buzzing problems

[specter73]

Hi everyone,

I built a mosfet integrated power amplifier but I have a buzzing problem and I'm wondering if anyone can help me to fix it.
I used a toroidal transformer for the power supply and an adequate levelling capacity,so it doesn't depend on this.

I'm not able to find the cause of the buzzing except that the inputs are fairly close to the transformer for space reasons.

How can I fix this?
Is there anyone that had the same problem and fixed it successfully?

Shielded cables?
metal case for transformer or inputs?
separating the negative ground of the inputs from the rest?

Thanks for any helpful hint

Sincerely

[AndrewT]

Hi,
check the ground from input RCA to PCB.
Chassis safety earth to mains earth.
Transformer centre tap to smoothing cap common.
Smoothing cap common to central star ground.
Speaker return and Thiel network (Zobel) to Central star ground (CSG).
PCB decoupling to CSG.
PCB signal ground and global NFB ground to CSG.
Mains earth to CSG or Mains earth to input RCA ground. You may need to try either to find a quiet option.
DO NOT make the transformer centre tap your CSG.
DO NOT make the smoothing cap common your CSG.

[wintermute]

Hi Specter,

How loud is the buzzing?? need to be close to the speakers to hear it?? is it only in the bass, or in the mid/high freq too?

I've had a buzzing problem with my mosfet amp for years (can hear it in the woofer, mid and tweeter of my 3 ways), never completely gotten rid of it, but a lead shield over the torroidal made a big difference. If you can temporarily move the transformer outside of the case it should give you an idea if the problem is the transformer radiating noise or not.

edit: also what part of the circuit is directly above the torroidal (if any)?

Tony.

[specter73]

Hi guys,

Thanks for your kind replies.

Answering to AndrewT:

I have some problem understanding your abbrevations:
Could you please explain me what's PCB? And CSG? And NFB?
And what do you mean with "centre tap" and "cap common"?
And "central star ground"?
Thanks for your kind clarifications.

Answering to Wintermute:

I thought too to move the transformer checking the difference and I'll do it,but anyway I need to fix the problem since the amplifiers was born in this way and a different placement of the transformer means to remake it completely starting from the scratch!!!
As regard to your question, just the preamplifier/input selector board is beside the transformer, fairly close.

Thanks and regards

[sss]

maybe your bias current is too high

[AndrewT]

Hi Specter,
PCB = printed circuit board
NFB = negative feedback
Centre tap is the transformer secondary connection that is common to both output windings i.e. 25 - 0 - 25, the 0 tapping is the centre tap. In a transformer with separate windings 0 - 25 and 0 - 25. one 0 and one 25 can be connected together to form a centre tap.
Cap common is the common connection between two series connected capacitors. In your Power Supply Unit (PSU) you have a capacitor for the positive supply rail and a second capacitor for the negative supply rail. The common connection becomes your feed to the CSG.
When on writes word (W) then that means that in all following text then the abreviation W will stand for word, so when I write Central Star Ground (CSG) then in all text CSG means Central Star Ground which in turn means the central location where all the ground wires meet to give a common voltage reference for each of the circuits fed from each ground wire.

[specter73]

AndrewT

Thanks for your explaination .
I'll verify the connections,and if the case I'll follow your indications .

Thanks a lot, for the moment.I'll let you know.

Regards

[sagarverma]

plz see the i/p supply decoupling caps.they should b as close as possible to the pwr i/p of the amp.
i had the same prob with all my amps,but this small attention has minimised the hmmmm so much so that its audible only when i put my head into the speakers.

[specter73]

<<<plz see the i/p supply decoupling caps.they should b as close as possible to the pwr i/p of the amp.>>>

What do you mean?Close to the transformer?

[sagarverma]

nope.
place 100nf caps at that point on the pcb where the dc power supply just enters the components(power transistors,opamps etc)
for feeble noise,use 10nf ,for hoarse noise,use 100nf.i.e shriekier the noise smaller the cap
one leg of cap on power line and other to ground with each power line. or one leg on one line and other leg on other line(in case of dual power line i.e +-15v etc)