o-scope & gains questions

[RemlapaN 10+ year member]

ok, i did the whole search thing, read some stuff on bcae, and its all making sense. However, a few questions:

I have a small handheld scope, which i plan on using to set amp gains. I found the clip point of the HU (playing sine waves and watching the scope).

The amp i am using is a 4ch amp, 2ch's for mid-range, 2chs bridged for a pair of 10" subs. THe scope im using says it can calculate watts if you tell it the impedance, so i did this. It has two settings for RMS Watts: AC, or AC+DC. Which should I use? AC always seems to read kinda low, AC+DC always seems to read kinda high.

Basically, the amp can do 800wRMS (or so it claims) at 2ohms, but my subs only handle 150wRMS each, so i need to set my gains to keep it near 300wRMS @ 4ohms (since the subs are wired in parallel).

Is it as simple as playing a 50Hz tone, and setting the gains to 34.64volts? ( sqrt( 4ohms * 300watts) )

Click to read more...

 

[RemlapaN 10+ year member]

4 ohm subs, two of em wired in parallel. But in reality, whats the difference if i set it for 300w @ 2ohms, or 150w @ 4ohms, its the same reading.. ahh, i just realized i calculated it wrong above //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif thanks for pointing my faulty math out. Make that 25V at the terminals.

 

[RemlapaN 10+ year member]

one more question:

Is there any way using a scope to see what your crossovers are set at? I set them by ear first, and then tweaked them alittle more, but I was wondering if you can use a scope to determine precisely where the rolloff begins? (Maybe using a square wave or something else?)

 

[RemlapaN 10+ year member]

LOL, yeah i was doing that, but as i was sitting there changing tracks to get different test tones playing, i was thinking "There HAS to be an easier way"... so i take it the short answer is "yes there is another way, but its not worth the time or price of equipment to do it"

 

[noob with an RS 10+ year member]

how much of a voltage drop should i look for to determine that i am at the crossover point?

 

[noob with an RS 10+ year member]

ic ic.. so if im using a LPF that is 24db/ocateve and a HPF that is 12db/octave would i rather overlap or underlap?

 

[noob with an RS 10+ year member]

doesn't this graph show the slopes for 1kHz crossover points? the web page says its 650 hz?? im confused

 

[noob with an RS 10+ year member]

doesn't this graph show the slopes for 1kHz crossover points? the web page says its 650 hz?? im confused //content.invisioncic.com/y282845/emoticons/confused.gif.e820e0216602db4765798ac39d28caa9.gif

its from this web page:

http://www.bcae1.com/xovrslop.htm

 

[maylar 10+ year member]

I believe the crossover point is -3 dB when talking power, or -6 dB when talking voltages. That's the 50% point. I much prefer to use a scope for that, since some DMM's have very crappy frequency response. My Sears DMM starts rolling off fast at 900 Hz.