Tuesday, August 8, 2017

New Super Tweeters! and some more adjustments

I didn't know how long it was going to take to get new super tweeters online.  I am kind of slow about putting together new stuff.  Even old stuff which just needs to be set up, can wait months, just to get set up.

But the new super tweeters went up quickly last weekend, then survived a second unexpected (but required) important modification.

Actually I put up the first Dynaudio D21AF the previous weekend, replacing the one measurably resonant Elac 4PI in the left channel.  That was a huge dosage of relief.  Much cleaner highs, with each cymbal sounding unique.

There was clearly a lot of asymmetry, having the wood boxed one directional tweeter on one side, and the shiny metal encased omnidirectional ribbon tweeter on the other side.  Though it still seemed having both supertweeters turned on was far better than having them off, and much more so than ever before in a long time.  I figured I could live with the assymetry, maybe, for a few more weeks.

I didn't set up the new super tweeter on the right side firstly because that side is near the entrance to the living room, where I and many other people are constantly brushing against (or hanging on to) the  Elac supertweeter, held in place with a clamp.  Until I get the new stands put together, I won't have any way of holding up the LS3/5A cabinets which now "host" the D21AF tweeters (part of my experiments 1986-2004 with highly modified "LS3/5A" speakers...including the new tweeter).

I first figured that in order to have the D21AF inside a LS3/5A cabinet on the right side, without getting constantly knocked over and possibly damaging any or all of the new super expensive and super unobtanium components (the speaker box, the B110 woofer, the D21AF tweeter).  The new stand has a "split" post which means I could wrap a series of nylon tie straps under the stand platform and over the speaker, securing the speaker to the stand.  The new stands would also be lower, which I figured would mean less chance of getting knocked over.

But I took another look at this problem Friday night.  It appeared to me that having the speaker lower than 40 inches would not make it less likely to get knocked over.  If anything, a speaker placed at 32 inches on top of the shorter stands (which will require a weekend of assembly now) would be *more* likely to get knocked over.  The higher speaker is actually not in the way of swinging hands.

Only then I sought an answer to the essential question online.  How does one hold a speaker to a stand so it doesn't get knocked over?  I hadn't faced the problem before because I had only used stands in the bedroom near the corners, where there was no chance of the speaker getting knocked over.

And the answer is, as I should have known: Use Blu Tac !

Just a matter of months ago I had renewed my supply of Blu Tac (for which supermarket tac made by Dap and others is useless, useless, useless and only blu tac works correctly).  So I had several unopened packs of Blu Tac ready to go.

This turned out to be pretty easy.  Except that I first blu tac'd the wrong end of the speaker onto the right 40 inch stand, and had to peel it off (almost completely), wipe clean, and I decided also to use some of the moisturizing Pledge, sprayed on a cloth in the kitchen (and the overspray temporarily made the kitchen floor slippery, maybe this should be done outside).  Finally I had the speaker blu tac'd into place and hooked up with a 0.5uF capacitor as on the left side (the actual 6dB "cutoff" into an 8 ohm load would be 40,000 Hz...but since the level is set reasonably well at 20kHz, this means that there is an extra boost of 6dB at 40,000, which might even be somewhat helpful).  This seems to work well.

But now it was clear that the lack of super tweeting toward the back wall was flattening the image.  Where the image should be behind the plane of the speakers sounded strangely sucked out, the image want to be only in the plane of the speakers or more forward.  I really really wanted to go ahead with adding the Vifa NE19VTS tweeters I had purchased recently to the back of my supertweeters.  After  a quick survey of tweeters below $1000, and in cloth domes which dampen the high frequency resonances, the NE19VTS is a stand out winner with nearly flat--actually slightly elevated--response to 40kHz, which actually looks remarkably similar to the D21AF on the charts.  The more expensive cloth domes, especially including the 19mm Eton suggested as replacement for D21AF, just don't reach 40kHz.

I had imagined this as an elaborate wood working project, making a small box for the D21AF in front, and the NE19VTS in back.  Such a box would need to be very non-resonant.  It should be big enough to house and protect the two tweeters, and have enough space for crossover components, but otherwise be as small as possible.

Well it turns out I already have the D21AF mounted in about as non-resonant a box as I could ever make...the LS3/5A cabinet.  It is somewhat too big for the new intended purpose, and has a useless B110 driver mounted up top which surely isn't an advantage now.  But otherwise, it works, and it might take me hundreds of hours to come up with something better (I'm no wood worker, though I have access to a maker space).  Hundreds of hours I'm not likely to have until I retire in 5 years.  And, lets face it, there are more important things in stereo, let alone life, than perfectly sized supertweeters.

But I would be extremely loath to modify the LS3/5A cabinet more than I have done already (to mount the D21AF, I very slightly enlarged the inner circumference of the hole for the original D27, which could still be re-mounted at some point, and I have two NOS T27's ready to go for that day).

