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Old 9th January 2018, 07:10 PM   #1901
AcoustatAnswerMan is offline AcoustatAnswerMan  United States
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Originally Posted by DeltaStar View Post
What is the actual xo point of the 121 interface, I figure it to be around 1200hz and 6db, but I could be way off on this.

The MK-121 does not have a traditional "crossover", like you might find dividing the signal between a woofer and tweeter. The LF transformer is run full-range, and there is a high-pass filter feeding the HF transformer. As the LF transformer begins to naturally roll-off, the HF transformer takes over. There is actually considerable overlap of operation between the two transformers, so one can't give a specific frequency for the transition.
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Old 9th January 2018, 08:47 PM   #1902
bolserst is offline bolserst  United States
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Measurements showing the large overlap range were posted here:
All Acoustat panels can give

You can see that the Cmod shown in Attachment#3 does a good job of reducing the LF energy the HF transformer has to handle compared to early circuit shown in Attachment#2.
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Old 9th January 2018, 09:39 PM   #1903
charlesp210 is offline charlesp210  United States
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Default Super Tweeters

Originally Posted by Jrimer View Post
Has anyone ever tried a super tweeter with any Acoustats? I know the Townshend Super Tweeter was supposed to work well with Quads. I'm wondering if this would fill out the top end that can sometimes seem lacking.
Yes, I have most often been using a super tweeter. Though, not because I have ever felt the top end was lacking. My general impression on my 1+1's is that the high end was "too much." After several years, I discovered the main answer is to listen slightly off axis, as Acoustat originally recommended (the recommendation I read was to point the speakers somewhat toed-in relative to the listening position) except that I toe the speakers somewhat to the outside of the listening position, still not quite parallel to the front and back walls, however. I find listening about 10-15 degrees off axis to give the most correct sonic balance, though it measures (1/6 phone analyzer with pink noise) a trifle rolled off. This is what nearly all speakers do, it seems a slightly rolled off response (as measured) is what generally sounds correct. Many well liked speakers roll off the highs FAR more.

I was also making the "too much highs" situation worse by connecting to the wrong transformer winding, giving me proportionately more highs and less bass. This may not be good for reliability reasons as well. It seems best both for sonic and reliability reasons to connect the LF transformer to the correct winding.

Also, being a capacitive load, the high frequency damping factor of the amplifier is extremely crucial in determining the sound. I use an amplifier (Krell FPB 300) with a very high damping factor everywhere including the highs, and power response to 300kHz. If I use a "simpler, less feedback" amplifier, such as my newly acquired Hafler 9300 TransNova, there is some loss in the highs. Using the Hafler, which I've only started trying, the best frequency balance might be straight on axis. As of now I strongly prefer the sound of the Krell. On paper, however, I see much in the 9300 design to like...it is the simplest and most effective power amplifier design I've ever seen, and it needs hardly any feedback to get distortion down to the 0.002% level.

I also like the slightly off axis listening position because it varies less as you move your head around at the listening position. The difference between 12 degrees off axis and 13 degrees off axis is negligible, however the difference between 0 degrees and 1 degrees is huge. So being off axis removes the head vice.

I listened to a friend's 2+2's last year, and discovered their sound on-axis is very much like the sound of my 1+1's at 15 degrees off axis. And that may be because the actual electrostatic panels inside the 2+2's are canted off axis by about the same amount as what I'm doing.

And then, there is the high frequency adjustment in the Acoustats themselves, which I've never felt a need to tinker with.

I do super tweeters because I'm a true believer in supersonic affects. I've heard systems such as Plasmatronics and heard legends about the Iverson Corona speaker back in the day when it actually existed, and ribbons such as the Sequerra having response to 100kHz. My goal nowadays is to alway have response to 40kHz, not because I will hear more highs, but because it seems to do other good things, like add more detail to the bass especially. This is a controversial area, but there have been a number of one-off published experiments showing benefits to extended HF response. I want to be on the leading edge of proving it makes a difference, even though proof (through double blind testing, for example) is shakey at this time. Many argue that what's needed is the better transient response (or slew rate, etc) that you get from a higher bandwidth system.

With my Acoustats, I measure virtually flat response on axis to 19khz, falling off above that, and that is consistent with many published measurements. That's fine and good so far as what I can reliably hear, but not to meet my design goals.

Anyway, if you are going to use a super tweeter it's pretty obvious you can't use a one-sided supertweeter like the Sequerra (though, I haven't tried). It's easy to imagine that a dipolar tweeter might work best. Along those lines, a dipolar ribbon might be the ticket, hung off the side of the speaker at listening height.

What I actually did for many years was to use an omnidirectional ribbon tweeter, and Elac 4pi. It seemed to do all the things I wanted. Bass seemed tighter. I generally crossed over the Elac at 19khz-20khz with 4th or 8th order DSP crossover, in addition to it's own built-in crossover (which in my model is non-defeatable). Crossed over like this, I heard either nothing or extremely little coming out of the super tweeters, yet it seemed to have a positive effect I didn't want to go without.

My Elacs 4pi's were damaged by several high power accidents over the years, and the aluminum ribbon has gotten quite warped (as they often do with thick true ribbons) and I became afraid last year that I was hearing a slight metallic sound below the crossover point caused mechanically.

