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Old 12th October 2013, 10:26 PM   #1
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Default Convert stereo signal into moni with y-adapter. Safe?

Hi
I have a mono sub. With my b1, i have a stereo out to go into my subwoofer that has only a one line level input, hence the use of a y adapter. Can i use a y adapter to sum up both channel so both are played by my sub?

Last edited by murphythecat8; 12th October 2013 at 10:31 PM.
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Old 13th October 2013, 12:24 AM   #2
gootee is offline gootee  United States
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NO!

http://www.rane.com/note109.html

You must NOT short the outputs together like that (with a Y adapter).

You need to make a summing box. It's only three resistors. See Figure 2 at the link above.

Your stereo grounds go straight through, to make the mono output ground. Each stereo output needs a resistor in series with it (Rane shows 475 Ohms). Those resistors both connect to one end of a 20k resistor, which is the mono output "+" point. The other end of the 20k resistor connects to the output ground. Use 1% resistors, or match them yourself. I guess you could use 2x 1 kOhm resistors in parallel, to make 500 Ohm resistors, to use in place of the 475 Ohm resistors, if you have a bunch of 1k resistors on hand and don't want to wait.
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Old 13th October 2013, 01:04 AM   #3
sreten is offline sreten  United Kingdom
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Hi,

Simply you need to make the proper cable. The best way is 3 phono
plugs and some cable. Put a 10K resistor in series with + on the two
phonos for the sub output and join them together to drive the + of
the mono cable to the subwoofer. All the earths are common.
The far end plug is wired as normal.

Under no circumstances join them directly, resistors are needed.

Might sound a little odd but its the best way. It will prevent the
capacitance of the cable loading the driving stage, and the treble
loss over the cable will be no real issue for a subwoofer input.

I know this having tried a sub with my Audiolab that had a stereo input
and internal summing. The preamp out normally just drove the power
amplifier directly. The long stereo sub cable destroyed the top end.

rgds, sreten.

None of those Rane circuits are the right way to do it in this case.
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Last edited by sreten; 13th October 2013 at 01:17 AM.
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Old 13th October 2013, 02:26 AM   #4
gootee is offline gootee  United States
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I have not tried it myself.

@sreten: Your way ends up as the same topology as figure 2 in the Rane document, except that the 20k resistor to ground that Rane has is replaced by whatever the input resistor is, inside the powered sub's amplifier.

And you are using 10k instead of Rane's 475 Ohms.

Each channel has a voltage divider, formed by the series resistor and the shunt resistance to ground. Rane's has 475 Ohms in series and the shunt is 20k in parallel with the sub amp's input R. Yours has 10k in series and the shunt is the sub's input R.

It seems like Rane's is likely to suffer much less attenuation than yours.

Is there any compelling reason to use such a high series resistance? Does it give better channel separation? Is it easier to drive? Do those matter very much? Or is it something else?
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Old 13th October 2013, 05:46 AM   #5
djk is offline djk
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The PASS B1 has a series 1K resistor on the output, you may use a Y-connector on it.

Other models will need some sort of resistor(s).
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Old 13th October 2013, 03:16 PM   #6
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Quote:
Originally Posted by djk View Post
The PASS B1 has a series 1K resistor on the output, you may use a Y-connector on it.

Other models will need some sort of resistor(s).
ok great, thanks.
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Old 13th October 2013, 06:38 PM   #7
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by gootee View Post
I

Is there any compelling reason to use such a high series resistance? Does it give better
channel separation? Is it easier to drive? Do those matter very much? Or is it something else?
Hi,

Yes. Not throwing the baby out with the bathwater. My Audiolab
has preout /powerin phonos but they are internally linked meaning
you can use all 4 of them to connect something else to the pre-out.
(i.e. you can bi amp and drive a stereo sub using the 4 sockets.)

(removing the internal makes them a standard out /in loop.)

The power amplifier load is very easy, however driving long
cables isn't and connecting a subwoofer destroyed treble.

You don't want low loading and you don't want low impedance
across the two channels (like 1K or 2K total) either. You want
an arrangement that minimizes any sonic effects on the preamp
whilst providing a suitably adequate signal for the subwoofer.

Losses in the arrangement are not a problem for the active sub.

Of course you could also buffer the output for the subwoofer,
then most of my concerns would then become pretty irrelevant.

The details determine the best approach.

rgds, sreten.
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Last edited by sreten; 13th October 2013 at 06:41 PM.
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Old 14th October 2013, 03:11 AM   #8
gootee is offline gootee  United States
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Thanks for taking the time to craft a detailed response!
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