Y-Connecting l + r amp out channels into one mono without resistors

[SaladDressing]

One question: Why can it fry the amp? Please be technical but basic.

 

[charleski]

Your speaker has, say, a 4ohm impedance. The output impedance of the amplifier is generally (for solid-state) a lot lower than this, let's say it's a tenth of the speaker at 0.4ohms.
Current = voltage/resistance. The lower the resistance, the higher the current that will pass.
If you connect the outputs together, then each output channel sees a load composed of the speaker in parallel with the other output channel. 1/Rt = 1/R1 +1/R2, so the load on each output is 0.36ohms and they'll be passing ten times the current they were designed for.

 

[BDWoody]

One question: Why can it fry the amp? Please be technical but basic.

Found a solid answer here:

Why are resistors necessary in a summing audio circuit?

I am researching DIY stereo audio mixers (2 input, 1 out) -- there's lots of info out there -- but all the passive solutions (e.g., DIY Multi-Input Audio) include resistors, and I don't understand ...

electronics.stackexchange.com

 

[holla]

To add to what charleski said, and to oversimplify:

Suppose your sound is all on the L channel. So the amp is trying to generate, for example, 2 volts on the L channel and 0 volts on the right. If you connect them directly together, it can't do that: the two output points become one point, and that point can't be at both 0V and 2V.

When an amplifier output is at 0V and the amp tries to generate 2V, it does so by pushing out current. (Pushing current will make the voltage rise.) So the L channel is going to keep pushing out current, trying to make the voltage rise. The R channel is going to try to suck that current in, reducing the voltage. They can't both win. The end result is that very large amounts of current flow from the L channel to the R.

 

[Astoneroad]

I have a similar, but different question. I just bought my first active speaker (KRK Rokit Classic 5) and am learning the very basics. It is connected via RCA connector from my Topping DX3 Pro+. Since I'm only using one speaker and running it from either the L or R output, am I hearing only the L or R stereo output or a mono version? I'm not very technical, but an aging music lover getting excited about the new age of hi fi that is unfolding here. Thanks for insight.

 

[Zek]

The end result is

...white smoke - every device works on white smoke, when the white smoke comes out - the device no longer works...

 

[SaladDressing]

Your speaker has, say, a 4ohm impedance. The output impedance of the amplifier is generally (for solid-state) a lot lower than this, let's say it's a tenth of the speaker at 0.4ohms.
Current = voltage/resistance. The lower the resistance, the higher the current that will pass.
If you connect the outputs together, then each output channel sees a load composed of the speaker in parallel with the other output channel. 1/Rt = 1/R1 +1/R2, so the load on each output is 0.36ohms and they'll be passing ten times the current they were designed for.

Thank you for the detailed explanation! Sounds logical. Now, how does this apply say between a preamp and the power amp? Is it even worse because (as far as I understand it) input impedance of power amps are quite high, while the output impedance of a pre amp is much lower, thus making the current not 10 times larger but maybe 100 times larger or more. Is that correct?

 

[SaladDressing]

okay well I presented the maths with a sketch to my boss and well he said the following: "I did that before though and it didn't pose any problems. We have to look up the spec sheet of the amp we are using later. There definitely are amps capable of doing that.". To me that sounds like total bs, but well, it's his money not mine.
I am also just the intern so... someone who's concerns arent respected anyway lmao

 

[somebodyelse]

I have a similar, but different question. I just bought my first active speaker (KRK Rokit Classic 5) and am learning the very basics. It is connected via RCA connector from my Topping DX3 Pro+. Since I'm only using one speaker and running it from either the L or R output, am I hearing only the L or R stereo output or a mono version? I'm not very technical, but an aging music lover getting excited about the new age of hi fi that is unfolding here. Thanks for insight.

Assuming you are n't using a plugin that downmixes the stereo to mono, you are hearing the left or right channel. Depending on the music this may or may not be a problem. A plugin example would be 'Stereo Tools' in PulseEffects.

 

[Astoneroad]

...white smoke - every device works on white smoke, when the white smoke comes out - the device no longer works...

 

[charleski]

Thank you for the detailed explanation! Sounds logical. Now, how does this apply say between a preamp and the power amp? Is it even worse because (as far as I understand it) input impedance of power amps are quite high, while the output impedance of a pre amp is much lower, thus making the current not 10 times larger but maybe 100 times larger or more. Is that correct?

While everything you say is true, the output impedance of a preamp is generally a lot higher (100ohms or more) than that of a power amp. Also, the voltages being produced are lower since a power amp will generally increase voltage by 25dB or so. Both these factors mean that the amount of current you’re trying to shunt through the opposite output stage is a lot lower, and thus you can often get by with this sort of fudge.

 

[SaladDressing]

While everything you say is true, the output impedance of a preamp is generally a lot higher (100ohms or more) than that of a power amp. Also, the voltages being produced are lower since a power amp will generally increase voltage by 25dB or so. Both these factors mean that the amount of current you’re trying to shunt through the opposite output stage is a lot lower, and thus you can often get by with this sort of fudge.

Ahh I see, that's interesting. One more specualtion: If a signal from one channel "bleeds" into the output of another, doesn't that mean some stuff gets cancelled out because of phase reversal? So the Signal coming out as mono in the end is a bit meh? If I have resistors it doesn't bleed, or bleed is very low so the signal gets only affected in a minor way, so the end mono signal is "clean". Is that right?

