Neve 1290 gnd wiring, Whatever i do i'm getting hum!!!!

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Spencerleehorton

Well-known member
Joined
May 12, 2012
Messages
4,032
Location
Felixstowe, Suffolk, UK
Hi All,

Me again!!!! i have 8 Neve preamps that i have always thought worked fine, but as i have been using the API's and nothing else.
Now i've gone back and tested all my Neve 1290's and they all hum?
I have done the usual trawl through the treads and fine a post which seem to specify exactly how to wire the 1290 and have spotted a few mistakes i have made.
I have wired them back up and still getting hum?
my IEC gnd tab is connected and checked to case, i have also checked when i put the top on that this is also becoming part of the circuit.
my toroidal is then connected to IEC and then to an adapted SSL 9k PSU pcb delivering 24v and 48v, i have tested and i'm getting both voltages fine. check!!!
from the other information i have seen and checking the schematic i have connected pin 1 from XLR input to GND on pcb this then connects through 10ohm (i only had a 12ohm) resistor 1W to the IEC gnd connection.
pin 1 on output XLR is disconnected, and the tabs which sometimes are on the XLR sockets are not connected to pin 1.

I have checked and when i remove gnd from PSU pcb and measure from input pin 1 there is no connection to IEC GND.
I have also checked that i am getting 12ohm from input pin1 to IEC GND.

there is still hum? any ideas?

thanks in advance.

regards

Spence.
 
Are you sure your ground hum is not resultant from a loop in the hook-up to the rest of the system?  Try lifting the mechanical ground and see what happens.
 
when i use the same channel the same leads but use my API preamps its clean as a whistle, and all neve's have the same sound?
They are all wired the same way i know, but there must be something i'm missing?
 
>from the other information i have seen and checking the schematic i have connected pin 1 from XLR input to GND on pcb this then connects through 10ohm (i only had a 12ohm) resistor 1W to the IEC gnd connection.

This might be your problem. PCB ground needs to go back directly to 0v at the rectifier filter caps, particularly if you are applying phantom at the mic inputs. You don't want the 10-ohms separating circuit  current from GND. The 0v rail should be referenced to chassis ground separately, and if you have phantom power it should be connected to chassis close to the XLRs.

 
so by my understanding of it, from C7 or C8 or a combined value of 4400uf, from this cap i need to feed a 0v to the PSU and a connection to the IEC star gnd but have a 10ohm/1W, is using a 12ohm/1W ok?

Im not understanding what you mean about the 48v to chassis magneto?

Ive disconnected the 48v and i still have hum, and its in the 120hz region.

I should also state that im using 2 x EZ1290 pcbs and the other 6 i have made replica's of the original PCB, all units make the same hum!!!!???
 
Spencerleehorton said:
sorry magneto i'm not understanding what you have said, maybe i'm having a bad day!!!
Not sure I understood either but the way I would say it is that XLR input pin 1 should be connected to the chassis as close as possible to where it is mounted. That ground also needs to connect to the 48V filter cap (because the 48V return will be through the chassis) and the screens of the transformers but otherwise that's it. The mic pre circuitry should have it's own separate ground running back to the PS filter cap ground point [1]. I'm not sure if that's how it was actually done in a real console (hard to tell from looking at those old modular schems) but that's how I would do it. Actually I have done it that way in my Neve channel strip it's pretty quiet [2]. Modern XLR connectors often have a metal spike builtin to one of the screw holes that stabs the panel connecting pin 1 directly to the panel. Of course, then the panel has to be ultimately connected to earth ground which is connected to PS ground (not always a given with anondized aluminum enclosures which can develop resistance from corrosion over time). There should definitely be no resistance in those path. If there is and you have 48V, you could definitely get some serious hum from that. Anyways, I think magneto's point was that if you have 12 ohms in that path, that could easily explain the hum. Use a DMM to measure resistance between pin 1 of input and earth ground (with the unit powered off). It should be less than 0.2 ohms maybe.

