# Bandpass Gain



## Synthsayer (Dec 19, 2007)

My question is whether there could be some bandpass gain created when using a 2 or 3 way passive speaker combined with an Audio Video Reciever which has a built in 2 way active crossover for use with LFE or subwoofer systems?

Having worked with 3 way active and passive crossovers for some time I have become familiar with bandpass gain and ways of controling it. 

Most of my projects lately have been 2 or 3 way systems for my modest home theater system and use passive networks. Depending on the driver and circuit topology, bandpass gain in three way networks can take quite some time to fix. 

I don't have much in the way of sophisticated test equipment and I use most of the available design tools written for Excel or online calulators.

What I have noticed is that when I set up one of my speakers with a normal 2 channel preamp and amp with no tone controls, the overall tonal balance is good. In some cases it is very good. The project I am working on now is made up of 2 ways for a typical 5.1 setup. They are crossed over at 2.5kHz using Second Order LR filter with close tolerance components of decent quality. When I take the same speakers, and connect them to my Harman AVR 154 there is a noticeable drop in the high end. Voices sound okay but the tweeters seem to drop off a lot.


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## GranteedEV (Aug 8, 2010)

Bandpass gain is an issue with 3-way and especially 4 way speakers because the tweeter and woofer are both expected to blend to the midrange for one coherent response. 

So we can think of the woofer/midrange crossover and midrange/tweeter crossovers as separate entities which affect the midrange equally, and the goal becomes to get a summed response which is flat. The easiest way to do this for a passive speaker is to adhere to the decade rule of keeping the tweeter/mid crossover a decade higher than the bass/midrange crossover frequency. Either way, the point is, wherever you may want the midrange to be down in level, you also want the tweeter to be attenuated if it is making any contribution to the sound, which is often ignored.

Now consider this

A filter between a sub and a speaker is not a filter between the midbass and speaker. It is literally a filter that comes before any filters. The high pass filter used on the mains is thus summed and not driver dependant. Therefore you should not have any problems. So even if your mains have something like, say, a woofer/midrange crossover at 200hz which sums to flat, it is NOT an issue because the "sub" crossover is not a subwoofer/[woofer] crossover" but rather a [subwoofer] / [woofer/midrange/tweeter] crossover.

Now without knowing more about your setup and design, we can't help you fix the problem. If the tweeter seems weaker with the AVR than the separates amp, it could be many things... a phase angle/impedance issue, a setup/autoEQ problem, a room acoustics issue, faulty amplifiers... there;s a lot of things which could be responsible.


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## Synthsayer (Dec 19, 2007)

I guess my question could have been: How does the high pass filter section of a common AVR affect the performance of the woofer when frequencies below 60-150 HZ are removed? Is there any affect other than possibly increased power handling because the low end has been greatly reduced thus reducing excursion, etc.

Maybe another thing that causes the little treble drop problem I notice is the 'virtual surround' modes of my AVR. There are the standard Dolby Pro Logic Music/Movie/3 Speaker/4speaker as well as a plethora of DTS, Logic 7 of different types, and on and on...

Is it true that the different surround modes have there own unique ways of 'virtualizing' the surround field which may cause high frequency output, especially from surround channels, to vary? That was the impression I got from reading back throught he despription of Surround Modes in the Harman AVR154 operation manual.

Another thing that causes me furstration from an audiophile/sound buff viewpoint is this: I watch a lot of news, live sports events, and other assorted television programs provided via cable TV service. One channel will have good strong sound levels with great bass and treble sound that we might expect from today's higher quality electronics, while another channel/network will sound thin and lifeless and have a weak signal strength. I think The U.S. Calm Act was supposed to help combat the wide variences in volume between a TV program and commercials, but didn't have anything to do with any standards regarding dynamic range, overall volume or sound quality. When I use audio sources such as a laptop, IPod, DVD/CD player, the treble drop is much less noticeable.

Thank you for the help and have anice day.


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