Stereo Fourth-order active lowpass filter for subwoofer / Bass boost

Discussion in 'The Projects Forum' started by mcanever, Dec 7, 2007.

  1. mcanever

    Thread Starter New Member

    Apr 11, 2007
    7
    0
    Hi all, i'm an italian guy, noob in theory and I see that in this forum there are a lot of highly skilled people...
    Yesterday I thought that it would be a nice idea to make a little circuit that filters high frequencies from an audio source (line level, for example a TV audio-out) and outputs only lower ones, so that any amplified pair of speakers with line-in can be used as a subwoofer or better bass-boost. The idea started because my Samsung R series LCD TV is cool but has @#%$& bass. I have a lot of old but still very good pc-speakers, so I thought I could use a pair of these to do the trick.
    After Googling around a bit I put down a schematic, that you can see in the attachment below.
    The filter configuration is unity-gain Sallen Key type, fourth order, with a Q of about 0.5.

    Now is What I ask to you guys...

    Is the transformerless p.s. I show in the schematic suitable for such an application?

    Do I have to connect power supply's ground to audio ground? (I think so...:))

    Do you think it will work? :D

    Any advices on the best op-amp of the ones I listed? The list is based on part-availabilty in my area...

    Thank you very much in advance..

    P.S. Feel free to spread and use this schematics... I've drawn it with visio but if you need the "source" file feel free to ask!
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    First, please refrain from using profanity as there are youngsters on this Board.

    I strongly advise against using the "transformerless" supply; if the capacitor fails (shorted) you will fry everything, along with exposing yourself to line voltage.

    The transformer power supply would be much better - but there are no voltage regulators shown. This will cause distortion due to the low frequencies you'll be processing. You could use LM78xx and LM79xx for postive and negative regulation, respectively (xx being your selected supply voltage; 09, 10, 12, etc) - or use adjustable regulators like LM317's and LM337's. Be certain to use bypass capacitors on the regulator outputs (0.1uF or larger.) You COULD use the 9v batteries - but that will get expensive if you use your sound system very often.

    Sallen-Key filters have a fairly sharp cutoff, but the passband and stopband do not have a flat response. Instead, I'd suggest using a Butterworth design. This will ensure that your passband is flat, and the stopband response is flat as well. The cutoff will also be a smooth transition, but by adding stages, you can sharpen the transition.

    Some example Butterworth active filters are here:
    http://alignment.hep.brandeis.edu/Electronics/A2037/HTML/Active_LPFs.jpg

    Some other resources here:
    http://alignment.hep.brandeis.edu/Electronics/A2037/Filter.html

    Google:
    http://www.google.com/search?hl=en&q=Butterworth+active+filter+design

    Beware of those that start off saying "Butterworth", but then change to a Sallen-Key design - you won't get the same flat response from a Sallen-Key.

    Chebyshev designs have a VERY sharp cutoff - but poles are everywhere, and therefore unsuited for audio.

    Be sure to use 0.1uF capacitors on your op amp's power pins; keep the leads as short as possible, and preferably use tantalum.

    LF353 or TL072 op amps will work fine. You might also look at the LT1057, a relatively new offering from Linear Technology. It has something the LF353 and the TL072 doesn't have - phase reversal protection. Datasheet is available from their website:
    http://www.linear.com

    The LT1057 is a dual version of their LT1007. The LT1058 is a quad version, but it may exhibit phase reversal under certain circumstances; therefore you should stick with the LT1007 or LT1057 - if you so choose to go that route.
     
