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Old 8th December 2009, 08:43 PM   #1
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Default active xo help

I like making speakers but Im frustrated at having to buy big expensive passive crossover parts every time. It really limits my ability to experiment and have fun.
I have a couple of different cheap amps so I think multi amping is a good route for me to take. With that in mind, Im looking to make up a preamp/active crossover.

The want list:

-Simple. I like simple. It also matches my skill set and knowledge… Im not an audio engineer. I'd trade off a lot for simple.
-Cheap. As part of the building/experimenting/learning/messing around process, I expect it to go up in smoke multiple times. Also Im poor.
-Flexible. I want to use it for a bunch of projects. Changable crossover points. Different amps. Easy to swap out parts (see above)
-The parts have to be easy to find. If I can walk down to the local radio shack and grab up what I need that would be great.
-All the sources that Ill be using are fairly strong so I think Im looking at a buffer rather than gain.
-For no rational reason, I like the idea of jfets. So all jfets (unless someone can tell me why thats a bad idea).
-I can live with 12db slopes.
-Some sort of simple level control for each output.

The "not looking for" list:

-Worlds best anything.
-Sublime, perfect, " I heard things in my music collection I had never heard before..", sounds. Good is good enough.
-Full of amazing features.
-kit/prebuilt.

I envision the whole shebang to look something like this.

Source selection-->volume attenuator-->jfet buffer
-->pllxo--> jfet buffer--> attenuator-->amp1
-->pllxo--> jfet buffer--> attenuator-->amp2
-->pllxo--> jfet buffer--> attenuator-->amp3

Seems like a lot of buffers. Do I need them all? I guess if they are simple then its just a bunch of the same simple buffers. Step and repeat.
I currently have a nice 50k stepped attenuator and a good +/-18v power supply. If I could use them in this, well, that would be a bonus.

Can anybody help point me in the right direction? Offer any advice? Tell me why this is a terrible idea? Tell me audio is best left to the professionals?

MrKramer
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Old 9th December 2009, 12:15 AM   #2
godfrey is offline godfrey  South Africa
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Hi MrKramer

Good idea! Active crossovers give you more flexibility and probably better sound in the end.

Any objections to op-amps? They're easy to design filters with.

Simple circuits with discrete transistors or jfets can be done too, but probably have a bit higher distortion.

Do you have any experience with electronics? e.g. design / theory or construction / soldering etc?

Sorry, I'm not trying to be rude, just want to get a feel whether a rough sketch of a circuit will do or if you'll be needing detailed construction plans etc.

Regards - Godfrey
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Old 9th December 2009, 12:48 AM   #3
Key is offline Key  United States
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I'd say go with a PC that has 4 or 6 quailty outputs - most likely firewire based. Then use VSTs to fine tune and experiment. Nothing is can really beat the ease of dsp once you get the stuff you need.
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Old 9th December 2009, 12:52 AM   #4
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Check out this thread B1 Active Crossover

As far as I know still only spice modeled, so no one has built one yet, but it is what I am hoping to build.

If you want to go the pllxo with buffer route (something I also considered before the above thread popped up) then the B1 is a nice candidate for that as well, and there are people who have done it.

Tony.
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Old 9th December 2009, 01:14 AM   #5
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Quote:
Originally Posted by mrkramer View Post

-->pllxo--> jfet buffer--> attenuator-->amp1
So far this is the most effective method i have yet found... all too often the XO can get in the way of the music.

If the PLLXO does not load the following amp you can get away without the buffer. So far i have found that the need usually comes when you are trying to do a 2nd order HP filter into an amplifier with lower input impedance (i use a figure of 50 k as a minimum.

You also have to take consideration of the limits of a PLLXO. You can do 1st order or a droopy 2nd order. Of course you can run PLLXOs in series to get more complex filters. The buffers are then needed,

Click the image to open in full size.

Digital XOs are very versatile, but need to be a minimum of 24/96 and have good DACs with good analog output stages to even come close to the transparency of a simpler analog solution.

dave
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Old 9th December 2009, 02:47 AM   #6
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Wow, so many good responses. Thank you.
Ill read through that b1 active crossover thread tonight.

My skill level/knowledge is really low (but growing!). I can solder alright. I have built a simple chip amp and a power supply. I read anything I can on the subject. Thats about it.

I want to work with discreet parts on this one. Just feels more interesting. No better reason than that.
As far as a PC goes, I find that approach interesting but I cant really see myself doing it this time.

Dave- are you putting that in after a preamp? Where are you putting your volume attenuation? Before? after? both? Like the tape hold downs!

MrKramer
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Old 9th December 2009, 03:32 AM   #7
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Quote:
Originally Posted by mrkramer View Post

Dave- are you putting that in after a preamp? Where are you putting your volume attenuation? Before? after? both? Like the tape hold downs!
Preamp has a volume control. Each amp has a gain control.

dave
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Old 9th December 2009, 05:38 PM   #8
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I was thinking that if we can come up with something understandable and approachable here, maybe I could write up the build process. Take a few pics along the way (smoke and all!). There have got to be a lot of others out there who are new to this like me and want to learn more. I find even just seeing pics like the one you posted can lead to "Oh, thats what it looks like. I get it now."

MrKramer
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Old 9th December 2009, 09:46 PM   #9
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Interesting discussion, I'm in the same boat as you are MrKramer. Dave, do I understand this correctly then if you are using an amp with less than 50k input impedence you would use something like this:

source-> volume control -> pllxo -> buffer -> attenuator -> amp

and if you want something larger than 2cnd order pllxo it would look like this:

source-> volume control -> 2cnd order pllxo -> buffer ->
1st or 2cnd order pllxo -> buffer (if a low impedence amp is next) -> attenuator -> amp

very interesting thanks
Joe
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Old 9th December 2009, 11:09 PM   #10
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Yes... the volume control needs to have a low output impedance so that it doesn't affect the filter. ie it should be an active pre-amp of a buffered volume pot.

dave
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