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#16 Re: more speaker porn

Post by ed »

Cressy Snr wrote: Thu Jul 19, 2018 11:56 am
That dip might be a cancellation effect, or it might not. Have you tried reversing the phase of the tweeter relative to the main driver? That might work out additive with the dip, to bring the response flat with both drivers playing.
there is merit in what you say grasshopper......here is the plot of alpair on it's own and alpair with monacor. The dip is far more pronounced with the monacor in circuit........I will run a few plots with the phase reversed.....but then it's all about listening.
a10p-no-tweet.jpg
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#17 Re: more speaker porn

Post by ed »

this is giving me ample opportunity to go insane....

I measured the monacor at 10k with no cap...hey presto:
rbt95-spl-10k-nocap.jpg
and then did a sweep with the monacor phase reversed:
a10p-rbt95-phase-reverse.jpg
I'm feeling queezy...I'm going to put the phase the right way round and have a listen...I would have left it reversed but look whats happened to the 10k-15k patch when it's reversed.
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#18 Re: more speaker porn

Post by Nick »

What happens if you roll off the LF unit with a small inductor?
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#19 Re: more speaker porn

Post by ed »

If I took it down around the 5k mark and let the monacor do its thing as in the first monacor only plot then the range above say 8k where the monacor was working alone would be at least 4-6db below the alpair.....

and yes, I'm guessing/extrapolating from the plots I've got.

I took a risk and ran the monacor full range for a couple of plots and the results were v. positive...so I'm looking at that cap as a serious hindrance to my sanity. 1.5uf is not ideal by itself but it was left over from the 2nd order cap/choke which didn't work at all.

I've packed up for the moment having retrieved the screen monitor for the studio machine but I will come back to it.

been listening all afternoon with just the cap in and the content is good, but it's only a couple of hours so I won't nail any proclamations to the wall yet.
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#20 Re: more speaker porn

Post by Cressy Snr »

Hmm, the phase change sort of proved my hunch.
Pity the rest of the HF fell off a cliff, though I did say it might cure one thing and mess up something else.

I'd keep at it though. FWIW, I had a tottering time trying to integrate the RBT-95 with the Fostex 8 incher, on the big Mets, but once I got it there it was worth all the head scratching and frustration. IIRC I ended up with 2.2uF, but that's probably irrelevant, since you're using a completely different main driver. :)
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#21 Re: more speaker porn

Post by IslandPink »

I must have a proper read through this tomorrow, but I have great sympathy with your travails .
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#22 Re: more speaker porn

Post by chris661 »

Ed,

The >10kHz range is almost certain to change with listening height - the ribbon will have fairly narrow vertical dispersion up there, so you might find that changing the mic position by a few inches will get things closer to your liking.

Looks like the ribbon is 8ohm resistive, which is handy. 1.5uF on that will give a -3dB point at 13kHz-ish, so I'm not surprised that's messing things up for you - it'll still be a couple of dB down at 20kHz! Given the 6kHz dip on the Alpair and assuming you want something that's flat through that range, I'd aim at a 5kHz 2nd or 3rd order crossover on the ribbon, and then take advantage of the dip of the Alpair to get the lowpass section started before rolling off the treble on that unit.

For a 3rd order highpass at 5kHz (recommended for driver protection more than anything), you're looking at 2.65uF, 0.2mH, 8uF, from amplifier to driver.
The Alpair crossover will be more tricky, as you've got voice coil inductance to deal with. Best to measure impedance there, and maybe throw a Zobel at it if you've got the parts lying around.

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#23 Re: more speaker porn

Post by ed »

thanks for the input chaps

for the moment the speakers are going in the studio set up with the monacors out of circuit. I like what the alpair does on its own and I certainly don't want to upset or change that presentation at all. The alpair will not have a low pass filter, that was not the original intention. The original intention was simply to have it running full range with a minimal hi pass on the monacor. That original intention/aim remains.

I have listened, at length, and measured, and in my opinion the 2nd order filter at 9.6k sucks the life out of the presentation, big time. The 1.5uf cap on it's own sucks the life out of the presentation, and it's pretty clear that measuring the monacor spl at 10k with 1.5uf will deffo show a drop in efficiency. It's also pretty clear that running the monacor with a cap only filter at 13k ish upsets the alpair in the 6.5k region....I will pick this up again with the correct cap for the 10k 3db region but I think that's as far as I will go.

Chris, thanks for the input, I am well on top of filter theory but it's far too complicated for what I wanted here.
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