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402a clipping mids & 602b doing same


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#1 stevetwinfalls

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Posted 06 February 2006 - 08:50 PM

Hope somebody can give me some insight as to what is happening. I recently purchased Bose 901'series 6's to be run with a used xls402a....bose active eq in between yamaha reciever's pre-outs, and 402's inputs. This is being used as a home system (I've heard good things about Crown for home use) Problem being, this setup seems to illuminate the clip leds way too soon, and it seems to happen only to the mids, the bass is good and strong, no clip leds during heavy bass passages, and the highs stay crisp when this is happening. Bringing the level up will start the system into audible clipping of the mids only. My first train of thought was possibly an amp that was a little weak on power supply, good voltage in, 124-127 during clip, so that rules that out. next thought was maybe a weak amp, so lets rule out two things, it was replaced with a new xls602b, that should rule out not enough amp, and cure the problem of anything being wrong with a used amp. The 602 didn't cure the problem, same exact problem. I have tried to change my gains to see if the problem is prior to the amp, but the problem seems to be at the same output level regardless.  I did change wiring to the speakers, thinking inductance/capacitance may be creating a passive filter of some kind...yes my brain fade 18ga won't cut it, it was changed to 14ga 30 foot runs, with some improvement. Is it possible that the 901's change impedance through the midrange enough to sap a 602 dry that soon? There is definitly something that I'm not seeing with this, time to turn it over to the pro's. What's your opinion??
                                                 Steve

#2 DGlass

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Posted 13 February 2006 - 09:01 AM

It’s possible there is an Impedance anomaly with the 901 speakers but without an impedance curve for the speakers it would be hard to tell and they don’t publish that spec.

More likely it is related to the amplifier being overdriven by the Bose processor. The Bose speakers require some serious boosting a certain frequencies and the processor may be, at those levels, putting out a voltage that is too high for the input of the amplifiers. The input sensitivity of the XLS402 is 31db/1.025 volts and the 602 is 31db/1.26 volts which would be alright in most cases but I can’t say what their processor is doing.

#3 stevetwinfalls

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Posted 13 February 2006 - 09:18 PM

Thanks for the reply,  I finally received a service manual from bose on both the active eq, and the speakers. I see a section where they refer to the amount of boost, and cut at certain frequencies. 35 hz.9.8db. 100hz. 0db (reference) 225hz. -5.8db 650hz.-7.7db. 1.85khz. +.2db 3khz. +.5db, and finally 13khz.+12.2.  It would appear from the spec's on my source unit, that the input to the bose eq. would be variable from 0-1 volt. If I interpret db properly, at 13k. I in theory, could be asking the input of the 602 to accept 4 volts, which is indeed beyond it's rated spec by about 2.5 volts. Now if the amp was receiving enough input to drive it to clipping, it should be producing it's rated output power (producing it's 31db. gain)? If indeed that is the case, that's a serious amount of power to be asking those tiny little drivers to take. The 901 being a ported type of enclosure would exhibit an impedance drop at resonance, which does appear to be in the low midrange if I interpret their eq. curve properly.  
    Another theory, (this is what I had wanted to find out from the service manual) Is it possible that Bose would use a zener diode and cap as a power limiter? The reason I ask is that they publish in their literature use with amplifiers having a maximum power of 450 watts, but in their service manual they list amplifier power requirements as "unlimited in noncommercial applications" That type of limiter would be somewhat crude, but would certainly get the job done, along with testing amplifier protection circuits. It would appear if that is the case, the crown is functioning properly, exactly as designed.  
    I should be able to do an unequalized frequency sweep with an ac ammeter connected between the amp and speakers and be able to produce some sort of impedance curve shouldn't I??  
    I hope this gives you a little more info to go on, it seems that I'm burning off a lot of power somewhere, I just wish I knew where

#4 DGlass

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Posted 16 February 2006 - 04:40 PM

One thing else to look at is the efficiency rating for the speaker. Although Bose are good speakers, for certain applications, they have never been noted for their efficiency.
Efficiency, usually rated in percent, refers to how much acoustical output there is in relations to the amount of energy wasted as heat. Basically the lower the efficiency the less acoustical output there is for the amount of power put in.
Manufactures publish this specs as the devices “Sensitivity” with a  “db SPL at 1watt/1meter” rating.  It is measured using a specific testing procedure sending a 1 watt signal through the device and measuring the acoustical output at 1 meter with a calibrated SPL monitoring system.
So lets take a speaker, Brand X that has a Sensitivity of 97 db SPL (1w/1m) compared to a speaker, Brand Y, that has an efficiency of 94 db SPL (1w/1m). Brand X would be louder, as a matter of fact, it would be twice as loud as the Brand Y speaker as Brand X has 3db more output. If Brand Z were rated for 91 db SPL (1w/1m) it would be 6 db down in output with the same input level as Brand X. This would make the Brand X speaker sound 4 times as loud as Brand Z with the exact same input. Some people would then try to compensate for this mismatch by driving the input to amplifier of Brand Z higher which would only cause the amplifier to go into clipping sooner as the amp is already loaded the same; the Brand Z speaker is just less efficient.
Add to this to any frequency boosting that Brand Z is doing in their Active EQ and all I can say is you would need an amplifier with more headroom (more output power) to keep the amp from clipping.

