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  1. Sometime back I brought up the topic of modifying the AR-9 bass crossover network. Many thanks to Roy C for his comments and insight in this area. For those of you not familiar with this topic I have included the bass crossover schematic attachment My original question was based on speculation about what were the sonic effects of cutting loose the coil/cap that form a variable attenuation for this crossover. Holl noted in his design notes that these additional components contributed two aspects to the crossover function: 1) the combination of cap/coil added resistance at all frequencies higher than resonance thereby protecting the generally less than robust amplifiers of that era ('79 to '81) and 2) the extra R (as realized by these two components tended to "linearize" the bass performance in the general area of 30 Hz to 100 Hz (mid/upper bass). Note carefully that this cap is the infamous 2500 uF, about the size of a 16 ounce beer can, that so many people hate to replace (expensive, et cetera). I pondered the schematic for a considerable time and I could understand the basis of Holl's thinking. Amps of that era did not deal well with low impedances - and those two woofers in parallel formed an impedance that dipped as low as 2.5 ohms (back breakers to most receiver style amps). In addition the extra R would tend to keep a "mid/upper" bass rise in response from forming. My own analysis indicated that in THIS era of robust amps the extra impedance was NOT needed - my amps (Odyssey Audio Khartago Mono Blocs) are stable with a two ohm load - and those are not extremely expensive amplifiers. In addition the extra R contributed by these two components would tend to raise the Q of the circuit - thus making the bass above resonance more "wooly" and less precise. But I couldn't be sure. So I cut the darn things - right at the red mark in the drawing. Quick clip of the diagonal cutters and viola - the parts were out of circuit. I did however leave the parts inside the cabinet. (next time I go in there the entire crossover is "coming out" to be installed in an external box). So how does the resultant speaker sound? Pretty darn good - I will have an F/R plot for review sometime next week (friend has to come over and measure) but my subjective appraisal goes along these lines; All the bass has become "tighter" and is obviously being generated by a lower Q system - there was no "boom" before but now the bass is supple, tight and detailed. The bass in the region of 35 Hz to about 80 Hz has evidenced a significant increase in perceived volume - rather startling actually. Again the increase in perceived volume is not an increase in boom - but instead a lot more of a "tight" and agile bass response. If I am hearing - without any loss of quantity a higher quality bass, i.e. tighter and more focused (in the 80 to 160 range) then in truth the quantitative response (volume) HAS increased - boomy, high Q bass is typically perceived as louder - my bass is now lower Q but has maintained the same volume level (in the upper bass). The bass at or near the resonant point (28 Hz) has not particularly changed - so 25 Hz to 35 Hz is about the same - the deep notes, per those in "Dark Knight" actually seem to have slightly more volume (and those are low 20's) - with of course a more focused sound. This is a HIGHLY recommended modification. With one MAJOR CAVEAT. Do NOT - repeat do NOT perform this modification if you are using an amplifier from the "vintage" era. Those amps/receivers - no matter their size, shape or reputation cannot handle the low impedance of the two woofers in parallel. You will end up "smoking" your prized vintage amplifer/receiver. Only attempt this mod if you are certain that your amplifier is absolutely stable with a 2 ohm load. if you are not certain - then don't burn down your amp - and/or your home. The AR-9 was always highly praised for its great bass response (among many other aspects of its performance) - but that bass really becomes exceptional with this modification.
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