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Old 10th December 2002, 03:04 AM   #1
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Default Passive Bi-Amp Xover Project

A sad fate befell the Yamashita household this past weekend. What had been a solid sound image was now jumping back and forth from left to right. I ask my wife to listen with her cat like ears to see if she can tell what was wrong. After about 10 second of listening she asks, Where is the bass? as if she were ordering a hamburger from Wendys. Panic set in as I scurried back an forth between the stereo subs, left to right, change the cables, left to right, change the cables, and again until I narrowed it down to my Heathkit AD1702 electronic crossover. After 20+ years of service, it finally gave up the ghost (at least in one low-pass channel).

It was only a week ago that the owner of the local audio store told me I should go passive xover. Well that is what Im going to do (I hope). Theres not a lot of solid design information out there. Thanks to Fred (HH), Peter (HPotter), Dave (From beyond Pluto), and others from this forum and beyond who have provided the bulk of the info Ive been able to find on the subject of Passive Audio Preamps and Xovers.

Passive Preamp
i need and GOOD active XO
http://www.t-linespeakers.org/tech/f...ssiveHLxo.html
http://www.silcom.com/~aludwig/Sysdes/Bi_amping
http://users.pandora.be/airborne/download.html

Below is a photo of where I am today. Until I finish this project successfully, I will be without my subs so I am a bit more motivated then usual.

The plan is to create a 12db/oct low-pass for the subs and a 6db/oct hi-pass for the mid-bass. The sub amp is a NAD with ~20kohm input impedance and the mid-bass amp is a Bryston 4bST with an input impedance of ~50kohm. I will put a passive level control on the low-pass output and the hi-pass output will be what it is. This is because the subs are a full 6db more efficient the mid-basses. It is also a more straight-thru path for the higher frequencies.

I have a dual pole/6 position switch that I will wire to switch in various low-pass caps into the C2 position for each channel. The cap that is switched in will be in parallel with the main C2 so will add to the total value of C2. I think I will use a temporary pot in the hi-pass output to get a final shunt resistor value. I believe it will take some tweaking back and forth between the hi-pass and low-pass pot to get the xover where want it, which is in the 80 to 90Hz range.

My next step is to create a schematic (which I will post) to work to. Once all is right, and if I like what I hear, I will start looking at passive tri-amp (or maybe a single buffer output with battery power). Suggestions? Finally, when all the tweaking is done (is it ever?) repackage everything in a nice new case.

Any input that can be offered in this endeavor-o-mine would be greatly appreciated particularly since you had to wade through this lengthy scenario.

Thank You All,
Rodd Yamashita
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Old 10th December 2002, 03:18 AM   #2
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All that and I forgot to post the photo
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Old 10th December 2002, 03:33 AM   #3
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what kind of caps are the red ones? where did you get them?

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Old 10th December 2002, 03:44 AM   #4
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Quote:
Originally posted by BrianGT
what kind of caps are the red ones? where did you get them?
Hi Brian,

Their 15 year old Wima MKP 10% MFPP (but measured and marked) from Madisound. The big ones are .22uf@160V and the smaller ones are .1uf@ 160V.

Rodd Yamashita
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Old 10th December 2002, 01:04 PM   #5
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Grey started a crossover thread long time ago. Did you see it?
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Do something really well. See how much time it takes. It might be a product, a work of art, who knows? Then give it away cheaply, just because you feel that it should not cost so much, even if it took a lot of time and expensive materials to make it. - JC
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Old 10th December 2002, 02:01 PM   #6
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Quote:
Originally posted by Peter Daniel
Grey started a crossover thread long time ago. Did you see it?
Specifically, no Peter, but I will go back and check. I know, in many threads, there was a lot of off topic discussion that touch passive pre-amp circuits.

Gray was certianly included in my thanks.

Rodd Yamashita
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Old 10th December 2002, 02:38 PM   #7
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Here's the link: Active crossovers
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Do something really well. See how much time it takes. It might be a product, a work of art, who knows? Then give it away cheaply, just because you feel that it should not cost so much, even if it took a lot of time and expensive materials to make it. - JC
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Old 10th December 2002, 04:31 PM   #8
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Quote:
Originally posted by Peter Daniel
Here's the link: Active crossovers
Thanks Peter, but Gray is talking about Active bi-amp crossovers. I'm building a Passive Line Level Crossover as in the Peter Rettweiler article (PLLXO) on Dave's web site.

I should have a schematic done tonight.

Rodd Yamashita
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Old 11th December 2002, 12:45 AM   #9
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Here is the generic schematic for the passive line level xover. Channel 1 and 2 are the same. Ill now need to go through and calculate the values. Maybe by Thursday.

The 100kohm pot (PH1) will be replaced with a resistor once things are set.
RL3 will depend on the amps input impedance and the level adjustability.
Trimmer caps C1L will most likely be needed.

So far Ive spent less then $30 (since I had the caps and resistors). This should qualify as a frugal-phile project (what do you say Dave?).

Rodd Yamashita
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Old 22nd December 2002, 06:52 AM   #10
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Rodd Yamashita

Well Im running my system in a quasi-VOT setup with the TAD subs for the bass and the TAD 4001s coming in at about 600Hz. This has slowed the project somewhat, but I have made some progress on the crossover.

After some revisions and stuffing some proto-boards, I decided to revise the schematic. I will put some values in during next week sometime.

Im still contemplating a battery powered output buffer with discrete components, but Im not very strong on the design side so I still hoping for some input from you analog circuit guys.

Thanks in advance for you input.
Happy Holidays
Rodd Yamashita
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