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Old 22nd July 2015, 09:13 PM   #1
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Default Need 4PDT switch to switch two amp to drive one speaker.

I want to have a switch to switch different amps to drive the same speaker. Also to connect to a dummy load if not selected.

I know it has to be high current like 10A or more rating. I want to make sure I don't loss anything ( well as little as possible). I look at some 4PDT either to switch both channel. OR more importantly I can pair up two of the SPDT in parallel to have more contact area to get lower contact resistance. I don't even mind switching L and R channel separately.

I am looking at the 4PDT on ebay as they are a lot cheaper, I just pair up two switches in parallel.

Any other suggestions?

Also, to simplify the switches, I have to tie the grounds of both amps together. Any way to avoid hum as I cannot separate the two grounds with even very low value resistor.

Last edited by Alan0354; 22nd July 2015 at 09:16 PM.
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Old 23rd July 2015, 03:30 AM   #2
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Use a relay or two. Then you have a wide range of pole configurations and current ratings.
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Old 23rd July 2015, 07:09 AM   #3
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Quote:
Originally Posted by Alan0354 View Post
I want to have a switch to switch different amps to drive the same speaker. Also to connect to a dummy load if not selected.

I know it has to be high current like 10A or more rating. I want to make sure I don't loss anything ( well as little as possible). I look at some 4PDT either to switch both channel. OR more importantly I can pair up two of the SPDT in parallel to have more contact area to get lower contact resistance. I don't even mind switching L and R channel separately.

I am looking at the 4PDT on ebay as they are a lot cheaper, I just pair up two switches in parallel.

Any other suggestions?
I support the suggestion that you use relays.

You can wire up as many SPDT relays in parallel to obtain whatever contact configuration that you need. They are readily available for low prices and can be obtained with high quality contacts.

For example:

10pk 12 Volt 30 40 Amp Bosch Tyco Style SPDT Automotive Relay Harness Sockets | eBay

Use the low current switch of your choice to control them, and power their coils with one of the low-cost 12 DC power supplies that are readily available. Don't forget to put diodes (1N4001 or equivalent) across the relay coils to supress inductive kick-back during switching.

Do I need a flyback diode with an automotive relay? - Electrical Engineering Stack Exchange

Quote:
Also, to simplify the switches, I have to tie the grounds of both amps together.

Don't go there unless you want to fry your gear and frustrate yourself. A lot of modern audio gear just plain can't tolerate common grounds, particularly at speaker level.

Last edited by arnyk; 23rd July 2015 at 07:32 AM.
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Old 23rd July 2015, 08:31 AM   #4
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Thanks Arnyk

The relay seems to be a good choice.

I don't understand why it's dangerous to use common ground for two amps. The common is always earth grounded inside the amp, the circuit is not floating for safety reason. So the worst is having hum, not burning anything. Can you explain?
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Old 23rd July 2015, 11:33 AM   #5
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Automotive type relays often have the coil and one contact in common. So be a bit careful.
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Old 23rd July 2015, 12:01 PM   #6
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Quote:
Originally Posted by Alan0354 View Post
Thanks Arnyk

The relay seems to be a good choice.

I don't understand why it's dangerous to use common ground for two amps. The common is always earth grounded inside the amp, the circuit is not floating for safety reason. So the worst is having hum, not burning anything. Can you explain?
Two words: bridged amps.

They are very common in many markets such as automotive and pro audio.

In addition any common ground is a resource that can anchor a ground loop.
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Old 23rd July 2015, 04:25 PM   #7
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Quote:
Originally Posted by arnyk View Post
Two words: bridged amps.

They are very common in many markets such as automotive and pro audio.

In addition any common ground is a resource that can anchor a ground loop.
I see, so if it is not a bridge amp, it is safe?

Well, if I plug the two power amp in the same outlet, make sure they both have their ground connected to the earth ground( third prong), I should not have ground loop. There should not be current flow into the earth ground, it's only a reference and for safety.

I am not trying to make a universal selector switch, I only want to have fast A/B comparison between my Acurus(which I have schematic) and my new design.
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Old 23rd July 2015, 04:31 PM   #8
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4PDT ON/ON
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Old 25th July 2015, 12:15 PM   #9
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Don't try this with valve amplifiers, they don't like running without a load
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Old 25th July 2015, 10:48 PM   #10
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Quote:
Originally Posted by davidsrsb View Post
Don't try this with valve amplifiers, they don't like running without a load

I am going to switch load in for the amp that is not being selected to drive the speaker. Will never run without load.

In guitar amps, we put diodes at the primary of the OT to prevent the voltage from flying to avoid arcing to protect the OT and tube. I don't know how the capacitance of the diode affect the audiophile amp as they run in much higher frequency than guitar amp. We don't run over 5KHz in guitar amp.

Last edited by Alan0354; 25th July 2015 at 10:50 PM.
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