#5 stevetwinfalls

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Posted 19 February 2006 - 10:22 PM

I appreciate the little schpiel about efficiency, I can appreciate the theory behind it, and don't dispute one bit the accuracy. Had I not been aware of the inherent inefficiency of the brand in question I would've used a much lower powered home audio amplifier. 600 wpc is a lot of power to be using in one's front room. Over the weekend I did a few tests to see if there was indeed an impedance drop at certain frequencies, and as I thought from looking at the eq. curve there is indeed a notable impedance drop through the low-midrange. This was surmised through the use of an ac ammeter connected between the amp, and speakers, and doing a frequency sweep with a sine wave generator connected straight to the input of the amplifier. The eq. curve seems to very closely match  the impedance variations, so Bose has corrected for this in the eq. I was hoping to have an answer to this little anomaly without having to test it myself. I did note something interesting however. The 402a has power supply filter caps exactly in accordance with the schematic (3-4700uf) per supply rail, the 602b has LESS filter cap per supply rail (2-4700u) instead of the 4 per rail called for in the 602a schematic. I see in the variations of this series, that the designer stepped up the number of filter caps along with the expected output ie. 2per rail on the 202, 3 per rail on the 402, and 4 per rail on the 602. He must have had some logic in doing so. Being in the amplifier industry, you should know the importance of filter caps, and the multiple jobs they perform. It would seem that the 402a with a lower voltage supply rail, would handle transients, and difficult loads with more authority than the 602b due to the additional power supply caps stabilizing the rail voltage. Do I have an amp that didn't get the required number of filter caps, or did crown for some unknown reason decide that less is better?  Don't get me wrong, this is a very nice sounding setup, and the crown powers it very well, however, the  power output at clipping appears to be almost identical between the 402, and the 602, which is why I checked for input voltage drop prior to writing, actually it's why I'm writing.

#6 DGlass

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Posted 20 February 2006 - 10:40 AM

QUOTE(stevetwinfalls @ Feb 19 2006, 10:22 PM)
I appreciate the little schpiel about efficiency, I can appreciate the theory behind it, and don't dispute one bit the accuracy. Had I not been aware of the inherent inefficiency of the brand in question I would've used a much lower powered home audio amplifier. 600 wpc is a lot of power to be using in one's front room. Over the weekend I did a few tests to see if there was indeed an impedance drop at certain frequencies, and as I thought from looking at the eq. curve there is indeed a notable impedance drop through the low-midrange. This was surmised through the use of an ac ammeter connected between the amp, and speakers, and doing a frequency sweep with a sine wave generator connected straight to the input of the amplifier. The eq. curve seems to very closely match  the impedance variations, so Bose has corrected for this in the eq. I was hoping to have an answer to this little anomaly without having to test it myself. I did note something interesting however. The 402a has power supply filter caps exactly in accordance with the schematic (3-4700uf) per supply rail, the 602b has LESS filter cap per supply rail (2-4700u) instead of the 4 per rail called for in the 602a schematic. I see in the variations of this series, that the designer stepped up the number of filter caps along with the expected output ie. 2per rail on the 202, 3 per rail on the 402, and 4 per rail on the 602. He must have had some logic in doing so. Being in the amplifier industry, you should know the importance of filter caps, and the multiple jobs they perform. It would seem that the 402a with a lower voltage supply rail, would handle transients, and difficult loads with more authority than the 602b due to the additional power supply caps stabilizing the rail voltage. Do I have an amp that didn't get the required number of filter caps, or did crown for some unknown reason decide that less is better?  Don't get me wrong, this is a very nice sounding setup, and the crown powers it very well, however, the  power output at clipping appears to be almost identical between the 402, and the 602, which is why I checked for input voltage drop prior to writing, actually it's why I'm writing.
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The amplifiers are as they are designed. smile.gif

#7 boling00

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Posted 19 March 2006 - 09:55 PM

I cannot claim to understand much of the technical details mentioned in this post, but I appreciate the information. I am running a similar configuration as well (Bose 901s w/XLS-602) and I too noticed the clip LEDs illuminating in what appeared to be strange circumstances.

