Deang Posted March 31, 2008 Share Posted March 31, 2008 Shawn, at the top of Bob's plot I believe it says, "Input random noise at 500mv 1 watt output 8 ohms". Mark, right -- most pro guys find them to be on the light side. You'll see the same arguments you see here about power and how all watts aren't created equal. If you go to the QSC forum you can find threads with posts from guys who say anything less than 50lbs isn't worth putting in their rack. Most of these discussions relate to switching power supplies -- a lot of pro guys don't like them. I shouldn't have to point out how uselss that kind of information is to those of us connecting them to a 106dB loudspeaker sitting 12 feet away. When I watch movies the room is rocking pretty good, and I have yet to see even the first indicator on the meter light up. I don't care if the power supply squeals and groans at 200 watts -- I think I have yet to break the 40 wpc barrier with mine. I guess it's a crappy amp for portable sound reinforcement. Quote Link to comment Share on other sites More sharing options...
BEC Posted March 31, 2008 Author Share Posted March 31, 2008 Shawn, The test at 1db per major division is with the built in noise source of the HP spectrum analyzer. With resolution as high as 1 db per major division, sampling becomes a problem using a swept signal. Just the time involved to get a good number of samples at every frequency with the swept sine. The trace I posted was 1000 samples. 500 samples looks almost the same. 100 samples is a bit rougher. Of course, the more samples, the better the average. At some poiint though, chances of noise spike, ruining the stored samples makes you want to stop increasing the number of samples. Bob Quote Link to comment Share on other sites More sharing options...
sfogg Posted March 31, 2008 Share Posted March 31, 2008 Dean, "Shawn, at the top of Bob's plot I believe it says, "Input random noise at 500mv 1 watt output 8 ohms"." Thanks, I missed that. One possibility is that the variations may be from the random noise. I think Bob applied averaging to that measurement, which he should since noise will have a Crest factor. What I can't tell from that graph is if this is a transfer function or just a straight measurement of the noise. If it is a transfer function the graph is basically showing the difference between the input and the output, in which case the variations are being introduced by the amp. A transfer function may not work properly on the XTI though due to the latency through it. I'll have to try that when I receive mine to see what that looks like. If this is a straight measurement of the noise itself it could be the noise isn't flat or that the amp isn't flat or there wasn't enough averaging to even out the Crest factor. Measuring hundreds/thousands of points (size of the FFT) all at once with noise is a little different then measuring a couple of dozen points one at a time with an oscilator. Shawn Quote Link to comment Share on other sites More sharing options...
Deang Posted March 31, 2008 Share Posted March 31, 2008 "BTW, please man, don't take this personal. It's interesting, and that's all. And as Roy insisted, we ought to make it educational." I'm cool. "Ok, I get the argument. However, have you created an inconsistency for yourself? On the one hand, many argue that "only with 200W can you ever experience real music because of dynamics." (I think this has been argued many times here.) Then, you make an argument that you never pushed the amp past 40W. If so, a nice 40W amp would be sufficient, no?" I don't think I've ever said you need that much power in the house with horns, it's complete overkill. I"ve pretty much used the 5 to 1 rule since 1977, so the power requirement is really just relative to your source material and average volume level. Almost all of my music CD listening is done at less than a watt now, but I dig movies and DVD concerts where I do tend to jack things up a bit, but yeah -- 40 wpc would be plenty for me. But then I would have to build passives the DeanG way, which is about $1200 -- and then of course the amp. I did compare a single Crown with the first set of passives I built to a single Crown with the DSP doing the active filtering, and the latter really did sound better to me. There's no doubt in my mind that a set of cost no object passives and some really good tube gear would top what I'm running now -- I've said that many times. I think the amps are a tremendous value for someone taking the Jubilee plunge and still needing both filtering and amplification -- I think that particular combination sounds pretty good. I don't imagine any XTi user will stay with them indefinitely, well all of them expect me -- I have to wait until mine "squeal and groan" to a halt. "There is some meaning in real physical terms as to why an amp can't put out it's rated power in sustained ways. And, that meaning will impact what the amp does in PEAK situations too. Why do "audio" amp guys go to so much trouble making huge stiff linear supplies that will put out 200W all day long into a low impedance load, but the "pro" amp - which is touted at being so bulletproof and superior, won't put out it's rated power even for a minute? Surely, you find some of this a bit curious, do you not? I do." Well, it's certainly odd. I can't remember, did you do that with a 1kHZ test tone? The thing is rated at 275 wpc using a 1kHz tone, doesn't the available power drop if you do 20 - 20,000? Quote Link to comment Share on other sites More sharing options...
BEC Posted March 31, 2008 Author Share Posted March 31, 2008 Shawn, I am not using transfer function. Bob Quote Link to comment Share on other sites More sharing options...
Duke Spinner Posted March 31, 2008 Share Posted March 31, 2008 why ..?? mostly meaningless for Audio Quote Link to comment Share on other sites More sharing options...
