Add a Sub-Out to (almost) any audio system...  

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Douglas Blake
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08/06/2019 1:33 am  

For those who've been wondering why manufacturers don't include subwoofer outputs on some of their equipment ... I've been wondering that too.  It's almost too easy to do...

EasySubOut

Eyup, that's all there is to it.

If you have access to volume controlled line level outputs on a system with no sub out you can add it very easily... Just mount 5 RCA jacks in a small metal box, wire the left in to the left out, the right in to the right out, then add this small network of 1/4 watt resistors to feed the Sub-out.  Voila instant sub-out... for less than $10.00.

You can insert this in between a pre-amp and power amp, add it to the line output on your receiver, plug it into the monitor outputs on PA systems ... etc.   The newly created Sub-Out connection goes to the RCA input on your powered subwoofer.


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123Toid
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08/06/2019 1:34 am  

Now that is awesome!  I always wondered that and had no idea how to do it.  Thanks for the great tip!


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Douglas Blake
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08/06/2019 1:36 am  

Easier than falling off a log, my friend!

In fact this is such a butt simple project that you might want to put it up as a "how to" video on your YT Channel ... I'm betting you aren't the only one who's been wanting something like this.


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123Toid
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08/06/2019 1:39 am  

I am going to build this for sure.  And test it out.  I am guessing it is still considered a full-range output? So you would want a preamp on the sub to regulate the cutoff and volume. 


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Douglas Blake
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08/06/2019 1:40 am  

Most powered subs have built in crossovers to handle the filtering tasks, so a full range output is no problem.  

If you can get between a pre-amp and a power amp (for example) the pre-amp's volume control will also control the subwoofer volume for you.  When connecting to the line out on an integrated amp, just be sure you  are hooking up to connections that are volume controlled.  

 


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123Toid
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08/06/2019 1:42 am  

Absolutely agree. 


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Douglas Blake
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08/06/2019 1:53 am  

Since this is a full range output, another use is to feed a third "centre channel" using a full range amplifier and speaker to play with three channel stereo or to add a centre channel to a TV+Speakers setup.

In that case I would change the 6.8k resistor to 3.3k.


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Douglas Blake
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08/06/2019 6:23 am  

Some connection examples....

Preamp -> Sub Box -> power amp.

Computer -> TRS to RCA -> Sub Box -> Amplifier (use volume control in Computer)

Computer -> DAC -> Sub Box -> Amplifier (use volume control in computer)

Integrated Amp Pre-Out -> Sub Box

TV RCA Out -> Sub Box -> Amplifier (use volume control in TV)

And probably lots more.

With a passive subwoofer that has an internal crossover you can use something like the Nobsound subwoofer amp ( https://www.amazon.com/dp/B07D2X65HC ) or any number of plate amps available from various makers.

To use this to add a centre channel for stereo use the same connection ideas as above. In this case you would use the Nobsound full range mono amplifier or an equivalent ( https://www.amazon.com/dp/B07D118B2F ).

Note: Nobsound and many other Chip-Amp makers tend to provide inadequate power supplies. You will probably need to buy a better SMPS brick to get the best performance. 


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JonnyAudio
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08/06/2019 4:06 pm  
Posted by: Douglas Blake

For those who've been wondering why manufacturers don't include subwoofer outputs on some of their equipment ... I've been wondering that too.  It's almost too easy to do...

EasySubOut

Eyup, that's all there is to it.

If you have access to volume controlled line level outputs on a system with no sub out you can add it very easily... Just mount 5 RCA jacks in a small metal box, wire the left in to the left out, the right in to the right out, then add this small network of 1/4 watt resistors to feed the Sub-out.  Voila instant sub-out... for less than $10.00.

You can insert this in between a pre-amp and power amp, add it to the line output on your receiver, plug it into the monitor outputs on PA systems ... etc.   The newly created Sub-Out connection goes to the RCA input on your powered subwoofer.

I don't mean to be critical but you are missing 2 X 10uF 16v caps on the input as buffers for the mono output as you loose treble if you need a mono output to say a tape deck also gives better bass response in mono.

