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RE: Max. listening level ... have I hit a ceiling... (long)

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In Reply to: RE: Max. listening level ... have I hit a ceiling... (long) posted by bjh on May 28, 2007 at 18:43:32

Hi

It is very hard to tell “over the phone” what it is exactly but there are only a few possibilities.

Short term clipping appears to be inaudible unless you can compare to “with and without” where the unclipped sounds slightly more dynamic.

Longer term clipping (normally Voltage swing limiting within the amplifier) shows up as flat tops on the largest waveshapes. Being longer in time, these can sound like a click or a pop associated with transients. Gross clipping makes a raspy buzz etc.
Amplifiers can also current limit from an excess of current and / or load phase angle which is in the “Verboten” zone. This limiting is unpredictable; some popular early SS amplifiers actually went into HF oscillation when V/I limiting engaged (circuits based on the old RCA SS book, I’m looking at you).

Your best bet determining where you are is to get an old oscilloscope, figure out how it works (easy) and then look at the voltage coming out of your power amp.

Lastly, if your speakers really are 84dB 1W 1M, then you may well be running them out of gas. ALL voice coil drivers begin to depart form “linear” at about 1/10 of their rated power. In the 1980’s Doug Button (JBL) presented a paper at AES showing that as power capacity’s continue to climb, that the effects of power compression would eventually reach the point where an increase in input power actually resulted in a reduction of output sound level.

This kind of dynamic compression is driver dependant, the woofer, mid and tweeter all have different time constants so this is usually a case where by it self it is not very noticeable but is audible when you can compare with and without.
For the woofer though, VC heating can be related as the driver T&S parameters change wildly. Alternately, plain old over excursion can result in the VC former bottoming out.
This is usually a loud crack sound that gets your attention.
If you smack the coil too many times, it will be damaged.

Also, keep in mind the Radio shack meter on “fast” is not displaying peaks but rather is integrating over some (unspecified) time period.
Lets say your listening distance is only –6 dB down by distance from 1 meter and reach peaks of 96dB so at 1 meter its 102 dB peak (RS). Lets say the real peaks were only 6 dB greater than the RS meter so one needs 108 dB at 1 meter. With no power compression and 84dB sensitivity, it takes 250 Watts (+24dB over 1 Watt).
Your probably out of Gas.
Best,

Tom Danley

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