  3. mcanever

    Thread Starter New Member

    Apr 11, 2007
    7
    0
    Thanks a lot for the prompt reply. I'm sorry if I used "profanity", but to be honest I didn't even think that the word I used was so "evil". I'm italian and lots of my english words come from reading internet forums... I just thought it meant "bad"... :D

    Going back to my project I really appreciate your accurate explanations, and you're perfectly right on trasformerless power supply and its dangerousness. I put it as the last alternative only because I wanted the project to fit the smallest enclosure possible.
    Thank you for pointing out about regulation. I thought it was needed only under heavy loads. I'll correct the schematics and add the 7815 & 7915 with bypass capacitors. In the end i will go with the transformer solution, I'll try to find a tiny transformer!
    Regarding the filters I only need low-pass, not bandpass, and - I think - the Sallen Key design will work. In my first post I forgot to say that I based my design mainly on this very old page:

    http://ldsg.snippets.org/FILTERS/Pomann/crossover.htm

    It was the only one I could find that spoke explicitly about audio, and I used a design spreadsheet from this. Anyway I'll take a look to butterworth design!
    The LT1057 seems a very good product, I hope my supplier will get a reel of them soon! If he won't I'll go with the LF353.

    If I build the thing I'll make you know!
    Again, thank you!
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    As far as the language - this is a very "G-rated" forum; so they try to keep things pretty low key around here. ;)

    Sure, a Sallen-Key design will work - but simply won't have the flat frequency response a Butterworth has. Take a look at the first link I posted; the active low-pass Butterworth filter on the bottom left is non-inverting, has a gain of 1, 1KHz cutoff, with 36dB of attenuation per octave above that. A bit of tweaking of the R and C values, and you can put the cutoff frequency wherever you'd like. ;)

    Since it IS a preamp, your power supply requirements are going to be quite low; but for safety's sake you still need proper isolation from the mains, good filtering and good regulation. Any noise that creeps in will be amplified, as will distortion. You'll need to be careful to keep things balanced, and using your op amps in their most linear region.

    There are a number of vendors who sell flatpack PCB-mount transformers; they're quite compact.

    Something else you might consider is using a "wall-wart" or desktop type supply. You don't necessarily need +- 15V for line level signals, you could likely get by with a good bit less - just as long as your signal won't get too close to the rails. This is where variable voltage regulators come in handy ;)

    You could also consider using a power op amp to split a single supply to get a bipolar supply. Go to National Semiconductor's site:
    http://www.national.com
    and download the datasheet for an LM675. On the bottom of page 2, you'll see a schematic for doing just that. Of course, good filtering is essential no matter what supply configuration you use.

    If you get "stuck" c'mon back and holler for help. Or let us know if you do get it together and working.
     
  5. gootee

    Senior Member

    Apr 24, 2007
    447
    50
    Hi mcanever,

    You have been given a lot of good advice, already. One thing I can add: You might want to go to the Texas Instruments website and download the free FilterPro software. It will enable you to design active, opamp-based filters, very quickly and easily. FilterPro can design many types of filters, with up to ten poles each, and will provide response plots and schematics with part values.

    The FilterPro software's "homepage" is at:

    http://focus.ti.com/docs/toolsw/folders/print/filterpro.html

    You could also download the excellent, free LTspice circuit simulation software, from http://www.linear.com , for more-detailed simulation and plotting.

    - Tom Gootee

    http://www.fullnet.com/~tomg/index.html
     
  6. mcanever

    Thread Starter New Member

    Apr 11, 2007
    7
    0
    Thank you guys for all these info. It's much more than I expected. I think I'm gonna revisit all my design. I tried TI's software and it's really good for experimenting. It's a pity I can't put my hands on one of those excellent OPA series op-amps! :D

    Sure I'll make you know my results!
     
  7. gootee

    Senior Member

    Apr 24, 2007
    447
    50
    You can use almost any opamps, as I'm sure you know. But, be aware that for certain types of filters, FilterPro displays a minimum gain-bandwidth (GBW) product opamp-specification that is required of the opamps used in the filters, for proper performance.

    And you might still want to simulate your results with e.g. LTspice, just to make sure that the signal (vs. power supply) levels are all OK, everywhere, i.e. no clipping, etc. You could also, for example, add a circuit that better-models a real speaker. And you can have it calculate things like distortion, and do things like one-click plotting of the power dissipation of any component, et al.

    Have fun.

    - Tom Gootee

    http://www.fullnet.com/~tomg/index.html
     
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