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Old 4th September 2013, 02:44 PM   #1
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Default Active Crossover Adjustment

Hi guys, I have acquired a pair of active speakers that use a TL072 based active crossover. My biggest issue with them is the tweeter level is around 3dB too low, boosting 3dB on my PC's EQ really brings them to life but I want to adjust the crossover so the EQ isn't needed...

I plan on replacing the Tl072 with OPA2134 but I have schematics for this model & the next model up that uses identical drivers/power amp/box etc... The only difference is in the crossover...

Mine:

Click the image to open in full size.

The next model up:

Click the image to open in full size.

As you can see they use different Opamps & all of the surrounding components are different values... i guess they have completely different crossover slopes/levels etc ?

In a nutshell, would it be worthwhile replacing all of the components to values that match the next model up ? Or can you tell me how to increase tweeter output 3dB or so ?

Any help greatly appreciated
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Old 4th September 2013, 04:19 PM   #2
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It's the low-pass filter that has been raised in frequency. The high-pass is broadly the same response.

If you want more treble output you either need to attenuate the bass or boost the treble. You can't do it by fiddling with crossover frequencies. Also note that if you boost the treble, it has the effect of lowering the crossover point, so using the better models crossover values I fear would give you too much output at the crossover point.
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Old 4th September 2013, 06:12 PM   #3
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Ah okay, thanks for that... Seems obvious now you've said it !

I'll just change the Opamps & see what change that brings if any.

Thanks for the input
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Old 7th September 2013, 07:55 PM   #4
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I'm still waiting for the OPA2134's to be delivered but in the meantime decided to change the power amplifiers input coupling capacitors [they were 'AVF' branded electrolytics] The LF was 100uf & HF was 0.22uf so I changed the LF with a 47uf solid tantalum & the HF with a 0.47uf MKP which seems to have helped an awful lot.... A step in the right direction at least.
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Old 14th September 2013, 04:26 PM   #5
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There is a link on the Burning Amp Festival's website to a video of a quite excellent talk given by Douglas Self on active crossover design considerations. If you are contemplating any major revisions you should at least take a look at the video. It is quite interesting.

I found it! Here is the link: Videos | BurningAmp : The annual audio DIY event at Fort Mason, San Francisco CA for audiophiles who make their own equipment, components for amps, speakers, and parts.
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Old 14th September 2013, 04:59 PM   #6
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I doubt that you will see (ok hear) too much difference between the TL072 and the OPA, unless you have a really High End system. Used at unity gain the TL072 is a very capable audio op-amp.
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Old 16th September 2013, 03:34 PM   #7
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I don't consider my system to be terribly high end, but I noticed a fairly significant difference between TL072 and OPA2134. The OPA is much cleaner sounding, a layer of haze went away. This in an active system using Focal TC120TDX and 6W4264 driven by a pair of modified Pass A75s.

The easiest way to achieve your goal of increasing the relative level of the tweeter is to pad down the woofer. Break into the low pass line after the crossover and insert a pot or fixed attenuator consisting of two appropriate resistors for the desired attenuation.

Last edited by BobEllis; 16th September 2013 at 03:39 PM.
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