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-   -   Need help to kill a little hum (http://www.diyaudio.com/forums/aksa/162457-need-help-kill-little-hum.html)

atenolol50 3rd March 2010 02:24 AM

Need help to kill a little hum
 
I've had an AKSA55 for more than 5 years.

The amp is quiet when connected directly to the CD Player.
However there's a little hum when connected to a tube preamp, audible with ear 10cm from speaker.

I stumbled on this thread/post http://www.diyaudio.com/forums/aksa/...ml#post1953378 and decided to try the fix.
I joined both input RCA grounds and connected them to the GND pin on the Left Amp.
Now, no hum on the left speaker but the right speaker hums and gives out "Eeeee" screeching note from the tweeter.

Any tips / fixes?

Thanks.

gaetan8888 3rd March 2010 03:28 AM

Hello

Can you take a big close photo showing all the inside of your Aksa and post it here ?

We will look at it and tell you.

Which tube preamp do you have ?

Thank

Bye

Gaetan

AKSA 3rd March 2010 03:45 AM

When you connect signal to power ground, you bypass the back to back diodes and 10R resistor which ground lifts signal ground.

This can create earth loops, as can an RCA input whose ground unintentionally touches chassis - same deal.

If the amp is fine connected to a double insulated CD player, but hums slightly when connected to a tube preamp, I would be looking for another earth between signal ground of the tube preamp and chassis, or power.

The other thing you might factor in is that a slight hum audible only within 10cms of the driver is actually pretty good for use with a tube circuit, as they are generally much noisier than SS owing to the power supply and particularly the filament supply.

Long live your AKSA!

Hugh

atenolol50 3rd March 2010 04:46 PM

Here's a few pics, sorry about the lighting, its past midnight.
http://img692.imageshack.us/img692/6513/dsc00554vs.jpg

http://img714.imageshack.us/img714/2460/dsc00555x.jpg

http://img412.imageshack.us/img412/4641/dsc00556tz.jpg

http://img202.imageshack.us/img202/5095/dsc00557y.jpg

The power supply ground is connected directly to the mains (IEC) Gnd, i don't have a 10R lift resistor in there.
In my AKSA manual, there is no connection between power supply ground to mains.

gaetan8888 3rd March 2010 11:22 PM

1 Attachment(s)
Hello atenolol50

You should do the way I draw on my image.

The input plug ground tab are connect together and that's goes to the star ground.

The pcboard input ground should NOT be connect to the input wire ground, it's should be connect to the star ground.

Bye

Gaetan

AKSA 4th March 2010 08:44 AM

Ate,

Gaetan is not quite correct but he is very close. Signal input ground on the RCA input plug should be connected to signal input ground on the pcb, not to star earth.

If you have hum issues, connect a wire as Gaetan has indicated between chassis and star earth, with a 10R resistor and back to back diodes (A to K, and K to A) all in parallel spliced into the wire.

The signal input ground is already ground lifted with 10R/2 x diodes on the pcb. The additional ground lifting connection from star earth to chassis should fix your problem.

Thank you Gaetan!

Hugh

gaetan8888 4th March 2010 04:03 PM

Quote:

Originally Posted by AKSA (Post 2106658)
Ate,

Gaetan is not quite correct but he is very close. Signal input ground on the RCA input plug should be connected to signal input ground on the pcb, not to star earth.

If you have hum issues, connect a wire as Gaetan has indicated between chassis and star earth, with a 10R resistor and back to back diodes (A to K, and K to A) all in parallel spliced into the wire.

The signal input ground is already ground lifted with 10R/2 x diodes on the pcb. The additional ground lifting connection from star earth to chassis should fix your problem.

Thank you Gaetan!

Hugh


Hello Hugh

Yes, I forgot that in the Aksa the signal input ground is already ground lifted with 10R resistor on th pcb, so that's why I've suggest the ground connection and a 10r resistor connected to the star ground. Because wen I don't use a ground lifted resistor in a amp but one star ground for all including the input ground, I do as I draw in my post.

Sorry, my mistake.

Funny things happened, yesterday, wen I see my error I've deleted my post, it was delete, but this morning I see that it came back.

Bye

Gaetan

atenolol50 4th March 2010 04:46 PM

ta da! fixed :D

did as told.
rollback to original - Signal input ground on the RCA input plug connected to signal input ground on the PCB.
Connected the star ground to the IEC earth with a 10R.

No more hum. (ok, just a wee bit ... need to put my ear on the driver to hear it)
there's hiss, but thats tube signature :note:

Professor smith 4th March 2010 10:46 PM

what speakers are you using? how is the sound?

atenolol50 5th March 2010 05:50 AM

Quote:

Originally Posted by atenolol50 (Post 2107051)
ta da! fixed :D

did as told.
rollback to original - Signal input ground on the RCA input plug connected to signal input ground on the PCB.
Connected the star ground to the IEC earth with a 10R.

No more hum. (ok, just a wee bit ... need to put my ear on the driver to hear it)
there's hiss, but thats tube signature :note:

How can I forget. Thanks Hugh and Gaetan for the help.


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