mitsos 312 preamp di input excessive noise problem

[cmuller0420]

hello,

so here is my situation.  I have built some api 312 with mitsos boards and the mic pre portion has been working great (nice and quiet), however when I switch to the DI portion I have a hum that I can not get rid of.

Here is what I have done so far...

Since plugging in an instrument caused the large hum, I thought I would try to mimic a low impendance load.  Taking a 1/4 jack I soldered a 680 ohm resistor from the tip to the sleeve and inserted it, this did not fix the buzz, so I have ruled out external interferance or a cable problem.

I measured the voltage drop over the resistor and there was -1.2V...so I thought that maybe this DC voltage is my problem, however when I put a couple hundred nano-farad cap inline with the tip, that doesn't fix it. 

I rerouted the ground connection to a star ground point...still hasnt fixed

I have lifted the output ground from the channel...still hasnt fixed it

This is across both channels where I have the DI input.

The way  have this setup is coming from the DI send and return on the preamp pcb, I have it running to a spst switch to switch between the mic input or the di input, when using the mic input....it is perfectly quiet.

I also noticed that as I move my hand close to the jack the hum will increase...Now the wiring is running only a few inches and I have not used shielded cable, but the amount of noise is excessive over what I would assume would be induced noise.

At this point I am stumped, I have tried moving where ground terminates for the jacks with again no luck...any ideas or suggestions for narrowing down the problem would be great...stuck on this one!

Thanks

 

[Davo]

I don't know if the DI is active or not, but I recently had a similar problem with my 48v phantom on some racked modules... switch it on a buzzz
turns out that the voltage supplying the 48v lm317 was right at it's limit... when engaged, would drop to around 42v...
so that might be a starting point, to look at your psu regs

hope that helps,
dave

 

[mitsos]

well, I'm a bit too tired to think straight but here goes...

The DI jack just goes straight into the opamp + input, so there isn't much it could be. You should be using an isolated jack to avoid ground loops and a switching one at that, so you can get rid of the switch you are using. Is that really SPST? Or do you mean SPDT? How do you have it wired?

I have tried various grounding methods on mine, they've worked fairly equally.  My prefered way is to use a shielded mic cable, with one conductor connecting the TIP to the opamp, the other conductor connecting the switching part of the jack to the transformer side of the DI input. Then I take the shield and connect it between the sleeve and the most convenient ground point on the PCB, usually this is the ground side of either the load R (R12), the zobel (CZ), or C12.

You can also try to take the sleeve direct to your main audio ground if that's how you are grounding the unit. I usually ground everything in one channel together and run all the channels grounds together from power ground point on each PCB. 

HTH!

 

[cmuller0420]

Problem solved....were the power transformer secondary common was connected, I had trimmed the wires back abit and the solid wire had a bitof a coating on it....it seems that I did not have a solid connection, when  I removed the PSU and was wiggling the wires I found this out.

Thanks for the input!!

 

[mitsos]

weird that this only gave you a problem with the DI, but glad you sorted it out!