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merlinx76 8th February 2005 06:29 PM

My subwoofer Crossover & LT
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Would you guys be able to look over my circuit design and tell me if it's ok.

The circuit consists of an input buffer, Linkwitz transform and two 2nd order LPF's, the second of which can be bypassed.

Subsonic filtering was considered when I chose C1 and C10. Each gives me a cutoff freq of about 8 or 9hz so I should be 6db down at that point right? (this of course assumes a 10k input impedance on my sub amp)

The switch would be a DPDT and for the pots I will try to find a single 4 ganged pot to control both filters.

Should I also include small caps on the power leads of the opamps? If so, what value?

Also, what opamps should I use?

richie00boy 8th February 2005 06:37 PM

Really great design there. Very much like what I do myself :) The supply decoupling caps depend on the impedance of the supply and the inductance (read length) of the wires. 100nF seems like a workable value for most small circuits.

As for the op-amps, TL07x series or OPA134 series are good and not too expensive.

merlinx76 8th February 2005 06:39 PM

I should add that for the linkwitz transform i used:



F0 and Q0 were only derived using WinISD and I plan to measure the actual response later on to be sure of these values.... If I'm off by too much I can just change the component values for the LT.

merlinx76 9th February 2005 01:48 AM

Should I use a regulated power supply for this circuit? My guess is yes.

I have a 24 VCT traffo . If I go regulated do I just get +12V and - 12V regulators? or should I separate the secondaries and use 2 +12V regulators? Which way is preferable? Because I have heard people talk about separating the secondaries before and I can get at them easily enough.

richie00boy 9th February 2005 07:06 AM

You do not have to use a regulated PSU for this circuit as it is low frequency, so as long as you have reasonable smoothing on your unregulated supply it will be fine. However, you will need to use a transformer of something like 12-0-12 or you will blow up your op-amps.

If you want to use a regulated PSU you should use 15 V regulators or LM317/337 set to 17 V. If you want to just use positive regulators for both rails you must have a transformer with totally separate secondaries. The only advantage with this setup is that you can achieve theoretically identical rail voltages. In practice though this won't happen due to component tolderances so will be no better than +/- regs, and the difference is irrelevant as you have couoling caps in your circuit anyway.

merlinx76 9th February 2005 03:01 PM

I managed to find almost everything I need lying around in spare parts. The only thing I don't have are the opamps mentioned earlier. I do have TL082's though. They are low noise bifet similar to the 072's... would they work ok?


richie00boy 9th February 2005 03:17 PM

Yes, they would be OK in this aplication. They are just a more general purpose version.

merlinx76 9th February 2005 07:59 PM

For the 2nd LPF section, if the switch is bypassing it there is no DC path to the non-inverting input. What's gonna happen if I have the switch set to bypass (2nd order) and I change it to 4th order with my sub amp on? I am thinking it could make a pretty big thump.

Edit: Also, my transformer is rated at 12-0-12 but with no load it's 14-0-14 so I'll be getting slightly over +/- 19VDC after the rectifier. The chips are rated for +/- 18V. Do you think it will be a problem?

richie00boy 9th February 2005 09:51 PM

Ah, I didn't get the switching part when I gave it a quick look over. It may thump. What I do is just drive both buffers all the time and use just a single changeover switch to choose the output between connecting to the 2nd-order point or the 4th-order point.

You may have a voltage problem. Build it with some chip sockets and measure the voltage on the rails when in use. Keep an eye on the temperature of the chips. If it's more than 18V and you may fear chip damage you can just put some new ones in and add some extra diodes into the supply to drop some voltage.

thomas997 10th February 2005 03:10 AM

Just use some sockets for the op-amps, then you can switch out and compare different ones if you would like.

I would personally go with TL072s.

btw Im assuming you know how to connect those pots right? (wiper to left or right side, depending on what its function is).


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