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8 pill 2SC2879 mobile

Discussion in 'Amplifiers' started by donemad, May 16, 2011.

  1. donemad Active Member

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    I got a no name mobile box that has 8 sc2879 in it and need to know what kind of amps I need to operate this? They had a boomer 400 with 4 MRF455 pushing it, I think thats a little to much! Would 2 reg. automoble batt. power it? How many watts should I be looking at? ........Thanks for any info...


     
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  2. eagle1911 Active Member

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    I'd say an absolute max of 800W PEP is to be expected from that amp. You're looking at 25A peak collector current per device @ 800W, so I'd say you'll need at least 200A peak input plus a little headroom.
     
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  3. MrSuburban Guest

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    He will never see 25a per device if he does they will be just like a fuse.

    800 watts Pep is just loafing with 8 -2879's.
     
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  4. eagle1911 Active Member

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    I figure he should have the amperage stated in the datasheet as max Ic per device, but I agree that in real world usage things might be a little different. Best to have more available than less..

    Also, the devices are rated per their datasheet to 100W each@28MHz, so once you push things past that point your output cleanliness will be shot to hell. I will never recommend overdriving transistors to anyone, even though it might be common practice. Personally I disagree with doing so.
     
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  5. donemad Active Member

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    Are you saying I need a 200 amp power supply to power this? Do you recommend no driver at all! And what ga. wire do I need?
     
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  6. Shockwave Sr. Member

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    To determine the maximum current a multiple transistor amp will draw requires knowing the difference between Push-Pull and Single Ended configurations. Nearly all solid state amps that combine the power of more then one device utilize the Push-Pull configuration with broadband transformers.

    The system is quite similar to older audio amplifiers that used a pair of NPN finals with transformer coupling. Think about the sinewave we are trying to amplify linearly. It swings both negatively and positively across the zero baseline. If you only have a power source and transistors of a single polarity, how do we amplify the other half of the sinewave in linear applications?

    The trick is the 180 degree phase inversion that both the input and output broadband transformers provide between the center taps. During the positive half of the cycle only one transistor in a bank of two are conducting. The same is true during the negative half cycle. It is important to understand that the only reason the opposing half cycle can be amplified is due to the phase inversion the transformers provide.

    If you follow this then it should also be clear that only one transistor in a Push-Pull bank is conducting at any given time. If you have a pair of 2SC2879's in this configuration, they should never draw more then 25 amps total even though they are rated at 25 amps each.
     
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  7. eagle1911 Active Member

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    ^That's what I was missing.. so the max current for an amp like that would really be the datasheet Ic multiplied by (total number of transistors/2).

    My reply was based solely on the datasheet numbers. Your real-world mileage may vary. As to more detailed answers I'll leave that up to those members here with firsthand experience with 8x2sc2879 amps as their real-world experience will be the best help to you. My quick research efforts thus far into what people's experience has been with 8x2879 amps (at work ATM) indicates that you *should* be able to run an amp like that on less than 200A, most probably around 100-120A tops. As far as the gauge of wire I'd say 4Ga.
     
    Last edited: May 16, 2011
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  8. VA3ES Old Buzzard

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    The nominal specs for 2SC2879's are nominally 100WPEP output with 10W PEP drive at 12.5VDC. 13.8 will give you more.
    8 2879's constitutes a hi-drive amp!!! Your looking at 80W PEP to drive it to full power. At 13.8 VDC I expect better performance. You should expect an increase of approximately 170W per volt increase. Thus you should see a 221 W PEP increase with 13.8V on the amp. In other words, 1021 W PEP out, with 80W PEP in. (Approximately.)

    If the amp is a true AB1 linear amp, you will see an almost straight line decrease in power with less drive. Assuming that the amp has 13 dB gain (nominal) or a gain of 20X, you can calculate your output accordingly. Understand that 13dB is the absolute maximum gain. Nominal gain will be from 10dB to 12dB. Worst case is 10dB, so you have a straight line power graph. 5W in is 50W out, etc. You will most likely see a lot more.

    The maximum current draw is indeed, 25A per device. That is with it fully driven and a maximum output. Therefore peak maximum current will be 200A. You will need many alternators, or some huge SOB power supply for that! :eek:
     
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  9. Shockwave Sr. Member

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    Assuming the amplifier could handle it (and it won't) a doubling of source voltage would increase output power by a factor of four. This is just Ohm's law since doubling voltage will also double current into the same load. Keep in mind the output impedance should remain virtually the same under these conditions and that volts times amps is watts.

    The 800 watt PEP amp would try and produce 3.2 KW PEP! Going from 12 to 18 volts should double power at 1.6 KW. In this situation each volt is producing about 133 watts of extra output. Every watt you try and squeeze out past the 800 will come at the expense of spectral purity due to rapidly increasing IMD. The people you talk to may not notice it but those using frequencies adjacent to yours probably will.

    Class AB1 is only given to tube amplifiers because the "1" indicates there will be grid current flowing. The equivalent class of bias for transistors would be AB. The drive suggestions and test for linearity that VA3ES suggests are right on target. The amplifier should only draw about 100 amps. Having 200 amps available is a great idea since it will reduce voltage drop and provide plenty of headroom for the positive peaks.
     
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  10. donemad Active Member

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    Thanks guys for the very helpful info....
     
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  11. nice to see some common sense and technically correct comments on driving amps/pills . but .... i dont think any one buys those class C amps to have a clean tx . they want their meter to swing as far to the right as possible and clarity is a afterthought , if its matters at all to them . and the fact that even doubling power has a negligible effect on their tx is ignored if favor of how far the needle swings .

    i know , i know .......... :bdh:
    couldn't help myself :love:
     
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  12. 1iwilly Active Member

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    well i can tell you from experience that i run a 2x8 class c box on 14.50 volts using a 29 classic i get 800 bird watts on 19 volts i see 1500 bird watts
    driver=2 2879 final=8-2879 on a test i did i only bleed 6 channels up and 6 down compare to guys that take the whole 120+ channels as a matter of fack shockwave has heard me on this box and i trust his judgement that my 2x8 is clean sounding compare o others out there i've notice that it all comes down on who ever built the box what cheap componets they use or short cuts they take.
    i love it when guys swear/claim their 8 pill does 2000 to 2200 bird watts if i can listen to my car stereo while talking on the pill and it doesn't come thru my speakers i consider that been some what of a clean box.??
     
    Last edited: May 16, 2011
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  13. GnG8d Well-Known Member

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    :w00t:
     
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  14. eagle1911 Active Member

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    "Only" 6 channels is still a whole lot of splatter.. a WHOLE lot. Not at all what I would call clean.. Perhaps a bit clean for an unfiltered class C box though.
     
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  15. my farts smell pretty good compared to a dead animal with maggots oozing out of it .
    compared to a rose they're not so nice .

    consider the context of what the comparison is based on .
     
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