If this is an unavoidable situation with this setup, what is the acceptable limit for allowing the amp to clip? I realize no clipping is ideal, but can the LED flicker occaionally without doing harm to any component? The rate of flicker seems to be less than once or twice per second. Will this do harm over time? Will it shorten the life of the amp?

I got the 901s for free so I'm going to use them in this configuration regardless of this situtation, so I'm just trying to find out how hard I can push this system without doing real damage.

BTW, I'm running this off of a personal computer with the PC hooked to the Bose equalizer, with a Rolls Promatch MB15 between the equalizer and the amp.

Thanks.

#8 stevetwinfalls

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Posted 21 March 2006 - 10:24 PM

Sorry to hear that you are having the same problem. Does your setup seem to have awesome no problem power for bass, but as the frequency raises the clip lights get worse? More listening and testing shows mine to be related to the treble, not the mids as I had first wrote. What volume level does yours seem to start? Prior to getting my first crown (402) I had the 901's hooked to my ht yamaha receiver with only a claimed 140 per channel, the highs seemed to reach the level that I expected from the bose, the mid punch was there, but you could definitely notice the lack of power during deep bass, not serious enough to say that it didn't power them satisfactory though. I had originally hoped by starting this post, that something could be figured out. I still am not sure whether bose has some kind of passive protection built into the 901, but would like to find out. I also am not sure whether something about the equalization for the 901 uses power in a range where the xls series is weak. Hopefully somebody can offer a better solution than throw more power at it. As far as the clip leds, you aren't going to hurt the amp. Crown's protection works, It'll shut the amp down before it gets hurt. The speakers on the other hand, yes can be hurt by clipping, that is what causes my concern.
     Steve

#9 DGlass

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Posted 23 March 2006 - 03:43 PM

QUOTE(stevetwinfalls @ Mar 21 2006, 10:24 PM)
Sorry to hear that you are having the same problem. Does your setup seem to have awesome no problem power for bass, but as the frequency raises the clip lights get worse? More listening and testing shows mine to be related to the treble, not the mids as I had first wrote. What volume level does yours seem to start? Prior to getting my first crown (402) I had the 901's hooked to my ht yamaha receiver with only a claimed 140 per channel, the highs seemed to reach the level that I expected from the bose, the mid punch was there, but you could definitely notice the lack of power during deep bass, not serious enough to say that it didn't power them satisfactory though. I had originally hoped by starting this post, that something could be figured out. I still am not sure whether bose has some kind of passive protection built into the 901, but would like to find out. I also am not sure whether something about the equalization for the 901 uses power in a range where the xls series is weak. Hopefully somebody can offer a better solution than throw more power at it. As far as the clip leds, you aren't going to hurt the amp. Crown's protection works, It'll shut the amp down before it gets hurt. The speakers on the other hand, yes can be hurt by clipping, that is what causes my concern.
     Steve
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The Bose has a smiley face EQ in that the lows and highs are bosted from the EQ. So it is totaly possible that the Mid/Hi output of the EQ could be boosted to a point of clipping the amp just like the low end has the possiblity of doing. Remember 3db of boost requires a lot more power from an amp.

#10 Russ Lund

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Posted 24 March 2006 - 12:41 PM

Face it guys Bose is overrated crap.There are many brands of loudspeakers out there that will out perform Bose.If you got em' for free what the *beep*.Any recording studio worth a *beep* doens'nt have Bose.I have a friend with a set of 901's and I had a pair of JBL 902vx.Mine sounded richer and crisper.The only thing I could say about his was the sound pattern was wider.But at the cost difference I could have another pair of speakers and tilt them out and still have money to throw a party with.Russ.

#11 boling00

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Posted 24 March 2006 - 03:02 PM

QUOTE(Russ Lund @ Mar 24 2006, 12:41 PM)
Face it guys Bose is overrated crap.There are many brands of loudspeakers out there that will out perform Bose.If you got em' for free what the *beep*.Any recording studio worth a *beep* doens'nt have Bose.I have a friend with a set of 901's and I had a pair of JBL 902vx.Mine sounded richer and crisper.The only thing I could say about his was the sound pattern was wider.But at the cost difference I could have another pair of speakers and tilt them out and still have money to throw a party with.Russ.
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Hi Russ, agree with the "crap" comment, but since I had all the components handy, I figured "why not use them". Definitely wouldn't have picked Bose on purpose. Anyway, I did a quick experiment and took the Bose equalizer out of the loop and the amp did not clip. It sounded horrible, but it confirms that the equalizer is the issue. Just going to live with it until I get a chance to swap out the speakers.

