thor.zmt
Well-known member
This is split of from a different thread:
Reducing hum, noise and ground loops?
Add additional measures for RFI hardening.
My personal suggestion is make sure that XLR cables separate Pin 1 and shell and connect the screen to shell and run a dedicated wire for pin 1. In other words treat the shell and screen as "chassis", pin 1 as "audio ground" and Pin 2/3 as signal.
Personally my signal cables are CAT6 STP, 3 pairs of wires for pin 1, one pair for 2/3 and all shields are connected to shell.
If using Jack or RCA connectors etc, which lack a separate chassis connection make sure they are isolated and fit a binding post for shields and connect the shields ona wiretail with a stackable spade connector.
Inside equipment keep chassis and audio grounds separate. Depending on original design it is often useful to add small value capacitors around amplifier IC's (op-amp's etc.).
1) Place 10pF C0G/NP0 capacitor from -in to output and from +in to gnd on any op-amp anywhere in the circuit, IF the Op-Amp is unity gain stable (NE5532, TL07X and most common audio targeted and audio use units are unity gain stable).
This should be as close as possible to the IC. SMT capacitors are strongly recommended. If you do have a follower (- & out linked) or the +in is linked directly to GND the respective capacitors can be omitted.
Placing a 10nF C0G/NP0 capacitor between +V and -V, again SMD is preferred, improves the RFI resistance and stability and this capacitor should be below the Op-Amp with the smallest possible loop.
Make sure to also have 100nF from +V/-V to GND directly at the +V/-V Pins. Again SMT and C0G/NP0 is recommended, X7R SMD or TH film capacitors are acceptable.
Resistors or inductors in power supply lines need to be carefully considered, however placing SMD Ferrite Beads (330 Ohm @ 100MHz) in series with the supply lines per IC is definitely a good idea, especially if not making a 4-Layer or more design with contiguous ground and power planes. Again, put them close to the IC.
Adding all these capacitors dramatically helps suppress RF ingress.
Power supply lines on single layer PCB's or veroboard make good Antennae's. As do signal traces.
Do not allow any "floating" metal. Make sure all metal has a low resistance/impedance to either chassis (mostly) or audio ground.
All input and output connectors should have RFI mitigation measures:
For inputs using a "PI" filter with:
100p to chassis -> 330R Ferrite Bead -> 100pF to chassis
For outputs use "T" Filters with:
330R Ferrite Bead -> 100pF to chassis -> 330R Ferrite Bead
For low voltage power supplies using a "PI" filter with:
100nF to chassis -> 330R Ferrite Bead -> 100nF to chassis
Use this on each connection that is not bonded "hard" and directly to the chassis.
All components should be SMT.
For PCB mounted sockets, ferrite beads can be added by cutting traces and bridging the cut with a ferrite bead. 0402 and even 0603 capacitors may be too small to solder reliably by hand, so use 1206. Selfadheasive copper tape can be used to create a local image plane. If the PCB mounted sockets link ground and Chassis, cut or remove the links or exchange them for version that break the connection.
For situations like XLR Chassis sockets and jacks that use wires to PCB's, it may be best to make a small PCB that can be directly fitted on the Socket and contains SMD 0603 or smaller format capacitors (smaller is better) and preferably the largest physical size you can find for Ferrite beads to create filters that drain any RF noise into the chassis and prevent it from entering the signal path.
Loop areas matter and ideally the small PCB gets a solid image plane connected to the chassis.
High Voltage (Mains) connections require great care to be safe. Using an IEC mains inlet with build in RF Filter replacing a standard Plastic item is a good choice. If refitting equipment with a fixed power cable, use a powercon socket (or IEC if you enjoy the required dremelling and cutting and filing) and fit a separate mains EMC filter preferably as a canned unit with metal case for chassis mounting.
Large areas of plastic that lead to RF ingress can be covered with either self adhesive copper foil or with PCB material.
All connections after the RFI blocker should be made using shielded cable, shield = chassis and using separate audio ground where required.
Cat 6e STP (shielded twisted pair) network cable makes excellent internal wiring and is also excellent cable for signal cables.
Connect Chassis (and earth) together with audio ground at the power supply "star point" using 100nF & 100R and connect a pair of anti parallel diodes rated at 25A RMS current.
Usually a 25A rated diode bridge block rectifier can be used (link +/- together, the two ~ pin's are used to chassis on one side and ground on the other. The requirements for earth bond testing is to drop less than 2.5V with 25A test current.
Make sure that the diode bridge complies with the voltage drop limit and all other connections are low resistance. This makes sure you have no earth loops and the device remains electrically safety regulation compliant.
In cases where for example a large display window with display is present, creating a faraday cage from PCB material and adding ferrites to the cables can improve the RF ingress situation.
A device refitted thusly and used with a correct cable that does not mix audio ground and chassis/screen and guards conscientiously against RF Ingress can often improve sound quality and SNR dramatically.
Add internal power supply modifications and possibly topology improvements to lower thermal and active device noise and you have a device suitable for employment in the 21st century and in systems with 120dB SNR.
