Voltage Regulator (7809) question

[Derringer]

I understand that wiring in a 9 volt regulator helps to rid the unit of unwanted noise from an unregulated power supply.
I also understand that it's ideal for a 9 volt regulator to "see" more than 9 volts on its input side.


I've built a pedal on commission.
If I include the 7809 regulator in the pedal, will there be any undesirable effects if the user plugs in a proper and regulated power supply?

I've tested this myself and have not had any issues. I would just like to know from those who know more than I.


Thanks

[R.G.]

I understand that wiring in a 9 volt regulator helps to rid the unit of unwanted noise from an unregulated power supply.

Linear regulators may be thought of as special purpose "opamps" which have an internal voltage reference and put out a steady DC voltage. As such, they have a certain rejection of noise and hum on an input power supply. This rejection is often in the range of 40db (i.e. noise/hum is reduced to 1/100th) but is frequency dependent. It's pretty good down at 60Hz and 120Hz, but declines to "sucks" at much higher frequencies near the top of audio.

I also understand that it's ideal for a 9 volt regulator to "see" more than 9 volts on its input side.

It's not ideal. It's mandatory. Every linear regulator puts out less voltage than it puts in. For the 7800 series, this is about 1.5-2.5V. So a 7809 run on 9V cannot put out more than about 7 to 7.5V depending on load. This difference between input and output voltage is called the dropout voltage, where it drops out of regulation and follows the input voltage down as it declines. When the regulator is running with less than its minimum input voltage (about 11V for a 7809) it has ZERO rejection of hum and noise, and could be replaced with a few diodes, which is what it approximates inside when it's saturated like that.

There are Low Dropout (LDO) regulators which have a much smaller dropout voltage (doh!), down to 100-200mV.

I've built a pedal on commission.
If I include the 7809 regulator in the pedal, will there be any undesirable effects if the user plugs in a proper and regulated power supply?

Depends on what you mean by "undesirable". It'll lose 1.5 -2.0V from the incoming voltage. So if he plugs in a 9.0V supply, it will make the pedal run on 7.0-7.5V. I don't know if that's undesirable or not.