Clipping question for skeptic, RGA or anyone...

[Number9]

Hello, I think that I read a post here once about clipping & that it's not always audible. I have a low powered amp (60wpc) & I think that someone wrote that clipping occurs quite often with low powered amps. Is there a range in which the nonaudible clipping occurs (like at 50 or 70% of the power)? Any info would be helpful. Thanks in advance, Michael

[skeptic]

In my current experiments with original Bose 901, possibly the most inefficient loudspeaker for bass performance ever offered to the public, I am regularly driving my amplifier into clipping. The speaker makes a very nasty popping sound. How do I know it's clipping. I peg the digital meters and I can hear DISTRESS. When I activate the below 40 hz cut on the Bose equalizer and play the same music at the same level, no popping sound. BTW, I use considerable additional bass boost beyond the 18 db at 30 hz created by the Bose equalizer. Fortunately, the amplifier driving the tweeters is coasting so no damage there. As for the Bose 901s they can take a lot more than this amplifier can dish out so no damage there either. (Anybody have specs on Marantz Receiver SR930--might be well over 100 wpc, the line fuse is 8 amps.)

Whether or not you have enough amplifier power for your needs depends on several factors. Here are some of them in no particular order.
How efficient your loudspeakers are
How loud you want to play them, in other words the loudest they will have to play
The size of your listening room
The acoustics of your listening room
The type of music you listen to

The maximum capacity of your loudspeaker is a limiting factor in how much power you need. A loudspeaker which has a capacity of 50 watts will not notice a difference between an amplifier of 100 watts or 500 watts (if that is their true maximum output.)

Sound systems should always be designed with these requirements in mind but home systems rarely are. There is a longstanding audiophile myth that somehow more available power is always better. It isn't so. Sometimes it's just a waste of money.

[Jimmy C]

...I was just wondering what you thought of the 901s, series 1. A friend of mine just inhereted (err... o.k... "bought") two pairs of 901s from his dad, one original, one second series, I believe. Point is, I'm gonna borrow them and see what all the fuss is about (positive AND negative) for myself. I can't say they sound good in his room, but he's using archaic, half-dead ancillaries along with a placement method Bose doesn't even recommend.

The old Bose cash cow has come up in more than one thread of yours... it seems you have some attachment to them.

Any set-up advice will be considered...

[skeptic]

There were some ingenious ideas incorporated into this speaker. Unfortunately when you add them all up, they come up short by modern expectations for audiophile loudspeakers. Some of the basic rationale for them seems to me to be ludicrous. Dr. Bose measured 11 percent direct sound, 89 percent reflected sound 19 feet from the performing stage of Boston Symphony Hall and devised a speaker which RADIATES about 89 percent reflected sound, 11 percent direct sound. That doesn't mean that this is the percentage you will hear. And of course there is no comparison between the relationship between direct and reflected sound fields produced by Boston Symphony hall with its huge dimensions, long reverberation times, carefully designed acoustics, and what you'll hear in your home. No relationship whatsoever. Hoever, there are other interesting ideas here. It was novel to eliminate the crossover network and use precision active equalization. It was novel to place the drivers on baffles that had no parallel sides eliminating internal standing waves. It was novel to use 9 drivers over the same range to eliminate second order resonances. It was novel to push the resonant frequency of the acoustic suspension drivers up deliberately to a pre determined point and then use the fact that they had a linear falloff below resonance and compensate for it with a complimentary linear rise in response as frequency lowers. Unfortuantely, the original design had at least one serious flaw which was only somewhat glaring in its day but has become more and more glaring in the ensuing 35 years and that is its inability to reproduce the top octave of sound. I have dealt with that by adding 4 polycarbonate tweeters per channel, three indirect firing and one direct firing crossed over with a simple first order 6 db per octave filter capacitor (2.7 mfd per tweeter) and bi amplifying it. I've also reduced the broad peak in the 250 to 500 hz range and further boosted the bass a few db. One of the systems had an air leak due to hardening of the putty between the wood cabinet and the metal frame of the drivers. This reduced the bass. I sealed all of them with clear GE silicone caulking. If you want to test yours, here's what I did. Remove the rear grill cloth, gently push three cones in slightly simultaneously being careful not to damage them and you should see all of the other cones move outward. If they don't there is an air leak and the extreme bass will not be reproduced. The drivers were also banging against the rear grill cloths. I've removed them and the staples which held them. I'm looking for another way to re-attach them and inch or so further from the drivers.

Series I and Series II were identical except for the following as far as I can tell. Series II had different equalization curves including more having a high end boost although the "flat" setting was identical. Series II had a small metal Bose medallion on the front. The drivers and enclosures were identical but the selection of drivers was done by computer instead of by hand. The CTS drivers used in these two versions were segregated into three groups and each system sold had drivers from only one of the groups so that they would match.

To make this speaker work properly you MUST have;

Tons of power
correct placement
willingness to experiment with additional tweeters.

If you get it right, and although I have only started playing with them about a couple of months ago, I 'm making great progress, this can be made into an outstanding system. Other people who have not experimented with them in this way will not understand this possibility.

The direct/reflecting principle will add life and presence to music in a way not possible with direct firing loudspeaker systems. That is why despite the shortcomings of this version and subsequent versions, so many people like them. It's too bad they didn't go much further with this idea. It still has a lot of potential.