After the incorrect polarity shown in the ARTA time record of my speakers (it appears that ARTA generates an out-of-polarity transient, and doesn't correct this by default in the Time Record display), the other curious issue raised in recent measurements is that my tweeters appear to peak just above 20kHz and then response plunges down after that. But I had chosen the Dynaudio D21AF tweeters precisely because they are specified to 40kHz and virtually flat to that point. What gives?
[Update: What I didn't discover until after writing this post: I had set the analog input sampling rate on the Tact to 44.1kHz, so obviously there was a brick wall filter at 20kHz, the effect of which I attributed falsely to the microphone. Normally I keep all sampling rates set to 96kHz, but on some previous occasion I had reset this one, which I rarely use except for testing purposes, to 44.1, or a power interruption occurred and it went back to the default. Moral: check everything, and I mean everything, even things you "never" change or forget about. And expect that a lot of your serious "findings" are really the result of technical errors.]
I have verified that the Focusrite Scarlett has essentially flat response at least to 23kHz in the 48kHz sampling mode. I have only just discovered that the Scarlett also supports higher sampling rates, up to 192kHz, so I would expect it to have a cutoff even higher than 23kHz.
But could it be that my D21AF tweeters have aged, or the response graph shown in the litereature was wrong?
I also measured the VIFA tweeters I used on the back side, which are supposed to have fairly flat response well above 30kHz. Those also appeared to show plunging response above 20kHz.
I suppose those might not actually meet their specs either.
I was lured into the belief that my testing microphone has response to at least 25kHz, because the calibration file provided by an independent calibrator goes that high. The microphone, btw, is a Dayton Audio EMM-6, calibrated by Cross Spectrum Labs. (The original NTI microphone was not available.)
So I pulled down my Elac 4PI tweeters from storage. These are supposed to have flat response to 35kHz, and they very nicely made, and it's possible I might start using them again. The new steeper crossovers I am using might prevent stimulating the metallic resonance around 6kHz. Rapping on the speakers, there is no indication of any metallic sound from the ribbon foil only a slight bell-like sound from the round top cover, which looks like it could be removed which might enable me to straighten the foil on one unit.
Once again, I put the EMM-6 within a few centimeters of the tweeters, right up to the foils. And measurement still showed plunging response above 20kHz.
I'd say I've proven that the plunging response is caused by my measurement microphone, despite the calibration to 25kHz (and I am using the calibration file in ARTA, which only shows a few dB loss at 25kHz). Online sources do show the EMM-6 generally rolling off fairly fast above 20kHz, but only about 10 or 15 dB by 30kHz.
In this review, the EMM-6 measures fairly closely to the Earthworks M30 up to about 30khz. But why does the Earthworks, supposedly flat to 30kHz, so why does the Earthworks show 10dB loss at 30kHz? These tests were done just like I am doing, stick the microphone as close as possible to a supertweeter.
This worries me in several ways. What if I were to get an M50, as I hope to do soon, and find it measures no different? I'm thinking what is going on here is that the electret element in the EMM-6 has deteriorated in such a way that the high frequency extension has fallen.
I have a few Bruel & Kjaer microphones with measured response up to 50kHz. But I've always wondered how to use them, and it now appears they require being mounted to a Bruel & Kjaer preamplifier wand which supplies the unusual 200V polarizing voltage. I do happen to have 2 funky surplus Bruel & Kjaer 2209 sound meters which have a built-in preamp and everything. One of them actually works. But for those, I had never figured out how to connect any cable to the output jacks which are most unusual. They look like banana sockets but are smaller. I once ordered a Bruel & Kjaer plug, a JP 0101, but it was too large. It was very expensive--it cost me about as much as the surplus meters themselves--and I felt devastated. At that point, years ago, I gave up.
Today I believe I have figured out that the plug required is the Bruel & Kjaer JP 006. This is actually documented in the 2209 manual but somehow I overlooked it. (It is not on the specifications page, but in the text describing the output and other jacks.) I have now ordered one of these rare plugs from eBay.
I am not lusting over an Earthworks M50. But given the cost, and the excess amount I've already spent on audio gear this year, I'm going to hold off until meeting some personal goals.
I put a fresh 9V battery in my Mitey Mike, and it's completely dead. I bought it over 15 years ago but never even took it out of the box until last year, when it turned up as I was cleaning my climate controlled storage building. To determine if problem was preamp or mike I angled the circuit board out and connected clips to the microphone and fed those to the Scarlett input, turned all the way up. Still no signal on shouting or tapping or running sweep tests. Finally, I noticed that just touching the microphone input terminals on the circuit board caused bit pulse. It looks like the amplifier is working, but the mike is dead. It could even be that I bought it this way and planned to replace the microphone, I recall seeing a loose tiny condenser about 10 years ago. But like many projects, everything just sat until the related pieces are hard to bring together.
Then I tested the "IMP" microphone which I bought with LAUD3. Online sources now describe this microphone as electret. I simply hooked it up the the Scarlet, with level turned all the way up, and ran sweeps, shouted, and tapped. No signal at all. It might have required polarizing voltage, but the Fiji card is only equipped with 5V, so that's likely what it would have needed. The 48V supplied by Scarlett might burn it out. In fact, I recall this issue coming up before, it could be that I incorrectly attached the microphone to the Fiji with the 5V turned on, when it should not have been turned on, so actually I burned it out way back then. Anyway, I dared not turn the 48V on now, for fear that would be way beyond it's safe operating region.
Likewise I'm somewhat afraid to hook up my Tact microphone to the Scarlett. I recall the Tact used some lower voltage, like 15V. 48V might fry it. I'm not going to risk this unobtanium microphone. I always found the Tact measurements confounding, but still interesting, and all of my earlier time alignments were based on them.
So now I've got to focus on the personal goal I've set for myself before getting an M50. That is, to read and perform the "Tutorial" for the HP audio spectrum analyzer that I bought this year. The unit is powerful but not totally intuitive or self explanatory, and otherwise will just sit there for decades like a lot of my stuff.
I've basically "done" the first pass on crossover adjustments AND time alignment. The fine details above 16kHz, for which I need a better microphone, are probably not that important. I'm certainly getting response to 20kHz if not far higher, and the peak at 20kHz I'm seeing may also be bogus, since I see that on my Elac supertweeters just as much if not more, and without the small value coupling cap intended to raise response at 40kHz by 6dB on the cloth dome tweeters (which would be a tiny amount compared with the 20kHz peak I've recently been seeing, but may well be microphone error).
No comments:
Post a Comment