buzz in micpre

[RoadrunnerOZ]

Thanks, i did release the capacitor. But i can still hear the noise no matter where i place it and regardless on where the wires are.
I will try the two wires per pole next.
im starting to wonder if the psu simply is not well thought out. It might be the phantom power that messes with things. When i disconnect the 48v outputs from the pcb it gets better.

Does it buzz with no input connected? Also is phantom switched or permanently connected?

 

[bruce seifried]

As someone pointed out earlier, you cannot share wires to the first filter cap. There should be four wires attached to that cap: two from the rectifiers, two going to the load. You need to change that before proceeding with anything else.

 

[RoadrunnerOZ]

As someone pointed out earlier, you cannot share wires to the first filter cap. There should be four wires attached to that cap: two from the rectifiers, two going to the load. You need to change that before proceeding with anything else.

I would guess by looking at the pic that the load is the voltage regulator system on the power board right where the leads are taken for that capacitor, pre voltage regulators. The regulators then will have their own capacitor(s) which then deliver to the audio board as the load.
Bit hard to say without a schematic.

 

[Audio1Man]

Look @ the DWG. CHARGE and DISCHARGE currents don't have common paths.
Your spacing of the CAP wires are changing the MAGNETIC FIELDS and HUM may increase or decrease due to coupling with other fields.
Duke

 

[RoadrunnerOZ]

Look @ the DWG. CHARGE and DISCHARGE currents don't have common paths.
Your spacing of the CAP wires are changing the MAGNETIC FIELDS and HUM may increase or decrease due to coupling with other fields.
Duke

But this all depends on what circuitry is on the PCB - there are two wires in -AC to rectifier, two wires out to the capacitor a pile of components on that board - regulator?, then two wires out DC - presumably regulated - are you suggesting to cut the print just post rectifier and use send from rectifier and return to post cut with 2 more cables from the capacitor? It would make sense.

 

[transient]

But this all depends on what circuitry is on the PCB - there are two wires in -AC to rectifier, two wires out to the capacitor a pile of components on that board - regulator?, then two wires out DC - presumably regulated - are you suggesting to cut the print just post rectifier and use send from rectifier and return to post cut with 2 more cables from the capacitor? It would make sense.

Agreed on this with RoadrunnerOZ. The 'charge and discharge paths' depend on the board layout. The lead-out wires to the capacitor as currently shown are just like having a capacitor with higher ESR. I still think RF or HF oscillation is a possibility - a classic symptom is attaching your meter and it suddenly goes away, as described earlier in the thread.

 

[RoadrunnerOZ]

I’m a bit suspicious that one of the LM395’s has gone - the left leg of these power IC’S is the base, which you said when you touched it with your meter probe kills the noise. As the impedance of your meter depending on which range you are on, or if it’s auto-ranging, will be in the region of 10MΩ or less so this will lightly ground whatever is coming in. This suggests that something in the regulator circuit is faulty - either one of the LM395’s is failing or a failed diode or capacitor, or an open resistor in the PSU. The LM395 is pretty hard to kill as it has built in overload protection but there’s a fair bit going on inside them and anything can fail. They are not that expensive - around AU$6.
There is what appears to be a green wirewound resistor as well - worth checking. It looks like there’s two small metal can transistors on that board also - hard to say how the circuit works without seeing how the power IC’s (LM395’s) are driven. Do you have a schematic for the PSU??
I’d be checking every diode and resistor on that board.

 

[jokeramik]

I dont trust the design at all. Nor the build. It was a prototype from some gear shop. The star ground did not even have connection to the chassis a complete mess. So there is a good chance that circuit is messy too. But im honestly not engineer enough to fix a flawed design…
The client so far used the pre with a condenser mic. No problems. Its with his dynamics and quiet sources that the buzz becomes obvious.

Connect the star ground with the chassis. What ist the result?
Best regards!

 

[RoadrunnerOZ]

It’s possibly because the condenser mics have higher output that the buzz is not noticeable as the gain would be turned down or the fact they are stand mounted and are not coming into contact with the body of the singer.