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Old 15th November 2011, 04:48 PM   #1
Frosteh is offline Frosteh  United States
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Default Help calculating amp required for Linkwitz Transform

I've got what I think is a pretty good design for a subwoofer (see Sealed sub with RSS315HF and Linkwitz Transform at 10Hz) but I'm not sure how powerful of an amp I'll need. I used WinISD to calculate all my specs, yet the apparent load power seems very low compared to what I've read low boost circuits need. Attached are pictures of WinISD's calculations of apparent load power, and signal input to achieve the maximum cone excursion (xmax + 10%) acceptable in a sub. I thought these low boost circuits required many more watts than rms for the sub, which is why I'm very confused as to how it only needs 160W (according to the graph.
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File Type: jpg cone excursion.JPG (96.9 KB, 107 views)
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Old 15th November 2011, 05:48 PM   #2
Frosteh is offline Frosteh  United States
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Just as a follow up, I did all the calculations by hand, and came out with the same voltage requirements, roughly 170 watts. This doesn't seem true, but my linkwitz transform only adds 7.55dB of gain, and the PEQ adds 4, resulting in just about 10.06dB of gain at 20Hz. Using that compared to speaker sensitivity and my desired listening level at about 98dB, I only get a load of 170W. How is this possible? I thought LT circuits sucked up massive amounts of power. Is this design really that good? If so, I'll gladly build my own high end, low power amp (200W) instead of trying to save by buying some behringer or peavey powerhouse.
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Old 15th November 2011, 06:00 PM   #3
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10 dB of gain sounds about right. At this level you would've originally been using only 17 W?
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Old 15th November 2011, 06:08 PM   #4
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Originally Posted by Frosteh View Post
Just as a follow up, I did all the calculations by hand, and came out with the same voltage requirements, roughly 170 watts. This doesn't seem true, but my linkwitz transform only adds 7.55dB of gain, and the PEQ adds 4, resulting in just about 10.06dB of gain at 20Hz. Using that compared to speaker sensitivity and my desired listening level at about 98dB, I only get a load of 170W. How is this possible? I thought LT circuits sucked up massive amounts of power. Is this design really that good? If so, I'll gladly build my own high end, low power amp (200W) instead of trying to save by buying some behringer or peavey powerhouse.
Ten dB of gain is the difference of 170 watts to 17 watts. Most would think that is a massive change, though somehow 100 to 1000 seems like more, even though it is the same.
If 170 watts does not exceed Xmax, and gets loud enough for you, no problem.

However, 98 dB at one meter will be only 92 dB at 2 meters, and 92 dB at 20 Hz sounds about as loud to your ear as 40 dB at 1000 Hz.

Maybe cabin gain will work in your favor, there is always hope .
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Old 15th November 2011, 07:39 PM   #5
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Hi,

WinISD has an apparent load (VA) tab that tells you what power is coming out at what frequencies - very useful for stuff like this. I only discovered it after building my first half-decent subwoofer, and now understand why it ran out of steam so quickly.

Chris
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Old 15th November 2011, 07:51 PM   #6
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3.5 meters of distance from the speaker is about 10db of loss in SPL's as well without cabin gain.

jer
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Old 15th November 2011, 08:17 PM   #7
Frosteh is offline Frosteh  United States
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Ok. I just wanted to make sure I was calculating correctly. And I had a feeling it was right but I wasn't sure. And I do get that it's going to be quiet, but that's why I built two of them. Somewhere in the thread I linked to earlier i have a graph of spl with two subs (around 106dB iirc).
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Old 15th November 2011, 09:14 PM   #8
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Originally Posted by Frosteh View Post
Ok. I just wanted to make sure I was calculating correctly. And I had a feeling it was right but I wasn't sure. And I do get that it's going to be quiet, but that's why I built two of them. Somewhere in the thread I linked to earlier i have a graph of spl with two subs (around 106dB iirc).
Now I remember, I replied #21, #35, #44, #48 to that thread of yours starting July 1st.
13 September you finally showed some boxes in that thread.
Now, seems you are just asking the same questions again, seemed to me I was repeating myself, and I was.

Time for you to forget AP Physics and start enjoying some music with LF, just add more subs if you need them, EQ to taste.

Cheers,

Art
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Old 15th November 2011, 10:34 PM   #9
Frosteh is offline Frosteh  United States
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Originally Posted by weltersys View Post
Now I remember, I replied #21, #35, #44, #48 to that thread of yours starting July 1st.
13 September you finally showed some boxes in that thread.
Now, seems you are just asking the same questions again, seemed to me I was repeating myself, and I was.

Time for you to forget AP Physics and start enjoying some music with LF, just add more subs if you need them, EQ to taste.

Cheers,

Art
A lot of the questions are similar, but I never recalled getting a definitive answer. I was still very skeptical about only needing around 200W per side considering the RMS power of the driver is 400W. I was planning on having to buy some huge expensive amp which is why I never finished them (budget issues). But now that I know they'll only cost around $200 to finish, I'm going to get started on them again. That thread got so long that I honestly forgot what had been discussed and what hadn't, so thanks for being patient with me!
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Old 15th November 2011, 11:10 PM   #10
tvrgeek is offline tvrgeek  United States
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10 dB is a lot to ask for out of a LT. I try not to exceed 6. If I remember, Siegfried cautioned everyone about getting carried away. Use the room, or just plain get a sufficient driver. My subs ( pair of 12" Peerless XLSS in 70L boxes) measure pretty weak out in free space, but are quite musical using the room boundaries. My cement floor helps. I prefer clean to more.
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