Neve 1290 Hiss HELP!

Newbie here.... First of all I really appreciate all of the expertise and support of this forum.  Second, I couldn't find a master 1290 thread, so I am posting this here.  I have built two Neve 1290 channels based on Martin's amazing guide.  I am using the JLM AC/DC PSU built outside of the 1U case I built the 1290s in.  I am getting hiss starting at the 4th position on the gain switch.  When both gain switches are at max, I get an even worse hiss.  I have checked everything Martin suggested: voltages, solder joints, resistor values, transformer wiring.  I am using all shielded cable.  Here is my first question.  Should pin1 of my input XLR be connected to chassis ground, therefore grounding pin6 and 10 of the input transformer?  Anything else I can look at?

Any help is appreciated!  I have been troubleshooting this guy for weeks now.... 

Thanks,
Brian

Do you have any way of measuring your signal to noise? All electronic circuits have a noise level, in this age of digital people seem to think noise doesn't exist, it does.

What have you got connected to the input? What have you got connected to the output?

Cheers

Ian

Here's the thread for this the 1290:

http://www.groupdiy.com/index.php?topic=22828.0

cheers

I will try and get an accurate signal to noise measurment, thats a good idea.  As for what I have connected to the input/output

input XLR, PIN1 - Shield(connected to chassis through mounting screws)
                PIN2 - Pole 2 on input trans
                PIN3 - Pole 5 on input trans

output XLR, PIN1 - not connected(not connected to chassis ground either)
                PIN2 - Pole 5 on output trans
                PIN3 - Pole 8 on output trans

bjknapp said:

input XLR, PIN1 - Shield(connected to chassis through mounting screws)
             

Click to expand...

Try it without terminating pin one to chassis.  Audio should terminate ONLY through a 10Ohm resistor which I believe is on the JLM PCB.

I have try not grounding input pin1 to chassis and then I start picking up radio stations, so that's not good. 

How is your pcb mounted to the chassis?

Double check that all pieces of your chassis make good electrical continuity between themselves as well as between all of the XLR connectors.

I've found (and it's stressed over and again in the dedicated help thread) for best shielding performance do not tie audio ground to the chassis via XLR connectors or elsewhere.

bjknapp said:

I will try and get an accurate signal to noise measurment, thats a good idea.  As for what I have connected to the input/output

input XLR, PIN1 - Shield(connected to chassis through mounting screws)
                PIN2 - Pole 2 on input trans
                PIN3 - Pole 5 on input trans

output XLR, PIN1 - not connected(not connected to chassis ground either)
                PIN2 - Pole 5 on output trans
                PIN3 - Pole 8 on output trans

Click to expand...

Sorry, what I meant was what have you got plugged into these two connectors?

Cheers

ian

MicDaddy - My PCB is mounted on nylon standoffs.  I have made sure that all of my panels have good electrical continuity, it took some scraping of paint!  It is the 1U version of the aluminum chassis Martin suggested in his BOM.  I will double check my XLRs and make sure they are getting good connection to the chassis.  When I disconnect XLR PIN1 from chassis, I pickup a radio station....

Ian - I am connecting an SM7B on the input XLR and an XLR/TRS cable to my DAW on the output.

bjknapp said:

When I disconnect XLR PIN1 from chassis, I pickup a radio station....

Click to expand...

For me this would suggest something isn't wired correctly or there is some other underlying issue. 
Post details/pictures in the dedicated help thread so more folks will see it.

bjknapp said:

input XLR, PIN1 - Shield(connected to chassis through mounting screws)

Click to expand...

should pin 1 of input xlr be connected to pin 6 of input transformer?

short the input; does the noise go away

It was my understanding that the drain wire from the mic cable in 1290ez goes from input pin 1 to input transformer pole 6, then connects to pole ten, then to 1290 pcb (labeled 10468 SHLD/10).  I honestly just lego-ed it together with the great instructions and am trying to wrap my head around the grounding of it too! ;D

OK, so I have found that one of my input XLRs was not grounding to the chassis quite right.  That got rid of the "radio station" interference.  With that solved AND having disconnected the input XLR pin1 from chassis ground, it's MUCH quieter now.  I also do not get the horrid "super hiss" when both channels are at max gain.  I would like to measure the signal to noise ratio, and see if I'm now in the ball park.  There is still SOME hiss. (I know Neve's are know for that though)  Do any of you have a good way or a link for me to measure signal to noise ratio?

I really appreciate all of the help from you guys.  Sometimes I feel totally lost and frustrated with troubleshooting, it's nice to bounce things off of this forum!

THANK YOU!
Brian

bjknapp said:

Ian - I am connecting an SM7B on the input XLR and an XLR/TRS cable to my DAW on the output.

Click to expand...

So when you talk into the mic can you hear it OK on the DAW?

Cheers

ian

Great News!!  bjknapp  ;D

I too had issues (my own fault) where the XLRs were not making contact with the chassis, which caused both 60Hz hum when touching a microphone as well as oscillations at higher gains.  My first attempt to 'remedy' was to tie audio ground to the chassis.  Tthough this did clean up the oscillations at higher gains, I still noticed a slight hum when touching an XLR or a mic, as well as static intermittent noise when adjusting gain/trim or other switches on the front panel.  After spending some time with the DMM, checking for solid continuity, I found that there was none between the rear panel XLRS.  Some paint scraping and reinstalled the XLR and the pres are my favorite by far.

There is some 'noise floor' but I wouldn't even call this hiss, it's only noticeable with nothing on the input and the gain taken waaaaaay up.  Still, with this 'Neve Haze' these pres are much 'quieter' than any other in my rack.