Deang Posted March 31, 2008 Share Posted March 31, 2008 Heh, sorry Mark, gotta a little carried away there. Definitely some head scratching stuff going on. Shawn gets his in the next day or so, if he throws his hat in with the head scratchers -- I'm selling them. I think slew rate might be a factor. Wasn't that a big thing with Luxman and Sansui in the early 80's -- some of those amps sounded pretty good. Quote Link to comment Share on other sites More sharing options...
Duke Spinner Posted March 31, 2008 Share Posted March 31, 2008 more than 30 micro volts/second ..any modern amp oughta do that .. More ...is meaningless for Audio .. were not using MicroWave Technology here .. Quote Link to comment Share on other sites More sharing options...
Don Richard Posted April 2, 2008 Share Posted April 2, 2008 How is it meaningless? The transient attacks in music are slower than 20V/usec, the "fastest" speakers are also. Don Quote Link to comment Share on other sites More sharing options...
Duke Spinner Posted April 2, 2008 Share Posted April 2, 2008 thank you, Don .. hadn't got back to this thread .. as I stated .. more than 30mv/s .. seems meaningless Quote Link to comment Share on other sites More sharing options...
Don Richard Posted April 2, 2008 Share Posted April 2, 2008 Where did the figure 20V/uS come from? I should have said that an amplifier with 20V/uS slew rate can reproduce any musical transient without distortion. Came from PWK in an old Dope From Hope. If you feed a square wave into a loudspeaker you won't get anything resembling a square output, unless you use mic positioning tricks. I saw that 2 weekends ago when a friend SMAARTed an installed system that he was aligning. Output @ 200 HZ resembled a clipped triangle wave drawn by a 5 year old. Amp output(QSC) had rounded corners, looked reasonably square. Don Quote Link to comment Share on other sites More sharing options...
tigerwoodKhorns Posted April 2, 2008 Share Posted April 2, 2008 Mark, I'm sure that I am not the first to post this, but nice new avatar and system profile. Quote Link to comment Share on other sites More sharing options...
Rivendell61 Posted April 2, 2008 Share Posted April 2, 2008 I guess there is no interest in measuring the slew rate? Raw slew rate by itself--high or low--is not reliably useful re performance/sonic info.An amplifier with a high slew rate may have elevated slew rate distortion at lower levels vs a slower circuit. What we really trying to discover is: how linear is a particular amp at the fastest slewing frequency?A twin tone IMD (CCIF/ITU-R 18.5 kHz and 19.5 kHz) = an appropriate diagnostic tool.If that result is good.....amplifier slew rate is fine. Mark Quote Link to comment Share on other sites More sharing options...
Don Richard Posted April 2, 2008 Share Posted April 2, 2008 I'd never suggest putting square waves into speakers of course. Correct. Absolutely never ever put square waves into a tweeter. He was sending about 1 watt @ 200 HZ into full range speakers rated for 800WRMS. Don Quote Link to comment Share on other sites More sharing options...
sfogg Posted April 3, 2008 Share Posted April 3, 2008 OK, had a little bit of time with the Crown on the bench tonight. First things first... my Crown seems to have about a 3dB boost at 20kHz as compared to 1kHz. The FR starts heading up at abot 5kHz. I checked this with DSP OFF and with DSP OFF and the Limiter turned off too as set by the front panel. I want to plug in the computer to verify all the settings are flat though, it is totally possible there could be a bug in the interface. Also for those biamping Jubs with the Crown and Roys numbers it is possible this was accounted for when he worked up the measurements for the Crowns. This boost was consistent at 1v output, 3.3v output and 9.1v out all referenced to 1kHz. Next up some distortion measurements.... Shawn Quote Link to comment Share on other sites More sharing options...
sfogg Posted April 4, 2008 Share Posted April 4, 2008 Tried looking at distortion at 1v output into 8 ohm load. This is about 0.12w output. 500hz is the fundamental all the other markers are at harmonics of 500hz. The second harmonic is down almost 75dB, that is low distortion. Shawn Quote Link to comment Share on other sites More sharing options...
sfogg Posted April 4, 2008 Share Posted April 4, 2008 However if you look at this on a scope it looks nasty. Quote Link to comment Share on other sites More sharing options...
sfogg Posted April 4, 2008 Share Posted April 4, 2008 .... because of noise in the output. Same thing but applying averaging in the scope to pull the signal out of the noise. Signal itself looks fine again... Quote Link to comment Share on other sites More sharing options...
sfogg Posted April 4, 2008 Share Posted April 4, 2008 Same thing but at 1kHz. Harmonics are a little higher here.... Quote Link to comment Share on other sites More sharing options...
sfogg Posted April 4, 2008 Share Posted April 4, 2008 On the scope in real time.... Quote Link to comment Share on other sites More sharing options...
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