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Old 22nd November 2012, 11:25 PM   #1
JForged is offline JForged  United States
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Default Looking to make a mono sub from a stereo output while keeping stereo speakers.

Hi

I'm hoping that I can get some help on this.

This is a diagram of what i'm wanting to do.

Click the image to open in full size.



I'm pretty limited on space as this is going to be occupying a desktop as such I would like to have a small box for all of the active stuff and then three separate boxes. 1 stereo set of 3" tang bamboo full range drivers and 1 woofer that can carry the low end of each stereo channel.

Is it possible to do this with one cross-over? I would think doing that I would lose the stereoness and end up with two mono channels.


Anyone have any ideas on how I can take the lower end of each stereo channel into one woofer with a minimal amount of space required?
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Old 22nd November 2012, 11:59 PM   #2
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yes, it's possible and easy

to join the L+R use 10k resistor or a little bit higher, from here use a low pass active filter

if you want you can make 2 high pass filters for the front speaker too

i'm doing this for my gainclone, if you ned i have the pcb layout for a 2 way high pass filter 24db/oct + a mono exit to plug in the low pass filter

the pcb is pretty small
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Old 23rd November 2012, 01:03 AM   #3
DUG is offline DUG  Canada
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I think the OP wanted a solution at the speaker level not at the signal level.

See jpg for speaker level power combiner

Signals common to both channels will show up at the sub the same as an active signal (line level) mixer.

Advantages:

no power amp needed

Disadvantages:

(or requirements)

XL of the power combiner transformer (centre tap transformer) should be high 50ohms???) at the lowest frequency.

The inductor self resonance should be high enough to not become capacitive at frequencies that the R & L amps are generating.

The inductor should be able to (without saturating) withstand any difference in R vs L amplifier output. (stereo signals)

Has to handle the power you are going to put through it.

This is probably why this technique is used at RF frequencies and not AF.

Easier to build/buy an amplifier.

Hope that helps.
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Old 23rd November 2012, 02:05 AM   #4
JForged is offline JForged  United States
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Pedro - I would be interested to see the pcb layout and the components needed. You mentioned it was active....what's the power requirement?


Dug - I am interested by this idea but need some more information.

I am using a 25 watt max amp to power 3 x 8 ohm speakers. Two full ranges and one woofer. I want to xover the 2 x 3" full ranges so that they do not have to handle anything too low and use the 8" woofer for the low frequencies.

I assume the transformer in your diagram is an audio level one. What values of the inductors/capacitors and transformer. Or at least what formula should I use?
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Old 23rd November 2012, 02:12 AM   #5
DUG is offline DUG  Canada
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What frequency range did you want the sub to handle?
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Old 23rd November 2012, 02:33 AM   #6
DUG is offline DUG  Canada
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If you wanted to try without designing/building a transformer:

Use the SECONDARY of a 60Hz power transformer. (leave the primary open...not connected to anything)

At your 25W levels:

for down to 60Hz operation a 40VACct at 5A
for down to 30Hz operation a 80VACct at 5A
for down to 20Hz operation a 120VACct at 5A

I would not expect a 60Hz laminated core power transformer to operate much above a few 100 Hz but some people have stated that toroids operate at higher frequencies.

If you look at the sizes of these transformers you will see why this is not a popular solution.

For the sub low pass filter consult someone more knowledgeable than I.

The inductor for the sub low pass filter might be better placed in the feeds to the transformer to help isolate the R & L channels from each other at non-sub frequencies. The cap can stay where it is.

You might also want to put a fuse in one of the transformer feeds...just in case.

Have fun and let us know if it works.
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Old 23rd November 2012, 02:37 AM   #7
JForged is offline JForged  United States
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Quote:
Originally Posted by DUG View Post
What frequency range did you want the sub to handle?
Probably around 30 to 600 hz.
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Old 23rd November 2012, 03:04 AM   #8
JForged is offline JForged  United States
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Quote:
Originally Posted by DUG View Post
If you wanted to try without designing/building a transformer:

Use the SECONDARY of a 60Hz power transformer. (leave the primary open...not connected to anything)

At your 25W levels:

for down to 60Hz operation a 40VACct at 5A
for down to 30Hz operation a 80VACct at 5A
for down to 20Hz operation a 120VACct at 5A

I would not expect a 60Hz laminated core power transformer to operate much above a few 100 Hz but some people have stated that toroids operate at higher frequencies.

If you look at the sizes of these transformers you will see why this is not a popular solution.

For the sub low pass filter consult someone more knowledgeable than I.

The inductor for the sub low pass filter might be better placed in the feeds to the transformer to help isolate the R & L channels from each other at non-sub frequencies. The cap can stay where it is.

You might also want to put a fuse in one of the transformer feeds...just in case.

Have fun and let us know if it works.

Thanks for the suggestion Dug but the cost and weight of that iron will probably put that in the horizon. Seems like something fun to try some time when the amp driving it is more worth it.
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Old 23rd November 2012, 10:42 AM   #9
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Quote:
Originally Posted by JForged View Post
Probably around 30 to 600 hz.
600Hz is to high, cut the sub on a a maximum of 120HZ

do you already have the speakers? What models they are?

Quote:
Originally Posted by JForged View Post
Pedro - I would be interested to see the pcb layout and the components needed. You mentioned it was active....what's the power requirement?
When i get home i will post it, i have made the pcb but not assembled and tested, the cut is 70hz

The board need +-12 or +-15
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Old 23rd November 2012, 11:09 PM   #10
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Dual voice coil sub, 1 coil to each channel.
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