My first serious design = 5.1 Ch - Amp Switch

[Lundaa]

Hi Everybody

So I finally came around to register on Dave's site, I have only been watching the bloody show for the last couple of years.
Love the teardowns and "everyones favorite segment" MAILBAG! 

I have been messing around the idea of making a relay-operated switch, to switch between my retro stereo-setup and a more modern 5.1 setup, on the same speakers.

And this is what I have come up with so far.

I have some concerns about noise, but I am a novice designer and would like to hear what this excellent community has to say.

This might have come in the wrong fourm ... if an OP moves the topic I have no issues with that. 

[mariush]

Will both amplifiers function properly without speakers? Do you let the amplifiers continue working without speakers?  Some amplifiers aren't happy if/when you suddenly remove the speakers.

I would think something like turn off / unplug amplifier 1 , wait 1s, switch relays, wait 1s, turn on amplifier 2 would make more sense. So that means either a microcontroller or some time delay circuit / logic.
With a microcontroller, you would also be able to use bi-stable relays, so that once they're switched, you don't need to keep sending power to the relays.
Actually even without a microcontroller, you could probably have a slide switch  [amp 1]  -- off - [ amp 2]  or something like that, and depending on the slide switch position you would reverse the polarity to the relays and flip them as needed.

I'd also add a capacitor at the input, something like 100uF 25v... also don't be cheap and add a diode on each relay primary winding.

[Boscoe]

In all honesty I think turning the amps off before switching the speakers is complete overkill - if the amplifier isn’t happy being disconnected from the speakers it shouldn’t be sold.

I used to work in a major audio brand electronics department. We had loads of amplifiers (including competitors) and none of the engineers thought twice about plugging / unplugging £10k+ of speakers into those amps while they were on…

[tooki]

While switching while on won’t damage the amps, the popping it can cause if there’s a signal isn’t good for the speakers.

I see a few issues with the design:
1. No clamping/flyback diodes on the relay coils.
2. The selected relays have 2A contacts. That may not be enough. For example, 100W into an 8 ohm load has a peak current of 3.5A, into 4 ohms 5A. While the average current is far lower, the contacts need to be able to handle the peak current, should they be switched at that exact moment.
3. Don’t draw connections across components in a schematic. Rotate and flip components as needed, use ports if necessary.

[Lundaa]

Thank you for your input. 

I've modified the curcuit with your recommendations, and it now looks like this.

I don't think my amplifiers would have any issues with the speakers being unplugged, since I can only listen to one at a time. 
I would probably not be switching input while playing loud music or anything.

I have considered the relays being to small, and not being able to manage high wattage.
The ones in my current build can manage 2A - OMRON G5V-2 https://omronfs.omron.com/en_US/ecb/products/pdf/en-g5v2.pdf

It's on my TODO-list :-)

Sorry about the schematic, its my first time using Ki-CAD, I'm still learning all the features 

[Lundaa]

Latest release.

[NiHaoMike]

In all honesty I think turning the amps off before switching the speakers is complete overkill - if the amplifier isn’t happy being disconnected from the speakers it shouldn’t be sold.

Some tube amps can be damaged trying to run them with no load and some poorly designed modern amplifiers have the same problem. Easy workaround is to use more relays to switch the power to the amplifiers.

[mariush]

You may want to label that GND instead of -12V ... because especially in audio, it's not uncommon to have split power supplies (+12v, -12v, GND)

Also, you may want to put the leds somewhere more visible, for example to the left of each output connector, so if you choose to put this thing in a case, you would see the led by the output connector.
Moving the resistor and led closer to the outputs would also allow you to have a custom footprint for each relay, maybe have a footprint with 2 sizes of relays.
Also diode for each relay I'd want to put to the side of the relay so that if there's need to install wider relay, diode and resistors won't block the relay. 

[Lundaa]

Great stuff from all of you ! 

The newest version here has been extended and upgraded to support 5A load.

I've moved some things around and I am pretty pleased with my design. 

[magic]

No reason to use schottkys on those relay coils.

Is there such a thing as break-before-make and make-before-break relays? I suppose you would strongly prefer the former...

[mariush]

It probably doesn't make any difference, but give some thought about whether the layout will be simpler if you arrange the relays so that the secondary windings are close to each other... for example:
 

[  primary  -- secondary ]  [ secondary -- primary ] [  primary  -- secondary ]  [ secondary -- primary ][  primary  -- secondary ]  [ secondary -- primary ]

     || 12v                                                        \\  //  12v                                                           \\  //  12v                                                      || 12v


It may allow you to use wider traces for the 12v at the bottom, and you have 3 groups of secondary winding
You could probably also route the 12v straight through the centers of all relays and use the whole copper fill as ground.

[JohanH]

No reason to use schottkys on those relay coils.

Is there such a thing as break-before-make and make-before-break relays? I suppose you would strongly prefer the former...

Indeed, regular small signal diodes (e.g. 1N4148) will do. Shottkys have reverse leakage and aren't the best fit in all scenarios (not that it would matter here, but no reason to use either).

All common relays are essentially break-before-make. Larger relays, or contactors are mostly on-off type (single throw). If you wanted make-before-break, you could use two single throw relays with some timing logic.