RFI and shielding help needed

Hi all,

I'm posting in SS hope this is the right venue.

I have made a phonostage - a solid state 2 stage one, some of you may know it, its the phonoclone. Link to more info:

http://www.diyaudio.com/forums/showthread.php?s=&threadid=57398&perpage=25&pagenumber=1


Its a superb performer - but a few of us have noted that its particularly sensitive to RFI problems - and at least in my case, it is the interconnect from the phonoclone to the amp/preamp that is acting as an antenna. Careful routing of the interconnect is needed to minimise the RFI. If you go to the last few posts of that thread you will see one member having big problems with it.

So I'm wondering, what tricks can be used to lessen this type of interference (interconnect is shielded)?

Can you do something like put a cap across the outputs or something?

Balanced is not an option here!


Fran
 
Going back to the decades before CD's, RFI and other stuff getting into the phono input was perhaps the single most common audiophile problem next to surface noise. At least it was mine. Here are few things I recall:

Some interconnects were better than others. This had no correspondence to price. I think (but I'm not sure) that foil shoelding worked better than woven strands since when bent, strands can open up a gap.

Re-routing the interconnects can help sometimes. May just a an inch, sometimes several feet. Unless you have expensive instruments, this is a trail-and-error task.

Depending on construction of the component at each end, the shield may or may not actually be connected to earth rather than just internal 0-volt reference. I'm not sure there is a fixed rule about which is the right way plus what kills the RFI may give you ground loop buzz in exchange.

Sometimes no reasonable solution existed, especially when the neighbor was a CB radio fan.
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Hi, If R.F. is getting into the input it might be worth trying a low value resistor of say 33ohms in series with the inverting input of first op-amp together with a small ferrite bead around the leg of the resistor. Try something like this for starters. If it works try replacing the resistor with a wire link+bead. The bead should be as near the op-amp input as possible. This has the effect of increasing the impedance to high frequencys.
EDIT , I have just looked at the photo in your link, if constructed like this just thread the signal lead, the one from the centre pin of the phono socket inputs through a bead, one for each channel.
Regards Karl
 
Very high gain amplifiers -- here are some hints -- but you might want to take a look at the excellent tutorials on grounding on both the Analog Devices and Texas Instruments websites.

1) bypass the power supply pins of the opamps with 100nF ceramic caps.
2) a ground strap between the chasis of the phono amp to preamplifier, as well as the usual ground strap between phono preamplifier and turntable.
3) are the IC's socketed? if you are using a tin plated sockets replace them with ones using gold plating -- they don't cost a lot more but are worth it in this application. You can also get snappable machined sockets -- these would have been used in the high quality instrumentation from Hewlett Packard, Boonton and others back in the 1980's.
 
Which cable do you use to connect RIAA to the preamp and with what kind of shielding?
Balanced (cable, not connection) might be an option here, with directional soldering. You can use + and - leads for the signal and solder the shield at only one end. Normaly, that end should be on the source side, but you can actually reverse it and decide when you get less interference. The third option is to solder the shield on both ends.
 
Thanks again.

jackinnj:

I will check but I think the PS into the opamps is already bypassed.

I don't have grounding connection to the preamp - I will try this, but the case/circuit is grounded to earth

Gold sockets are used.

I think the opamps are either OP27 or 37 - would need to double check but I think either will work.

Willi Studer:

Copper braid on the interconnect - one end connected to ground. I do have some cable that is 3 core with foil shield and I could try that - although it sounds poor. I have also tried a number of cables, both homemade adn commercial and they make very little difference.

Thanks for the help so far,

Fran
 
You need to keep RF out of the box. I'll assume you have a metal box. If you seal the doors of a house in a flood, you stand a chance, but once the water comes in, all the bailing in the world won't help. What you need is to address RF right at the connectors. Small caps from the signal lines right to the chassis. The input signal is mostly current at very low voltage, perfect for a ferrite bead. I'd say put the ferrite bead right near the connector, just after the cap to the chassis. You want to create a filter right at the connector, not let the RF get any distance into the box on the wiring. Do something similar on the output. Remember that the usual phono input will have an isolated ground, brought back to an internal ground point. If this is the case, you have another RF leakage problem. Put the same cap to chassis followed by ferrite bead on the ground. Pick up a copy of Ott's noise book if you can find it, hopefully used, as new books are getting out of sight.
 

