Center speaker connected to stereo [Left+Right] - Impedance

Thread Starter

2Hexornot2Hex

Joined Apr 16, 2020
54
Hi gurus,

I want to connect a center speaker (that's usually connected to a dedicated output in surround system) to a stereo system (amplifier) that has only Left/Right outputs - for listening music.
In some forums I encountered a proposition of connecting the CenterSpeaker to the '+' terminals of L/R channels, like this:
center speaker schematics.pngIn my amplifier's case, the '-' of Left and Right channels are connected.
Each speaker has 8 Ohms Impedance.

1. how does this added Center Speaker connection affects the Impedance for each Left/Right channel
(will it be lower/greater/the same as 8Ohms)
2. In case you're familiar with this center speaker issue (connection to stereo/music systems) - is this good/bad connection ? any other suggestions ?


Thanks in advance.
 

nsaspook

Joined Aug 27, 2009
8,786
1. It's the dreaded differential pseudo center speaker hack (lower impedance but only for differential L+R signals) that should use series resistors on the center to limit Impedance changes like the old "dynaquad" boxes did.
https://web.archive.org/web/20120617074926/http://www.the-planet.org/dynaco/Misc/Quadaptor.pdf

2. I would't do it for stereo music. Typically the center channel is for dialog in multi-channel X.1 configurations so you would want a center designed for voice clarity. For a typical stereo music mix the added differential center will IMO muddy the source image, not make it better.
 
Last edited:

Audioguru again

Joined Oct 21, 2019
3,675
A center channel is supposed to play when both channels are the same and are producing mono.
The circuit shown does the opposite, it produces nothing when both channels are the same but produces an output only when the channels are playing different sounds, are playing far away echoes (channels are out-of-phase) or when one channel plays nothing.
 

crutschow

Joined Mar 14, 2008
27,956
Here's a excerpt from an old Dynaco amplifier manual about using four speakers:
Note that the speaker connected from Hot-to-Hot, as the TS shows, is for the back ambience, not center channel.
Also the two amps must share a common output ground.

1630789966969.png1630790108871.png
 

nsaspook

Joined Aug 27, 2009
8,786
If the music was mixed for stereo then that's the way it should be played back IMO. I've got a fancy AV/Media room with multi-channel X.1 processing for tracks designed for multi-channel audio and movies. When I listen to records it's in the analog L+R mode with minimal processing. I worked for RS during the QUAD era and listened to most of the extra 'dimension' gimmick boxes made during that era.
11952872864_b210472363_z.jpg
Most of this type of junk was totally useless for good sound from a stereo source. A good sounding room with proper treatments, speaker placement and minimal processing works wonders to improve stereo sound.
 

Yaakov

Joined Jan 27, 2019
3,609
Properly recorded stereo produces a “virtual” center channel without a transducer. Trying to derive a center channel for a transducer from the stereo image is always going to be counterproductive.

Good imaging depends on interaction of exactly two transducer systems properly placed so the constructive and destructive interference can reproduce that the microphones (in the case of a live recording) or the mixing console (in the case of studio work) sent to the recorder.

Sophisticated active signal processing can make some improvements by compensating fo the room and transducer inadequacies, and possibly even producing a useful center signal for a physical transducer but the mixing of the LR stereo signals is best done in the air, acoustically, with your ears working out the meaning of the sound stage.
 

Audioguru again

Joined Oct 21, 2019
3,675
I think that speakers in series ruin the damping of resonance by the extremely low output impedance of the amplifier. Then the bass sounds "boomy" and the frequency response is messed up. Look at "amplifier damping factor" in Google.
 

Thread Starter

2Hexornot2Hex

Joined Apr 16, 2020
54
How well did that work?
When I hookup the Front Speaker (as per your suggestion), the Left/Right speakers volume is decreased and it's less then the FrontSpeaker's volume)
So the Front is being heard "too much".
I guess it makes sense, since the Front gets the Voltage+Current from Left and from Right.
So probably somehow the Front's volume needs to be adjusted.
Any ideas ? (my Front/Left/Right are 4Ohms each)

(In my case, this setup isn't intended to be listened in a specific room spot..)
 

Thread Starter

2Hexornot2Hex

Joined Apr 16, 2020
54
Connect a low value resistor across the center (front) speaker to bypass some of the signal.
You could use a low value rheostat/pot (e.g. 5Ω) or try some low value resistors (a few ohms).
The usual 0.25W resistor won't be good ? Should I put several Watts resistor ?
 

Thread Starter

2Hexornot2Hex

Joined Apr 16, 2020
54
I think that speakers in series ruin the damping of resonance by the extremely low output impedance of the amplifier. Then the bass sounds "boomy" and the frequency response is messed up. Look at "amplifier damping factor" in Google.
My amplifier is intended for 8Ohm speakers.
Each of my current speakers is 4Ohms (I know, I know...just don't yell at me on this).
So this Front speaker in series is filling up the 'missing' impedance.
From what I read on Damping Factor [DF] - As I understand, in my case, since the final impedance is closer to 8 Ohms, looks like the DF is fine - am I right ?
 

Audioguru again

Joined Oct 21, 2019
3,675
A resistance in series with a speaker allows it to resonate like a bongo drum. An amplifier that has a good damping factor has an output impedance of 0.04 ohms or less to prevent the speaker from resonating.
If music should play "tap, tap tap" then a poor damping factor make it play "bong, bong bong".

Your speaker is 4 ohms at 400Hz. But it is about 31 ohms at its resonance. Then in series with an identical speaker the damping of the resonance is almost nothing.
 

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Thread Starter

2Hexornot2Hex

Joined Apr 16, 2020
54
A resistance in series with a speaker allows it to resonate like a bongo drum. An amplifier that has a good damping factor has an output impedance of 0.04 ohms or less to prevent the speaker from resonating.
If music should play "tap, tap tap" then a poor damping factor make it play "bong, bong bong".

Your speaker is 4 ohms at 400Hz. But it is about 31 ohms at its resonance. Then in series with an identical speaker the damping of the resonance is almost nothing.
Good to know of this Damping Factor. Thanks for enlightening.
In most of the forums/explanation sites, the series/parallel speaker configurations are covered (too much) without mentioning the Damping Factor - it's probably for more advanced audio 'hobbits'
 

Audioguru again

Joined Oct 21, 2019
3,675
A cheap sound system usually sounds awful. A stereo system with a center speaker is usually high fidelity.
A high fidelity system plays sounds properly.

Years ago a cheap TV played sounds poorly, usually with bong, bong bass because they had a poor damping factor.
Guitar speakers are frequently connected in series to make a poor damping factor because the guitar people like the distortion it produces.

Even my cheap clock radio sounds high fidelity because I increased the value of a few coupling capacitors and added a 2-way speaker I designed and built.
 

Thread Starter

2Hexornot2Hex

Joined Apr 16, 2020
54
Generally at typical listening levels (not loud rock) the speaker power is no more than a few watts.
A 2W resistor in parallel should be more than sufficient.
During a "crescendo" in the music or a ball game score then the 0.25W resistor will unsolder itself.
Will a wirewound resistor be good for this ? (Since it's a coil (inductance...) and we're talking audio here)
 
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