Passive line level x-over - CAN IT BE DONE? - diyAudio
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Old 24th June 2009, 02:29 PM   #1
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Default Passive line level x-over - CAN IT BE DONE?

Some say yes others no!

What is your take?

Please state the reason for your answer.

I'd like to know the precondition for a successful PLLXO, like:
Pre-amp output impedance.
Power-amp input impedance.
Complexity of the x-over.
Etc etc.

Please jump into the fray. I'm listening.

My goal is to drive a combo of midbass/midrange woofer and horn with my Radford STA25 tube-amp and the 15" bass section with solid state or tube.

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Old 24th June 2009, 03:08 PM   #2
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I believe Marchand makes a passive line level.
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Old 24th June 2009, 03:09 PM   #3
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Old 24th June 2009, 03:38 PM   #4
Dr_EM is offline Dr_EM  United Kingdom
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Default Re: Passive line level x-over - CAN IT BE DONE?

Originally posted by hasselbaink
I'd like to know the precondition for a successful PLLXO, like:
Pre-amp output impedance.
Power-amp input impedance.
Complexity of the x-over.
Etc etc.
I've not done it myself, but at a guess

Pre-amp output impedance - <100ohms
Power-amp input impedance - around 100k
Complexity of the x-over - 12db filters for 2 way should be achieveable quite readily.

Matching of levels will be harder with PLLXO than full active as impedance must always be considered. Also, features like notch filters will be much harder to implement
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Old 24th June 2009, 03:39 PM   #5
Pano is offline Pano  United States
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You can do some simple stuff well.
See here:
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Old 24th June 2009, 04:59 PM   #6
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A PLLXO composed of resitors & caps works pretty well everywhere if 1st order. One has to adjust the R values to suit the impedances of your devices (ideally you use the input impedance of the amp as part of your PLLXO). 2nd order is possible, the slopes are quite droopy. That the second section requires Rs that are on the order of 10x as large as the 1st section, a 2nd order is much more restrictive. Your preamp needs to have the grunt to drive the 2 series R in the low pass, and sufficiently low output impeedance to not be loaded down by the load provided by the 1st section of the HP. and the input impedance of the amp needs to be high enuff to help satisfy the 10x requirement (again best to use the input R of the HP as part of the circuit). In my experience, an input impedance of less than 50k on the HP will reqiuire a buffer (as long as you have enuff gain in the LP section, a single buffer on the output of the HP covers off most situations.

The Marchand uses caps & chokes. This allows higher orders (4th is probably practical max) I beleive that the load built--in is 5k. That makes for a requirement of quite low output impedance of the pre-amp. The chokes in these have to be very large values, and it is hard to get them small physcially & large valued without compromises in their characteristics. (ie the Marchand is probably well priced given the challenge of sourcing those chokes)

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Old 24th June 2009, 05:12 PM   #7
adason is offline adason  United States
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I am using passive line level signal splitter at 150 Hz buffered on the outputs. The line level splitter was from Dan Hildebrands page, which is long gone. I do feel that RC filters present quite a signal loss, therefore would like to try merchangs LC one day. They claim only -1dB loss!

dave, do you know what is signal attenuation with 1st order RC crossover?

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Old 24th June 2009, 06:58 PM   #8
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I have built into a BOSOZ pre-amp a line level passive crossover. the BOSOZ drives the filter, and the filter drives the amps directly with out a buffer. I used those panasonic polypropylene capacitors and some air core inductors. values are small enough that capacitors and inductors were not a problem. I used a CAD electronics simulator to get the rough values, then I swept the filters with a signal generator and measured the results to get the frequencies right on. (I had to make adjustments in the inductance and capacitance to get the frequencies just right.) I'm careful to do the sweeping with the filters between the BOSOZ and the Aleph 3 amps so that impedences are correct. I was even able to do some shaping of the response curves by experimenting.

Loss isn't too much and you can compensate for levels using the internal gain adjustments on the BOSOZ...which does not effect output impendance of the pre-amp.
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Old 24th June 2009, 07:03 PM   #9
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I made the filters on a plug in board so I could try different capacitor values, inductance values and experimenting with more than one stage.

The results are very good...much better than the active crossovers I had been using which I thought killed the highly musical nature of the BOSOZ pre-amp and the Aleph 3 amps.
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Old 24th June 2009, 07:48 PM   #10
konut is offline konut  United States
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I'm using the a pair of Marchands, 24db per octave at 70hz highpass, being fed from Creek OBH-12 remote passive line level attenuator/switcher which is being fed from a Bryston BDA-1 for most of my sources. The Bryston has a 50 ohm output impedance. I had tried an other DAC that had a 560 ohm out. Details here
DACs and Passive Pre Question
The sub has it's own, active, lowpass.
Because of the passive nature of my setup I need high gain amps. Currently they are 36db gain X-Amps from AV123. Amps are plugged in to the wall. Everything else is plugged into a Pure AV conditioner. No hum, noise, or hiss. Very satisfied with this system. I've used active Xovers in the past and they've always imposed themselves.
"If we knew what it was we were doing, it would not be called research, would it?" A. Einstein
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