 

[Astoneroad]

Assuming you are n't using a plugin that downmixes the stereo to mono, you are hearing the left or right channel. Depending on the music this may or may not be a problem. A plugin example would be 'Stereo Tools' in PulseEffects.

Thank you. "Depending upon the music", how so? My question now is nearly the same as the OP, can I use a Y cable from both the L and R on my DAC into the single RCA input, but on an active speaker? Is the answer the same, but for different reasons, or will this feed both signals without problems. Will this sound better than either L or R alone into a single speaker? This is a new experiment/toy for me, rather than squeeze every possible bit from it, but if a $6 Y cable gets better... then what the hell... otherwise your answer saves me from that little rabbit hole.

 

[DVDdoug]

I have a similar, but different question. I just bought my first active speaker (KRK Rokit Classic 5) and am learning the very basics. It is connected via RCA connector from my Topping DX3 Pro+. Since I'm only using one speaker and running it from either the L or R output, am I hearing only the L or R stereo output or a mono version? I'm not very technical, but an aging music lover getting excited about the new age of hi fi that is unfolding here. Thanks for insight.

Same basic issue... The source impedance (usually not specified) is much lower than the rated load impedance and you should never "short" two outputs together. With line-level outputs you probably won't fry the Topping but you could get distortion or some other strange* effects and you are stressing the device.

If you are not an electronics hobbyist or you don't want to build something yourself you can get a small mixer for around $100.

It's OK to connect two inputs together... It's OK to connect two power amps to one preamp with a Y-splitter, etc.



*I did something like this when I was young and didn't know any better. I got a side-channel killer... The "center" (vocals & bass and other sounds identical in both channels) came-through but anything on the left or right was reduced or eliminated. Different hardware may give different results, and it's something you shouldn't do! Interestingly, it's easy to make center-channel "vocal remover" in hardware or software (using subtraction) but it's not so easy to make something that throws-away the sides (unless you do something "bad" like short the outputs together).

P.S.
All of the analog & digital formats and hardware are mono-stereo compatible. i.e. You can play a mono record on a stereo phonograph and the sound will come-out of both speakers. Or you can play a stereo record on a mono phonograph and both channels will be combined. If you play a stereo MP3 on your mono smart phone speaker you hear both channels. If you play a mono MP3 on a stereo system (or with headphones/earphones on your phone) the sound will come out of both speakers (or both earpieces).

But the Topping is stereo-in and stereo-out and the best solution would be a pair of stereo speakers.

 

[thegeton]

This is what I use for summing L+R to mono as an input into a subwoofer.

 

[Speedskater]

The old go-to RaneNote on the topic:

Why Not Wye?
Dennis Bohn, Rane Corporation
RaneNote 109 written 1991; last revised 4/04

Splitting Signals
Subwoofing in Mono
Unbalanced Summing
Balanced Summing
Output Impedances

Introduction
Wye-connectors (or "Y"-connectors, if you prefer) should never have been created.
Anything that can be hooked-up wrong, will be. You-know-who said that, and she was right...

RANE Commercial - Knowledge Base

Rane Commercial

www.ranecommercial.com

 

[somebodyelse]

Thank you. "Depending upon the music", how so?

Some music doesn't make much use of stereo so you'll barely notice the difference if you're listening to just the left or right. You might find some instruments or vocals a little louder or quieter than you expect if they're panned a bit to left or right, but not enough to disturb casual listening. Other music uses it a lot and you'll really notice the difference. Think dueling guitars panned hard left and right, where you'll only hear one of them if you don't mix to mono, or something being panned around for effect like in EXP on Axis: Bold as Love where it'll get louder and quieter rather than moving around in stereo, or staying more or less constant if mixed to mono. I ran into this when one of my pc speakers was out of commission for a couple of days, hit a jarring one a few tracks in and went searching for a tool to mix to mono.

My question now is nearly the same as the OP, can I use a Y cable from both the L and R on my DAC into the single RCA input, but on an active speaker? Is the answer the same, but for different reasons, or will this feed both signals without problems. Will this sound better than either L or R alone into a single speaker? This is a new experiment/toy for me, rather than squeeze every possible bit from it, but if a $6 Y cable gets better... then what the hell... otherwise your answer saves me from that little rabbit hole.

Much the same. See the RANE link from @Speedskater above, or do it in software if you can.

 

[charleski]

Ahh I see, that's interesting. One more specualtion: If a signal from one channel "bleeds" into the output of another, doesn't that mean some stuff gets cancelled out because of phase reversal? So the Signal coming out as mono in the end is a bit meh? If I have resistors it doesn't bleed, or bleed is very low so the signal gets only affected in a minor way, so the end mono signal is "clean". Is that right?

Actually what you’re doing is converting the signal to Mid-Side and throwing away the Side signal. This is a different way of representing stereo signals (and with some recording techniques it’s how the stereo info is originated).
Mid = L + R
Side = L - R
and thus
Left = (M + S) / 2
Right = (M - S) / 2

The best way to do this is with summing amplifiers which have extremely high input impedances. But if you can’t do that, then tying them together through resistors will do. The aim here is simply to protect the transistors by limiting the amount of current that flows.