Ptownkid
Well-known member
I'm trying to find info on making a speaker switch for my two sets of monitors. I've found some useful info but they all seem too complicated fro what i'm looking to do.
Cheers
Cheers
Need to make a simple a/b switch for two sets of speakers
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chipmanv
Member
Try a big DPDT "ice cube tray" type power relay fired by a 2n2222 transistor. Use a relay rated for at least 10 amps.
Ptownkid
Well-known member
please elaborate...
chipmanv
Member
sorry, I meant use a 4PDT relay, meaning four pole double throw, so you can use one relay for a stereo signal. You don't need the transistor if you're only switching two pairs of speakers. Just connect one side of the coil to V+ and the other to ground via a switch. The power relays I've used are rated for only 3A, and I've never had a problem with them in a permanent installation in a commercial studio with speakers taking a severe beating with 400W amps. Doesn't make sense according to ohms law, but never had a fried relay for breakfast.
> The power relays I've used are rated for only 3A, and I've never had a problem with them in a permanent installation in a commercial studio with speakers taking a severe beating with 400W amps. Doesn't make sense according to ohms law, but never had a fried relay for breakfast.
Where does Ohm's Law come into it? "3A" does not say any resistance.
In a relay or switch, the rating is usually the "carry" rating. You can run 3A continuously. You may be able to run 6A for one second, 0A for one second, averaging 3A: the contacts on big relays/switches don't overheat instantly.
In audio we have peak powers much higher than average powers. If the average speaker current were 3A, the peak speaker current is probably 15A, but for only a few milliseconds. 15A current in 8Ω is 1,800 Watts. Actually an "8Ω" speaker is well above 8Ω over most of the bass, where a lot of the power happens. So you would need many KiloWatts of peak speaker power to burn 3A contacts.
The contact rating also suggests the make/break current. It is easy to carry current when contacts are closed. But when contacts open or close on a reactive load, things get tough. The classic problem in relays is opening an inductive load: that 3A wants to keep flowing even after the contacts open. It will kick-up a high voltage and arc-over the gap. Say 3A flowing with 120V pressure: that's 360 Watts of heat in a 1/8" dot of metal. It gets hot fast. In power relays you can try to pull the contacts apart so far so fast that the arc is broken. In AC power you also have the fact that an arc will want to die at the next zero-crossing, no more than 8-10 milliseconds in the future. In speakers, we often don't switch while at full power, we have more zero-crossings to break an arc, we don't have such high voltages as power lines, and speakers are not as reactive as motors.
All that said: unless you need remote control, you can A/B speakers with an ordinary DPDT switch. You don't have to break both wires on each speaker, you can leave the black terminals connected. Run the amp's left red terminal to the center pin on the left side of a DPDT switch. Run the top left switch terminal to the left A speaker red terminal, the bottom left switch terminal to the left B speaker red terminal. Wire the right side of the switch the same but with right-side amp and speaker connections. The main drawback to this plan is the 12 fat wires (6 speaker cables) hanging out the back of the switch: you have to mount the switch on a brick or in a rack to keep the wires from pulling it off the back of the desk. And all them wires have to run from the amp, to your hand, and off to the speakers. Using a relay lets you put the power switching convenient to amp and speakers, short cable runs, no fat wires hanging off your desk. Two thin wires from relay and power-pack run to the desk and your hand, and you can use some pretty-switch instead of a heavy-duty toggle.
Where does Ohm's Law come into it? "3A" does not say any resistance.
In a relay or switch, the rating is usually the "carry" rating. You can run 3A continuously. You may be able to run 6A for one second, 0A for one second, averaging 3A: the contacts on big relays/switches don't overheat instantly.
In audio we have peak powers much higher than average powers. If the average speaker current were 3A, the peak speaker current is probably 15A, but for only a few milliseconds. 15A current in 8Ω is 1,800 Watts. Actually an "8Ω" speaker is well above 8Ω over most of the bass, where a lot of the power happens. So you would need many KiloWatts of peak speaker power to burn 3A contacts.
The contact rating also suggests the make/break current. It is easy to carry current when contacts are closed. But when contacts open or close on a reactive load, things get tough. The classic problem in relays is opening an inductive load: that 3A wants to keep flowing even after the contacts open. It will kick-up a high voltage and arc-over the gap. Say 3A flowing with 120V pressure: that's 360 Watts of heat in a 1/8" dot of metal. It gets hot fast. In power relays you can try to pull the contacts apart so far so fast that the arc is broken. In AC power you also have the fact that an arc will want to die at the next zero-crossing, no more than 8-10 milliseconds in the future. In speakers, we often don't switch while at full power, we have more zero-crossings to break an arc, we don't have such high voltages as power lines, and speakers are not as reactive as motors.
All that said: unless you need remote control, you can A/B speakers with an ordinary DPDT switch. You don't have to break both wires on each speaker, you can leave the black terminals connected. Run the amp's left red terminal to the center pin on the left side of a DPDT switch. Run the top left switch terminal to the left A speaker red terminal, the bottom left switch terminal to the left B speaker red terminal. Wire the right side of the switch the same but with right-side amp and speaker connections. The main drawback to this plan is the 12 fat wires (6 speaker cables) hanging out the back of the switch: you have to mount the switch on a brick or in a rack to keep the wires from pulling it off the back of the desk. And all them wires have to run from the amp, to your hand, and off to the speakers. Using a relay lets you put the power switching convenient to amp and speakers, short cable runs, no fat wires hanging off your desk. Two thin wires from relay and power-pack run to the desk and your hand, and you can use some pretty-switch instead of a heavy-duty toggle.
Ptownkid
Well-known member
Ok, i'm getting confused now. You said that there'd be a whole whack of wires coming out of the switch...both sets of mt speakers are powered, so i need something with 2 sets of left/right outs and a switch to flip between them.
Is this what you're talking about as well?
Is this what you're talking about as well?
Ptownkid
Well-known member
anyone...?
chipmanv
Member
If the speakers are powered, that makes it much easier. You can use a four pole double throw switch, 4PDT. You don't need anything heavy duty since it's switching line level before the power amplifier input. The four signals to the input of the switch would be left signal balanced +/- and right signal +/-. The switched outputs would be spkr A +/- left and right. and spkr B +/- left and right. Tie the shields together for the balanced audio cables around the switch. You could mount the switch in a box with xlr ins/outs.
BYacey
Well-known member
Make sure you use a relay with good quality contacts. At low current levels such as line level or even worse mic level, the contacts tend to become rectifiers when they develop a layer of oxide, causing distortion or a complete loss of signal.
