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Old 16th November 2003, 08:46 PM   #1
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Default Pass Diy High Low Pass Please

Has anyone done any ground work or built up their own xover using Jfets or whatever.

I thought a generic 2-3 way mother PCB with the power supply and input/output buffers and plug in daughter boards for the filtering offering 1-4th order / bessel, butterworth/ LR would make the project almost univeral.

I am curious to see what Mr Pass comes up with (on Christmas Eve would be nice).

Ian
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Old 16th November 2003, 09:02 PM   #2
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Here is an obsolete commercial offering that used plug in boards (no longer available).

This design is fairly unique in that it has a full descrete bi polar input buffer and output buffer for the hi pass section.

The hi pass section uses all descrete active stages and the low pass uses the LM 833 chip. The high pass plug in board has all the passive filter parts , while the low pass boards have the chip and passive parts. (pity know one has any boards for these units, they were designed for the JBL XLP-200, and apparently DTS bought up all the remaining stock of DX-1 crossovers after the XLP -200 was discontinued).

Ian
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Old 16th November 2003, 09:05 PM   #3
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Everything is pretty much explained in XVR manual http://www.passlabs.com/pdf/XVR1MAN1.PDF

Even the schematics are given out (check pages 30 and 31)
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Old 16th November 2003, 09:10 PM   #4
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I've got such a circuit, complete with variable Q, selectable frequencies, etc. etc. etc. It's discrete, with JFET inputs. I've been holding off in order to let Nelson do his thing. I'm sure that people are more interested in his design.

Grey
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Old 16th November 2003, 09:11 PM   #5
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I see what you mean Peter but that is the masters ..masterpiece and a very complete unit.

I think the hard part is how to arrange it in practical terms with boards and stuff.

Here is the schematic to the unit I mentioned above.

Ian
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Old 16th November 2003, 09:15 PM   #6
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Forgot to mention--I've got PCB layouts, too.

Grey
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Old 16th November 2003, 09:21 PM   #7
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Grey,

Your a wonder...have you ever thought about working with/for Mr Pass.

Has Mr Pass seen your creation...it may receive his blessing!

Ian
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Old 17th November 2003, 12:27 AM   #8
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I spent a long time debating whether to go Sallen-Key or multiple feedback. A loooong time. I've still got this silly idea in my head that I'd like my system to be balanced from phono all the way through to the outputs. Why? Just 'cause.
Balanced phono isn't that much of a problem because you don't change anything; the RIAA curve is what it is and once you set the filter, you leave it alone. As long as you don't have one of those cartridges that internally grounds one of the pins, you're set to go. Line stages are obviously not a problem. Amps...well, there's always the Aleph-X, which was part of my overall program of converting to balanced. Or you could just put together a straight X amp, without the Aleph part. Nuthin' to it. But there's still that pesky crossover to deal with.
The problem is that Sallen-Key doesn't lend itself to balanced design. It can be done with multiple feedback circuits, but then you lose the ability to easily change Q, etc.
Oh, bother!
It's one of those tradeoff things that you face in the real world.
For the time being, I'm doing Sallen-Key, unbalanced (the same thing Nelson's doing, according to the manual). I may decide to go back and take a look at the multiple feedback question later if I get time. The brute force approach would be to use a Sallen-Key filter to set the design parameters--the frequency and Q--then design a fixed multiple feedback circuit to do the job. But clearly there's a lot of duplication of effort involved. Ugh.
To answer your question, I've corresponded with Nelson about one or two small points, but he hasn't seen the whole shootin' match. Given that he's got the NP LP/HP somewhere in the pipeline, I'll defer to his circuit unless it looks like it'll be a year or more till it comes out. Then maybe I'll post something to tide people over until the 'real thing' arrives.
Oh...and the other question. I'm sure Nelson's getting by without me. I'm just one little feller out in the hinterlands with a few parts to play with. (And with more imagination than sense, according to some.) But, hey, it keeps me off the streets and out of trouble.

Grey
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Old 17th November 2003, 01:26 AM   #9
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I see your point of view:

"The problem is that Sallen-Key doesn't lend itself to balanced design. It can be done with multiple feedback circuits, but then you lose the ability to easily change Q, etc.
Oh, bother!"

So, it depends on whether you want to experiment on the fly, and from reading Mr Pass XRR1 Crossover Bible (which is surely the most comprehensive manual on crossovers this side of the black stump), this is a must do if your a audio Nut like the rest of Us.

Imho, I see no reason not to use the Sallen-Key passage and then an inverting buffer as Mr Pass has used here (and also in the Ono) . It certainly rationalises the guts of the crossover, the inverting -ve signal does not stand in the way of the original +ve feed.

Attached is an edited version of the block diagram of te XVR1 manual (for my own educational purposes only).

At least in pro audio (and the Aleph Series improve performance), the aim of Balanced signal input and outputs is really for best CMRR i.e. less noise and distortion, in other words eliminate the lousy earth loops that make bi amping in unbalanced a crime. It is only required at the in and outputs, not the processing.

So for bi/tri and quad amping balanced operation for the inputs and outputs is a given just to keep the hum out of the picture

I recall the multiple feedback stuff is widely used in state variable filters with multiple high order slopes (like Rane Audio's AC22)

"Oh...and the other question. I'm sure Nelson's getting by without me. I'm just one little feller out in the hinterlands with a few parts to play with. (And with more imagination than sense, according to some.) But, hey, it keeps me off the streets and out of trouble."

Grey, I think you under estimate your commitment to the diy audio community.

That fact that you've had your own Passover of sorts going for yonkers kinda makes the rest of us "mere mortals" in the diyaudio Kingdom.

There are probably a few peanuts in the gallery who might challenge that statement, but we need peanuts don't we.

Lets wait and see what the Master has in the wings. Time for a nice Cab Sav...

Ian



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Old 17th November 2003, 02:30 AM   #10
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Bringing in a balanced signal, then dropping a phase bothers me aesthetically. Then you have to recreate the missing phase on the way out. It's a workable compromise but, still, I grumble.
State variable filters and such can be done--but they aren't optimal for typical audio purposes. We rarely, if ever, need high Q filters, and that's really their raison d'etre.
The whole active crossover thing has generally been taken to one extreme or another. Either the reader is told not to worry about the details and treat the circuit as a black box, or is told that they must master mutiple layers of obfusicating theory and mathematics in order to get the job done. There is, in fact, a middle road. Long ago I began a thread on active crossovers. I started with the basic formulas and such and was building up in complexity. People didn't seem too interested at the time, so I went on to other things. Now, what with various Alephs, Aleph-Xs, chip amps, and the like accumulating on shelves as their owners move on to the newest amp-of-the-month, perhaps people have more harware on hand and might be interested in pursuing the topic further.

Grey
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