Change/Bypass High Pass Filter(s)

Discussion in 'The Projects Forum' started by boaterdad, Feb 7, 2014.

  1. boaterdad

    Thread Starter New Member

    Feb 7, 2014
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    Hello, forum noob here. I'm a physicist but do mechanics, not electronics, so go easy on me. :)

    I searched these forums and read through what seemed appropriate, like this thread.

    My son and I are putting together a relatively affordable stereo system for his wakeboat, an older Ski Nautique. The system will consist of the following:

    200W x 4 ch. @ ? ohms amp driving four speakers in the boat
    125W x 2 ch. @ 4 ohms amp driving two tower speakers
    self-amplified subwoofer

    My question is about the first item. The amp is an XTP Trifecta 4200. It is being "re-purposed". XTP was designing a system with this amp and tower speakers which had midwoofers and HLCD horn tweeters driven by separate channels of the amp. So channels 1 and 2 were to drive the midwoofers, and 3 and 4 the tweeters. The speakers were never mass produced, and the amps sold off. I actually got 2 new amps for 27 bucks each at an auction.

    The manual is awful. I don't know if the 200W is peak or RMS (don't really care), and if the amp is rated for 4 ohms or 2 ohms minimum (do care).

    Channels 1 and 2 have a high pass filter (HPF) that is adjustable between 70Hz and 300Hz, which works great for our use. Channels 3 and 4, meant to drive the tweeters, have a HPF adjustable between 2300Hz and 3000Hz, which is of course terrible for our use, where we are trying to drive combination speakers, each with a midwoofer and dome tweeter (and I assume their own crossover inside).


    What I have done on the test bench:

    1. Tried bridging. Bridged 3 and 4 still go through 2300Hz HPF.

    2. So now running two 4 ohm speakers in parallel on each of channels 1 and 2. Not sure if the 2 ohm impedance will burn out the amp long term. Oh well, I have a spare amp.


    HERE'S WHAT I WOULD LIKE TO DO:

    I want to change the channel 3 and 4 HPF to be just like the channel 1 and 2 HPF. Move it down to 70Hz to 300Hz.

    OR, move it to a fixed 100Hz HPF.

    OR, just bypass it completely.


    PROBLEMS:

    a) I cannot locate a circuit diagram.

    b) I know what HPF circuit diagrams look like on paper... beyond that, I have no idea what I'm doing.


    I guess my questions are... how would I identify what components make up the high pass filter(s) for channels 3 and 4, and... what would I do to change them (e.g. different R and C values?) or to bypass the filter(s). I know it's tough without a circuit diagram, but maybe some general guidance could be given, or I could open it up and take some pictures...


    Do I have a prayer? If not, I'll leave the speakers in parallel and cross my fingers. It's a shame to ride two channels hard while two are loafing, though, just because of that doggone HPF.

    Thanks in advance.

    Steve
     
  2. kkazem

    Active Member

    Jul 23, 2009
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    Why don't you just use the biamp the way it was meant to be used. Open the speaker cabinets and run separate pair wires for the mid and tweeter to the biamp. The trouble with trying to modify the amp is that the xover might be in a low level preamp stage and not at the amp output. This would be hard for an expert like me without a schematic. Regards, Kamran Kazem.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    I agree with Kamran. The best way would be to go into the speaker cabinets and wire the speakers separately to the amps as the amps were intended to be used. That actually should give better sound the running everything together.
     
  4. boaterdad

    Thread Starter New Member

    Feb 7, 2014
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    Ok, I feel pretty dumb now.

    Thank you both, that is obviously the smart solution. So I would run the front left (FL) mid and RL mid in parallel on ch 1, FR & RR mid on ch 2, and the corresponding tweeters in parallel on ch 3 & 4.

    A question for you... the speakers are outdoor speakers with hard dome tweeters. One pair rated at 300W, the other at 400W. I think the mids will handle ch 1 & 2 of the amp ok, but ch 3 & 4 were designed for HLCD tweeters, meant to project the sound back to the rider (which I think is kind of rude to other boaters). Do you think the hard dome tweeters will take the power or will we blow them? I don't know how much power normally flows through the mids and through the tweeters based on the speaker crossover.

