PE Plate Amp Mods

Discussion in 'Home Theater Projects' started by BenSC, Aug 21, 2003.

  1. BenSC

    BenSC Stunt Coordinator

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    Hey guys,

    I've tried to find this information in the search, but I can only find "how" to mod the amp vs. why. I'm picking up the 250W amp, and will be running a slot ported 3ft^3 box with a Dayton 12" DVC in it. Do I need to mod the amp for this particular setup? And if so, what does it do for me? Change the boost level (which I beleive is set to off default).


    Thanks,
    Ben
     
  2. AndersP

    AndersP Stunt Coordinator

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    I would have listened to it first before starting digging in to the amp. Maybe you like it as it is?

    It´s always a good idea to know what to change ( and how to ) before you do it right?

    The next best thing would be to have someone ( don´t count me in for the moment ) running a sim on that box of yours. The dimensions of the port would be of vital interest for that person and exact inner volume of the box as well.

    The best thing is to measure it.
     
  3. BenSC

    BenSC Stunt Coordinator

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    I haven't built it yet. I've got the parts coming in tommorow. With the amp mounted inside and the braces, interior of the box is 3ft^3 on the nose. The port is 18x1, tuning it to 23htz, port mach is in the green. The design is also downfiring, since Adire said the shiva gave its best linear dispersion downfiring, and the Dayton is so similar. I put the whole thing together using WinISD.

    I'll definatly be assembling it before opening the amp. I figure since this is such a popular combination, and there are so many posts about how to mod it, that it has to have some benefit. I'm just trying to figure out what that is.

    Thanks.
     
  4. ColinM

    ColinM Cinematographer

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    For me, it's better to have an amp with no EQ built in, as I have an EQ in the signal before the amp anyway.

    But beyond just altering the EQ, the amp mod can also dial in the rumble filter, a good thing for ported systems. This can be done while also maintaining the zero EQ - Some more examples of zero boost configurations:
    R27=130k, R26=62k, Fc=17.7Hz
    R27=150k, R26=75k, Fc=15.0Hz
    R27=180k, R26=91k, Fc=12.5Hz

    FC = the start of the rumble filter, too low for your app.

    I got these figures some time ago, from this board.
     
  5. Ryan Schnacke

    Ryan Schnacke Supporting Actor

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    Generally the idea is that you want the rumble filter to reduce the amplifier's output below your tuning point. This is because cone excursion goes up very quickly below your tuning point. It doesn't take much power to bottom out the woofer down there.

    The 300-794 amp comes with a configuration that provides no boost and a corner frequency of 19.4Hz. No boost is definitely a good thing for a properly designed vented sub (and yours looks like a good design). In simulation with 270 watts I'm seeing this design hit its excursion limits at about 18.75Hz so maybe the stock filter point would be okay. But this is only a 2nd order filter (12dB/octave). If you find that you're bottoming it out then I would try moving the filter's corner frequency up closer to the tuning point. Here's a couple of numbers for you:

    R27 = 109k, R26 = 54k, Fc = 20.75Hz
    R27 = 100k, R26 = 50k, Fc = 22.5Hz

    Notice that any resistor combination where
    R27 = 2 * R26
    gives you a no-boost (Q=0.71) configuration.

    BTW, I've got a 12" DVC in 3.5 cu ft tuned to 22Hz powered by the 300-794 amp. It works great with the stock filter.

    I've modified one of these plate amps before and I have a warning for you. If you choose to disconnect the wire bus running from the pre-amp board to the amp board then DO NOT let the speaker output wires touch each other!!! The huge power supply caps on the amp board can hold a huge amount of charge even after being unplugged for DAYS! With the wire bus disconnected the input is floating and could be driving the outputs to the rails. If they touch you'll get a big spark and a busted plate amp. I learned the hard way. You get to learn the easy way - from my experience.
     
  6. BenSC

    BenSC Stunt Coordinator

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    Time to revive the thread now that I've got the sub going. I know this amp is SUPPOSED to come with no boost, but I swear at 30htz it goes wacko. Using my SPL meter I read about 5-10db higher from like 25htz up to 40htz. It's really annoying when watching movies. Some of it I'm sure is due to room acoustics, but that's a huge jump, and it is sustained no matter where I am in the room.

    So, I'm gona open this thing up and take a look. Thanks for the warning on the speaker wires. I also haven't had any bottoming out problems, Though I might try this anyway:

    R27 = 109k, R26 = 54k, Fc = 20.75Hz

    It's not really audible beyond that point anyway, and I can feel that note plenty well without needing to feel below it. Better safe than sorry. The real thing I want is to just make sure there is no boost on it.
     
  7. Ryan Schnacke

    Ryan Schnacke Supporting Actor

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    Well when I reviewed your first post I noticed that you said you have "the 250W amp" but you didn't mention whether it was the 300-794 or the 300-796. These two are exaclty the same except the 300-796 has 4.4dB of boost at 31.3Hz so its possible that you don't have a flat filter.

    But its also very easy for in-room response to vary 5-10dB from the standing wave patterns. The fact that its sustained throughout the room does suggest its something other than standing waves though.
     
  8. BenSC

    BenSC Stunt Coordinator

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    I ordered the 794. Though is there a way to look at the amp and tell if I got the right one? I don't think I have the box anymore.

    I'm going to go back and remeasure tonight to make sure it was only a 5-10db gain. I was playing last night and while say 60htz sounded nice and loud, 30htz was making the lights in the bathroom across the hall rattle.

    I'll try some more placement issues, though I'd rather not have to move the sub since it's so big and the room is limited for space, perhaps a BFD will be in my future.

    Is there anything about my enclosure design that could be causing the gain? WinISD reports that volume at that port size to be an even curve.
     
  9. Ryan Schnacke

    Ryan Schnacke Supporting Actor

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    I don't think there's a real easy way to tell the diff. The only thing I can think of is to check the resistor values by reading their color coding. A quick search online will show you how to decode them.

    Another possibility is that your sub is out-of-phase with your main speakers. This would cause a big loss of sound in the frequencies where both the sub and the mains are playing. This is focused at your crossover frequency if you've got it set to "all speakers = small" on your receiver. But if you're running your mains large then it could be reducing the frequency response over a wider range.

    Interestingly I'm pretty sure that I had to turn my phase control to 180 degrees to get in phase with the mains. I'm pretty sure I didn't get the wiring backwards. But I'm not 100% sure. In the end I decided to reverse the wiring rather than add 180 degrees of delay into the circuit.
     
  10. Greg Peniston

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    Would removing the R27 and R26 resistors eliminate the rumble filter altogether, or would there be adverse effects?
     
  11. BenSC

    BenSC Stunt Coordinator

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    I'll check the phase, but I doubt that's it. My main's blow (DIY Bose). They can't reproduce below like 220htz.
     

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