Stereo to mono doubles the center? - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 11 Old 07-22-2017, 07:11 PM - Thread Starter
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Stereo to mono doubles the center?

Sorry if this is the wrong section

So I've made a little setup based on some research I've done. I have a single Polk T50 being fed by a cheap Lepy 2020A amp. The source is a bluetooth receiver with it's output Y-cabled to mono. I know this setup might destroy the bluetooth receiver, but I'm willing to take that risk.

Here's the problem, summing stereo to mono makes the center image significantly louder than the left or right. Playing a simple youtube test shows the center being significantly louder than the left or right.

I've done as much googling as I can stand, and no one seems to be talking about this, does anyone have any ideas how to fix this?
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post #2 of 11 Old 07-22-2017, 09:30 PM
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If you unplug say the left channel from the y-cable does the right channel signal then get louder from the speaker?
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post #3 of 11 Old 07-23-2017, 07:17 AM
 
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Originally Posted by hibyprime View Post
summing stereo to mono makes the center image significantly louder than the left or right
If you have only one speaker how can you have a left or right?
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post #4 of 11 Old 07-23-2017, 02:04 PM - Thread Starter
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Originally Posted by mtg90 View Post
If you unplug say the left channel from the y-cable does the right channel signal then get louder from the speaker?
I had to do a bunch of listening to try to understand what the sound is doing when I unplug one of the channels. If I unplug the left channel, it sounds like anything that has hard-panned left doesn't change at all, but anything that is panned center gets quieter (along with losing all the right-panned sounds).

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Originally Posted by Bill Fitzmaurice View Post
If you have only one speaker how can you have a left or right?
So the output doesn't have left or right, but the input does. What I'm saying is that when the Y-cable sums the left and right into a single channel, it seems to increase the volume of the center. This all occurs on one output channel. Does that make sense?


I'm starting to think this is actually what should be happening. If you think about it, centered audio is a waveform that's duplicated on the left and right channels. If you sum those two waveforms, you'll get an increased volume. Whereas if the left and right channels are playing a different waveform, there won't be any volume increase. This is weird and hurts my brain a bit.
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post #5 of 11 Old 07-23-2017, 02:34 PM
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Here is a simple Line Level Stereo to Mono Converter - Left/Right In and two Mono Out -

http://www.mcmelectronics.com/product/50-20795

A "Y" Cable is NOT the answer, whether low level amps or Power Amps, you can NOT tie two outputs together without risk of shorting them.

One Output can feed Two inputs, but Two Outputs can NOT feed One Input, at least not safely.

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post #6 of 11 Old 07-23-2017, 03:18 PM
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The above MCM Stereo to Mono likely uses a Transformer to not only convert but to provide electrical isolation between the circuits.

There are ways to do Passive Stereo to Mono Conversion -



Or this -

http://johnhpalmer.com/?m=201602

Scroll down to the second to the last project on this page.



The goal is to isolate the two outputs from each other.

For sake of simplicity and consistency, let's assume we use 680ohm resistors, which is a standard size. In either case, the signal path between the left and right channels has 2x680 = 1360 ohms, therefore the output channels can not short together.

The Input to the Mono Circuit is likely in the range of 10,000 ohms to 50,000 ohms, so the 1k or 2k ohms in the passive summing circuit is insignificant. In the last Circuit with the "X" Pattern, 1360 ohms relative to 10,000 ohms means about 13% of the signal is lost in the circuit. If we apply the same principle to the first circuit above, then 6.8% of the signal is lost in the circuit. However, if the imput impedance of the circuit you are feeding is 47kohms, then in the first "X" Case, 2.8% of the signal is lost, in the second case, 1.45% of the signal is lost. All these are acceptable.

Standard resistor values that could be used in these Passive Circuits would be 470 ohm, 560 ohms, 620 ohm, 680 ohm. You choice as long as they are all the same.