tube 300

Member
2005-08-10 9:57 pm
Hello i am the other person with noise problems
i hear voices :)
and also hum that is more or less when i move or touch the cable or sometimes come close to the cable ore case.
It is the same amp as Fran build and i have only problems with the input side
and i tried every ground and earth connection i think.
it is written in the last posts of this phonoclone toppic.

on this pages are some pics of the case i build :
http://www.diyaudio.com/forums/showthread.php?s=&threadid=57398&perpage=25&pagenumber=32

Thanks a lot so far
Robert
 
woodturner-fran said:
Hi ... and at least in my case, it is the interconnect from the phonoclone to the amp/preamp that is acting as an antenna...
Fran

Well, there have been many good ideas and suggestions :D My first thought though was: What makes you think your problem is with those interconnects? And when you say you have an antena? What indicators are you aware of that tell you, you have an antena? :xeye:
 
Why I think its with those interconnects is that the noise lessens/increases as I move them around. Moving the tonearm cable (inputs) around makes no difference. I( called it an antenna cos it reminded me of adjusting "cats ears" to get a decent picture on an old TV.

In saying that my problem is hum - whereas tube 300's problem is that he is picking up radio stuff.

If I get a chance tomorrow night i will try the caps on the inputs as suggested above. I have some 330pF ones that might suit OK.


Fran
 
Start by taking a look at the output with the oscilloscope. If you see a sine like component well above 20Khz, then some op-amp is oscillating. If you see a "wild" signal with components well above 20Khz, the circuit is picking up RF. If you see clean 50/60Hz, the circuit is picking up one type of humm. If you see noisy 100/120Hz then it's picking up another type of humm...

Plenty of things can go wrong...
 

Onvinyl

Member
2002-08-02 10:41 am
Germany
Hi,
the phonoclone is indeed picky with/at RF!

With both thin twisted copper wire, and cardas wire, both not shielded, I had relativly loud hum, almost unacceptable.

With vintage Tonearm cable from SAEC, fitted on a SAEC arm, I detected an internal wiring problem with one channel: while one channel is superquiet now, (almost unhearable), the other channel is clearly audible (but not as severe as with the unshielded cables)

Rüdiger
 

tube 300

Member
2005-08-10 9:57 pm
Hello i tried today a capacitor on the input of the OP27
the value is printed n27 (not 27n) is this 270pf?
The radio is totaly gone and the cable dont react on moving or touching this is a lot better so far.
Hum is not so loud as before but also to much i can hear it trough the music when i play a record.
But i did a giant step forward today thanks for the capacitor advise.
And Thanks Fran for asking it here

Have anyone some advise for lowering the hum ?
i wired the amplifier as the second diagram
case to input ground
Robert

[IMGDEAD]http://i25.tinypic.com/rsapg0.gif[/IMGDEAD]
 

tube 300

Member
2005-08-10 9:57 pm
hi Fran
Itried already a few things.
When i disconnect 1 rca ground the hum get much lower
and on listening level allmost quiet.
When i disconnect both the hum is back and also toching the case is hearing then.
I tried also the above com to case diagram and that is how i play now the hum is the same level as 1 rca connected
and i can a little move and turn the Phonoclone case for lowest hum.
moving the case have not much influence but a can lower the hum a fraction.
At the moment i have the quiest hum level i ever have with the Phonoclone
i think it is 10 times lower than before
and made giant steps today.
On normal listening level i can play records now with allmost no hum
:happy1:
When i play i little harder the hum is still a little to much i can hear it when the music stops .
Robert
 
Jackinnj: I actually have 2 of these things (!) - one is in a metal case and the other in a wooden case. PS is separate from both. Both behave the same. FWIW, I added in the caps tonight on one of them and got the same results as tube 300. I don't have the same hum as tube300 though, probably partly because my gain is lower (cart output higher). It looks like this is a good solution and a mod well worth doing.

Many sincere thanks to all here who helped :c_flag: :up:

The way I did it:

Get a spare socket and solder the cap to that, put that into the one on the PCB and then sit the opamp into the new socket. Easy and removable and no danger of damage to the existing PCB.


When tube300 gets his hum sorted out I will reference this thread back to the main phonoclone thread in analogue.

Fran


tube300 - good to hear that! Keep going and you will get the hum down to v small levels. I know my carts are higher output than yours, but hum isn't realy an issue for me anymore. Only really audible when I turn the preamp up much higher than normal.