Naim NAP90 - loud hum when no input connected

gilwe

Member
2007-11-07 3:49 pm
Hi all !

My NAP90 sounds great (once fixed) but I have a small issue. I noticed that if I connect the DIN-4 input cable to it, and no input source is present (i.e the cable is disconnected on the other, RCA side) a very loud buzz/hum is heard. I wonder why it happens and how can I work-around it ? Is it true that both input left and right terminals should be shorted (i.e hot should be connected to ground) when no input is present ? I currently connect it to an input/output passive selector and when the output switch is set to off I get this loud hum/buzz noise... Thanks for any advice !
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
Hi
Without connection to a suitably grounded impedance, like the amplifier is designed for, the input leads are prone to detecting RF, hum and noise from whatever source they are near. They are an "aerial" for recieved EMR from anywhere. However, amplifier input sensitivity is relatively low and not usually so sensitive in modern equipment with unconnected RCA leads fitted.

It's likely that the passive selector switch device has no or a poor signal grounding connection that suits Naim wiring internally or the lead itself is incorrectly terminated, so the input to the amp will have an open circuit ground connection. This is a problem.

Remove the input leads from the amp and check whether this is the problem or it is within the amplifier. In the latter case, it will then be an amplifier input grounding issue that you may like to check on a Naim forum or at least Google some pics on the "net" to find the correct arrangement, if it has been tampered with in the past.
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
When I take off the cable the amp is quiet. Should the ground lead of the 4 din cable connector should be connected to the sleeve of the connector ?
Well, this shows that the problem is with the lead connections. It may be easier to sort out.

I admit that I don't know the specific pin connections anymore. Mine is converted to RCA connectors but original Naim connections should be visible from the wiring inside the NAP90.

Certainly, the shield braid around the cable should be connected to ground, usually chassis ground, leaving the other 4 wire connections to L+ and L ground, R+ and R ground - those grounds being separate from chassis or safety ground.

I have some vague memories about Naim feeding 24V DC on one wire to power the preamp so the signal grounds were a single wire, common connection but that is from even longer ago - I'm not even sure of the model no. :eek: , That system was also adopted (without the DC power) by other early UK manufacturers like Quad.

However, assuming you have an adaptor lead to connect to RCA outputs, there will be no shield braid connection at the RCA end, though it shouldn't be a problem as the signal grounds should still be correct, even if floating. See that you do have a ground connection of the shield at the DIN (amplifier) end, though. An obvious question now; didn't this hum and noise problem always exist or are you now using a different cable/set-up to previous?

There should not be an unconnected lead plugged into the powered amplifier. A preamp or such should be connected whenever the amplifier is powered so the issue of grounding the inputs shouldn't arise. If you are experimenting, grounding or placing a resistor of 1-10k across the inputs could be a wise precaution against nasty surprises.
 

gilwe

Member
2007-11-07 3:49 pm
Hi.

the cable I'm using is a modified stereo RCA cable, meaning that it has two RCA plugs on one side, and a 4 DIN on the other. I didn't notice any braid, just the ground wires in of each channel. Both ground wires are connected together at the DIN end, being also connected to the DIN's ground pin and to the DIN's metal housing. Should I use a different type of cable (one with braid?) and how should the braid be connected at the RCA side ?

The cable is connected to a passive volume switch so when the output is not switched to the NAP, the hum will be there. Should I make the switch placing a resistor across the 2 channels when it is not switched to any input ?

Thanks for any advice !
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
OK. I'm getting the picture. I see that the common signal ground wire is a general feature of Naim 4-DIN leads. So now, I understand we have 2 conventional shielded leads, the ground wires already being braids surrounding the inner (signal) leads, L and R channel - that's if the cable is a modified, typical RCA type Interconnect. All grounds as you say, are combined at the DIN plug.....hummm! The DIN shell should not be.

Being connected also to the DIN shell then means signal ground is likely connected to chassis ground which in turn is to the mains protective earth. This means a likely hum loop if any other part of your equipment is also connected to mains protective earth. I can't say this will definitely be the case in your NAP90, which may have been modified too but I think you could simply check this with a continuity beeper if your multimeter has one or just read for low resistance between the DIN socket outer shell and chassis ground and the mains ground (with the power lead removed, of course!)

With plain RCA leads (Naim SNAIC and similar leads were quite different), there is no longer need to maintain grounding to the shell of the DIN connector, as the signal ground leads form the shield braids. Making assumptions; if the signal grounds are connected even at high impedance to a chassis or power ground, they can then inject hum into upstream devices like the volume control and CD or Media players too.

The Naim cables were always part of the Naim trip - defiantly anachronistic but very satisfying when all put together right :)
 
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Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
So you still have hum to be dealt with in the amplifier? Check the ground leads to each amplifier input on the PCB.
They appear to be separate + and ground PCB traces from both channels in pics on the web but it is a bit puzzling
because chassis ground wiring is also connected to each input ground. Chassis ground seems already connected
directly to the socket shield. That's a small loop, though i think with that PCB mounted socket, there may not
be another connection to the chassis there.

What is the actual situation? Do the signal grounds rejoin to 1 at the DIN socket?