Problem with hum/buzz in Pearl Phono

[Buhl]

Just finished a Pearl Phono (sry, but no camera at the time....)
Ive read several places that the circuit made no noise, or very little - well, when turned up, i have some hum or buzz, that is a fair amount higher than the "hiss" (wich is very low...).

Could this be because the circuit is build in a wooden box ??
PSU is in a steel housing, 4 feet away....

Could it be grounding ? have fiddeled around with this, and have made a star ground with a speaker terminal, with grounds from input, output, and turntable meeting.....is this a good way to do it ??
Have arranged the psu wire and the audio wire like in Waynes article, but in pass gallery I se that some wire the C15/Gnd/jumper point to grounds of the input/output ???

Cheers !
Hans

 

[Ren]

I had some initial hum problems with the Pearl as well, but got it sorted out with some help from this forum... it was grounding.

What worked for me was to keep signal grounds, PS ground, and earth grounds ALL isolated from the chassis, except at one "star" ground point.

Turntable grounding to the phono pre seems to help in some cases and hurt in some cases. That might be the easiest thing to try first... just disconnect the tt ground wire with the preamp volume turned to minimum.

Ren

 

[bocka]

Hi Hans,

shielding is very important when dealing with low voltages. So a wooden enclosure is not a good solution. Use a metal one, connect the chassis from ONE point to ground. Avoid ground loops. This can be done with a star point. Examine where high current flows, mainly the power supply return path. Do not mix this ground with the signal ground.

 

[Buhl]

Thanks for the replys - I will shield the inside of the box with copper (or cobber ??) - and then ground the shielding to the rest of the grounds at just one point.

But - speaking of starpoint - when I make a starpoint, where both grounds from input and output sockets meet, along with the ground from the TT arm, I get more noise (possibly ground loop) then if I only connect the TT ground to just one(1) of the input phono grounds....???

Exactly what should be connected to the starpoint, and what should be left alone ??

Also, you talk about powersupply ground (return path) - should it be connected to the signal - no ! - rigth ??

Is it possible that all this will go away with a metal lined chassis ??

Oh, just another thing, that one of you very wise may be able to answer - I initially use a trafo with 30-0-30 - and then used the 0 for
AC 0 with two bridges, so I had 2 times 40 volt - but this makes havoc when connected to the RIAA - its like the voltages gets added up, so I measure over 70 volts on the voltage input on the RIAA board - after 2 minutes the trafo starts to hum noticeably - I now use the same phase and the zero on the trafo for both bridges in the PSU - but can anybody tell me what went wrong ???

Cheers !
Hans

 

[steenoe]

Oh, just another thing, that one of you very wise may be able to answer - I initially use a trafo with 30-0-30 - and then used the 0 for

Is that a centertapped trafo? If yes you cant have two independant supplies from that. The two secondaries will be out of phase, connected to the same 0v (or GND, whatever) You need two separate sets of secondaries for that.

Steen.

BTW, sometimes it is possible two separate the centertap and get two complete sets of secondaries.

 

[Buhl]

Is that a centertapped trafo? If yes you cant have two independant supplies from that. The two secondaries will be out of phase, connected to the same 0v (or GND, whatever) You need two separate sets of secondaries for that.

Steen.

BTW, sometimes it is possible two separate the centertap and get two complete sets of secondaries.

Hej Steen !
Yes, it is centertapped - if I understand what you mean..... it only has 3 secondary wires coming out of it, 30 volt, 0 volt and 30 volt - so I used the 0 volt on both bridges, and thus had two seperate supplys @ 40 volt DC after filtering. But, when connected to the RIAA, where the grounds meet - or the meet in the preamp, it makes a hell of a noise (buzz) - a monster ground loop i think ? and then I measure over 70 volt on each supply.......

Hans

 

[bocka]

Also, you talk about powersupply ground (return path) - should it be connected to the signal - no ! - rigth ??

Hi Hans,

think in systems. The Power Supply is one system, each side of the amp is a system and sometimes a lot more depending on your project. Every system creates noise and is noise sensitive. For example when you charge or discharge the storage caps in your power supply current through the caps and every device (diodes, trafo etc.) which is directly connected the the caps flows. Unfortunately copper wires have no 0 ohms impedance. When this current flows in the ground (return) path, a voltage is created and injected in your amp.

A star point is one solution to make this return path 0 ohms. This means, you have to connect every connection to one point (brute force method). Or better think and build in systems and connect the ground of every system (power supply, left and right ground of your amp, input ground, output ground, chassis) to one star point. This is the very simplified answer to get rid of the hum. And don't forget schielding: Connect the shield to ground AND earth.

Cheers

 

[Buhl]

Hum/Buzz - almost solved.....

I thank everybody answering this thread, I have learned a lot - and will be using the descriptions when building my own Pearl in a metal chassis. The first Pearl, that I have built i a wooden case, belongs to one of my friends, and I have still not found a better way to ground it, than to connect ground from the TT to one of the input phone ground as only grounding - all others are isolated. I have spent several hours trying everything, but this seems to make the least noise - mabye because it is a wooden case.....

By the way - my friend, who is very much in to analog sound and turntables says that the Pearl is perhabs THE best RIAA he has ever owend, and prefers it over the Ono - this beeing more "relaxed" and still VERY open and detailed, especially in the upper middlerange. The stereo perspective he says, is very good, with a definition of higth that is rarely heard. The Ono did how ever make less noise - but again, this is a wooden cabinet.

Cheers !

Hans

 

[steenoe]

a monster ground loop i think ?

Nope, thats the trafo trying to make two +40 volts with the same GND. You will propably have a trafo "melt down" if you let it run for some time Regarding the hum, I still think it is possible to have a wooden case. Try to connect the GND from the turntable to the GND of the PSU's. Not in the PSU case, but where the grounds meet at the Pearl boards! There have to be a way out of that problem. Can you draw a schematic of what you actually have done? I have a separate GND connection for the arm, running out from the starground. No hum whatsoever. It's not connected to the chassis or the RCA's.

Steen

 

[Buhl]

Hej Steen
Here is a block layout of what I have done. You say connect the TT ground wire to where the ground wire from the PSU enters the board, you mean where the wire is soldered to the PCB ? - should this be done on just one of the boards ?

I hope you can use the block layout......


1000 tak for hjælpen !

/Hans

 

[steenoe]

Try to connect the TT GND wire to GND(0V) at the XLR connection.
Just connect it to one of them at first. If that doesn't do it, try both. If that still doesn't help, you might want to try a real Stargrounding. Somebody posted a drawing some time ago that was very good. I will try and find it.

Steen.

I found it within the edit time It was jh6you that made the drawing. It kind of makes it very clear, how the starground is made!
starground

 

[Panelhead]

Metal Lined Wooden Case

I used a metal lined metal box for mine. It is extremely quiet even with an added input gain stage to do mc cartridges.
The metal used is mu-metal. It may not shield as well as copper, but is real stiff and heavy.
Your noise is most likely a transformer issue. You need a dual secondary transformer to do the supplies. The center tapped, is okay for a single dual rail. But not good as two single rail supplies.
With the low current draw of the Pearl it could be wired as a single supply using half the output. But the best would be to get a dual secondary transformer.


George

 

[andyr]

Avoid ground loops. This can be done with a star point. Examine where high current flows, mainly the power supply return path. Do not mix this ground with the signal ground.

What do you define as the "power supply return path"?

AIUI, 2 wires go from each PS to each Pearl PCB (in a dual-mono setup) - the Gnd and the +ve DC rail.

Andy