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reapersdirge 24th August 2012 02:41 AM

stupid basic input question...
hi all, im really new to the amp dyi caper.
im putting a 2.1 power amp together.
i've a really simple questions about the inputs, the left and right channel come from the pre amp.
if i am making a a left and right channel per speaker. how do i intergrate the sub channel into setup?
how should i get a signal from the left and right speaker modules to the sub module portion of the amp.
sorry if its retarded im just not sure at all on this one. thanks all :)

glennb 24th August 2012 02:52 AM

You need to mix the L & R signals (preferrably before the L & R power amps) and then pass the mix through a low pass filter (this may be in the sub module). You also need some control over the sub volume (and preferrably the corner frequency).

The L & R power amps should also have a high pass filter before them, which complements the frequency curve of the sub's low pass filter. You may think this is not necessary if the main speakers have a c**p low frequency response, but you don't really want lots of LF signal going to them, it could be hell on the woofer cones.


picowallspeaker 24th August 2012 02:52 AM

The sub (woofer , not subsonic ) channel is one since frequencies below about
say 100 Hz are omnidirectional . So it is made by summing L+R ( mixer ) and
applying a lowpass filter . Usually it employs some operational amplifiers
for both mix & low pass .

reapersdirge 24th August 2012 03:16 AM

got you gents i thought that might be the case.
ive made provision for the sub volume control.
there is a crossover on the sub so we should be right there.
many thanks i think i will let you know what im going to do shortly.
im just not sure how i should get a combined signal prior to the L and R... hmmm i say.

maybe just run a parallel signal off the input to the sub and split it off to left and right maybe???

picowallspeaker 24th August 2012 03:42 AM

You say that the sub has a crossover , so it must be passive , because you 'feed' it with the output of the amplifier .
Hypotethically , you can use just the L or R channel because mostly if not all
recordings contain a a monophonic signal in the very bass region .
But that's a story :rolleyes: from the ages of vinyl , to keep the needle on the track by preventing non-homogeneous excursions bilaterally .
Nowadays with digital media you can have 100 dB channels separation
down to 20 Hz .
So , basically a pair of resistors ( from 10 K to 50 K Ω ) joining the L and the R.

Ian Finch 24th August 2012 07:15 AM

Yes, Picowallspeaker is right. If you Google a schematic or two of subwoofer amplifiers (plate amps), you often see a stereo input which accepts a normal twin RCA male-male lead from the main power amplifier. Then there are just a 10k resistor from each socket centre pin, joined as a "Y" adaptor and that junction leads to the input and LP filter of the Subwoofer - painfully simple!

If the output of the amplifier is a single socket, a "Y"adaptor will already be fitted in the amplifier.

reapersdirge 24th August 2012 02:11 PM

it really helps when you ask the right question. thank you all.
and yes that's what i have at the moment picowallspeaker...
im curious, and i will ask, i dont need to incorporate the resistors in my power amp sub module do i, as they are all ready in the passive crossover, yes? or something like that is occuring.
also i think ive picked an error in my construction too. seems sub outputs should be rca's. better head on over to the woofer section and get a bit more intel, thank you all again.

Ian Finch 24th August 2012 09:34 PM


Originally Posted by reapersdirge ( curious, and i will ask, i dont need to incorporate the resistors in my power amp sub module do i, as they are all ready in the passive crossover, yes? or something like that is occuring.....

You will need to fit isolating resistors (say 10k) from the preamp output in both cases - simply in series with each subwoofer output for a stereo pair but subsequently joined for a single lead connection. That is all that is necessary to connect a typical subwoofer, where the amplifier, level and turnover controls are all included.

sreten 24th August 2012 10:06 PM


Most subs will accept stereo line or speaker level inputs and mix them to mono.

Most subs at speaker level incorporate simple satellite filters, 1st order.

Your clearly not putting together a 2.1 amplifier, get your terminology right.

rgds, sreten.

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