Thor
Reducing hum, noise and ground loops?
Add additional measures for RFI hardening.
My personal suggestion is make sure that XLR cables separate Pin 1 and shell and connect the screen to shell and run a dedicated wire for pin 1. In other words treat the shell and screen as "chassis", pin 1 as "audio ground" and Pin 2/3 as signal.
Personally my signal cables are CAT6 STP, 3 pairs of wires for pin 1, one pair for 2/3 and all shields are connected to shell.
If using Jack or RCA connectors etc, which lack a separate chassis connection make sure they are isolated and fit a binding post for shields and connect the shields ona wiretail with a stackable spade connector.
Inside equipment keep chassis and audio grounds separate. Depending on original design it is often useful to add small value capacitors around amplifier IC's (op-amp's etc.).
1) Place 10pF C0G/NP0 capacitor from -in to output and from +in to gnd on any op-amp anywhere in the circuit, IF the Op-Amp is unity gain stable (NE5532, TL07X and most common audio targeted and audio use units are unity gain stable).
This should be as close as possible to the IC. SMT capacitors are strongly recommended. If you do have a follower (- & out linked) or the +in is linked directly to GND the respective capacitors can be omitted.
Placing a 10nF C0G/NP0 capacitor between +V and -V, again SMD is preferred, improves the RFI resistance and stability and this capacitor should be below the Op-Amp with the smallest possible loop.
Make sure to also have 100nF from +V/-V to GND directly at the +V/-V Pins. Again SMT and C0G/NP0 is recommended, X7R SMD or TH film capacitors are acceptable.
Resistors or inductors in power supply lines need to be carefully considered, however placing SMD Ferrite Beads (330 Ohm @ 100MHz) in series with the supply lines per IC is definitely a good idea, especially if not making a 4-Layer or more design with contiguous ground and power planes. Again, put them close to the IC.
Adding all these capacitors dramatically helps suppress RF ingress.
Power supply lines on single layer PCB's or veroboard make good Antennae's. As do signal traces.
Do not allow any "floating" metal. Make sure all metal has a low resistance/impedance to either chassis (mostly) or audio ground.
All input and output connectors should have RFI mitigation measures:
For inputs using a "PI" filter with:
100p to chassis -> 330R Ferrite Bead -> 100pF to chassis
For outputs use "T" Filters with:
330R Ferrite Bead -> 100pF to chassis -> 330R Ferrite Bead
For low voltage power supplies using a "PI" filter with:
100nF to chassis -> 330R Ferrite Bead -> 100nF to chassis
Use this on each connection that is not bonded "hard" and directly to the chassis.
All components should be SMT.
For PCB mounted sockets, ferrite beads can be added by cutting traces and bridging the cut with a ferrite bead. 0402 and even 0603 capacitors may be too small to solder reliably by hand, so use 1206. Selfadheasive copper tape can be used to create a local image plane. If the PCB mounted sockets link ground and Chassis, cut or remove the links or exchange them for version that break the connection.
For situations like XLR Chassis sockets and jacks that use wires to PCB's, it may be best to make a small PCB that can be directly fitted on the Socket and contains SMD 0603 or smaller format capacitors (smaller is better) and preferably the largest physical size you can find for Ferrite beads to create filters that drain any RF noise into the chassis and prevent it from entering the signal path.
Loop areas matter and ideally the small PCB gets a solid image plane connected to the chassis.
High Voltage (Mains) connections require great care to be safe. Using an IEC mains inlet with build in RF Filter replacing a standard Plastic item is a good choice. If refitting equipment with a fixed power cable, use a powercon socket (or IEC if you enjoy the required dremelling and cutting and filing) and fit a separate mains EMC filter preferably as a canned unit with metal case for chassis mounting.
Large areas of plastic that lead to RF ingress can be covered with either self adhesive copper foil or with PCB material.
All connections after the RFI blocker should be made using shielded cable, shield = chassis and using separate audio ground where required.
Cat 6e STP (shielded twisted pair) network cable makes excellent internal wiring and is also excellent cable for signal cables.
Connect Chassis (and earth) together with audio ground at the power supply "star point" using 100nF & 100R and connect a pair of anti parallel diodes rated at 25A RMS current.
Usually a 25A rated diode bridge block rectifier can be used (link +/- together, the two ~ pin's are used to chassis on one side and ground on the other. The requirements for earth bond testing is to drop less than 2.5V with 25A test current.
Make sure that the diode bridge complies with the voltage drop limit and all other connections are low resistance. This makes sure you have no earth loops and the device remains electrically safety regulation compliant.
In cases where for example a large display window with display is present, creating a faraday cage from PCB material and adding ferrites to the cables can improve the RF ingress situation.
A device refitted thusly and used with a correct cable that does not mix audio ground and chassis/screen and guards conscientiously against RF Ingress can often improve sound quality and SNR dramatically.
Add internal power supply modifications and possibly topology improvements to lower thermal and active device noise and you have a device suitable for employment in the 21st century and in systems with 120dB SNR.
Thor
Last edited:
