Mic pres being Hissy!

[Deepdark]

Hi

I made a 4 channel Access 312 pre with an external power supply (psu board and toroidal from 5fish audio). The 4 channels became more hissy all of a sudden, and it's worst with the 48v engaged. Because all board act the same, then the trouble should come from the power supply. Is it possible it comes from one regulator (LM317/337)? Any way to test it out with a scope to find the culprit without having to replace all 3 of them? And, is it common for regulator to act like this over time, and regular switching on and off?

 

[JohnRoberts]

Anything is possible.... when you switch the PS off, does the hiss go away until the PS rails finally collapse?

Might be a regulator but all mic preamps have some hiss...

JR

 

[Deepdark]

Thanks John!

In fact, it became noisier than usual, so this is why I'm suspecting something in the power supply, since all 4 boards act the same (all of them are noisier)

 

[JohnRoberts]

Does adding or removing capacitance from the rail affect the behavior?

With a scope you can look at the regulated rail... it should be clean with no HF signal riding on top of the DC.

JR

 

[Deepdark]

Just found it's the phantom power rail. Will have to look at it. adding capacitance, you mean adding a cap?

 

[Monte McGuire]

If the box became more hissy all of the sudden, and it's the P48 supply, then first make sure that the P48 regulator is working properly. More capacitance might help, and it's certainly a good debugging test, but why band-aid a broken regulator - a component or solder joint or connector etc. could have failed.

 

[Deepdark]

by the way, is it common for a LM317 to broke, I mean, I read that they aren't the most solid regulator, but never had any trouble with them. This is just for my own knowledge. any better replacement?

 

[PRR]

317 is solid, within ratings.

They are not rated for 48V. Yes, you can contrive to drop 70V to 48V and it looks like just 22V while running. But what about start-up and shut-down/short surges??

Many people have moved to more elaborate circuit or higher voltage regulators.