I wouldn't even like to make a small hole in the back, near the top center, for a single small screw to hold in the Vifa tweeter, and the tiny size of the mounting ring on the Vifa suggests that wouldn't work very well either.  For a number of years in the late 1970's and early 1980's I used tweeters mounted with one screw to flat piece of particleboard which sat atop my Advent speakers.  As with the LS3/5A, I hated the Original Advent tweeters.

I agonized for quite awhile over the possibility of making a small screw hole.  But I feared in might cause air leakage, and for sure it would reduce the resale value of my LS3/5A's, should I ever have time to put everything back in place enough to resell them.

But now that I could see how strong Blu Tac is, I figured I could just Blu Tac the tiny Vifa tweeters to the top of the LS3/5A cabinet.  And sure enough, it works and seems quite strong and perfectly stable.

At first, I just connected them to the same 0.5uF capacitor which cuts out the non-ultrasonic from the D21AF.  But I immediately measured and noticed a large drop in output on both sides.  It took a few minutes for the idea to hit me--of course I am changing the load from 8 ohms to 3 ohms (4 and 8 ohm tweeters in parallel) with the same capacitor, so the cutoff is now 3 times higher, or that much more loss (8dB or so) because of the way the high pass is attenuating below the "cutoff" frequency.

While I think I normally have pretty good electronic intuitions, this one had just blasted past me, though I was thinking a lot about the effect of the "3 ohm" load on my Parasound HCA-1000A amplifier.  But it should be able to handle any load with low enough output and this is very very low output, and it isn't tnat unreasonable of a load for such an amplifier either, and actually the tweeters themselves are getting higher and higher impedance...the ultimate limiting factor, the voltage attenuation of the capacitors merely being *additional load*.  So the only place where the capacitor drops to a low value like 2 ohms is going to be close to 100 kHz, and at that point the tweeters themselves will have way above normal impedance because of voice coil inductance, so their parallel impedance will be much higher, perhaps around 8 ohms.  So, there is no real "load" problem here, despite having two tweeters in parallel, at least as far as being a problem for the amplifier, the capacitor attenuation network takes care of that, and there basically wouldn't be anyway.

But I needed to add capacitance.  It seemed a better way was to simply use a second capacitor for the new Vifa's, which I probably should have thought of right off.

Since the Vifa has lower impedance, it would help, I figured, to have a higher capacitor value, such as 1 ohm, but I didn't have a nice matching pair of 1uF poly caps in my junk box, just a second pair of the 0.5uF's, so I decided to go with those.

Very carefully I've done all the new soldering, after the attachment of the right super tweeter to the stand with Blu Tac, actually in the living room, while the cat was not around, and without spilling the least bit of solder (except at one point a wire dripped the tiniest amount behind the speaker, which I cleaned up).

In fact, I even did all the second capacitor soldering with the music playing, and got treated to some tiny sparks.  In retrospect, I wonder if that's the best way.  The currents must have been tiny though.  Nothing seemed to have been harmed and the solder joints seemed as good as I have ever done, using my relatively new (NOS) classic soldering iron I bought from recommendations at DIYAudio.

And when it was done, it was wonderful.  Though the back level is indeed a few dB lower because of the load, it nevertheless works wonderfully, and there almost seems to be an omnidirectional effect when I measure with my iPhone RTA app.  Having two tweeters, front and back, somehow produces sound all around at 20kHz.  Possibly mostly reflected sound, but it decreases with distance from the speaker.

The ultimate effect is far better than the semi-broken Elacs 4PI's  (To compete now, the Elacs will need to be fixed.)  The 3D time machine is back.

I proceeded to re-adjust the DSP to make the RTA spectrum smoother and flatter.  I moved around the control frequency above 2k and around 6k, for an utterly smooth rolloff above 3kHz.  I added two new filter frequencies 630 and 500, and first went after the bigger 630, but later found that if I fixed 500, I could then tweak 630 just a little.  I was afraid to do this just on the advice of a 1/6 octave RTA, but that is better than what many have used and still do.  I prefer to set these things using oscillator to find actual center frequency, but I didn't have time for that, though I was intending to do so.

Noticing that my GEQ of deep bass to a 3dB rise at 20kHz (in the face of many  many PEQ"s which cancel in this very area) wasn't helping at higher bass frequencies--which were depressed, I raised the entire subwoofer level 4dB instead, which greatly flattened the bass response, but then lowered to only 2.5 dB higher.

These and other changes led to the smoothest RTA I may have ever seen.

And it has sounded wonderful, playing CD's, FM and LP's.

More transparent than ever before.

Turning the panels off when playing pink noise, the ultrasonics seem to make a huge effect on the bass from the subs.  With the super tweeters, the bass sounds thunderous and authoritative.  Without the super tweeters, the bass sounds artificial.

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