Chinese Ear834 preamp trouble

[Bulletseever]

Hi all,

Brandnew to this forum. Ive been a phono listener for years and enthusiastic guitar pedal builder. So I thought let's try and build a tube preamp for my TT. And that's where trouble starts.

I've build a Chinese Ear834 clone, but I'm getting 50hz hum when playing. The preamp does work. My system is a naim nait xs2 with naim flatcap, they all get power from the same emi filtered power source. My TT is a Sony ps-x6. Normally I use a naim stageline n solid state preamp. But I thought it was fun to try and build a 12ax7 preamp based on the ear834. Ordered a pcb and transformer and put it together. When I engage the preamp I get a loud 50hz hum, it gets less when I detach my TT from the input, but is still present. I think it has something to do with a ground loop. Could you guys give me advise on what I'm doing wrong with my grounding scheme and build?

On the last supplied picture there are 4 numbers.
1: transformer to chassis and chassis to IEC ground
2: TT ground to chassis
3: pcb ground to chasis from 4 which is groundpoint on the PCB

PCB holders are non conductive plastic.
Input and output wires from Rca are shielded.

I'd really like to know your opinions and appriciate some help.


With regards.

 

[nerdorama]

I would suggest you should mount the pc board 180 degrees from the orientation shown. You have the power transformer at one end and the power supply at the other. This requires the heater and power connections to run under the board. The AC connections should be as short as possible and as far from the high gain audio circuits as possible. Just a first thought.

 

[Bulletseever]

Thanks nerdorama, will try this first. See where it gets me. What do you think of de the ground points?

 

[nerdorama]

Hard to say. I think I would want some isolation between the audio ground and chassis. You can try disconnecting the audio ground from chassis as an experiment. If that helps, you can install a ground loop breaker, i.e., a resistor in parallel with a capacitor and possibly some back to back diodes.

Earthing (Grounding) Your Hi-Fi - Tricks and Techniques

Earthing (Grounding) Your Hi-Fi. Detailed explanations of the safety and sonic aspects of proper earthing (grounding) techniques

www.sound-au.com

scroll down to the loop breaker discussion.

 

[rothwellaudio]

I think I would want some isolation between the audio ground and chassis.

Yes. Connecting the chassis to mains earth is a requirement for safety. The audio circuit's 0V does not need to be connected to the chassis at all, and doing so is likely to cause problems. However, connecting 0V to the chassis provides shielding, so that's why people do it.
Yes, read the link in the post above.

 

[Lavane]

Hard to say. I think I would want some isolation between the audio ground and chassis. You can try disconnecting the audio ground from chassis as an experiment. If that helps, you can install a ground loop breaker, i.e., a resistor in parallel with a capacitor and possibly some back to back diodes.

Earthing (Grounding) Your Hi-Fi - Tricks and Techniques

Earthing (Grounding) Your Hi-Fi. Detailed explanations of the safety and sonic aspects of proper earthing (grounding) techniques

www.sound-au.com

scroll down to the loop breaker discussion.

I agree. That was the first two things I noticed when I looked over the pictures in the OP's first post. The power supply is orientated at the wrong end of the chassis, and the RCA jacks aren't isolated from the chassis. Usually those RCA jacks come with plastic washers to isolate them from the chassis.

 

[Bulletseever]

Thnx all, I read the earthing post. It's very well written.

Will give an update over the next days, and do the following:
1 rotate pcb 180degrees and shorten power leads
2 remove pcb ground from chassis,, so no connection to ground from the pcb whatsoever right?

The RCA plugs are indeed isolated from the chassis by plastic washers, didn't send you a clear picture from the back.

Two more questions, if removing the pcb ground from the chassis works, should I install the ground loop breaker circuit or leave it disconnected. And If I should install the breaker, which kind of resistor, cap, and diode configuration do you recommend?

 

[Bulletseever]

So, the story continues. I rotated the board 180 degrees, cut the power cables as short as possible and disconnected the pcb ground from chassis. I must say the hum is much and much less, but still very much audible. The crazy thing is, when I only connect the right side of the preamp to my system it is dead quiet. When I plug in the left side, hum starts. It's still 50hz. When I disconnect the right side it doesn't change. Also when I connect the pcb to chassis hum gets worse. And when I put on the enclosure roof it also gets worse.

Any more advise? Thanks a lot you guys. You've been a great help so far.

Think we're getting there.

 

[rothwellaudio]

Are you doing this with something connected to the input? I would test for hum initially with shorting plugs in the inputs.

 

[Lavane]

Also would connecting the ground lugs with jumper wires on the RCA jacks help?

 

[Bulletseever]

I was testing it without something in the input and with my tt connected. If nothing is connected there is no hum.

Will jump the grounding lugs of the rca plugs

 

[rothwellaudio]

If nothing is connected there is no hum.

That suggests to me that the problems isn't the phonostage, it's the turntable.

 

[Bulletseever]

My thoughts exactly Rothwell. So I checked all connections to the cartridge,but everything seems to be fine. I also have no trouble when using my other solid state naim stageline phono preamp. So it's weird...

 

[Bulletseever]

So I feel like a complete idiot haha. Changed the cartridge on my tt and it was dead silent. Turns out it was a bad ground connection on the left channel of the cartridge holder.

So, what have I learned. Always check preamp first without input connected.

I've connected the pcb ground to chassis and there is absolutely no hum.

Didn't use a ground loop breaker, it isn't necessary.

Thanks all for the great advice.

Next week will start a new post on my Sony ps-x6 turntable. It doesn't want to auto return.

 

[nerdorama]

I think you still made some true improvements in your phono stage by rotating the pc board and improving the wiring layout. Glad you have it figured out.

 

[Hajidub]

I'm not a builder but have questions for the experts, though the OP has solved his questions. My Chi clone EAR has metal shielding between the P/S and the signal portion of the board. I can crank mine up to max with zero noise. Would the OP benefit from fitting one between the 2? Thanks.

 

[Lavane]

It can help if there is an issue with hum. But not always. There can be other reasons for hum that the shield won't fix, as in the OP's situation.