[CButterworth]

After a long battle with 60Hz hum.  I can now enjoy listening to my AKSA55 N and Aikido preamp.  However, there is still a little, very faint residual 60Hz hum.  I reduced the louder hum using 20 ohm ground-lift / ying-yang diode network in the preamp.  As I remember the AKSA already has something like 10 ohms of lift.

OK, so I prewarm my preamp - 30 seconds to get the tube heaters warm before the B+ voltage is applied.  Only after then do I switch on the AKSA.  When I first switch on the AKSA, I get a louder 60Hz hum, which goes quiet after a minute or two.  My first thought was that it is the tubes warming up.  However, if I leave the preamp on for over 5 minutes, then switch on the AKSA, I still get the louder 60Hz hum, which then fades.

So, should I hunt for the cure in my AKSA?

Any ideas will be gratefully appreciated.

Charlie

[Geoff-AU]

the very first thing I would do is test the AKSA with the pre-amp disconnected, and if you still get hum then try it with shorted RCAs on the input (ie, join signal to ground on the back of the AKSA).

That will tell you whether the hum is upstream or downstream of the input to the amp.

I had the briefest of reads of your other thread about the same issue - how did you go with your star earth and chassis earth?  IIRC the correct grounding method for an amplifier is to have your star earth join all the common points of the amplifier's circuits at one point, and then join your mains earth to the equipment chassis.  The mains earth and star earth are not connected!

What this means is that your star earth becomes a solid point of common potential for the various amplifier circuits.  The chassis is grounded, which sucks away external interference and shields your amplifier.  And because there is no connection between star earth and mains earth, there is less risk of the chassis picking up interference and injecting it onto the amplifier circuit or speaker common output.

Also, of less importance for immediate listening bliss and more importance for continued existence (and future listening bliss), connecting the chassis to mains earth means an internal wiring fault does not become a shock hazard for the user.

[AKSA]

Charlie,

Geoff is right on the money - his input is both correct and valued.

Check that a disconnected input DOES actually produce hum at the speaker......  If it does not, then the problem is not the amp, but rather it is upstream.

One point I'd make;  some amps need star ground connected to chassis/IEC ground, some don't.  Try both;  one will deliver less hum overall.

Thanks Geoff!

Cheers,

Hugh

[CButterworth]

Geoff, Hugh,

I only get the extremely low level hum from the AKSA (by itself) that I got when I used the passive preamp.  This begs the question of the ground-loop being in the preamp.  Right now, I have the star-earth connected to the chassis via one of the IEC bolts.  I am using the ground lift that Rod Elliot has on his website (sound.westhost.com/earthing.htm).  Except that I solder the capacitor and 20 ohms directly onto the bridge rectifier.  This made a huge difference in the level of hum.

I vaguely remember Ginger discussing the positioning of the ground bolts.  My preamp has two chasses - a top plate which hold all the components, and a back plate that has the plug sockets.  Both a connected via braided and bolted cable.  Maybe the star-earth / ground connected should be made directly to the IEC ground - right now it is bolted to the back chassis IEC mounting bolt.  This same bolt connects to the top chassis.  I am using a mounting bolt for one of the transformers (either the 125-0-125V or the 6.3V).  Maybe this is a source of hum???

What is interesting is that the hum-level drops significantly after the AKSA is switched on, even if the preamp has been warmed-up for a considerable time, implying that the hum maybe due to an interaction of the AKSA and preamp.

I suppose that I shouldn't be too hung-up on it because, when warmed-up the level is very tolerable.

Anyway,  thanks for your help.  Hugh, I was sorry to read about the 6moons trouble.  It seems that both you and the 6moons editor remained amicable.  It's always a pity when such misunderstandings happen.  It has always been a pleasure dealing with you - my AKSA purchase and Nirvana upgrade, and my numerous questions.  Your fast and detailed responses and willingness to help was quite noticeable when I had building problems with my Aikido Preamp PCB which I purchased directly from the amp designer at TubeCad.

Regards,
Charlie

[AKSA]

Charlie,

Hum is a black art, and entire books are written on this arcane subject.  Needless to say, this is not eye-popping stuff, but there is no doubt that a little hum is very irritating.

When confronted by an earth problem, I use something of a practised approach:

1.  Short inputs to see if hum comes from the component or upstream.
2.  Connect star earth to chassis/IEC power ground with speakers connected to see if it reduces hum.
3.  Bypass any floating resistors (typically 10R) with a jump lead just to be sure hum is not actually caused by the small resistor.
4.  Place small caps, typically 0.47uF, from star earth to pots and signal grounds in case hum is reduced.
5.  Disconnect signal grounds between source and power amps and recouple with a 1uF cap.  If this affects hum, then it's clear the problem is a loop, and the source needs to be looked at.  Often the signal ground is also ground lifted to power ground, in which case disconnecting star earth on the amp from power ground can do the trick.

I've given up trying to precisely identify why there is hum;  I'm focussed on preventing it, and the above approach seems to be very successful.  Essentially any voltage developed between signal ground and star ground will be amplified by the amp, and this happens because the charge pulses coming off the rectifiers and passing to the filter caps can be as high at 50 amps.  A tiny resistance, say 1 milliohms (0.001 ohms!!), will then result in a voltage drop of 50 millivolts, which when amplified by a gain of 35 will give you 1.75 Volts at the speaker.  This is LOUD, and you will hear it very clearly.  So it should be obvious that the resistance from the filter caps to the rectifiers is critically important, and any signal earths should NOT be place anywhere near this high current path, lest they be magnified by the gain of the amp.  Hope this helps!

Yes, the 6Moons situation was not nice for either of us.  I deeply regret it happening, I do not want enemies or even broken associations.  Much of this business depends on the intangible of goodwill, and once lost, it can't be recovered.

Cheers,

Hugh