Nothing on the market looked affordable, so I decided to make my own supertweeter using a pair of D21AF's left over from the speaker I used (and mostly loved) from 1988-2009. But it was clear immediately that I needed to add a back tweeter also, to make it bipolar. I wish I had purchased all the D21AF's I could have in the 1980's for $49, that is the most amazing tweeter ever IMO and surpasses even the various Esotar and Esotek replacements in flatness from 20-40kHz because it's a 21mm speaker and not the 28mm used by later models which now run over $1000. Meanwhile I am not impressed by the supersonic response of many currently highly regarded tweeters, including the famed Scanspeaks. Judging from manufacturer info alone. After a week of examining all the online information, I decided to get a pair of Vifa NE19VTS. You might not like this if you are trying to crossover to a woofer, but in it's supersonic range 20-40khz it is about as good as you can buy from any dynamic driver. AND, they are cheep! I was only shocked to see how small they are compared with my D21's that have about the same frequency range.

Anyway, I very much like my home brew supertweeter using D21AF's in front and Vifa NE19VTS in back. It works even better than my Elac 4PI, and adds no audible colorations at all. In operation, I hear nothing coming out of it at all, and yet I can measure the supersonic response all around it. I have used it positioned at 42 inches high though I think it would be better lower at just slightly above ear level. I place it on the outside of my 1+1's.
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Old 10th January 2018, 10:08 PM   #1904
mraudioguru is offline mraudioguru  United States
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Originally Posted by tyu View Post
Could you report back on the sound change that you hear.. by going to the C-mod from the stock setup?...An which one you like?..
After all with the C-mod we are adding a crossover to a interfaces that the Stock 121 transfourers never had.......
thanks for your time
Absolutely, I will update this once I've completed the "C" mod install. Still vacillating on the Cap purchase, but everything else is here. Putting on new grille cloth also. These should look pretty much brand new when I'm done...
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Old 12th January 2018, 09:33 PM   #1905
charlesp210 is offline charlesp210  United States
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Default Now looks like LF transformer needs replacement after all

I repeated my latest test on both LF and HF circuits in isolation. Running the LF transformer by itself causes my amp to heat up abnormally, whereas the HF does not. So I'm now working on the LF transformer is bad theory. I changed the speaker wire recently so I'm going to test going back to what I had used in previous months, but that would not explain an HF/LF difference. It looks now like I will have a definitive result by Saturday in time to start the LF transformer replacement if needed (I have one on hand that I ordered last year) over the rest of the 3 day weekend, bringing to a close 6 months of testing.

But this "amplifier overheating" problem is something I had not seen until recently. The original problem with this speaker was that it caused my amplifier to shutdown in 60-120 minutes, fairly consistently. That particular problem seemed to have been fixed for a couple months now by replacing the factory electrolytic capacitor with a brand new 100V bipolar electrolytic. That was easy to do, and cheap, so I just tried it, and it seemed to make things better.

After I made that capacitor replacement a couple months ago, I ran the one speaker without issues for a week, then decided all was normal and put both speakers back online. Then, after another week, I had another amp shutdown. This was bad, because I didn't know which speaker had caused it.

That kicked off a week of trying to get one or the other of the two speakers to fail by itself. Nothing failed. So then rather than just playing music, I decided to try a seriously difficult test. There is a SpeakerPop app for iphone, and there is (or was) a test signal which consists of a series of full range (up to digital max) step-like pulses every few seconds. The pulses only go positive, and hold the postive level for about half of the total interval. I don't know if the creators realized how dangerous a signal this is for a true hifi system. I immediately realized how dangerous it was, but gingerly used it at low levels to start.

Well, for a serious test I tried ramping it up. When I first started this kind of testing, I still had my 100 Hz high pass filter (4th order LR) inline. I quickly found that I could turn the signal up to 0dB on my system (which corresponds to 200W peak at 4 ohms or something like that) without any problems. And I tested both speakers with this deadly signal for hours and nothing bad happened. As usual, the interface box was stone cold (it never heats up as long as there is a 100Hz high pass filter on the Acoustat 1+1's).

So then I tried ramping it up a bit more. I switched off the 100 Hz high pass filter, and ran the test again. In the left speaker, I was running the LF transformer by itself (just as when the unexplained shutdown had occurred). Since I had run the previous test for hours without any issues I wasn't paying full attention. After about 40 minutes the left speaker blew a 3A fuse (I'm using 3A fuses now, as some recommend with high power amplifiers).

When the fuse blew, I checked the interface box, and it had heated up enormously, maybe to 110F or higher. I should have realized that the LF content of this test signal would stress the transformer badly.

In a sort of panic, I tried this same signal on the other speaker with both transformers connected. It immediately blew the 3A fuse on that speaker. Uhoh. I decided not to use this test at 0dB on the speakers full range anymore. However to ensure I had not damaged the speakers, I turned the 100 Hz high pass filter back on, and ran the test at the full level for an hour without any issues, and the interface boxes were stone cold.

Anyway, it's possible that this test either *caused* the present amplifier overheating problem, or it simply made the underlying LF transformer problem worse. Given the preceding unexplained shutdown, and the current overheating problem, my belief now is that the test only made an existing problem worse.

Anyway, if someone starts seeing issues like I have, my first suggesting would be to replace the electrolytic with another suitably rated electrolytic. That seemed to help if not completely fix one of my speakers. It's possible that fix by itself would have been sufficient had I not proceeded to do some high stress testing.


I used a flashlight last night and also determined that the "upside down" way I mount the interface cover with binding posts on top does in fact still line up with the screw holes. So, judging by the screw holes, the designer permits you to mount the cover either way.

I'd correct my two earlier posts this week with these latest findings except they seem to be too old to change now.
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