but, will i get the issues listed at the beginning of this thread for driving a 16 ohm load in BTL mode, when i use 8 ohm speakers in SE mode?
the more i try to understand the PFFB document the more nervous i get about screwing something up.. the lower gain might trip me up, im not sure how much of an output signal i can get from the minidsp, and in single ended mode, the amp output is gonna be rather modest, without reducing the gain further. i need to work out what kind of output i can get from the amp in SE mode into an 8 ohm load with decent distortion figures.
All this is unfortunately rather over my head. i started this project building a single subwoofer.. playing with winisd was fun!
now its a full active system, with amps i need to actually understand how to mod it seems..
hope i didnt bite off more than i can chew.
the more i try to understand the PFFB document the more nervous i get about screwing something up.. the lower gain might trip me up, im not sure how much of an output signal i can get from the minidsp, and in single ended mode, the amp output is gonna be rather modest, without reducing the gain further. i need to work out what kind of output i can get from the amp in SE mode into an 8 ohm load with decent distortion figures.
All this is unfortunately rather over my head. i started this project building a single subwoofer.. playing with winisd was fun!
now its a full active system, with amps i need to actually understand how to mod it seems..
hope i didnt bite off more than i can chew.
It's a "disease", and we're all "sufferers" of it 😀 "Hi, i'm Chris, and i'm a DIY-er..."
Let's look in the datasheet (again)... http://www.ti.com/lit/ds/symlink/tpa3255.pdf
Fig.17 on page 12. At a 50v supply voltage, we should get just short of 70w @ 1% THD into a 4 ohm load.
P = U^2 / R; let's round that up to 70 = U^2 / 4, so U^2 is 280, agreed? If we divide that by 8 (which is the "new" load resistance), we get about 35 watts, which sounds about right. That should be more than plenty to get 130Hz and up plenty loud, i'd think.
Specs for the Alpair 7 Gen3 say it's roughly 85dB @ 1w, and (at least in theory) 35w should get you up to around about 101dB (88dB@2w, 91dB@4w, 94dB@8w, 97dB@16w, 100dB@32w). That being said, they're "nominally" 20w, but since music isn't full-power sine waves, and they'll be high-passed anyway, it's all good 😉
Re: MiniDSP max output, i'd imagine that should be somewhere in the spec sheet.
That being said, you might still get some peaking of the output filter, but i'm not 100% sure how grave of a concern that is, in this case. I'm not positive whether it works that way, but with these 4" fullranges having a bit of a droop at the top end, that might actually "even out", to some extent 🙂
Let's look in the datasheet (again)... http://www.ti.com/lit/ds/symlink/tpa3255.pdf
Fig.17 on page 12. At a 50v supply voltage, we should get just short of 70w @ 1% THD into a 4 ohm load.
P = U^2 / R; let's round that up to 70 = U^2 / 4, so U^2 is 280, agreed? If we divide that by 8 (which is the "new" load resistance), we get about 35 watts, which sounds about right. That should be more than plenty to get 130Hz and up plenty loud, i'd think.
Specs for the Alpair 7 Gen3 say it's roughly 85dB @ 1w, and (at least in theory) 35w should get you up to around about 101dB (88dB@2w, 91dB@4w, 94dB@8w, 97dB@16w, 100dB@32w). That being said, they're "nominally" 20w, but since music isn't full-power sine waves, and they'll be high-passed anyway, it's all good 😉
Re: MiniDSP max output, i'd imagine that should be somewhere in the spec sheet.
That being said, you might still get some peaking of the output filter, but i'm not 100% sure how grave of a concern that is, in this case. I'm not positive whether it works that way, but with these 4" fullranges having a bit of a droop at the top end, that might actually "even out", to some extent 🙂
thank you very much for the help.. i wasnt sure if i could just half the 4 ohm power rating at a given distortion level to get the 8 ohm one...
so, i will basically see how i go without the PFFB stuff.. its giving me a headache. if i find issues i will consider adding it.
so, i will basically see how i go without the PFFB stuff.. its giving me a headache. if i find issues i will consider adding it.
Well yeah, instead of "putting the cart before the horse", it might be worth seeing if the "horse" might be good enough on its own 😉 (ie. skipping the feedback mod headaches)
If using that calculator for the output filter of a full-bridge amplifier, remember to input the load resistance as half the actual value (like 4R if calculating for an 8R load).
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