Steve/bluewizard

Last edited by bluewizard; 07-30-2017 at 07:53 PM.
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post #7 of 11 Old 07-23-2017, 03:38 PM
 
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Quote:
Originally Posted by hibyprime View Post
If you think about it, centered audio is a waveform that's duplicated on the left and right channels.
Not when it's a x.1 source, decoded with an x.1 AVR.
If it's a stereo source and you're combining the two channels to mono that won't cause an increase in level. That's because the level is set by voltage swing. Assuming the voltage swing of the two channels is roughly equal combining them in parallel doesn't increase the voltage swing, so it doesn't increase the volume. Connecting the outputs with a Y could be causing the voltage swing to increase. That's one of the reasons why you shouldn't Y outputs.
http://www.rane.com/note109.html
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post #8 of 11 Old 07-23-2017, 03:59 PM - Thread Starter
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Originally Posted by Bill Fitzmaurice View Post
Not when it's a x.1 source, decoded with an x.1 AVR.
If it's a stereo source and you're combining the two channels to mono that won't cause an increase in level. That's because the level is set by voltage swing. Assuming the voltage swing of the two channels is roughly equal combining them in parallel doesn't increase the voltage swing, so it doesn't increase the volume. Connecting the outputs with a Y could be causing the voltage swing to increase. That's one of the reasons why you shouldn't Y outputs.
Sorry, I was talking specifically about 2-channel stereo. 5.1 or similar would definitely create the center image differently.

If it was doing what I think it was doing, the inputs would somehow have to be in series to increase the voltage, not in parallel like a Y cable. Thanks for clearing up my confusion there. I'm not an EE by a long shot.

So, if you're correct, then basically using a Y cable is ****ing up the bluetooth receiver's output in a non-obvious way.

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Originally Posted by bluewizard View Post
The above MCM Stereo to Mono likely uses a Transformer to not only convert but to provide electrical isolation between the circuits.
...snip...
Steve/bluewizard
Great information, I'll definitely look into that.


Overall guys, thanks for the help. I've learned a lot in the last 24 hours. I'll ditch the Y cable and look into something more built for the purpose.
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post #9 of 11 Old 07-25-2017, 07:45 AM
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Post Summing

Passive summing, which is what the OP did with a "Y" cable, is not advisable. It will also not give you the desired audible output.

If you have the exact same signal with the exact same phase relationship in two channels and you sum those channels together, the value of the output will double. If you are using a digital audio meter, the increase will be +6 dBFS relative to either channel on its own.

If you have two different signals and you sum them together, the value of the output will be +3 dBFS greater than either channel on its own. This is where the 3dB Pan Law comes from. A mono source panned to the center will be reduced by -3dB at the panpot. This is so that the perceived volume is the same when a single source is being reproduced by two speakers as it is when it is reproduced by a single speaker.
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post #10 of 11 Old 07-26-2017, 06:21 AM - Thread Starter
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Originally Posted by Chief Technician View Post
Passive summing, which is what the OP did with a "Y" cable, is not advisable. It will also not give you the desired audible output.

If you have the exact same signal with the exact same phase relationship in two channels and you sum those channels together, the value of the output will double. If you are using a digital audio meter, the increase will be +6 dBFS relative to either channel on its own.

If you have two different signals and you sum them together, the value of the output will be +3 dBFS greater than either channel on its own. This is where the 3dB Pan Law comes from. A mono source panned to the center will be reduced by -3dB at the panpot. This is so that the perceived volume is the same when a single source is being reproduced by two speakers as it is when it is reproduced by a single speaker.
Holy crap. I'm not crazy. Thanks Chief. I can't post links, but just google Pan Law and there's tons of stuff about this!

Do you have any idea how I would go about converting this to an "active" system? Or is that even possible? It looks like everyone who is talking about pan law is working with master tracks.
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post #11 of 11 Old 07-28-2017, 05:52 AM
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Quote:
Originally Posted by hibyprime View Post
Holy crap. I'm not crazy. Thanks Chief. I can't post links, but just google Pan Law and there's tons of stuff about this!
You are welcome!

Quote:
Originally Posted by hibyprime View Post
Do you have any idea how I would go about converting this to an "active" system? Or is that even possible? It looks like everyone who is talking about pan law is working with master tracks.
Take a look at Wye Not Wye.
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