5VPSU just for LEDs

[Guest]

I saw this in some John Hardy Documentation:


If you look at the schematic there is a separate 5v PSU for the LEDs for the switches in the project
Is this to keep the current in the circuit or is it to stop the circuit getting "dirty" current (I apologise for my newbie lack of understanding here)

www.johnhardyco.com/pdf/990.pdf
Look at the MPC1 schematic on Page 6

Also one thing is bugging me - which switch do I use and how do I wire it...

Thinking on my feet here - are my assumptions right...
I have a SPST switch to switch a cicuit in and out
If I know want a LED to show this circuit is in or out AND i use a LED PSU to feed the LED
Then which switch do I use and how do I wire it?
Is it as simple as a DPST switch which pushes the LED circuit in at the same time as the audio circuit...

 

[Samuel Groner]

Is this to keep the current in the circuit or is it to stop the circuit getting "dirty" current

If you don't have an extra supply (or at least ground), you may hear the LEDs switching on and off.

Samuel

 

[John Hardy]

I keep a separate ground-return path for the 5V supply so that the ground-return currents of the LEDs do not interact with the ground-return currents of the audio signals that pass through a separate ground-return path.

Look at the gain pot of the MPC-1 card. It is connected between the inverting input of the 990 op-amp and ground (it is a bit more complex because of the two-section pot and a couple of fixed resistors, and the "HIGH GAIN" switch to choose ranges, but the net result is a resistance between the inverting input and ground). The ground-return path for that resistance must go directly to the central-point audio-ground without any interaction with other parts of any circuit. There is always a small, but measurable amount of resistance in such a ground-return path, no matter how thick or wide the copper pattern is on the p.c. board. If some other circuit shares that path, it will cause a small voltage to develop as the current that is being returned from that circuit passes through that small resistance. Each LED that turns on or off causes about 10mA of current to flow (or stop flowing), resulting in a small voltage appearing (or disappearing) across that small ground resistance. That voltage becomes part of the audio signal path if the ground-return paths are shared. The audio circuit SHOULD be seeing zero volts along the ground-return path, but is suddenly sees several millivolts. This can be audible.

I hope that makes some sense. Thanks.

John Hardy
The John Hardy Co.
www.johnhardyco.com

 

[Flatpicker]

[quote author="John Hardy"]I hope that makes some sense. Thanks...[/quote]Makes a LOT of sense and thanks to you!!!! :thumb:

 

[Guest]

Blimey - I wasn't expecting the designer of the product to reply
Many thanks John