Mains hum for sure?


2014-06-22 6:45 am
My p2p opamp preamp hums, mostly likely 60hz but i havent been able to verify it.

The hum goes away when the ac is unplugged or the power is turned off all the while the opamps are still operational and making sound from the reserve charge from the psu caps.

The psu is rather well built for my standards. :p

Double bridge rectifier> 22000uf smoothing> salas ccs> mosfet gyrator> 220uf smoothing

Based on symptoms described should i assume that the hum can be fixed from the ac side?


Paid Member
2007-09-15 8:14 am
60Hz hum will appear as a very deep and pure tone.

If there is harshness or it has a buzzing rasping quality then its more likely to be 120Hz and spiky in nature caused by grounding/wiring problems allowing contamination of the clean audio grounds by reservoir cap charging pulses.
A theoretical comment:
A gyrator is not an inductor simulator. A gyrator is a 2 port device operation described here:

It can be used to transform 1 impedance to an other, including capacitance to inductance, but the resulting 2 pole is not a gyrator, its an application of a gyrator.

Furthermore: the description written here: is false, because second equation of the gyrator doesn't apply if second port is defined by the statement "it is terminated with the capacitor" (and because it is not a real 2 port device, only a degenerated one with 3 connection points, but this is the smallest problem).

Ugh.. it looks embarassingly bad- input wires ei transformer.... untwisted wires here n there...

This is why you have to share it. If you keep the problem a secret, then who will be able to tell you the solution?
dont want to pollute the forum so i'll just add on here another question i have- the gyrator i'm using is this circuit- resistors are scaled down to 100ohm for 15v and i've subbed irf610 for 840, and 9610 for the negative supply. have i done everything right?

i've only swapped the mosfet and the cap polarity for the neg supply...

Grundlagen, Einführung, Berechnung, Theorie,Platine Bausatz, Bauplan Schaltplan Mosfet Gyrator

I checked this link, it´s written in my mother tongue, so it´s easy for me to understand (at least) the words.
The writer claims: This circuit can replace a series inductor, mainly used for filtering in tube stuff psu.
I´m not sure if this is true, but i must admit that I´m too lazy to analyse this circuit.
There also is no equation of the "inductors value"

Pafi, you are right, the principle of a gyrator is to transduce one impedance to another.
I know only gyrators related to ground, so they can´t be used in series.

Donovas: I wonder why you use this gyrator circuit in a psu for opamp supply, what benefits do you expect?
I also can´t figure out what resistor values you changed or what´s the issue with your MOSFETs.
Like Pafi said, it would be very helpful if you can post your psu circuit.