Resolved: Stereo to Mono Summing For Subwoofer

[haskellcurrying]

So I'm (mostly) retiring my Denon HEOS Link for a Topping E30.

Unfortunately doing so has gotten me into a bit of a pickle. Currently, the Link has a Sub/LFE output that is summed in the digital domain. That LFE output is pumped into my single channel DSPeaker antimode and then finally to my sub. Obviously the E30 doesn't have a sub out, or even a second set of output channels, so my plan was to sum the line-level L+R output of my amp and feed that to the antimode.

Turns out passive analog summing is weird.

It looks like one can use a basic stereo to mono summing circuit like this:
https://www.amazon.com/OSD-Audio-SM...coding=UTF8&psc=1&refRID=6YBAH35A6GAV0GJZDN6F

or a variable level mixer like this:
https://www.amazon.com/Portable-Cha...coding=UTF8&psc=1&refRID=6YBAH35A6GAV0GJZDN6F

That said, looking at some diagrams/board shots for these summing circuits, if the resistance added to each channel before summing isn't bang on, the summing will result in a channel bias.

Now I know they make 1% error resistor, but I don't know what's being used in any of these low end devices. I could buy a nice mixer, but that seems like serious overkill and probably out of my budget. I could probably buy a nicer audio processor that has proper sub/lfe out, but again I've got a 2.1 setup and the E30 already fits the bill otherwise. Plus I got it for $100 locally.

So what do I do?

 

[GimeDsp]

What amp and what sub do you have?

 

[haskellcurrying]

Nothing remotely fancy.

An emotiva a-300 and a cadence csx-15 mark II.

EDIT because I'm feeling old: Both are almost 10 years old now!!

 

[digitalfrost]

Not sure if this helps you, but the LFE is not supposed to mixed into the stereo channels at all. If you want to downmix LFE for stereo output, the correct ratio is 1.58 (+10dB standard LFE boost -6dB because of additive bass output from the two stereo speakers or from remixing in bass management = +4dB = 1.58).

See also: "In all downmixes, the LFE channel is not included." https://developer.dolby.com/globalassets/professional/documents/dolby-metadata-guide.pdf

 

[GimeDsp]

Not sure if this helps you, but the LFE is not supposed to mixed into the stereo channels at all. If you want to downmix LFE for stereo output, the correct ratio is 1.58 (+10dB standard LFE boost -6dB because of additive bass output from the two stereo speakers or from remixing in bass management = +4dB = 1.58).

I believe he just needs to split rca then sum those into mono to feed a sub.
Since LFE is a discrete channel a 2 channel dac like his e30 wont have it.
And the summed to mono isnt low pass till it hits the sub.

 

[Chrispy]

Nothing remotely fancy.

An emotiva a-300 and a cadence csx-15 mark II.

EDIT because I'm feeling old: Both are almost 10 years old now!!

What about rationalizing it's time for a new sub?

 

[haskellcurrying]

I believe he just needs to split rca then sum those into mono to feed a sub.
Since LFE is a discrete channel a 2 channel dac like his e30 wont have it.
And the summed to mono isnt low pass till it hits the sub.

What GimeDsp said. E30 only has a single set of stereo outputs which isn't a big deal because my amp has line level outputs. The subwoofer has a built-in crossover, so i just have to sum the outputs of the two channels.

 

[GimeDsp]

The simplest would be a summing network
Preferably a box with 2 rca input and 3 output.

Problem is spending money without enough benefit. Mains are still full range.

It makes most sense to get a solution with an xover or even dsp.

But if you want the data I'll talk to my friend and give him the output impedance of you dac and input of your amp, hell sau what value resistor to use.

 

[haskellcurrying]

What about rationalizing it's time for a new sub?

I wouldn't at all mind getting a new sub, but then I'd have to get a new antimode/subeq and now we are looking at a 1k+ investment total!

I have a rather old antimode that only takes in a single input and output:
https://www.hometheatershack.com/attachments/antimode-10-jpg.16856/

 

[haskellcurrying]

The simplest would be a summing network
Preferably a box with 2 rca input and 3 output.

Problem is spending money without enough benefit. Mains are still full range.

It makes most sense to get a solution with an xover or even dsp.

But id you want the dats ill talk to my friend and give him the output impedance of you dac and input of your amp, hell sau what value resistor to use.

the antimode already descretizes/digitizes whats passed in to eq. If i put in another DSP infront of it, I'm sure I'll be introducing even more noise. If you check the ASR review of the newer antimode units, you'll notice they are pretty piss poor DACs... converting twice sounds like it is asking for trouble.

Plus I'm trying to stay as cheap as a bird chirping.

EDIT: Also do tell your friend! I'd love the help!

 

[haskellcurrying]

This one has posted specs:
http://www.muxboxs.com/products/143.html

and this one is from a brand I have actually heard of:
https://www.monacor.com/products/pa-technology/signal-processing/signal-optimisation/smc-1/

Anyone have experience with either?

..the two have very different input impedances; 600 ohm vs 4700 ohm. I've been told 600 ohm is generally considered pro audio grade, but I really don't know what is what.

 

[haskellcurrying]

For posterity - I think I've gotten to the bottom of this.


DSPeaker has a diagram for just such a situation:
http://www.dspeaker.com/en/support/using-stereo-pre-out-with-8033.shtml

In it, it list the resistor value range as 5k-10k ohm.

Monoprice has a summing circuit that fits the bill:
https://downloads.monoprice.com/files/manuals/600122_QIG_190920.pdf

If you scroll to the bottom of that document, they give you a proper diagram of their summing circuit. In it they list 5.6K ohm %1 resistors (if you aren't toggling the -20 db switch) and even have inductors for isolation.

You'll obviously need a XLR to RCA adapter, but that is hardly the end of the world.


Thanks everyone who chimed in and helped!

 

[patrick]

Thanks for sharing, @haskellcurrying!