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Old 20th April 2016, 08:59 AM   #1
donovas is offline donovas  South Korea
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Default Mains hum for sure?

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?
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Old 20th April 2016, 09:14 AM   #2
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schematic and photos please.
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Old 20th April 2016, 09:23 AM   #3
Mooly is offline Mooly  United Kingdom
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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.
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Old 20th April 2016, 10:26 AM   #4
AndrewT is offline AndrewT  Scotland
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Almost certainly a wiring fault.
Post pics and drawings explaining the pics.
And your circuit diagrams.
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Old 20th April 2016, 10:40 AM   #5
donovas is offline donovas  South Korea
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Ugh.. it looks embarassingly bad- input wires next.to ei transformer.... untwisted wires here n there...

But the hum sounds exactly like this tho

https://en.m.wikipedia.org/wiki/File..._hum_60_Hz.ogg
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Old 20th April 2016, 10:43 AM   #6
donovas is offline donovas  South Korea
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Ill build another with a better layout but ac disconnected- no hum, hum 60hz in nature... i do hope its ac related problem which im told is easier to fix.
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Old 20th April 2016, 02:41 PM   #7
donovas is offline donovas  South Korea
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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...

http://www.loetstelle.net/projekte/gyrator/gyrator.php
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Old 20th April 2016, 08:43 PM   #8
Pafi is offline Pafi  Hungary
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A theoretical comment:
A gyrator is not an inductor simulator. A gyrator is a 2 port device operation described here:
https://en.wikipedia.org/wiki/Gyrator#Behaviour

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: https://en.wikipedia.org/wiki/Gyrato...lated_inductor 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).

Quote:
Ugh.. it looks embarassingly bad- input wires next.to 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?
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Old 20th April 2016, 10:53 PM   #9
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Quote:
Originally Posted by donovas View Post
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, itīs written in my mother tongue, so itīs 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.
Iīm not sure if this is true, but i must admit that Iīm 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 canīt 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 canīt figure out what resistor values you changed or whatīs the issue with your MOSFETs.
Like Pafi said, it would be very helpful if you can post your psu circuit.
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