Powering up/down Amp

ptah

Member
2006-08-07 8:48 pm
I recently built an amp with four channels and configured the power leads from the supply through two DPDT switches allowing each pair of amp circuits to be switched on/off independently. In other words each DPDT has one positive and one negative lead coming in and going out (they are then split at the fuse header to supply the two circuits).

When I switch each pair off a popping noise can be heard in the speakers.

I wanted to try this - The +/- leads connected to the fuse header tie to the middle two lugs on the DPDT. The +/- leads connected to the supply tie to the two lower lugs. The other two lugs are unused. When the switch is off the contacts bridge between the fuse rail and the two empty lugs.

Would connecting a capacitor across the two empty lugs have a useful dampening effect when the switch is in the off position since the capacitor would see the +/- rails of the amps (isolated from the supply)?
 
You might want to measure the DC level change or the spike seen on the output to see if any fixes help. Do DPDT switches actually switch both poles at exactly the same time? Can one pole bounce a little more than another? I don't know but might want to look at the switch changes and see if the supplies are not being powered up at the same time causing some DC offset.

-SL
 

ptah

Member
2006-08-07 8:48 pm
SL - One DPDT began sticking so I sacrificed it. Although rated for 5A at 120V the contacts were burning. The rockers are not linked. I clearly made a mistake thinking that I could switch both rails simultaneously. As you point out, they are not likely to make/break contact together. I believe the approach will be to switch the AC side of the rectifier. I will need to install one more rectifier and rewire the four reservoir caps to place two on each rectifier. Can't get some thing for nothing!
 

The Donk!

Member
2008-02-24 4:48 am
Nordic said:
That is correct, Switches with DC handleing capacity are very expensive items... Swicthes do not like switching DC....

Why is that..

I'm no noob to this stuff, and it doesn't make sense.

Maybe in AC and flow going from the two contacts would not go in just one direction.

Not electron flow. Particles from the actual contacts.

You know..... The stuff you see when you burn contacts

:D
 

ptah

Member
2006-08-07 8:48 pm
I'm not sure why switching DC is more difficult. Perhaps more total power since DC is constant voltage. Anyway, adding another rectifier and switching the AC sides cured the problem. Good to have a place to hash out these problems, thanks. I'll post a picture or two when complete.
 
ptah said:
I'm not sure why switching DC is more difficult. Perhaps more total power since DC is constant voltage. Anyway, adding another rectifier and switching the AC sides cured the problem. Good to have a place to hash out these problems, thanks. I'll post a picture or two when complete.

Because ANY reactive component of the load (AND source) will generate an arc at the contacts when they open. This may quickly destroy (burn / weld) said contacts.

For AC this is only briefly true as the voltage dies to zero within milliseconds.
 

poynton

Member
2005-03-10 11:57 pm
UK
cliff said:


Because ANY reactive component of the load (AND source) will generate an arc at the contacts when they open. This may quickly destroy (burn / weld) said contacts.

For AC this is only briefly true as the voltage dies to zero within milliseconds.

This is exactly the reason.

Some years ago. I had a flat screen monitor powered from a power block supplying 18v at 5.5A. The switch had to be replaced as it had melted due to heat from the arcing. A little dirt on the contacts aggravates the arcing.
The best switches for DC work have self-cleaning contacts which wipe themselves on make or break
 
AndrewT said:
the old fashioned points in an automobile ignition broke the battery DC to generate the voltage fed through the coil.
These used tungsten to resist burning from the arc and still they pitted and had to be changed regularly.


EchoWars said:
'Regularly' meaning billions of cycles later...

That is what the "condenser" was for! - remember those?

Shunts the inital dV across the contact breaker, much reducing / eliminating the arc.