    Thanks. I'm learning!
     
  5. crutschow

    Expert

    Mar 14, 2008
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    The power proportionally goes to each speaker and is determined by the music and crossover frequencies. The new setup won't apply any more power to the tweeters relative to the midrange then using the speakers in their original configuration would if the crossover frequencies are comparable.
     
  6. boaterdad

    Thread Starter New Member

    Feb 7, 2014
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    Thanks again for the replies.

    I'll take a look at one of the speaker's internals and see if it's easy to access the mid and tweeter wires, to bypass the speaker internal crossover circuit. If so, the idea would be to do active bi-amping, and I would parallel the front & rear left mids from amp ch 1, front & rear right mids from amp ch 2, front & rear left tweeters from amp ch 3, and front & rear right tweeters from amp ch 4. This would allow ch 3&4 to handle the high frequencies and so drive ch 1&2 less for a given volume.

    I have since come across a very interesting bi-amping article, which is a little bit self-contradictory: it touts the benefits of active bi-amping, but states that the lower frequencies contain most of the power. So I'm wondering how much of the power goes to the mids, and how much to the tweeters... i.e. is it worth re-wiring the speakers and running double cables if only 1/8 or 1/4 of the wattage is offloaded from ch 1&2 to ch 3&4 for the tweeters.

    Any idea what percentage of the power typically goes to the mids and what percentage typically goes to the hard dome tweeters? These mids are 6" diameter and smaller; the lows are going to the separately amped subwoofer.

    Of course, maybe using the amp bi-amped as it was designed will sound better, although the setup really sounds very good already. I'd just like to utilize all four channels and not overdrive the two.

    Thanks, I really appreciate you guys sharing your knowledge.

    Steve
     
  7. boaterdad

    Thread Starter New Member

    Feb 7, 2014
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    In addition to my question above (how much power to mids vs tweeters), I have another question--I came across something I didn't expect when I opened up one of the speaker cabinets.

    [​IMG]

    [​IMG]

    The incoming white is the positive and the black is the negative.


    (edit: my prior interpretation of the wiring connections was just plain dumb)

    I am having trouble matching the solder points in the photo above to the points in this simple HPF circuit:

    [​IMG]

    Help! lol

    Thanks.

    .
     
    Last edited: Feb 8, 2014
  8. boaterdad

    Thread Starter New Member

    Feb 7, 2014
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    .

    I think I found some info regarding mids vs tweeters power. I located these on another interesting forum:

    Quote 1:
    "The music spectrum is as follows:
    about 50% below 300hz
    about 75% below 2500Hz
    about 95% below 10k"

    Quote 2:
    "As a bit of a guide, in pro audio concert PA, a 3-way system crossed at 250Hz and 1.2kHz usually runs mids = 3x tweeter power, subs = 3x mids power."


    I realize these are gross generalities, but they are a starting point.

    So the first quote tells me that my tweeters (given their HPF) see about 25% of the power. This is roughly consistent with the second quote.

    We don't have final settings yet, but we've been setting the mid HPF around 170Hz, and of course it rolls off, so maybe the mids don't see about the bottom 25% of the music power (the separate subwoofer makes up for this). Then the mids would see about the top 75% of the music power, and the tweeters about the top 25%, and so "mids = 3x tweeter power" seems very reasonable.

    So then bi-amping should offload roughly 25% of the load from ch1&2 to ch3&4.

    Does that all sound about right?


    .
     
  9. boaterdad

    Thread Starter New Member

    Feb 7, 2014
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    .

    This is the best I can come up with for trying to match the circuits above... I know this is terribly wrong..........


    [​IMG]
     
  10. boaterdad

    Thread Starter New Member

    Feb 7, 2014
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    0
    .

    Here is my Guess # 2 as to what they are doing... can the resistor be put before the capacitor like that?


    [​IMG]
     
  11. crutschow

    Expert

    Mar 14, 2008
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    You connection in post #10 shorts out the signal to the tweeter. I suspect point B is just an isolated terminal used as a solder point, and the resistor and capacitor are in series with the tweeter. You can measure the connection between the three terminals with an ohmmeter to determine which are connected.

