Pearl Two

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Shielded Cable in Pearl II

Hi, I searched before posting. Didn't find an answer in 10 returned posts.

I'd like to know, when using a shielded cable inside the Pearl II, where does the shield terminate at the input and output? Trying to avoid noise and hum.

I was getting hum with an unshielded wires, so at the output, I installed a shielded mic cable and connected the shield at the RCA output, not the board side.

First, is this correct? It works, but doesn't mean it's correct.

I'd like to add same type of cable to input side. Where would be the best place to ground the shield, RCA input side or input on the board?

I found some thin studio grade Mogami balanced wire with shield and would like to out it to good use. :)

Thanks!

Vince
 
What do you mean by "bad sound"? Can you adjust P1 for 0 volts on the test pad?

Hi Kevin

The bad sound is a combination of distortion, buzzing and low volume of the actual music. I suspect that the earthing arrangment is also a contributing factor but the difference in voltage at R16 of 12V when it should be 0V must be contributing also.
Which test pad are you reffering to Pad3? if so this is 0V.

On the subject of earthing my pre amp is in the same case as the PSU however its a 2U rack case and they are at opposit ends of the case. I have tried earthing the pre amp direct to mains earth and also through the 35V bridge rectifier as suggested. Both methods dont remove the buzzing/distortion. What method of earthing would you suggest when the PSU is in the same case as the pre amps?

Kind Regards
Dominic
 
If you can't adjust all the high voltage away with the pot, and you probably won't be able to, replace the ZVP3310.

Be extremely careful in handling, they are very static sensitive and easy to zap. It's not unusual at all to have killed one in construction, about half of the people building the Pearl 2 have done so. :)

Ok thanks 6L6 i shall give that a shot tonight when i get home from work.

Kind Regards
Dominic
 
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Bring the shields to the main ground lug and see if it quiets everything down. It should.

6L6,

It's dead quiet right now, I just don't want to introduce noise by shielding it incorrectly.

Do you mean the lugs on all the RCAs? In other words, no shield connection on the board side?

Thanks,

Vince

Just realized you may not have been talking to me...
 

6L6

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Vince, yes, I was talking to you... :)

The logical place (well, in my mind anyway...) for the shield is the ground lug of the chassis, not the RCA. The PCB center ground is there, along with the chassis and the PSU zero point. So in theroy at least, that should be the 0 of the the potentials, and a good place to attach the shields.

However -- If everything is quiet now, don't mess with it. For example my Pearl 1 has shielded twisted pair for the signal with the shield not connected to anything, and it's silent. (It's also in a steel chassis...)

My biggest rule of DIY is once a project is hum free, it's done. Don't mess with it anymore, you might make things worse, and just enjoy!
 
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Ok thanks 6L6 i shall give that a shot tonight when i get home from work.

Kind Regards
Dominic

I replaced the ZVP and it didn't make any difference at first. I connected the pre amps up to the amp and turned P1 until it made the least noise possible. At least I can here the music now but there is still a lot of buzzing and I can tell some of the music is distorted maybe clipping. I will try various earthing arrangements to see if that eliminates the buzzing.

Kind Regards
Dominic
 
Dominic,

I left R20, C7, C15, and C22 out.
I jumped R15.
This seems to be the recommended set-up.

Kevin

I have the following;
R20 - Empty
C7 - 10pF
C15 - Empty
C22 - 100pF
R15 - Jumper

I will try it with those caps removed.

FYI I designed my own PCB layout and etched it myself so I don't have test pads.

I'm currently reading up on earthing/grounding to see if I can rearrange my setup better. I will post some pictures soon.

Kind Regards
Dominic
 
Dominic,

C7 is the one people were having problems with (causing oscillation). If you search this thread using the characters C7 you'll see what I mean.

Leave C22 in.

Kevin

I removed C7 and it has made a difference but there is still alot of buzzing etc so after having a good read up on earthing im going to try a few things tonight and will let you know how i get on. Fingers crossed :p

Kind Regards
Dominic
 

Thanks 6L6. The only problem is my PSU is in the same case as the preamp. I have now tried various methods of grounding and it hasn't made any difference. I'm now thinking that the preamp is oscillating. Below is an export of my oscilloscope with nothing connected to the input or output. I think I'm going to just have to replace parts of the circuit bit by bit. Process of elimination :h_ache:





 
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ZTX450 Oscillation

The ZTX450 that I was using as a constant voltage source for a BF862 JFET buffer oscillated until I put in a 270 ohm base resistor. Q10 & Q11 have 220 ohm resistors but the super critical constant voltage ZTX450 for Q3 has none. I suggest trying a base resistor on Q3.

I also installed a TL431 voltage regulator operating at 3ma with a voltage regulating pot to get the exact voltage/voltage drop I was looking for to the ZTX450 base on my buffer. The ZTX450 draws enough current to change the expected voltage from a simple resistor divider voltage source.