Audio A/B; A+B switch help

Thread Starter

Lunakyra

Joined Feb 26, 2017
9
This is my first electronics project so after much reading and comparing other A/B switch schematics I decided to jump right in. I am trying to supply two rooms (with two speakers each) with audio from a receiver that only has room for two speakers (left and a right). So basically i'm trying to split two lefts and two rights from one left and one right. I need to be able to switch between the two rooms or have them both on at the same time. The latter is a deviation from any other A/B schematic I have come across thus far and is the reason i'm really asking for insight. I am pretty sure what I have drawn up will work but I am also pretty sure there is an easier way to accomplish it. I'm hoping I can cut it down from two 4pst switches into two dpst switches or something but I cant figure out how. I understand that there will be a volume drop if I have both rooms playing at the same time but unless it is drastic i would rather build a passive splitter. Hopefully someone here will point out any flaws in my design or let me know of a way to trim it down a bit. Please don't hesitate to ask for clarification and I look forward to your responses.
 

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crutschow

Joined Mar 14, 2008
23,295
All you need is one DPDT switch to control one lead to each speaker.
The other leads can be common to the two speakers as shown below:
upload_2017-2-27_0-23-19.png
 

AlbertHall

Joined Jun 4, 2014
8,310
All you need is one DPDT switch to control one lead to each speaker.
But that doesn't allow both rooms to be connected as requested by TS.
Also we don't know whether the amplifier speaker outputs have a common ground. If they use a bridge output then they won't have a common ground.
 

crutschow

Joined Mar 14, 2008
23,295
But that doesn't allow both rooms to be connected as requested by TS
True.
I missed that he wanted them both on at the same time.'
That can be readily done with another DPST switch.

But to do that he has to determine if the amp can drive two speakers in parallel without damage.
Any type of passive resistive splitter will degrade the sound
Also we don't know whether the amplifier speaker outputs have a common ground. If they use a bridge output then they won't have a common ground.
That's not a problem in my circuit.
Only the two speakers have a common ground, not the amps.
 

djsfantasi

Joined Apr 11, 2010
5,573
Here is a simple schematic for the situation you describe. It requires a 3P6T switch, which is available in both slide switch and rotary switch form. The diagram only shows half of the speakers; the right channel for example. The switch shown only has three positions. Hence, you need three more positions for the left speakers for a total of...drum roll... six positions.

Two positions switch between speakers A & B. Note that the other position (shown at the top of the schematic) has no connections. However, when in the third selection (throw), The third set of contacts connect the - of speaker A to the + of the speaker B. And the other throws connect the amplifier + signal to the speaker A + and the amplifier - signal to speaker B -.
SpeakerSelect.PNG
 

Thread Starter

Lunakyra

Joined Feb 26, 2017
9
Also we don't know whether the amplifier speaker outputs have a common ground. If they use a bridge output then they won't have a common ground.
I had to look up what a bridge was. And from what i understand no, i have not bridged the speakers (its just connectint two amps to a set of speakers right?) As long as i didnt personally bridge them am I safe from not sharing a common ground? Or do some recievers just not have a common ground and i have to watch out for that?
 

AlbertHall

Joined Jun 4, 2014
8,310
It is about how the amplifier works not anything you have done. Some amplifiers use a bridge output which means that neither of the speaker wires is grounded and you cannot connect them together.
 

Thread Starter

Lunakyra

Joined Feb 26, 2017
9
But to do that he has to determine if the amp can drive two speakers in parallel without damage.
So I need to make sure the impedance will still be within safe levels? I believe thats what the concern is. Is there a formula for it or something? I tried looking it up bit really did not grasp a simple way to just check if the amp output can safely be split between multiple speakers in parallel.
 

Thread Starter

Lunakyra

Joined Feb 26, 2017
9
It is about how the amplifier works not anything you have done. Some amplifiers use a bridge output which means that neither of the speaker wires is grounded and you cannot connect them together.
How would i check if my reciever is bridged? And would that only matter if i connect the grounds from the two initial output channels? If i just ground them both separately would i be safe and it wouldnt matter anyways?
 

AlbertHall

Joined Jun 4, 2014
8,310
How would i check if my reciever is bridged? And would that only matter if i connect the grounds from the two initial output channels? If i just ground them both separately would i be safe and it wouldnt matter anyways?
If you have a multimeter, you could check for continuity between the speaker terminals and the chassis of the amp. If you find continuity then the outputs are not bridged.
 

Thread Starter

Lunakyra

Joined Feb 26, 2017
9
reciever_switch2.png
Assuming the amp is not bridged would this be fine then? I'm pretty sure this will let me turn on both A/B separately while still being able to listen to them at the same time. Is there anything I am missing though?
 