    So for you biamp connections just remove the both wires between the two speakers (unless your biamps have a common output in which case you can leave the white wire) and connect each speaker to its respective amp. You can leave the resistor and capacitor attached as long as one end is open, if you may want to go back to using the crossover some day.

    You division of power between the various frequency bands seems reasonable. The high frequencies generally carry very little power in most music.
     
  12. boaterdad

    Thread Starter New Member

    Feb 7, 2014
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    .

    Thanks for the reply.


    > "You connection in post #10 shorts out the signal to the tweeter."

    Yes, great point. lol... good thing I'm not designing circuits for a living.


    > "I suspect point B is just an isolated terminal used as a solder point, and the resistor and capacitor are in series with the tweeter."

    I guess that makes sense. Is that sort of a "poor man's HPF"?


    > "You division of power between the various frequency bands seems reasonable. The high frequencies generally carry very little power in most music."

    Yes, after doing a lot more reading about this, I think the estimate of the tweeters carrying one-third as much power as the mids is probably quite high. I get the impression 10% is probably a better number.

    So given that, we did a thorough listening test. My son and I went through a lot of typical music that will be played on that boat. We adjusted the Ch 1&2 HPF. We also played with the equalizer on the MP3 a little bit, but set it back to flat for a more valid comparison. We found that the highs sound fine, very good actually. The main issue with any missing highs is in a few hard-to-find songs that have too much MP3 compression. We had my wife check it out, and she thought the highs sounded great.

    From what I read, bi-amping can provide true benefits with very good equipment in a good acoustic setting. This will be on a wakeboat, with loud kids on it, surrounded by lots of other boats making noise on the water.

    So after thinking about it, rather than modify the four speakers to bypass their crossovers, I'm just going to go with the original set up. I'm pretty lousy with a solder gun and I'm sure to mess something up anyway.

    The bi-amping wouldn't really accomplish what I was after, which was to split the power load across 4 instead of 2 channels. Each midrange channel amp would still have a pair of midrange speakers in parallel it.

    So now, my question would be...

    Is it even worth opening up the amp and taking some pictures to try to figure out what components comprise the HPF on Ch 3&4? Those would be the ones I'd love to change to match the way Ch 1&2 is set up. If it would be an easy desolder and solder job, I could find someone to do that part for me.

    Or should I just run a pair of speakers in parallel on Ch 1, and a pair in parallel on Ch 2, and be done with it? The speakers are 4 ohms each, so the amp is then seeing 2 ohms. Again, I have not seen a published ohms rating for the amp. We ran it for half an hour or so pretty loud in that configuration and nothing got hot or even warm. But we're in a 70 degF room, not on a boat in 100 degF heat with high humidity.

    I would value your guys' thoughts on this. Thank you.

    Steve
     
  13. crutschow

    Expert

    Mar 14, 2008
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    Yes, it's a simple high pass filter with the resistor in series with the tweeter impedance providing the series R and the capacitor providing the series C.

    If it works okay in your test then I think it will be fine to run that way. Even at at "pretty load" volume you probably are only putting a few watts average into the speakers. Did you feel the temperature of all the heatsinks and other heat generating components after it had run for the half hour? Optionally you could put both speakers in series to give an 8 ohm load (if they are identical speakers).
     
  14. boaterdad

    Thread Starter New Member

    Feb 7, 2014
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    Great input, thanks.

    Yes, I did monitor the power lines from the battery to the amp, and all of the metal on the amplifier housing, including the long heat sink fins. But I will charge the 12V battery and run the test again.

    For the installation, based on amperage charts, I'm planning to use 6 gauge wire from the battery to the circuit breaker and then to the distrbution blocks (and then for the negative return to the battery), and then 8 gauge wire from the distrbution blocks to/from the three amps (tower speakers, subwoofer, and the one we've been discussing here, which is for the in-boat speakers). I plan to use 16 gauge wire for the remote switch lines. All of this is stranded copper, not copper clad aluminum. And I've been convinced that tinned wire isn't needed unless it's going to lie in water in the bilge or be used in a saltwater environment (no and no).

    So anyway, I want to pick up some 8 gauge wire to use from the battery to the amp for this test. I'll post the results.

    Thanks.
     
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