Why -3db trim? - Home Theater Forum and Systems - HomeTheaterShack.com

 
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post #1 of 7 Old 10-06-11, 10:05 PM Thread Starter
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Jeremy
 
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Why -3db trim?

Quote:
e) Using the Manual Speaker Setup menu of your receiver or pre/pro, play the speaker level-matching test
tone and adjust each speaker to read 75 dB (“5” on the meter with the range switch set to 70) using the speaker Trim controls found in the menu for each channel. When playing the test tone for the subwoofer, use the VOLUME control on the STA-800D/STA-1000D to raise or lower the level of the subwoofer to read 75 dB leaving the subwoofer Trim control in the AVR or pre/pro set to -3 dB. (Note the meter may fluctuate by several dB when measuring the subwoofer. This is normal and what you are seeing are room modes in play. Use the average of the swings to determine the volume of the subwoofer.)


f) From this point forward to add to or decrease the level of your subwoofer(s), use the subwoofer Trim in
your receiver. In some cases the level of your subwoofer Trim may exceed +/- 5 dB after tweaking your sub level to taste. If this occurs, use your SPL meter and a combination of the subwoofer Trim in your AVR or pre/pro and the VOLUME control on the STA-800D/STA-1000D to get the subwoofer Trim back to -3 dB while reading your desired subwoofer level on the SPL meter.
I was reading the manual and can you tell me why this is necessary to have -3db trim within the receiver?
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post #2 of 7 Old 10-07-11, 02:34 AM
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Re: Why -3db trim?

Output headroom? Sounds a bit weird, though.


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post #3 of 7 Old 10-07-11, 07:51 AM
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Ed
 
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Re: Why -3db trim?

The tiny op-amp in the AVR subwoofer pre-out can be clipped/distorted if the control range is pushed too far into the positive region. We've measured THD levels >1% from AVR sub pre-outs and have seen a clipped waveform on the o-scope when the control range is set to at/near maximum.

Setting the sub channel level to somewhere between -5 and 0 provides a clean signal to the subwoofer amp, and also allows some upward adjustability while still keeping the signal clean.

Ed Mullen
Director - Technology and Customer Service
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post #4 of 7 Old 10-07-11, 08:00 AM
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Re: Why -3db trim?

At the same time, IF your output can handle it, you have introduced a 3-5dB higher noise floor.


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post #5 of 7 Old 10-07-11, 11:20 AM Thread Starter
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Jeremy
 
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Re: Why -3db trim?

Does it matter if the cable is plugged into left or right RCA jack?

My ST-1000D volume is now at about -13db, with the AVR trim at -3db to get 75db test tone, does that sound reasonable?

Out of the box the amp default is -20db but at -20 it didn't have much volume to it.. kind of new to me coming from analog dial controls (and old rule of thumb like not to exceed 1 o clock etc)

Last edited by PoorSignal; 10-07-11 at 11:30 AM.
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post #6 of 7 Old 10-07-11, 11:37 AM
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Re: Why -3db trim?

Quote:
PoorSignal wrote: View Post
Does it matter if the cable is plugged into left or right RCA jack?

My ST-1000D volume is now at about -13db, with the AVR trim at -3db to get 75db test tone, does that sound reasonable?

Out of the box the amp default is -20db but at -20 it didn't have much volume to it.. kind of new to me coming from analog dial controls (and old rule of thumb like not to exceed 1 o clock etc)
Either L or R input is fine - they are summed internally.

The subwoofer gain setting seems reasonable for that AVR sub channel level, but naturally you should be level matching with an SPL meter or auto-set-up in the AVR. If you do auto-set-up, make sure to override any Large/full-range speaker size settings and set them back to Small with a crossover appropriate for their respective bass capabilities.

Ed Mullen
Director - Technology and Customer Service
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post #7 of 7 Old 11-30-12, 07:41 PM
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Re: Why -3db trim?

Excellent advice. Can I apply this to the Emotiva X Ref 12 as well?
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