[SOLVED] Advice needed - rotary switch for audio

[Ghostfather]

I'm a relative newbie to electronics, and I'm trying to figure out which rotary switch I need in a set of speakers that I'm building.

The speakers are powered bij an internal monoblock amplifier, supplying 150w at 8 ohms to the crossover network. I want to be able to switch between the internal amplifier and an external amplifier (via speaker terminals and speaker wire, max 150w). Basically an A/B switch, thus 2 position, and two decks.

What I also need (and this is the basis of my question) is that when one of the amplifiers is disconnected, it will be shunted through a dummy 8 ohm 150 watt load. Am planning on using a wirewound chassis mounted resistor for this, just to protect the amplifier burning out or being damaged if there is no load. It shouldn't usually be necessary, but people make mistakes. Just trying to make it idiot-proof, as well as protect expensive audio amplifiers.

Switch rating should be maybe min. 0.25 amp (0.15 expected), and 5v (1.25 expected).

Inputs will be two speaker wires coming from either the internal monoblock amplifier, or via two speaker wires from an external amplifier via the backpanel terminals on the speaker.
Outputs will be two speaker wires going to the crossover network in the speaker.
When Amplifier A is conected, it goes straight to the crossover, and Amplifier B gets shunted to the 8 ohm dummy resistor.
When Amplifier B is connected, it goes straight to the crossover, and Amplifier A gets shunted to the 8 ohm dummy resistor.

If separate 8 ohm 150w dummy resistors are needed, then so be it. They are somewhat expensive, but much less than the amplifiers they are protecting.

What kind of rotary switch do I need, how many decks, how many poles? Just two positions (A/B switch) functionality. Shorting or non-shorting contacts? And how do I wire this up? There are a dizzying number of rotary switches to choose from, and I'm really lost on this.

Any help is appreciated. Thanks in advance.

 

[KlausST]

Hi,

I see absolutely no need for a dummy 8 Ohms load. It´s a waste of money, power and just generates a lot of heat.
No load is less stress for an amplifier.
Nobody would run a car with full throttle and using the brake not to get faster.

P = I x I x R --> solve for I
I = sqrt(P / R)
I = sqrt(150 W / 8 Ohms) = 4.33A

How do you come to 0.25A?

What`s the 5V (1.25V ) rating? How do you come to this value?

For each channel use an SPDT switch. Break before make.

Klaus

 

[Ghostfather]

I see absolutely no need for a dummy 8 Ohms load. It´s a waste of money, power and just generates a lot of heat.

I've been warned specifically by the maker of the monoblock amplifiers NOT to operate them without a load, while there is a signal on the input, as they WILL be damaged.

This is the amplifier circuit, DC coupled, fully balanced dual feedback modulator. I don't have the expertise to say whether it's true or not, but when the designer says that running this amplifier without a load will damage them and invalidate the warranty, I assume he knows better.

P = I x I x R --> solve for I
I = sqrt(P / R)
I = sqrt(150 W / 8 Ohms) = 4.33A

How do you come to 0.25A?

What`s the 5V (1.25V ) rating? How do you come to this value?

I stand corrected, I was reading typical values from a spec sheet.
Using Ohm's law equations, it should be 4.33A and about 35v at full 150w load

That means a beefier switch, but there is still a dizzying variety to choose from.
Thank you for pointing that out.

For each channel use an SPDT switch. Break before make.

I will be using rotary switches, and switching both channels at the same time, not with two switches. Rotary switches are most often used in audio equipment for this purpose.

Sorry if I'm coming across as bull-headed on this, but I'm just puzzled by the inner workings of rotary switches, as well as how to wire them for what I want. I intend to use a rotary switch.

 

[KlausST]

Hi,

About the amplifier:
You show a differential (bridged, BTL) Class D amplifier with "undamped" low pass output filter.
I assume the problem is the undamped output filter. It has a high Q resonance. With a speaker connected the Q will be smaller resulting in lower resonant peak.
If this is the case then the volume does not matter.
While I now see why the amplifier could be damaged ... I still think that's the job of the manufacturer to prevent damage.

******
A switch is a switch. It does not matter if it is a rotary switch, a rocker switch or a latching push button switch....
What matters are the contacts and their arrangement.
A SPDT has a COM an NC and a NO contact.
Two SPDT in one switch is a DPDT.
4 SPDT form a 4PDT .. this is what you need for two channels.

All COM go to the speakers, all NC to the one amplifier, all NO to the other amplifier.

Klaus

 

[Ghostfather]

Thanks, a 4PDT switch seems logical. I've learned that for rotary switches that means 2 positions, 2 poles with 4 decks.

With a rotary switch it seems possible to do complexer switching than with toggles, but I'll stick to keeping it simple.