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Old 6th February 2007, 12:58 PM   #11
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I have a question (maybe several) about active/line level crossovers.

Is it reasonable to make them passive? The treble could just use a high value input cap 1500pf(?) which would cut out all the bass to that amp, and the bass could use a cap to ground, which would dump all the treble and act as a low-pass for the other. Would this be good enough? It's certainly simple and cheap.

But then again, with biamping you would be driving two amps with a single source, changing the impedance and lowering the sound level (right?). So using a buffer/op-amp would help with that. But, finding good sounding buffer circuits that run on a 12v single supply seems to be an issue, especially for non-engineers like myself who haven't got the chops to design them for ourselves.

Thoughts?

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Old 6th February 2007, 02:21 PM   #12
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Default active crossovers for biamping

here is a very small active line level crossover adjustable from 50 Hz to 2.5 kHz.

rolls crossover

There is also the ibump on makezine.com for DIY. There is a schematic for the project, you may be able to bring up frequencies to suit your needs.

Tom, from quaketronics, that sells the ibump says that changing the six capacitors will yield different frequencies.

For lower than stock subwoofer freqs he recommended these:

Capacitor value: 0.068uF
Corner frequency: 99Hz
Digi-Key part number: P4523-ND

Capacitor value: 0.082uF
Corner frequency: 82Hz
Digi-Key part number: P4524-ND

Capacitor value: 0.1uF
Corner frequency: 67Hz
Digi-Key part number: P4525-ND

He arrived at the values by:
"The formula for the capacitor value is:
C = 6.715/F
where F is the corner frequency in Hz and C is the value in uF.
Or, the other way around works:
F= 6.715/C
This is handy because capacitors are only easy to get in certain values, such as 0.068uF, which is the value that we use. Other values that are available are 0.082uF and 0.1uF. Lucky for you the 0.082uF value gives a corner frequency of 82Hz, which should be close enough. The capacitors that you can easily get usually have a 5% or 10% tolerance. Use the 5% parts if you can get them."

ibump
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Old 6th February 2007, 09:10 PM   #13
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Quote:
Originally posted by Buckapound
Would this be good enough? It's certainly simple and cheap.
Good enough for what? - would be the question. Not tying to be smart, just that you gotta know how you want to use it.

Crossovers are, or should be, designed for the drivers and box you are using them with. One crossover is not going to work the same way with another speaker. Active or passive.

What you have described about is a simple 1st order active crossover. Yes, it will work well if the drivers and box alllow that type of crossover. Just pick the frequency you need.

Most analog active crossovers are built into the feedback loop of an opamp. That gives you a second order filter - 12dB per octave. They are easy to design and build. When you need a steeper crossover, you start to add these opamp filters in series. Flters like htis can also be made with tubes.

One cool thing about the T-Amps (and Hypex IIRC) is that the input stage is what? An opamp! So you can build your 12dB filter right into the feedback loop. Just add a cap and resistor or two and you have your crossover. Easy!

dgoodier That little Rolls crossover is cute! I had not seen it before. No telling how it sounds, as it has some obvious compromises. Only a single 12V supply (as mentioned by Buckapound earlier) and the simple twist knob frequency select. But it could be a fun place to start.

The iBump I've seen before, it's a nice circuit. I saw the schematic somewhere.

Back to Buckapound. Using a smaller cap in the T-Amp channel for the treble is something I've done a few times. Not really a crossover, but it does keep some of the bass out of that channel. Why amplify what you don't need? Means you can use a smaller, cheaper, better cap, too.
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Old 9th February 2007, 06:41 PM   #14
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i'm very interested in this because i was planning a triamp setup using compression drivers for the mids and highs. as their efficiencies are 105 and 107 dB I concluded that 5 watts would be plenty of power for them...so i will probably either go with a pair of 41hz Amp32PS or modded 5066's...one thing i am concerned about is gain matching and i am wondering if there is a simple way to adjust gain on either of these amps. has anyone had any experience with this?
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Old 9th February 2007, 09:42 PM   #15
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In this thread, Motherone talks about a resistor that adjusts the gain:
http://www.diyaudio.com/forums/showt...248#post567248
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Old 9th February 2007, 11:02 PM   #16
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You want the gain to be "nominal" that is, both the input and feedback resistors are the same value.

I can't remember if there is an option for that on the AMP3. There is on the AMP6. The gain is a fixed little higher on the Sonic.

If you have a true fully active system what you will be doing is running these amps wide open and controlling the individual section (hi/mid/lo) levels at the active crossover. Your preamp does overall volume control and input selection.

If you are just Bi-amping/Tri-amping with passive crossovers at the speaker, then you might use the volume controls on the amp to adjust the relative section levels. Your preamp still serves as overall volume control.

Make sense? We need diagrams, don't we?
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Old 9th February 2007, 11:23 PM   #17
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But if the amps are identical.......do we need to worry about separate volume controls or adjusting the gain? Can't we just use one volume control to control everything?
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Old 10th February 2007, 06:56 AM   #18
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well, i do intend to match levels mostly through the outputs on the crossover but the crossover i planned to use has a boost/cut range of only 6 dB, so if the range were to be exceeded(due to the wide disparity in driver sensitivity as my woofer is 96 dB powered by 40 watts) i thought another option would be to pad the input of the T-amps. however i have read that adjusting the actual gain of the amp is better than using a pot or a fixed resistor pot. is this true? sorry if this is a stupid question...
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Old 31st January 2013, 09:46 PM   #19
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Smile Bi Amping or Bi-Wiring

Hey guys, what you are describing is Bi-Wiring not Bi-Amping.
You still have the passive crossover in the circuit along with all the nasty things it imposes on the signal. Bi-Amping uses an electronic crossover between the pre-amp and power amps allowing the removal of the nasty passive crossover.
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Old 1st February 2013, 11:52 AM   #20
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Quote:
Originally Posted by rgrickerson View Post
Hey guys, what you are describing is Bi-Wiring not Bi-Amping.
Not at all, 2 amps = bi-amping (bi==2 ) which can either use a passive or active crossover whichever you fancy. Biwiring is just two sets of wire not two amps and is only going to work with a (split) passive Xover.

Quote:
Originally Posted by rgrickerson View Post
You still have the passive crossover in the circuit along with all the nasty things it imposes on the signal. Bi-Amping uses an electronic crossover between the pre-amp and power amps allowing the removal of the nasty passive crossover.
Change 'uses' to 'can use' and it becomes correct.
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