active xo help

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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
 
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
 
Just another Moderator
Joined 2003
Paid Member
Check out this thread http://www.diyaudio.com/forums/pass-labs/156094-b1-active-crossover-3.html

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.
 
frugal-phile™
Joined 2001
Paid Member
-->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,

133499d1242283080-thread-tysen-variations-fast-tysen-pllxo.jpg


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
 
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
 
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
 
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
 
Check the Marchand web site. They have crossover modules that come standard as 24 db/octave but can be modified for lower order crossovers. The crossover frequency is easily changed. They also have power supplies and kits. I've built a couple of crossovers based upon the XM-1 and XM-9 modules and have been pleased with the results.
 
Alright, been reading and trying to understand this better.

First off it seems to me that "passive line level crossover" is a somewhat misleading name. I think the only reason to call it that is because, unlike say a Salen Key filter, you could isolate the passive parts and put them in a box. In isolation it occurs to me that this is just an RC filter which as I understand it is a simply a frequency variable voltage divider. It also seems we could replace the R with an inductor. Apparently the reason is financial as the values are off the charts. Andrew T had said in another thread that for the filter to work it needs 0 input impedance and infinite output impedance. Obviously this cannot, in our real world, be true. However it means that the calculations assume it. So what we are looking for is constant very low in and constant very high out.
Since I want to change around amps and therefore cant rely on amp impedance in, my implementation of this filter requires an included buffer on both sides. Correct?

If the filter requires 0 in infinite out then does that explain why higher than 1st order gets "droopy"? The 2 filters for 2nd order don't both see the same impedance?
My understanding is that a salen key filter includes as part of the circuit, its own buffer. It also uses a feedback loop. Not sure I completely understand how it really works.

Would I get similar looking results with these:

Buffer-->1st order RC-->buffer-->1st order RC-->buffer-->

VS

Buffer-->salen key-->

Both "non droopy" textbook 2nd order?
Anybody compared the 2?

MrKramer
 
frugal-phile™
Joined 2001
Paid Member
Yes, a PLLXO is just RC filters... called passive because there are no active componenets.

The device driving it has to have low output impedance and the device after high input impedance. Buffers (making it a buffered PLLXO) get you around this -- one could think of the buffers as adding the required input and output impedances to what comes before and after.

Even with buffers a 2nd order is droopy, because it is not active... in an active 2nd order, there is a feedback loop "lifting up" the response to whatever you target.

An LC XO would not have these issues, but the inductors required are huge. I'd love to find a source. You still have to worry about input and output impedances of what comes after & before. In the case of the Marchand, there is a 5k load at the output, so it is mostly the need for a low output impedance pre-amp.

dave

dave
 
look up the DCX2496 threads.
If you have this as an experimenters tool then you can dial in just about any filter, notch, EQ all with varying Q as well as Freq and then just read off the Freq & Q data and copy it in discrete analogue filters for your active system.

It saves a lot of time and a lot in components.

The one thing that DCX can do that is difficult in discrete analogue is time delay.

Sell on the DCX after you have completed your project.
 
Like I said in my first post, Id like to stay away from prebuilt stuff.
It seems to me that if Im making something like this buffer-->rc-->buffer circuit, I should be able to change frequency pretty easy. Also seems that if the buffer is something like a simple jfet source follower, it should be fairly basic and cheap.
Am I barking up the wrong tree here? Seems those prebuilt crossovers are immensely popular...

MrKramer
 
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