#12 stevetwinfalls

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Posted 25 March 2006 - 02:42 AM

this wasn't intended to turn into another internet bose bash session, it was an honest effort to find solutions to a problem.  I've seen enough bose bashing lately to last several lifetimes. I have put something together that sounds good to me, and that is what is important to me.  I have asked several very simple questions that anybody who has spent any time in the pro audio industry should be able to answer. these problems should be almost commonplace when setting up multi line array cabs with several drivers per box, bass cabs with severe impedance drops, and unpredictable resonances that add to the impedance variations. I have also offered the results of my measurements to try to solve this problem.
  Mr Glass: sorry to say this, but how long did it take you to figure out the smiley face eq. thing?  In my second post on this problem I had listed the eq curve that bose uses with the 901's, you replied with the efficiency topic. I have even went as far as to run a dynamic impedance test to find out what impedance I am driving at various frequencies, You didn't seem interested in the numbers. Between the eq. curve, the impedance curve, and a known efficiency measurement we have a very close estimation of what an amplifier of a given power SHOULD do. I have asked a question that you should be very knowledgable in, and that is in regards to various driver protection strategies, is it that you don't know, or don't realize that it could be a contributing factor? Given your answers to my questions on basic amplifier theory, we can almost be certain that my next amplifier purchase won't be crown. I am a firm believer that a warranty, no matter how long, isn't worth the paper it's written on if  the support is worthless. You may indeed manufacture a top notch product, but if you're not up to helping solve a problem that involves your product, then to me your product is worthless. Bose has sent me a complete checkout procedure on their speakers, lets see you send me one on your amp.    
    To the Bose Bashers: In researching this problem, I have seen more misinformation from individuals looking for instant gratification than I care to talk about.  Yes, in a studio setting  Bose is the wrong product for the application, the reflected sound principle would be a severe handicap to what is being done, and referring to studio use is a definite cheap shot, and shows an ignorance of the intended use of the 901. The 901 is definitely not the product to have if you are not willing to be patient about putting a lot of thought and some sweat into your setup. Bose feels that placement is important enough to spell out very specifically how, and where to put the 901. (yes for those illiterates there are pictures) I have heard several sets of higher end Bose that were very impressive, and lots of lower end Bose that sound like crap.  
     A little background on myself: I am vision impaired, and have been so since birth,(no vision in my right eye) and as a result my hearing has become quite acute to time space cues. I am able to walk through pitch darkness and tell where obstacles are by listening to echos of normal room noises, Tall bridges bug me because there is no sound reflection from the floor or ground, making me feel as if nothing is under my feet.  
Bose, When set up properly are some of the only speakers that are able to remove the auditory cues, and almost make me feel as if I am really there. To me the walls disappear. in my opinion, standard cabinets are a little too directional, the highs seem to be too "located" almost like mini stages all to themselves. I can tell exactly where in the room the drivers are located. The mids and bass seem to be coming from someplace totally different from the time space cues (highs)  
     In closing, To me pure sound pressure isn't important, presence, ambience, and tonal balance are. Should the individual who also had the same problem as myself decide to spend a little less time "Bose bashing" and a little more time enjoying his system, and setting it up properly I may be inclined to share some of what I have learned about setting the 901's up to demand a little less power in the range that causes the amp to clip. I enjoy my 901's very much, someone else might not. That's their right. Some circuits don't work well together. The 901, and xls combination might be one of those cases. They worked great with the yamaha, and so-so with the crown, who knows?? might be the fault of the bose, might be the fault of the crown. Sure would be nice to know for sure. It's not worth my time to try to sort a problem out with people whose minds are already made up.

#13 DGlass

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Posted 12 April 2006 - 04:06 PM

My apologies for the delay in responding as I have been out of the office overseas for two weeks.
There was no intent on my part to bash any product. As for the others if they had gotten into a Bose Bashing Session than I would have cut them off. However they are intitled to their opinions just as you are.
Now for the reputting of my knowledge or the quality of our product.
This forum is not the place nor will I say anthing else on the matter other than if it continues I will cut you off.

#14 boling00

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Posted 20 October 2006 - 04:48 PM

I finally got back to playing with the Bose 901s w/XLS-602 combo and tried several different PC sound cards. I found an old Philips sound card that virtually eliminated the clipping...and the audio quality is VERY respectable. Don't know why this worked but it did.

One very weird thing I found when I was experimenting is that I could plug my iPod into this contraption and it sounded great and did not clip AT ALL. Go figure.

Anyway, thanks for everyone's help and comments.