My subwoofer Crossover & LT

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
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?
 

Attachments

  • lpf.jpg
    lpf.jpg
    50.2 KB · Views: 590
I should add that for the linkwitz transform i used:

F0=40
Q0=.61

F1=25
Q1=.61

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.
 
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.
 
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.
 
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?
 
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.
 
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).

:)
 
I will definately use sockets and I will definately get TL072's to try when I have something else to order with 'em.

What do you mean with the pots?

Measuring them they go from 0 - 10k in the first half and 10k-50k in the second half... but with my 10k resistor in series things seem to work out not all that bad (40hz min, 117hz halfway, 230hz max for the corner freq.)

edit: I did fix those hastily connected pots and one of the opamps was backwards too :xeye:

UPDATE
 

Attachments

  • lpf.jpg
    lpf.jpg
    45.7 KB · Views: 218
Yeah... its not linear

I believe it was a balance control on the receiver it came off of. A neat little extra feature is that it has 2 knobs each controlling 2 pots... they turn together but can be turned separately so I could set both LPF's differently if I wanted.

The amp they came off of was full of such goodies. It was a really old "quadraphonic" kenwood amp.

a few goodies:

2X 4-ganged 50k pots
4-ganged 100k pot
a few dual ganged pots
push button switches: SPST,DPDT, 4-pole double throw and 8-pole double throw
and a couple huge rotary selector switches.
 
richie00boy said:
I don't get what he means about the pot either. From your measurements it sounds like you have a log pot, is that correct?

What I mean is if you, for example, have a volume pot in there somewhere. You want it so that turning the knob clockwise will increase the volume.

But I guess its not that important, stuff like cutoff can be either way, as long as its labelled.

Looks good
 
But I'm using it as a variable resistor...The worst I could do is have it backwards which would reverse which way I have to go to increase frequency. All I gotta do is hook it up so it's at 50k when turned all the way to the left. It's not like a log pot as a volume control where if it's backwards the control would be almost useless.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.