2 plus 2, Speakers cause amplifier clipping

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Alan Hoffman2007-08-08 22:09

I am the original owner of a pair of Acoustat 2 + 2\'s with MK-121 Medallion Transformers. I am presently using Bryston monobloc amplifiers model 7B-ST rated at 500 wpc. Even with this power available, I still experience momentary clipping of the amplifiers when the music is played at “live concert level” (probably 100-105 db on peaks). I listen mostly to classical orchestral music. The Acoustats are rolled off at 80 Hz (6 db/octave) with the lower frequencies channeled to a Velodyne HGS-18 subwoofer. The Bryston amplifiers can be run in series (higher voltage for 3-8 ohm loads) or parallel modes (higher current for 1-3 ohm loads). Specific tests of my amplifiers yielded at clipping 630 watts in series mode and 615 watts in parallel mode (8 ohms at 2 KHz). This should be more than sufficient to drive the Acoustats to 115 db levels, especially since they are rolled off at 80 Hz. I ran the amplifiers in both modes and there was no improvement in the parallel mode over the series mode. In fact, clipping occurred at lower volume levels in the parallel mode. The amplifiers are connected to the crossover in the subwoofer via unbalanced Monster cable RCA connectors that are 20 feet long due to space limitations. The speaker cables are also Monster cable and are 12 feet in length. I would appreciate your comments as to why this problem is occurring and how to solve it. Thank you in advance for your reply. Alan Hoffman

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Andy Szabo2007-08-10 23:58

You certainly have enough watts to drive the 2+2 to its maximum output, and therein may lie the problem. You may be clipping the amp occassionally, but what you are probably hearing is minor \'crackling\' of the speaker, indicating it has reached its maximum output. This causes no damage to the panels themselves (unlike some ESL\'s). You may also have a problem in the step-up transformers. They can develop problems at high-voltage levels, causing an arc inside the transformer. This does damage the transformer, and will get worse (and eventually fail)over time if you continue to play at high levels. If your problem has developed recently, this may be the case. Most people find the 2+2 capable of producing acceptable SPL levels, so you may be asking too much of the speaker, especially if you are in a very large room. Crossing-over at 80 Hz will certainly increase the speaker\'s effective dynamic range, but the speaker still has its limits. You might try using a steeper slope than 6 dB/octave - at the present crossover slope you are still getting quite a bit of low frequency energy into the speaker: the signal is only 6 dB \'down\' at 40 Hz. I would suggest a 12 dB or 18 dB/ocatve slope, and/or possibly raising the crossover frequency somewhat.

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