Power Filtering questions

[smitty]

Hello Everyone,

Just looking for some "wisdom" about power filtering...

I have built a valvecaster (my first diy stompbox)  sounds great but is a bit "hissy"

So far I have a 2200uf Cap across the + and - of the DC jack... 

Also, I have a 7812 voltage regulator w/ no filtering cap,   is it common to put a filter cap across the
output and ground on the voltage regulator?   its an TOpackage Lm7812   Could the cap be strapped
right on the "legs" of the Voltage Regulator?

thanks for the help,

smitty

[Johan]

hiss is generally from the amplifying and not the powersupply. powersupply's hum ( does it go "ssssssss" or "mmmmmm"? )
78/79 series regulators usually have a big cap before and a small value cap after the regulator. a big one after the regulator would mess with the regulator.
as a round number to aim for when choosing filtercaps for audiocircuits. 10uF/1mA is a good rule...so your 2200uF would be good for up to 200mA current draw...but rules dont allways apply, so expect exceptions..

I never played around with the valvecaster, so I cant advice on that particular circuit

J

[smitty]

hiss is generally from the amplifying and not the powersupply. powersupply's hum ( does it go "ssssssss" or "mmmmmm"? )
78/79 series regulators usually have a big cap before and a small value cap after the regulator. a big one after the regulator would mess with the regulator.
as a round number to aim for when choosing filtercaps for audiocircuits. 10uF/1mA is a good rule...so your 2200uF would be good for up to 200mA current draw...but rules dont allways apply, so expect exceptions..

I never played around with the valvecaster, so I cant advice on that particular circuit

J

The sound is definately a "sssssssssssss" ....    thanks for your response it was very useful...   I might try the small value cap on the
VR.   Can I just tie the cap to the Output leg and Ground leg of the VR?

thanks again,

smitty

[amptramp]

Yes, output to ground is the correct connection for the cap.

Tubes have a problem: they run at 6.25 times the absolute room temperature, so they have the square root of that or 2.5 times the noise of a semiconductor circuit operating at the same voltage and current.  Add the high cathode resistor of the input stage and it almost looks like it was intended to hiss.  The tubes are run at very low current - even if the tube had a plate to ground short, there would only be 54.5 microamps flowing.  At these low currents, the noise performance is going to be worse than it would be when operating at its design current.  Try minimizing the cathode resistance (which should be a pot) of the first stage to reduce noise.