disclaimers apply to this post's contents.
Yeah, could well be. I'll keep at it to see what I find. May end up with a thin paper shim under 1 side.
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