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

[Spencerleehorton]

Yes this seems to be a minefield!!!

Just got off the phone with a few companies who can make the cans , will see what price they come back with!!!!!

Sowters  cans measure at 0.8mm so this is a common thickness.

 

[metalb00b00]

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.

I have a different grounding scheme.

When using a toroidal power transformer, I connect the primary side wires to Live and Neutral tabs. I have not seen or encountered any which primary side also has an earth wire, yet.

Everything that must be earthed (metal chassis, connector metal body, etc.) are connected to the Earth tab.  I never use it for audio ground and digital ground, instead they all go to one point, which is the ground pin of the power supply output.
So far, no hum yet, and no 10 ohm 1W resistor was ever needed.

 

[Spencerleehorton]

mmm, so your saying try removing 10ohm resistor connection from 0v to chassis.

I will try this as this is how i had it before with no 10ohm.

i have noticed that the CH2 nearest to the psu is humming  and CH1 is not, so i might move the psu out of the box all together or try positioning everything further away.

My other alternative is my toroid has a slight hum to it i've noticed, so i might buy a few more toroidal's and re build the psu in all of them.

 

[Spencerleehorton]

Mu metal is so expensive!!!

Anyone have any DIY info on making good cheap shielding cans?

Ive heard that some food cans work?

and then there is the annealing it to bring back the magnetic properties? any ideas how this is done also?

 

[Spencerleehorton]

what current does each channel of 1290 pull?

 

[Ilya]

what current does each channel of 1290 pull?

This depends on your trimmer setting for 2N3055. But in general a single channel consumes 100-130mA.

 

[Spencerleehorton]

great stuff thats exactly what i had estimated, so for 2 channels i would need 24v 0.5A should cover everything and 4 channels 0.83A.

1. I'm going to change the toroid in my 2 channel A version and move everything around to try get rid of the noise on channel 2.
If its still noisy i'll try an external PSU.
2. on my other 2 channel B version it hums like  bastard!!! i think this one needs a PSU rebuild and i need to put it in a better case, so i'll order a new 1U rack and start again with this one.
3. My 4 channel version i think i need to move the PSU out of the box and then test.

something tells me with doing all this testing the Neve stuff is very susceptible to EMI and RF over other designs so much more care and attention is need to get good results.
I was too sloppy in putting these together.

 

[Ilya]

something tells me with doing all this testing the Neve stuff is very susceptible to EMI and RF over other designs so much more care and attention is need to get good results.

I'd say it's very sensitive to ground issues. My 312 build is much less problematic and has virtually no traces of PSU rectifed ripple (100 or 120 Hz hum). The 1290, on the contrary, is very sensitive to this. I often get strange things like increase of 100 Hz hum if using the same PSU for several channels. For example, if 1 channel has some -100 dBFS hum component, 2 channels will have around -90 dBFS hum component (in both channels), and 3 channels will have -80 dBFS hum component (all 3 channels). I have separate regulation and additional filtering on each channel, but it doesn't help much. various grounding bodges (like taking some points of the circuit directly to star GND point) may work, but often they make a single channel better at the expence of the others. I'm using  original cards layout and I blame it for ineffcient ground path routing. But I still need to check that theory. And yes, if you put PCBs close to the power transformer, the unit will hum as crazy.

 

[metalb00b00]

mmm, so your saying try removing 10ohm resistor connection from 0v to chassis.

Nope.

I said I never use the AC main earth connection for my circuit grounding.

 

[Spencerleehorton]

but you said

Everything that must be earthed (metal chassis, connector metal body, etc.) are connected to the Earth tab.  I never use it for audio ground and digital ground, instead they all go to one point, which is the ground pin of the power supply output.
So far, no hum yet, and no 10 ohm 1W resistor was ever needed.

so you didn't use a 10ohm resistor from that statement, thats all i was saying.

 

[Spencerleehorton]

i still think we should all get our heads together and design and build the perfect psu for these so there is no EMI or RF

 

[Spencerleehorton]

I just had a major break through!!!

on my 1290 A the ch2 which was noisy, i have loads of laminations left over from some testing, M6.
So i though what if i cut them up and put them on the outside of the 24v toroidal and on the top and bottom.
Put some tape round to hold them all on and hey presto, no more noise!!!!

both channels completely clean, only thing i hear is the natural hiss when i turn it up, or until the mic feeds back!!!

 

[buildafriend]

are you talking about multiple channels in the same enclosure with the toroid in the same enclosure? If yes, try wiring the toroid from far away and see what happens. If your issue persists disconnect the cards one at a time from the power supply and audio and see if it go's away in the channels that remain until you get down to one channel, then move over to the channel next to it just to make sure it's not the last channel you went to.

you need to isolate where it's coming from otherwise you're just trying to look for solutions to something that you don't know the location or source of

 

[MagnetoSound]

Strange. Why would the mains toroid be radiating EMF at 100Hz?

I just had a major break through!!!

on my 1290 A the ch2 which was noisy, i have loads of laminations left over from some testing, M6.
So i though what if i cut them up and put them on the outside of the 24v toroidal and on the top and bottom.
Put some tape round to hold them all on and hey presto, no more noise!!!!

both channels completely clean, only thing i hear is the natural hiss when i turn it up, or until the mic feeds back!!!

 

[Spencerleehorton]

I have read in a thread about Neve preamps to shield the toroidal, I know you can buy the tape which goes around the outside but I just thought to do it with some M6 lams.

 

[Ilya]

Toroid radiates 50-60Hz. 100-120 Hz is PSU ripple which is completely different. Shielding the toroid should give you less 50Hz component, but you claim that shielding reduced the 100Hz component which is ripple and propagates through wiring, not via inductive/magnetic coupling.
I agree with Magneto - this is very odd.

 

[Spencerleehorton]

Ok maybe I was just hearing the hum wrong? It could have been 60hz and probably was, but now it’s all gone!

 

[Ilya]

Ok maybe I was just hearing the hum wrong? It could have been 60hz and probably was, but now it’s all gone!

This is when a proper measurement equipment will be very useful. At least a good sound card with capable software will give out a lot of valuable info.

 

[ruffrecords]

This is input txf on the Neve, sorry to be unclear, I’m getting a few mumetal sheets and will build some cans for them, then potting with some semi hard resin, should sort the job.

So, just to be totally clear, you are using an unshielded mic input transformer. Eaxactly which transformer are you using?

Cheers

Ian

 

[ruffrecords]

something tells me with doing all this testing the Neve stuff is very susceptible to EMI and RF over other designs so much more care and attention is need to get good results.

This is not true. Neve designs used mu-metal shielded input transformers. Neve designs enclosed the entire module in a screened box. Neve designs always had an external power supply.

None of these factors is magic. They are all good engineering practice. All the really good audio manufacturers do these.

Cheers

Ian