Input selector configuration

[Skorpio]

I want to be able to change between 3 input sources. If I chose a 4x3 selector switch I should be able to switch both the signal conductor and the return conductor, making shure the the unused equipments ground do not interfer with the selected source.

But what happens when this configuration is made in real life? Does it give problems if the signal conductor is switch on before the ground is established? A inputsignal with no ground/return could mean serious problems...or?

 

[AndrewT]

Hi,
if you are running a balanced signal into your Pass then you need both poles switched.

For unbalanced I would stick with switching just the signal pole.
Some amps do not like an unterminated signal line connected to an aerial lead. Could blow the amp and/or the speakers.

 

[Skorpio]

We are talking unbalanced signals and after the selector switch are series capacitor and resistor to ground.

Many "high-end" amps uses this configuration, switching the ground.....in which cases does it go wrong?

 

[AndrewT]

Hi,
you could use the spare poles to short the unused inputs to ground, IF the source can stand a nearly permanent short to ground.

Cuts crosstalk.

 

[Skorpio]

I don't think you understand or perhaps I don't put it right?

I want to switch both the signal and ground wire on each source to completely isolate the souces not in use (including ground currents).

But I fear that when switching from one source to another, in a very short while during rotating of the knob, the amplifier gets signal before ground is connected. And thereby making amplifier unstable?

A cut from ML homepage:

For even greater channel isolation, the Nº320S deactivates unused inputs to prevent interference from associated components. When an input is deactivated, the Nº320S disconnects the input signal and ground connection, eliminating ground loop noise between the Nº320S and the associated component. As a result, input signals pass through the Nº320S with exceptional freedom from interference due to other components.

 

[AndrewT]

Hi,
I understand what you want.
I do not know how to get there without risking the input of the amp.

If all your equipment is properly designed with isolation between audio ground and safety ground then there are no circulating currents between components.

eliminating ground loop noise

However many manufacturers ignore this practice. In that situation isolating the returns would be beneficial.

However if two sources or a source and an amp have both got audio ground connected to safety ground then isolation when deselected is not the issue. HUM, lots of it, when selected is the major issue.

There is a solution but I suspect the switch type is not readily available if at all.
The signal could be switched break before make and the ground could be switched make before break. That may give you what you need. A selector with the tracks specially made for this purpose could easily be manufactured. Similarly relays with delays could achieve the same.

How does your other equipment achieve it?

 

[mlloyd1]

Hi,
I understand what you want.
I do not know how to get there without risking the input of the amp.

If all your equipment is properly designed with isolation between audio ground and safety ground then there are no circulating currents between components.
However many manufacturers ignore this practice. In that situation isolating the returns would be beneficial.

However if two sources or a source and an amp have both got audio ground connected to safety ground then isolation when deselected is not the issue. HUM, lots of it, when selected is the major issue.

There is a solution but I suspect the switch type is not readily available if at all.
The signal could be switched break before make and the ground could be switched make before break. That may give you what you need. A selector with the tracks specially made for this purpose could easily be manufactured. Similarly relays with delays could achieve the same.

How does your other equipment achieve it?

 

[BrianDonegan]

You could use a make-before-break switch, in which case you risk dual input signals for a brief moment, rather than a complete signal break.

Another option is to use resistors (like 100k) to ground on each of the inputs, so they are never fully disconnected. This is what we did on our SS to prevent any popping.

Schematic

Shoudl be able to do this on a switch pretty easily, if you can live with the resistors, that is.

 

[mlloyd1]

yeah, i think we understand what you want.

doesn't ML use relays or MOSFET switches?
i think they also use a microcontroller to control it all. so, it would be easy for them to put some intelligence in to break ground connection last and reestablish ground connection first when changing inputs.

i've read that the Ayre K5xe switches grounds also (for the single ended inputs) via (MOS?)FET input source switches.

mlloyd1

Hi,
I understand what you want.
I do not know how to get there without risking the input of the amp.

If all your equipment is properly designed with isolation between audio ground and safety ground then there are no circulating currents between components.
However many manufacturers ignore this practice. In that situation isolating the returns would be beneficial.

However if two sources or a source and an amp have both got audio ground connected to safety ground then isolation when deselected is not the issue. HUM, lots of it, when selected is the major issue.

There is a solution but I suspect the switch type is not readily available if at all.
The signal could be switched break before make and the ground could be switched make before break. That may give you what you need. A selector with the tracks specially made for this purpose could easily be manufactured. Similarly relays with delays could achieve the same.

How does your other equipment achieve it?

 

[Skorpio]

You could use a make-before-break switch, in which case you risk dual input signals for a brief moment, rather than a complete signal break.

Another option is to use resistors (like 100k) to ground on each of the inputs, so they are never fully disconnected. This is what we did on our SS to prevent any popping.

Schematic

Shoudl be able to do this on a switch pretty easily, if you can live with the resistors, that is.

You mean switching both signal and ground in the selector and then have a 100k resistor from source ground to amplifier ground when selector is off?

 

[pooge]

However if two sources or a source and an amp have both got audio ground connected to safety ground then isolation when deselected is not the issue. HUM, lots of it, when selected is the major issue.

There is a solution but I suspect the switch type is not readily available if at all.
The signal could be switched break before make and the ground could be switched make before break. That may give you what you need. A selector with the tracks specially made for this purpose could easily be manufactured. Similarly relays with delays could achieve the same.

I favor this approach. It's fairly easy to custom order a switch like this. Making the grounds before braking while switching may prevent pops and hum while switching. Also, if the input jacks have grounds isolated from the chassis, connect the input jack grounds to the chassis via a capacitor or capacitor resistor combination with leads as short as possible to prevent the "antenna effect".