W3871S notch on amp instead of speaker? - diyAudio
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Old 26th January 2006, 02:50 AM   #1
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Default W3871S notch on amp instead of speaker?

I have built a small chipamp for work and im making matching speakers for it. they will be used with the amp. its just for laptop/mp3 player/minidisc/cd player. (sort of a "ipod dock" but for anything you like) i still want it to be pretty good saound but i am not expecting any miracles.

I have a pair of TB W3871S's I want to put in a pair of .13 cu ft sealed boxes similar to the zaph ones that are quite popular. I have a computer sub i may modify to use with it (only use the subs self power for the sub part, fed from input)

anyways I was wondering if there is any reason not to make an active opamp notch filter on the amps input rather than put the notch filter components in the speaker.

I see some pros:

1: it will let the amp drive the speaker without any extra components so more like the "pure" full range experience
2: it would be cheaper
3: I could make the amount of notch adjustable
4: much easier to change values of small components if i want to experiment rather than large inductors/resistors/cap inside the speakers
5: i can remove it if i decide to use different speakers (or switch)


and cons:

1: the speakers cant be used with any other amp and sound right (doent really matter in this case) could put active notch in seperate box that gos with the speakers (but you put it before the amp)
2: op amp(s) in the signal path (will be playing laptop or mp3 player etc cant do that much more harm)
3: I have to design a notch filter

can anyone give me a reason why i shouldnt do it this way apart from those cons above?

best regards
Neutron7
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Old 26th January 2006, 12:01 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

I don't think anyone could.

regarding cons 2), you could make a passive line level notch filter.

/sreten.
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Old 26th January 2006, 02:43 PM   #3
soongsc is offline soongsc  Taiwan
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The passive notch at the speaker interacts with the driver such that the response is not the same as the notch itself or implemented in a different location. You just might need to redesign and test and compare. I can be done any way.
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Old 28th January 2006, 05:05 AM   #4
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i have read many places that these drivers require a notch but nowhere does it seem to actually state the frequecy, width of notch and amount of attenuation

I am still deciding weather to use passive or active line level notch.
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Old 28th January 2006, 02:21 PM   #5
Salas is online now Salas  Greece
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If you build a 'similar' enclosure, then Zaph's notch recommendations dont stand. Its different diffraction that is going to change things. You can build exactly on of his boxes and extract your notch shape by subtracting his before and after frequency measurements.
Alternatively you can invest on a second pair of TBs and a pair of cheap tweeters and build this where no notch is used and has more output everywhere.
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Old 29th January 2006, 04:05 AM   #6
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well its not very similar at all. it has a very small frontal area 4.5x 7 externa)l and is very deep (for the size).

here is the standing wave calc for it.

http://www.vikash.info/audio/standin...9&threshold=50

i think ill have to try it with my friends parametric EQ and some measurements and listening till i decide what i need for it.

in the end
im pretty sure i will go for a buffer on the amp which is switchable to the active notch with the same gain. 2 actual different circuits with a switch. then i can go "straight through" with no filtering of any kind or put the notch on.

passive notch will just mean i have to put more gain on the amp and it will just add more distortion anyways. so i may as well use a quality opamp. i have a few

heres the baffle I worked on tonight.

looks like a simple bit of wood but its a bit of work making the flush mount and route curve on the inside (as mentioned by zaph)

it is very slightly offset (1/8 th) and the other speaker is mirror imaged.
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File Type: jpg speakerinbaffle.jpg (74.4 KB, 207 views)
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Old 29th January 2006, 04:22 AM   #7
Salas is online now Salas  Greece
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Good Idea to use the parametric if you dont have access to a measurement system.
As for going to lengths for optimally distributed internal resonances by choosing golden ratio dimensions etc, it is nice, but don't forget that the dampening materials we put in cabinets absorb the best part of midband standing waves if applied correctly, and thats their original purpose.
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Old 29th January 2006, 04:28 AM   #8
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but it cant hurt if the dampening materials have less work to do in the first place
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Old 29th January 2006, 01:50 PM   #9
Salas is online now Salas  Greece
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I said isn't bad anyway. Also felt pads in combination with wrapping air bubble sheets to opposite walls absorb and disperse internal waves without interactions with moving cone mass, keeping the bass notes crisp. Especially in small speakers, much stuffing can kill power and agility in LF.
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Old 29th January 2006, 01:50 PM   #10
soongsc is offline soongsc  Taiwan
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Quote:
Originally posted by neutron7
well its not very similar at all. it has a very small frontal area 4.5x 7 externa)l and is very deep (for the size).

here is the standing wave calc for it.

http://www.vikash.info/audio/standin...9&threshold=50

i think ill have to try it with my friends parametric EQ and some measurements and listening till i decide what i need for it.

in the end
im pretty sure i will go for a buffer on the amp which is switchable to the active notch with the same gain. 2 actual different circuits with a switch. then i can go "straight through" with no filtering of any kind or put the notch on.

passive notch will just mean i have to put more gain on the amp and it will just add more distortion anyways. so i may as well use a quality opamp. i have a few

heres the baffle I worked on tonight.

looks like a simple bit of wood but its a bit of work making the flush mount and route curve on the inside (as mentioned by zaph)

it is very slightly offset (1/8 th) and the other speaker is mirror imaged.
Recommendation:

Use Zaphs cap and resistor value, and run the following baffle simulator to see what kind of inductor you should use for your baffle. This should be a good start.

http://www.tolvan.com/edge/

For R1, you can alter it based on the actual impedance value of the system without much effect.
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