richieOOboy

[P901]

Good evening richieboy,
I wanted to thank you for the reply on my "AC Hum" posting but your settings would not allow direct reply email so I am using this method.

Your answer confirmed my thoughts, that I would not want to connect a capacitor from the AC main to the chassis.

I was unable to recieve any additional clarification from the gentleman that proposed this arrangement but his response could have been interpreted in another way.
In the way his response was worded he might have also implied that the capacitor could have gone across both AC incoming primary leads with no connection to the chassis. He used the term "ground" and here in the US one side of the AC line is "hot" and the other is termed "neutral" which is the grounded side, (at the utility transformer), giving us one line that has the potential above earth and the other AC line with its potential bonded to the earth value, effectively creating a "grounded" line.
If one were to go across the AC primary lines input to the power supply transformer would such a capacitor of the value(s) indicated, .02 - .05 uf be of a balanced capacitance to offset the objectionable 60hz hum?
If I remember my basic electronics there are circuits that balance an inductor and a capacitor of certain values to cancel unwanted frequencies. If I knew the inductance of my power supply transformer could I not find the associated capacitor that would cancel the 60hz hum ?
Or in reality, as the loads on the power supply fluctuate the transformers inductance is always shifting to the point where there would be no value of capacitor that would satisfy all conditions.
To let you know, the power supply caps were upgraded from 10,000uf 63v to 18000uf 100v ultra low leakage computer grade caps.
Thanks again for all your help, andrew

[richie00boy]

I prefer this method of dicussion as it allows others to join in/learn. I thought you should still be able to send me a PM, but I have never used them on here so not sure if they work.

The grounded-at-the-station neutral is also used in the UK.

Connecting a cap across live and neutral would do absolutely nothing at all with respect to hum. What you may be thinking is power factor correction. However, this is almost impossible to implement with a class-B amplifier.

The size of your caps looks fine to me and I would doubt that increasing them beyond 18,000uF per rail is going to bring much noticeable benefit.

I think you may just be being a tad too picky if you have to put your ear right up to the speaker to hear it It's possible that the amp is not laid out as best it could be with regard to mains being near sensitive circuitry, or it has poor power supply rejection ratio (PSRR).

I suppose you could disconnect all input leads AT THE AMP END and see if the hum is any different. If it is then your source could be the culprit, or just leads picking up hum.