AlbertHall

Joined Jun 4, 2014
8,310
That circuit is fine even if the amp does have a bridged output.
The only thing to watch is when both sets of speakers are connected that the impedance that the amp sees is halved. Let's say the amp is intended for 8Ω speakers. If the speakers are 8Ω when two are connected the amp will see only 4Ω. Exactly what happens then depends on the amplifier. It may be OK at low volume but at high volume, if it has a protection circuit it may shut down. If it doesn't have such protection it may be damaged.
 

djsfantasi

Joined Apr 11, 2010
5,573
That circuit is fine even if the amp does have a bridged output.
The only thing to watch is when both sets of speakers are connected that the impedance that the amp sees is halved. Let's say the amp is intended for 8Ω speakers. If the speakers are 8Ω when two are connected the amp will see only 4Ω. Exactly what happens then depends on the amplifier. It may be OK at low volume but at high volume, if it has a protection circuit it may shut down. If it doesn't have such protection it may be damaged.
How can that circuit be fine, when some speakers are missing a connection?

Secondly, it depends on the imoedances that the amplifier can deal with. Let's assume the speakers are 8 ohms. The last circuit placed them in parallel, presenting 4 ohms to the amplifier.

If that's too low, then a circuit can be devised to connect the speakers in series, presenting 16 ohms to the amp.

What isn't clear is where the TS wants the controls to be located. Centrally or one in each room?

The circuit in post #5 switches all leads, so bridging is not an issue. It is suited for central control and only requires one switch.

I can redo the schematic if this is not clear.
 

crutschow

Joined Mar 14, 2008
23,295
The circuit should be drawn with the amp red wire output going to the tip, and the black wire output going to the common of a single connector on each end, not two connectors.
 

Thread Starter

Lunakyra

Joined Feb 26, 2017
9
Do you want to do this with switches only, or are relays allowed?

ak
I honestly have no clue how to work with relays so im sticking to switches on this particular project untill i can read some more about them.
 

Thread Starter

Lunakyra

Joined Feb 26, 2017
9
How can that circuit be fine, when some speakers are missing a connection?

Secondly, it depends on the imoedances that the amplifier can deal with. Let's assume the speakers are 8 ohms. The last circuit placed them in parallel, presenting 4 ohms to the amplifier.

If that's too low, then a circuit can be devised to connect the speakers in series, presenting 16 ohms to the amp.

What isn't clear is where the TS wants the controls to be located. Centrally or one in each room?

The circuit in post #5 switches all leads, so bridging is not an issue. It is suited for central control and only requires one switch.

I can redo the schematic if this is not clear.
I want the switch to be centrally located. I do appreciate you asking for clarification though because i never consdered a switch in each room.

I honestly cant follow the schematic from post 5. I get that it is a single switch instead of two separate and it looks like 3 post but past that i dont get why it is shown 3 times. Id be happy to use it if it's easier but my concern is that it would be harder to come by than dpst switches and as a result more expensive.

After a bit of reading i now understand the importance of the speaker's resistance and ill just wire them in series instead of paralell. I have a yamaha rs-201. It wants 8 ohm speakers which i have and i dont want them to get halfed down to 4 ohms when both sets are turnes on and Id much rather take the volume hit. From what i understand im not going to hurt anything by increasing the resistance with the speakers wired in series right? As long as i dont crank the volume to compensate atleast.

The circuit should be drawn with the amp red wire output going to the tip, and the black wire output going to the common of a single connector on each end, not two connectors.
I guess i dont understand. Each speaker has a pos and a neg. Each connector is a banana plug connected to a single speaker wire so one connector would be pos and one would be neg. Wouldnt the ground attatched to the individual connectors be their own separate thing? I didnt draw ground due to space but am i drawing it all wrong? I thought it would be two jacks per speaker (one pos and one neg) but from what your saying i feel like im not doing it correctly.
 

djsfantasi

Joined Apr 11, 2010
5,573
Thanks for your response. The schematic was a little sloppy. I do wonder what you consider "expensive"? I don't know where you are but in the USA you can get the requisite switch for about $15 or less.

The trick is in knowing what 6P3T means. Poles are how many unique circuits can be switched. The left speakers require three poles and the same for the right speakers. Hence six poles.

Just considering the right speakers. one pole connects the amplifier to Room A, one pole connects to Room B and the third pole wire the speakers in series so both rooms play. The circuit is the same for the same reason for the left speakers.

You need three throws because you want the switch to do three different things. Play in Room A, play in Room B or play in both rooms. Three options; three throws.

I can try and clean up the schematic if you are interested. One switch will do it all.

Addendum: you may have to add some resistors to balance out the sound levels.
 

crutschow

Joined Mar 14, 2008
23,295
..............
I guess i dont understand. Each speaker has a pos and a neg. Each connector is a banana plug connected to a single speaker wire so one connector would be pos and one would be neg. Wouldnt the ground attatched to the individual connectors be their own separate thing? I didnt draw ground due to space but am i drawing it all wrong? I thought it would be two jacks per speaker (one pos and one neg) but from what your saying i feel like im not doing it correctly.
The jacks you show have both hot and common (or plus and minus), which would require only one connector per speaker.
If you use banana jacks then yes, you do need two connectors per speaker.
 
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