Stereo2Mono


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123Toid
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08/06/2019 4:56 pm  

I don't think that is critical at all.  Just helpful information.  This is looking really simple to do.


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JonnyAudio
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08/06/2019 5:40 pm  

Yes, well I studied the stereo to mono signal around 1995 & only did spice drawing around 2013 & my practical studies revealed how you need to separate the joining of the signal together like on a mixer input to an op-amp you have a capacitor & a resister to avoid the outputs shorting together (treble loss) before going into the input.

The resisters are higher because to only give a voltage drop of 1/10 of the input signal as the 6k3 would nearly half the input signal, you need as much signal as possible to feed powered amps etc.

Here is another circuit idea if you have an amp but not a powered sub box were the make the signal sub bass mono to the amp, then amp to speaker. You can only really do this to sub signal as when you want mid & tops signals like this you would really need to op-amp gain the signals back up after filtration.

Also never use Bi-Polar caps on the low level signals as they cause all sorts of sound problems, keep the caps polarized on the input caps only (as picture does not show polarity below)

Stereo2MonoSubFilterLineLevel

 

 


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Douglas Blake
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08/06/2019 9:36 pm  

You guys are both right.

But do keep in mind that we are not building a full-mono output, nor should we want to...   

If you do a spice analysis of my circuit you will discover that only signals that are encoded equally and in phase in both channels are reproduced properly. Anything that is out of phase or present only in 1 channel is reproduced at greatly reduced output.  In fact if you fed the two input channels the same signal 180 degrees out of phase the output would be 0.

Now lets think about what we are interested in ...

For the subwoofer application we only care about the bass and that is virtually always encoded in phase as two speaker mono in a stereo recording... it will be reproduced as presented. The physical subwoofer's crossover will kill anything over about 100hz anyway, so the rest of it is not an issue. 

For the centre channel application we don't want full mono there, either. We're looking to fill in the phantom stereo image without affecting the separation and in that case we only want to correctly reproduce information that is encoded , equally in both channels. Differences between the channels are also reproduced but at somewhat reduced volume. Yes, it is true that if I listen to the centre channel  alone, it is going to sound horrible ... but when heard , at reduced volume, along with the two speaker stereo it serves to bolster the centre part of the phantom image remarkably well.

In this case, we are listening to all three outputs at the same time. The capacitors in your circuits might  be a problem because they will introduce their own frequency dependent phase shift that can mess with proper phasing of the subwoofer or centre speaker.


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Douglas Blake
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09/06/2019 1:01 am  

For those following this with some interest, who are not familiar with Spice Modelling...

Spice is a software tool that assists in circuit design. It lets us draw a schematic and turn it on inside the computer to see what it does... or in the advanced case to be sure it's working correctly. This isn't an absolute replacement for breadboarding or prototyping but it does cut out a lot of the early guesswork and heavy math when designing a product.

This is the spice model for the SubOut...

Sub Out Spice

Within spice we can test characteristics like sine wave response, frequency response, noise and more. Here is the frequency response plot for the SubOut...

FreqResp

This is the Sine Wave response with both inputs in phase...

In Phase

Now at a 90 degree phase shift...

90 Degrees

Note how the output level dropped off.

And 120... 

120 Degrees

Note how the output dropped again.

And Finally at 180 degrees...

180 Degrees

And this time we get no output.

Finally here is the output with only one channel driven...

One Channel

If you want to play with Spice modelling you can get the free LTSpice program from Linear Devices at ...

https://www.analog.com/en/design-center/design-tools-and-calculators/ltspice-simulator.html


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Douglas Blake
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09/06/2019 11:36 am  
Posted by: 123Toid

I don't think that is critical at all.  Just helpful information.  This is looking really simple to do.

IF we are open to new ideas, the back and forth debate and discussion is one of the best learning tools we can hope for ... provided we don't take it personally.

I've always made a policy of Hat Flipping ... That is I try to switch from Teacher to Student whenever there is something new to learn.


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