More generally, as another poster pointed out, frequently noise problems are an artifect of some other piece of gear. So first, unplug *everything* that you possibly can and just hook up something to terminate the input with 150 ohms and a monitory maybe and maybe even supply power through the same power strip. That should be super quiet. If it's not then there's something wrong with the unit. If not, start to reconnect and generally divide and conquer.

[1] And if you want to be fancy about it, you might have a dedicated "quiet ground" for the inputs of just the gain stages and then use a regular ground for the PS bypass caps of the gain stages and everything else. And if we extrapolate that concept even further, you might have a dedicated ground for the return of the line driver. More specifically, the ground end of the 47 ohm should have it's own wire back to the PS filter cap ground. But some folks might think this is a little pedantic.

[2] My API channel strip is a few dB quieter. My Neve channel strip has a little bump in the noise floor but in the kHz range that I think might be because I accidentally reversed voltage on the gain circuit boards. I've been meaning to replace them and see but  it's still pretty quiet. I actually record with the gain set at 80dB and then just back away from the mic until the level is good. Probably an amateur thing to do but it sounds pretty good to me. The standard Neve pre can be very quiet at least if it is wired as described above in my case.
 
have just reconnected the tabs on input XLR to pin 1 and measured resistance, 0.5 to 0.6ohms?

i don't have a 48v filter cap and i cant remember where about's i would put it?

will do some digging!!
 
Spencerleehorton said:
48v filter cap?

i'm not seeing a filter cap for the 48v on the EZ1290 schematic?

i'll connected XLR pin 1 to chassis and see what difference it makes
I'm not familiar with the EZ1290 (I just look at schematics and do my own thing) but they probably put the 48V filter cap  on the PS board. It's slightly dubious that it has to go through probably a relatively long wire (that goes near the PS) and then return all through the chassis. But I don't think it would matter much because it should be quite low impedance.

Connecting XLR input pin 1 to the chassis is actually for shunting RF which would have nothing to do with low frequency hum. But if you have 10 ohms between pin 1 and earth ground and the mic you're using is 48V powered, that would definitely be a problem. But you just said you turned off 48V and still heard hum so that's not it (but measure that anyway - the hardest problems are usually multiple problems that mask each other so just fix anything you find wrong).

Usually when you're trying to track down noise like this, you try grounding signal inputs of each stage (where there is no DC of course!). If the noise goes away, you know the cause is upstream from that point. So try grounding the input of the output driver (again, the upstream end of the coupling cap because the downstream end has DC!). If that makes it quiet, try grounding the input of the second gain stage and so on working your way all the way out the end of the mic cable.

One final thing that is worth noting is that the Neve has 80dB of gain which is quite a lot. So if you just crank it you're going to hear noise that could very easily be characterized as hum. You would have to put some signal in, signing into it or playing guitar or whatever to get an idea of how hot the signal is. Obviously if you crank it to 80dB but the guitar is very hot, then you can just turn the gain down a little bit in which case you won't hear the hum. There's always hum. It's just a matter of how much hotter the content is.
 
Ah ok, yes, the 48v filter cap connects from the two 6.8k resistors and goes to pin 1 XLR and is 220uf 63v?

i found my API sheets and its on there but not on the EZ1290 info?

The hum i'm getting is audible after a couple of clicks of the gain
 
according to the EZ1290 guide it is saying there should be 3 connections to chassis (case) 1. IEC GND 2. PSU CAPS NEG to case 3. Audio GND through 10ohm to case.

is this my problem? my connection from IEC is going through 12ohm to audio GND?
 
i've now got 0.5ohm when i touch my MM end together and when i test from pin1 XLR to IEC gnd i get the same 0.5ohm.

its more of a buzz now rather than a hum, its there with or without the 48v?

I feeling  like iv'e tried everything but i'm sure i've missed something?

why wont they play ball!!!!
 
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