lpf on a rockfrod fosgate t1500-1bdCP

[Falcons 5,000+ posts]

okay, before people troll me on here, let me explain what im asking lol. i understand that when you turn the lpf up on the amp, the voltage to the speaker terminals increase, which is why you cannot turn the gain as far up when you have the lpf set higher. now my question is, why is that? if a lpf is just used to set what frequencies the sub plays, why does it affect the voltage going to the speaker terminals? for example, i could set my lpf to 45 hertz, and turn the gain to about halfway, and the system pounds like you wouldnt believe, however, if i set the lpf to 80, the system as a whole sounds better, but i can only turn the gain up to about a quarter. is it safe to set the lpf to 45 with the higher gains but keep the voltage the same on the dmm? i am by no means using the gain knob as a volume knob. just figured i wanna learn everything there is to know on this subject.

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[Falcons 5,000+ posts]

Bump!

 

[pimpnyou204 10+ year member]

Not a amp wizard but my guess is playing those 80hz songs tones etc the speaker is moving in 80 cycles a second and well moving a 30lb magnet is going yo cause some extra work. So it needs more power.. My guess anyways

 

[Falcons 5,000+ posts]

im just wondering cause if i can play 45 hertz and below (thats what i like to listen to) and it pound then im wondering if it it safe to do so with the gain up more.

 

[05trailblazer 10+ year member]

would have to be their to hear the difference

 

[Falcons 5,000+ posts]

since no one really gave me a definite answer i just split the difference and tuned it to 55-60 hertz and set the gains. then i played it for about 5 mins full tilt to see if anything heated up to quickly.

 

[BnGRacing 10+ year member]

The on board processing on RF amps is terrible. What you're hearing is the crossover section add in artifacts & do weird stuff with the signal in addition to bleeding (hearing voices out of subs). Really, they're gawd awful.

By turning down the gains or lowering the x-over point you're limiting the distortion added by the x-over you can hear. You're not sending any more power to the sub if you cross over higher despite what your DMM tells you. Impedance rises with frequency b/c the voice coil acts just like an inductor you'd find in a passive x-over. That amp may have some sort of circuit that regulates power output over a range of impedance. If the impedance rises, current goes down and voltage has to increase to compensate. That may be what's happening.

That amp may be very dirty too. D-Class amps all have LPFs around 2-300hz so you don't hear the switching noise. If that amp is extra noisy, you may be hearing some harmonics from the switching devices come out of your subs at higher x-over frequencies.

 

[Falcons 5,000+ posts]

So I should set my gains then my Lpf?? That doesn't seem right lol I think I'm a little confused

 

[kramer_212 10+ year member]

on the rf amps i just keep that bad boy below 45.

 

[Falcons 5,000+ posts]

So its fine having the lpf lower an the gain a little higher?

 

[Pborcich 10+ year member]

since no one really gave me a definite answer i just split the difference and tuned it to 55-60 hertz and set the gains. then i played it for about 5 mins full tilt to see if anything heated up to quickly.

This isnt a great idea... components inside the amp can heatup and fry before your amp ever even gets hot on the outside

 

[BnGRacing 10+ year member]

So its fine having the lpf lower an the gain a little higher?

The gain has nothing to do with crossover frequencies or what speakers the amp is hooked up to. It is a device that matches the input voltage coming through the RCAs to the output of the amp. Higher voltage inputs = lower gain. Keeping the gain down is a good thing as it reduces noise.

I have a hard time setting gains so there's no clipping if I use music. A 50-60hz sine wave/test tone is easy.

Crossovers are used b/c there is no driver on the market that can accurately produce the entire spectrum of sound (20-20k). Big speakers for the low stuff and tiny speakers for the high stuff. A crossover is used so you do not have several drivers of different size/spec playing the same music. It doesn't sound good nor is it linear. The biggest factor, IMO, is how the subwoofer integrates with the front speakers and what slope you're working with.

Fosgates use a Butterworth x-over, so I'd cross over the sub 1/2 to 1 octave below the x-over point of the front speakers. So if your front speakers are crossed over at 80hz; cross your sub over around 60. But at 60hz, you'll probably still hear some bleeding unless the subs are tucked away in the trunk and the seats are up. In my systems, I usually cross over the subwoofer between 40-50hz as my front speakers are pretty beefy and can handle 50hz and up (24dB linkwitz-riley x-overs; HP & LP is set the same as there's no bump).

 

[Falcons 5,000+ posts]

i set it to 60, played full tilt, and i delivered pizzas all day blastin and the amp didnt even get warm. i think im good //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif