Bass Boost on MCM 250 W Plate Amp

Discussion in 'Archived Threads 2001-2004' started by TommP, Apr 4, 2002.

  1. TommP

    TommP Extra

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    I just opened up the box. MCM Tech Support told me this amp does not have bass boost.

    Now that I've got this thing in front of me, how can I tell if it has a bass boost?

    -Tom
     
  2. Saurav

    Saurav Cinematographer

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    It has a manual which talks about this. If it's the same amp as the Parts Express amp, I know the PE page for the 120W version has a link to a PDF manual.
     
  3. TommP

    TommP Extra

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    Thanks. I found the PE Bass Boost Circuit Notes PDF, but I'll have to wait until I get home to remove the face plate and check the resistors.

    Has anyone else with the MCM amp done this yet?

    -Tom
     
  4. Ron-P

    Ron-P Producer

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    I just received mine and I have the PDF file just have not had time to pop the hood and take a look.
    Peace Out~[​IMG]
     
  5. Ben L C

    Ben L C Agent

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    I was wondering if anyone has taken the time to figure out if the MCM plate amp does in fact have the bass boost enabled? Just curious.

    Ben
     
  6. David_Rivshin

    David_Rivshin Second Unit

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    I was going to make a thread on this, but since it already exists I'll just chime in here [​IMG] First thing I did with my amp (before even using it, as is my habit [​IMG]) is take it apart and check on the bass boost specifically. Bad news is that it did indeed have bass boost set at 6db as 30Hz. R26 was 18kOhm and R27 was 180kOhm.
    As per another thread I tried removing the resistors entirely and seeing if it would still operate, hypothetically removing all boost and rumble filter. As far as I could tell it did not work, but did humm abit. Although that might have been something else in my settings at that point. Can someone who understands the circuit confirm whether removing the resistors all together should work at all, and if so what the result should be?
    I'm going to run to radio shack after work today and pick up an assorted pack of 1/2 Watt assorted carbon film resistors to bring it down to at least no boost. I'm still unclear as to what the rumble filter would end up being with suggested 56K and 120K resistors. Anyone have any clarification on that?
     
  7. Manuel Delaflor

    Manuel Delaflor Supporting Actor

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    "First thing I did with my amp (before even using it, as is my habit ) is take it apart and check on the bass boost specifically."

    Escuse my ignorance, but how do you do that?
     
  8. David_Rivshin

    David_Rivshin Second Unit

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  9. TommP

    TommP Extra

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  10. Mike Dr

    Mike Dr Stunt Coordinator

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    After tweaking my sub using avia and spectraPlus i did not notice a hump at 30hz in my particular setup ... so I'm assuming that removing teh boost would do nothing for me.. or actualyl make the 30hz region sound lower correct? so is there any benefit in removing bass boost if I have no audible spike there?

    thanks
     
  11. TommP

    TommP Extra

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    Can someone please explain how this boost will affect my sub's behavior at lower frequencies. Assuming the boost at 30 Hz doesn't create an unbearable hump, what effect will the rolloff have?

    I'm building a vented Tempest design, tuned to 16-17 Hz. Will these low freqencies be filtered out if I don't remove the bass boost?

    -Tom
     
  12. David_Rivshin

    David_Rivshin Second Unit

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    Tomm, the short answer is yes, I believe it will, and quite severly at that. Check this thread for alot of good info:
    http://www.hometheaterforum.com/htfo...threadid=47601
    It appears that even with the boost disabled you're down 3dB at 20Hz. With the default 6dB boost my guess is you're basically cutting frequencies below 25Hz at a relatively steep slope. I'll have to again beg for anyone who has more electonics knowledge than I do to give more definate answers as I'm not sure the numbers I've read/stated are correct.
     
  13. TommP

    TommP Extra

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    Not that I doubted you Dave, but I had to tear apart my amp at lunch today just to verify. R26=18Kohm and R27=180Kohm. Damn! Now how to replace the resistors?

    I've never done any circuit/electrical soldering before. I have a 150 W soldering iron (w/ a fairly large heat element). Is this too much to work on that little circuit board or will I fry the whole thing? What type of solder and flux do I need to install the replacement resistors?

    BTW thanks for the link Dave.

    -Tom
     
  14. Rick Guynn

    Rick Guynn Second Unit

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    Tom, 150W will run great risk of frying something. What you need is around 20-30W and a pencil-tip. See if any of your friends might have one [​IMG] Oh, and you might need to get some desoldering braid to take the old components out with.
    As for solder, just get some of the normal resin-core lead/tin type solder that you find at almost any electronics store.
    Rick
     
  15. David_Rivshin

    David_Rivshin Second Unit

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    Tomm,
    I have no problem with people verifying, infact in this case I was curious to know if everyone's amp's shipped the same, or if some shipping with and some without bass boost.
    Removing the resistors is actually pretty easy. I just took some desoldering braid, I think I had 1/4" wide, placed a slice over the solder joint and applied my low-wattage soldering iron to it, a second later the solder was gone. Infact the resistors just fell off the board onto my table [​IMG]
    Soldering them back on is abit more of a pain, since the have a nasty habit of wanting to fall off. I managed to get a good enough of a friction fit that I could manage to keep them in place while I soldered.
    I used just normal solder that I alwayts use. Probably got it from RadioShack ages ago, nothing special there [​IMG]
    -- Dave
     
  16. TommP

    TommP Extra

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    Thanks Dave and Rick. Looks like I'll be headed to Rat Shack after work to pick up some soldering supplies.
    See what happens when you try to save a buck. Now I have to dump all my savings from the amp into making it work the way I thought it would. At least I'll have a new low-wattage soldering iron to show for it.[​IMG]
    -Tom
     
  17. Mike Dr

    Mike Dr Stunt Coordinator

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    you guys still didnt answer the other question about sound quality .. if the response at the 30hz point is already low and not "boosted" (acoustically speaking) what is the down-side of this boost?
     
  18. Rick Thomas

    Rick Thomas Agent

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    Yes i just got mine from mcm and would also like to know

    Mike's question.Thanks
     
  19. TommP

    TommP Extra

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    Mike,

    I'm no expert, but in reading the thread that Dave posted a link to, I gather that the 6 dB @ 30 Hz boost will cause the rumble filter to cut out frequencies below 25 Hz very quickly. Dave estimated the filter to take effect at that frequency (25 Hz). The closest scenario I could find in the PE manual is a 5 dB boost @ 24-30 Hz, and it begins to filter out frequencies below 23 Hz. So I think that 25 Hz is a reasonable estimate for 6 dB boost @ 30 Hz.

    If your sub isn't capable of producing frequencies below 25 Hz, then it's probably not a big deal. But, in my case, I am building a sub that is tuned to 16-17 Hz and would like my amp to allow the sub to go that low.

    -Tom
     
  20. TommP

    TommP Extra

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