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Old 22nd October 2012, 04:55 AM   #1
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Location: Michigan
Default High Pass Filter

I'm confused on how to build a high pass filter for full range drivers - though not on the theory, just the implementation.

First, some background. I have a tube amp as a source, with tang band 4" bamboo speakers in a vented box tuned to 70Hz (a common build with these drivers), and I have a M&K VX-7 powered sub. This sub has no high pass filter, and I am using the speaker level inputs. This requires me to build a high pass filter to keep the lows out of my full range drivers. For anyone curious, yes, 20Hz tones are still being reproduced by the Tang Band 4" drivers....just very poorly and very quietly lol........20Hz at 55db Yay!

So, here is where I am stuck....

A first order slope with just a cap isn't good enough since 35Hz is only 3db down and still heavily distorts the driver. A second order filter is uber expensive. Typical 2nd order high pass filters at 70Hz require $56 caps and $200 worth of inductors per speaker!

I did find some theory about using a cap and a resistor for a first order filter and then putting two filters together in series, but this way still requires more then $100 in caps per speaker!

So.......what do I do? There must be something I am missing that you guys do to keep your 4" full range drivers from being killed by low bass......right???

BTW - I know about MiniDSP and would rather not use it....I am trying to keep everything DIY.

Help!

Steve

Last edited by soundwavesteve; 22nd October 2012 at 04:59 AM. Reason: Added MiniDSP line
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Old 23rd October 2012, 12:55 AM   #2
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Well, I just bought myself some 400uF caps to use as first order 50Hz high pass filters. I chose 50Hz so the 100-70Hz range wouldn't be too rolled off. If it doesn't work, looks like I am going to jump right in to electronic design and build myself some active second order crossovers....

Unless my ears can't tell the difference between non-polarized electrolytic caps and metal polypropylene caps...
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Old 23rd October 2012, 01:00 AM   #3
Davey is offline Davey  United States
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It makes little sense to implement such a low-frequency filter between amp and speakers. Large value (expensive) capacitors.

You should implement the filter before the amplifier, not after. This need not be an active filter, but possibly a passive filter with a couple of capacitors and resistors. Much smaller values and much cheaper. A superior alternative in all aspects.

Dave.
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Old 23rd October 2012, 01:03 AM   #4
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Hi soundwavesteve,

I'm in a similar situation. I have a set of 4" drivers in vented boxes tuned to 80Hz. When choosing the required caps for a first-order high-pass, you have to know the impedance of the signal you're trying to high-pass (the driver/cab combination).

And of course there will be two large peaks on either side of the Fb (in your case, 70Hz). And the peaks may go up to really high impedance values, e.g. 20, 30, 50 ohms.

So that impedance is a moving target -- in some places it's 8 ohms, in other places, so much higher. So I concluded recently that I would skip trying to high-pass passively, but I am definitely looking forward to your results. And I hope you find a way.
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Old 23rd October 2012, 03:58 AM   #5
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Davey - Thank you for your suggestion, but I can't implement the filter before the amp since the amp is feeding my subwoofer via the speaker level inputs - as noted in my first post. I do not wish to use the line level because I would like my sub to pick up the amp's sound to help it blend better.

RjBond3rd - Yes, I did chose my cap based on the actual measured impedance, but in practice it made little difference in the value I chose. I have a 6.8 Ohm resistor in parallel with the speaker which smooths over the peak - it's 10 ohms instead of 40 at 67Hz. Whether it's 3db down at 50 or 45 or 60 doesn't really matter since I am just trying to keep the bass out while my sub crosses over at 80.

Hopefully later this week I'll let you know how it actually worked out :-)
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Old 23rd October 2012, 04:24 AM   #6
Davey is offline Davey  United States
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Yes, I noted your sub-woofer input configuration. You should be feeding your sub-woofer line-level inputs and not the speaker-level inputs. This allows to line-level filter your main amp as I noted.
Your "blending" concern is not really an issue. I'm aware this is a preferred setup by some sub-woofer manufacturers, but it's a configuration that creates more problems than it solves......as you're finding out.

Cheers,

Dave.
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Old 11th November 2012, 11:54 PM   #7
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I just wanted to give an update. I ended not needing any high-pass filter. It turned out I was having a problem with the speaker terminals from my subwoofer. A ground connection from the terminal block was corroded and was allowing the audio signal to back-feed across channels. In other words, the left channel was bleeding over to all of the other terminals, and so was the right channel. So my speakers were playing both channels in both speakers - this was over driving them and mudding up the bass. Now that I fixed it, I can actually crank them up with all 8 watts from my tube amp and they sound great - my sub even sounds better now too.
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