Hello,
I built a simple analog mixer (inverting NE5532 opamp), and I noticed the following problem:
I run 10 coaxial cables to its inverting pin, that are supposed to carry the signals to be mixed. When I terminate them at the source end, it sounds and works fine. But if I leave them disconnected from the source end (just a cable into the - pin hanging in the air), they pick audible hum. The effect of one is negligible, but 10 of them together sound bad.
Poking around with a probe revealed something like modulated noise, maybe they just pick up RF interference and demodulate it into hum?
I tried the following with no success: I terminated the cable with 10k in series with 2,2uF capacitor, but it only got a tiny little bit better. I used that configuration because, roughly, this was the source output impedance when it worked fine.
The 10 cables are close together and touch the bottom chassis (I think it's made of zinc).
Is there any way of getting rid of this interference? Maybe shielded twisted pair would work better?
I built a simple analog mixer (inverting NE5532 opamp), and I noticed the following problem:
I run 10 coaxial cables to its inverting pin, that are supposed to carry the signals to be mixed. When I terminate them at the source end, it sounds and works fine. But if I leave them disconnected from the source end (just a cable into the - pin hanging in the air), they pick audible hum. The effect of one is negligible, but 10 of them together sound bad.
Poking around with a probe revealed something like modulated noise, maybe they just pick up RF interference and demodulate it into hum?
I tried the following with no success: I terminated the cable with 10k in series with 2,2uF capacitor, but it only got a tiny little bit better. I used that configuration because, roughly, this was the source output impedance when it worked fine.
The 10 cables are close together and touch the bottom chassis (I think it's made of zinc).
Is there any way of getting rid of this interference? Maybe shielded twisted pair would work better?
Leaving them "dangling in the air " is asking for RF pick up trouble , if that is the problem then a very small value ceramic capacitor at both ends to earth would help as its not likely to be mains pickup.
Also try to the coax shielding even if it is earthed.
I take it there are no quality coax connectors fitted just soldered ends ??
Whats the nominal cable impedance ?
Also try to the coax shielding even if it is earthed.
I take it there are no quality coax connectors fitted just soldered ends ??
Whats the nominal cable impedance ?
Did you think about the impact of cable capacitance on op-amp stability? Is there a small capacitor between the op-amp's output and negative input and/or a small resistor between the cables and the op-amp's negative input?
At the source end the coax is soldered, at the opamp's end it is fitted in a clamp pole.
The coax used is RG-174 type. (I think that means 50 Ohm impedance)
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Yes and no: I have used a small 220p capacitor between output and inverting input, but no series resistance before the cable reaches the opamp.
Then I'm barking up the wrong tree: it is not likely that the cables cause instability.
How and where did you connect the shields? Are there any mains transformers nearby?
How and where did you connect the shields? Are there any mains transformers nearby?
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I connected the shield at the opamp and source end, since it connects their grounds (the two circuits have isolated power supplies). There is a toroidal transformer nearby which has a magnetic belly-ring around its core (for extra magnetic shielding) and all outgoing cables are twisted and pushed as far away as possible form the audio parts. The transformer is about 5-10cm away from the pcbs, and its twisted cables are 8cm further away.
That is why I wonder! I had taken some precautions... The culprit is apparently something that escaped my experience so far.
I will try to terminate both ends with small ceramic caps to see if that helps, as duncan2 suggested.
Another experiment I performed was this: I simply connected a 2u2 capacitor at the source end and the result was a squeaking noise. I immediately disconnected it. I don't know if that run the opamp into instability or just picked up more interference.
I will try to terminate both ends with small ceramic caps to see if that helps, as duncan2 suggested.
Another experiment I performed was this: I simply connected a 2u2 capacitor at the source end and the result was a squeaking noise. I immediately disconnected it. I don't know if that run the opamp into instability or just picked up more interference.
I am still on the RF track , could you tell me ?
Is there any mobile phone stations nearby or DECT phones/baby alarms /near a router/modem / dimmer light switch etc ?
Is there any mobile phone stations nearby or DECT phones/baby alarms /near a router/modem / dimmer light switch etc ?
In both rooms that I tested the equipment, there where routers at least. The audible hum was obviously distorted 50 Hz (maybe rectified). The type of hum we guitarists are really familiar with 😛, the sound that comes from ground loops.
Let me mention that my metal housing is earthed, and audio ground is earthed too but outside the housing (the amplifier that follows my housing earths the signal's ground).
I will take your word for that then, musicians usually have good ears for tonal imbalances .
Greece uses the same supply as the UK but type "F" Shuko ( German designed plugs ) , these have earth contacts but could you elucidate--do you mean you have an American style external ground spike and that is your electric supply earth or do you have a separate utility company earth ?
Greece uses the same supply as the UK but type "F" Shuko ( German designed plugs ) , these have earth contacts but could you elucidate--do you mean you have an American style external ground spike and that is your electric supply earth or do you have a separate utility company earth ?
Did you say a "squeaking noise " when a capacitor was connected ??---I can assure you that's instability but that could be due to additional factors including sensitivity to added values of resistance/capacitance inductance which is inherent in capacitors.
It's not surprising that an extra 2.2 uF makes the op-amp instable. You have about -90 degrees phase shift in the loop gain from the op-amp's internal compensation and another -90 degrees from the feedback resistor and 2.2 uF, leaving a phase margin of about 0.
What's the value of the feedback resistor and what is the sum of the cable lengths, roughly?
What's the value of the feedback resistor and what is the sum of the cable lengths, roughly?
If I understand you correctly, my building's earth is a spike style one I presume..