Audio summing circuit

[mix4fix]

http://www.faqs.org/faqs/car-audio/part4/

5.16 How do I turn a stereo signal into a mono signal [BW]
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Creating a mono signal is often necessary when you are powering a
subwoofer by bridging the amplifier. Many people do not realize that
bridging an amplifier does not always provide a mono signal - many
amplifiers will simply use only one input channel, which means that the
subwoofer won't be receiving the full signal.

Some amplifiers have a switch that will allow you to combine the left
and right channels into a mono signal. Some signal processors and head
units provide a subwoofer-out channel that can be switched between
stereo and mono.

If you don't have this feature on any of your equipment, you will need
to provide a mono signal to the amplifier. The common thought is to use
a Y-adapter to "combine" the left and right channels. However, by
using a Y-adapter, you are actually summing the line voltages and
directly shorting the left and right channels at the head unit, which
could cause problems.

The correct way to create a mono signal is to cut off the ends of the
RCA cables, combine the signal grounds (the outer shield), and then use
a 1 kOhm (1/4 watt, 5% tolerance) resistor to each of the center
conductors. Solder and insulate the resistors so that you don't short
them prematurely, and then connect the two resistors together. Connect
the summed signal ground to the shield of the new RCA plug, and the
summed center conductor to the center pin of the RCA plug.

I always refer to this whenever the discussion of combining RCA signals comes up. It's car audio, but it's the same principal. As long as you have that 1/4 watt resister on the center pins, everything is good.

I built and still have a summing circuit I built long time ago. I used a radio Shack project box, multi-RCA panel, resisters, and some wire. I had it set up where the two middle pairs were stereo and the two outer pairs were summed. It's weird but I can never miss-connect anything. Stereo in, stereo out, and four mono connections.

I have a question for the experts: besides the resistors, is there any other component or design recommended that would make it a better circuit?

Last edited by mix4fix on April 14th, 2020, 3:05 pm, edited 1 time in total.

[Roscoe Primrose]

https://www.electronics-notes.com/artic ... lifier.php

[Pelliott321]

i still feel stereo subs are a better way to go unless the crossover for the subs is very low.
if you are using subs to supplement little monitors and need to crossover at 80 or 100 hz the single sub is very evident. I have stereo subs and crossover at 40. I think I can still hear a stereo image in the sub bass

[tomp]

Localization of a subwoofer below 100 Hz is very difficult to do. The main advantage of multiple subs is to place them at different positions in the room, minimizing room nodes.

[SoundMods]

I have a question for the experts: besides the resistors, is there any other component or design recommended that would make it a better circuit?[/quote]

Roscoe passed on the information you need. There are many ways to integrate sub-woofers. Attached are photos of my "He-Man" passive sub-woofer summing network.

The interconnect run is 8-meters to the corner loaded sub-woofer's integral amplifier. Cross-over is about 40-Hz.

The sub-woofer is capable of getting down to 18-Hz. with room shaking power.

Attachments

[pboser]

Edcor sells transformer based stereo to mono couplers in a variety of packaging, line level, balanced and unbalanced, and speaker level.
https://www.edcorusa.com/couplers

[mix4fix]

Every once in a while, I see someone trying to use a Y-cable to go from two-to-one with no summing of any kind. I have always used that link to educate someone. Plus, back then, I don't believe their was too many pre-built summing circuits. I have always trusted the 1K ohm, 1/4 watt resistance recommendations for years.

I am talking about design. I see 20K ohm resisters. Is that overall better, of just for your equipment?

[SoundMods]

Every once in a while, I see someone trying to use a Y-cable to go from two-to-one with no summing of any kind. I have always used that link to educate someone. Plus, back then, I don't believe their was too many pre-built summing circuits. I have always trusted the 1K ohm, 1/4 watt resistance recommendations for years.

I am talking about design. I see 20K ohm resisters. Is that overall better, of just for your equipment?

The benchmark is 10,000 ohms or larger for a summing network at line level (pre-amp). Using 1,000-ohm resistors allows a certain amount of Left/Right blending in your main stereo amplifier that you don't want.

Of course if you need isolation rather than what a passive network provides then using an op-amp is preferred and that methodology is contained in Roscoe's link.

Just about any transistor pre-amp will have +/- DC supply voltage that you can "piggy-back" on. Tube amps forces one to build a power supply for the summing amplifier if a passive network is not your "cup-of-tea."

My summing network got the job done with minimal effort using precision bobbins. I did it first with Dale RN-65 metal-film resistors and they somehow caused a pre-amp sound-quality problem. OK. So I pick nits.

If you are summing off of the low-impedance speaker outputs (4-16 ohms) then 1,000 ohms should be fine.

[dberning]

Speaking of combining channels for a mono subwoofer, I have a pair of Rodgers LS3-5As that I wanted to augment with something to get some decent bass. I put a woofer in a box and I am powering it passively through a pair of chokes from the amplifier channels. A rather crude crossover, but it works quite well. Probably an important reason it works as well as it does is that the Rodgers have very low efficiency and a halfway efficient woofer can have sufficient output to fill in what the Rodgers lack.

David

[mix4fix]

Speaking of combining channels for a mono subwoofer, I have a pair of Rodgers LS3-5As that I wanted to augment with something to get some decent bass. I put a woofer in a box and I am powering it passively through a pair of chokes from the amplifier channels. A rather crude crossover, but it works quite well. Probably an important reason it works as well as it does is that the Rodgers have very low efficiency and a halfway efficient woofer can have sufficient output to fill in what the Rodgers lack.

David

So you are summing passively? Any requirements on the components?