C84 Noises

[mrsteaks]

I've finished both of my C84's. They're both Rev 1.9.1. The second one has developed noise - a constant low level popping/rushing noise when the phantom power is on. I haven't checked the actual phantom power voltage while this going on, but from the sounds I think it is fluctuating. For it's first hour of life, it was dead quiet, and has developed this problem over time (two hours on it now), to where it is now doing it constantly. And it's getting louder.

The first C84, which is next to the power supply (slot one) is picking up a power supply buzz at high gain (50% and up). I've read all the posts on this and everything is as it should be. Chassis to module ground shows shorted (good ground), rotary switch is making good contact with the front panel, the A/C connector is grounded to the chassis, and every molex connector is good and tight. I could always move both C84's to slots away from the power supply, and place my T15's in slots one and two (hoping they don't get noisy!).

[mrsteaks]

I fired up the pre this AM, about an hour ago. I removed the top, and carefully repositioned the power supply harness, and in the process managed to reduce the power supply noise in the slot one C84. I put the lid back on, which reduced the noise even more. At max gain and max trim, it's still audible, as it is on the C84 in slot two (two is slightly quieter), but it's acceptable. Not AEA TRP dead quiet, but quiet.

As for the "rushing" noises on the C84 in slot two - nothing yet. Quiet, right now. Phantom power on or off. All of the T15's are quiet. Phantom power measures 47.1 volts from each module, and is stable. I don't have a mic with me to try (yesterday I used a Shure KSM9) right now, and I won't pronounce "all is well" until I try the thing with a mic. Later today I will. I've been testing the SCA unit into an old Mackie 1202 for monitoring, and I'll try another mixer and my HD24XR when I do the mic test. And, I'll report back.

[mrsteaks]

Well, the noisy C84 is still noisy, although it is intermittent, and seems to do it only when phantom power is applied. It sounds like a zipper with a rushing sound in the background when it does it's thing. It's unusable as a recording preamp in it's current state. This thread seems to indicate exactly the kind of problem I'm experiencing: http://seventhcirclestudios.com/SCA/php ... php?t=1159
I'll try my Royer ribbon into it (phantom off!) and see if it 1) passes signal and 2) if it does, if it makes noises.

The rest of the pres are still quite happy. Quiet, stable, and sounding very nice. I used a Shure KSM137 to test them, running them into my HD24XR and monitoring it's output through a Rane SM26B. The power supply noise in slot one when the module is at high gain is still audible, but it is very faint (-60db on my DAW), just slightly above the self-noise of the preamp. Not an issue. By the way, I did move the modules around in the chassis - I now have T15's in slots 1 through 4, and the C84's are in slots 5 and 6.

[tpryan]

Well, the noisy C84 is still noisy, although it is intermittent, and seems to do it only when phantom power is applied.

Are you sure that D1 through D4 are in the right way round?
Are you sure that you haven't swapped C5 or C7 with C23 or C30?

[mrsteaks]

Hi, Tim - short answer, yes to all. I checked it against the my other C84 and the BOM and all is where it should be.

As I said, the noise is intermittent - I emailed you a sample. As the unit heats up, it increases in the number of times it does it. It's not continuous.

With the phantom power off and a ribbon mic connected, the thing is dead quiet - not any extraneous noises at all. I've had it on for two hours, and it's silent.

[atiller]

Hi Tim,

Whatever became of this issue? I'm experiencing similar with one of two preamps I built recently (within the week).

Cheers,
Adam

[atiller]

Well, I'm thinking it is a DC problem. I'm going to reflow a few of the joints on this troublesome card to see if it helps at all. I think perhaps I was a bit too thrifty on the solder for my second attempt.

Tim, when you get a chance I would love to know if anything came of this issue? Things to check? A pattern perhaps (as I'm sure this is a common one)?

Cheers,
Adam

[mrsteaks]

No resolution just yet. Tim has the C84 and will let me know what he discovers. When he does, I'll post it (unless he beats me to it!).

[tpryan]

I would love to know if anything came of this issue? Things to check? A pattern perhaps (as I'm sure this is a common one)?

No, not common. Not a DC problem with phantom, either. Try removing 100pF caps C17 and C25.

[atiller]

Hey Tim,

Since I had posted last my phantom power noises have disappeared. Everything is dead quiet as of now (knock on wood).

If the issue does come back, and I am very paranoid about it, I will try removing those caps and I'll let you know!

Cheers,
Adam

[mrsteaks]

Tim told me the preamp didn't exhibit the symptoms I'd seen, but it did have a low level high frequency oscillation that he cured by removing C17 and C25.

At this point, I'm looking forward to putting the pre back to work!

Thanks, Tim!

[mrsteaks]

I used the pre last night and it is dead quiet. No problems. Apparently Tim found the cure!

Thanks, Tim!

[csavsquad]

I think that my problem (posted here: http://seventhcirclestudios.com/SCA/php ... php?t=1399) is very similar. Would you mind taking a listen to the audio file I posted and let me know if it's similar to what you were hearing?

Tim, when you say remove C17 and C25, do you literally mean just take them off the board?

[tpryan]

when you say remove C17 and C25, do you literally mean just take them off the board?

Yep