Amp crossover

[Mike]

I bult a amplifier with a LM3886T, and am planning on using it for a subwoofer. I need advice on what type of crossover to choose. Is a before or after amp crossover the best. whichever is the best, I need to know how to create it. If it is an after amp one, which I think would be the best, then what gauge wire would I need and how many turns, and around what type on material (air, magnet, etc). For before amp, what parts do I need and how are they assembled?<p>thanks, mike

[eyevancsu]

Head-Fi.orgHeadWizeFriends site for audio, including amps and xovers<p>try these sites, solely dedicated to audio including amplifiers, headphones, subs, speakers, xovers, xfeeds.<p>regards

[bruinbear714]

For subwoofer use, an active crossover (before amplifier) is preferred since the sub is insensitive to any high frequency noise (>1khz) introduced by the active circuits in the crossover.

[Mike]

I have tried an active crossover, but it doesn't work with my 3886 amp. It works with my 383 amp, but the sound out of the crossover isn't that great, so i would like to use passive. I would like to be able to adjust the frequency between 40 and 80hz. Can this be accomplished with an inductor and a pot? If so, what would the specs of the inductor be? I would like the inductor to be small, and use the least wire. <p>Thanks, Mike

[greg123]

Mike,<p>The crossover frequency is given by<p>R = 2*pi*F*L<p>R can be speaker impedance<p>Solve for L with your required Crossover Frequency.<p>Also, check this site out:<p>Crossover Spec's

[Mike]

I really don't get this. I have never wound a coil like this before. Has anybody accomplished the same thing I want to do, and could tell me the length of the coil, around what size diameter, how many layers, and what type of core? I guess I don't need a variable resistance if it would be hard to do, but somewhere between 60 and 100hz would be great.<p>Thanks, Mike

[greg123]

Mike,<p>Well for a Cross Over Frequency of 100 Hz, you would require an inductance of<p>R = 2*pi*F*L
L = 8 Ohms/(2*3.14*100)
L = 12.7 mH<p>Check out this link on winding inductors. Remember you can make an inductor by winding electrical wire around a bobbin. :p <p>Inductor Winding<p>Good Site Really Good....<p>Check these out and let me know.....<p>greg

[Mike]

Greg, in your equation, you used 8 ohms. My sub is a car sub, a 10 inch Lightning Audio P1.10.4, and is 4 ohms. Does that change the required inductance? Also, I have a catalog from a company called Parts Express, and they supply materials for building crossovers, plus the schematics. There are two types, a First Order 6db, and a Second Order 12db. Which is better? I can see that the first order uses the least parts, but does that sacrafice quality of sound? The catalog sells capacitors claiming they are for crossovers. Do I really need to use those capacitors, or can I just use a regular capacitor. Also, does it matter if it is polarized or not? <p>Thanks, Mike

[greg123]

Yes, it would change the required inductance. Re-work the formula for yourself to see what the new inductance is.<p>As far as first order and second order goes, the "order" is the number of filter stages. I'd perfer a second order filter, which as you mentioned offers a higher (or steeper) slope at gain crossover. All this means is that at the crossover frequency (or after) the output gain will drop at 12 dB/Oct.<p>Or is that a gain? (it's really late here...)<p>Also, yeah use the polarized caps. Stay away from ceramic. <p>You can do a google search for the best caps for crossovers. In my years of servicing speakers and sound systems...i've mostly see generic pol. caps.<p>greg

[Mike]

Greg, I just found out that I had the ground and out to amp mixed up on the volume control (ground was on the center of the control and out to amp was on the bottom. Switching those fixed 3 problems, the vol control not working right, a loud, annoying buzz when connected to certain sound sources (my laptop was one of them), and it now lets me use a passive line level x-over, so I am using that. The only problem left is that a small quiet buzz comes out of the subwoofer speaker when no sound is playing, but it is quiet enough to not be heard when sound is playing. Is that the amp? Or the x-over? Whatever it is, how do I fix it?<p>Thanks for all of your help, mike

[greg123]

Mike,<p>You've got a ground buzz. Created by a ground loop.<p> All you have to do is lift the ground on the source you have connected to you amp. i.e. remove the ground on the laptop power supply. Most commercial sound systems have a ground switch so you can easily remove the ground. Also, most mixers are not grounded to avoide the potential of a ground problem. <p>Greg

[Mike]

Greg,
Heres what I am doing now:
I have the input through a 3.5mm stereo headphone plug to rca adapter. One of the two rca connectors connects to the input. The 3.5mm connects to the headphone out on my boombox. Once finished, the tape out will connect to the input from my stereo using a singe cable with rca plugs on both sides. That sound will go through the X-over, and the x-over's output connects to the amp. How do I remove the ground from the source using either my boombox or my stereo?<p>Also, is there any gain adjustment on the 3886 like the 386?<p>Thanks, Mike

[Mike]

The buzz was caused by the crossover incorrectly connected. I fixed that. Also, the crossover I am using is a PLLXO. Heres the new schematic
right click, save as
The schematic is really a .bmp, but I had to rename it as a .wmf so that my web server allows it to be downloaded. Once downloaded, it will open with paint or any drawing program.<p>I found out that the .5uf cap across the input was needed to give me the correct frequency. I just guessed and checked on the values.<p>-Mike<p>[ July 09, 2003: Message edited by: mikea1962375 ]</p>

[bruinbear714]

I'm designing and building an active 3rd order lowpass crossover for my subwoofer... it'll be adjustable for -3dB between 42Hz and 75Hz without affecting the Q too greatly. Of course, I can tailor the adjustment range to anyone's liking. If you're interested, shoot me an email. I have the majority of the circuit finished - just waiting on Maxim for their samples of voltage conversion/regulators so I can do the power supply section.

[Mike]

I finished the project, and put it into a nice box. When I connected it to the tape out through the crossover that I posted in a previous reply, the volume of the speakers connected to the amp decreases, and they lose all of their bass. What is going on? When the project is done, it will connect to the headphone out, so its level adjusts with the amp's volume control. But what is going on right now with the loss of sound?<p>Thanks again, Mike