Topic: DMIC-20 *intermittent* hiss noise

[poorlyconditioned]

I've got two DMIC-20s.  One vanilla, and the second modded by me (gain knob replaced switches).

The crazy thing is there is a quite loud hissing noise.  Like self noise on the mic, but much louder.

This goes away when I touch the resistor array (grounding problems?).  It also sometimes goes away if I tighten the nuts on the BNC connectors on either side.

But it is intermittent.  Sometimes there, sometimes not.  Both units seem to have this problem at some time or another.

Any ideas?  This is driving me crazy!

Thanks,
  Richard

[Church-Audio]

I've got two DMIC-20s.  One vanilla, and the second modded by me (gain knob replaced switches).

The crazy thing is there is a quite loud hissing noise.  Like self noise on the mic, but much louder.

This goes away when I touch the resistor array (grounding problems?).  It also sometimes goes away if I tighten the nuts on the BNC connectors on either side.

But it is intermittent.  Sometimes there, sometimes not.  Both units seem to have this problem at some time or another.

Any ideas?  This is driving me crazy!

Thanks,
  Richard

Its very hard to say with out looking at the circuit and hearing a sample of the noise it could be a problem with ground but you dont usually get a hiss with a bad ground you get a hum. I suspect it could be a problem in the feedback loop somewhere I would check for bad solder joints. I would also take a good look at the opamps and see if one of them is going south on you. This can be a bad opamp I have had many opamps go bad on consoles and they always produce the same type of sound a loud hiss.

I would also check the power supply to all the chips and make sure its in the ballpark. I would also look at making a better gound connection to the bnc connectors maybe soldering a separate ground lead to them? I would like to see the circuit. I would scope the outputs and inputs of each section and see at what stage its making this noise. I always start at the first stage and work my way down to the output. I am sure you have thought of most of this. I would also consider decoupling the A-D converter from the preamp section and see if its your A-D converter. It could be that your converter chip is bad or on its way out.

[Chuck]

I got a Rolls MX28 recently that I planned to use as a line mixer in the field. When you bring up the gain on any of the inputs you get clear reception of a radio station through the outputs! Also when the chassis ground wire is connected it humms like crazy. I traced the radio station interference down to a feedback loop in the output op-amp. The chassis ground thing is bad when the ground is lifted, put worse when it's connected.

[Church-Audio]

I got a Rolls MX28 recently that I planned to use as a line mixer in the field. When you bring up the gain on any of the inputs you get clear reception of a radio station through the outputs! Also when the chassis ground wire is connected it humms like crazy. I traced the radio station interference down to a feedback loop in the output op-amp. The chassis ground thing is bad when the ground is lifted, put worse when it's connected.

Try putting a 3pf cap in the feedback loop of the preamp... And as for the ground try a resistor between the chassis and the ground on the preamp.. See if that helps any value around 100ohms. See what it does for you. Maybe it will work maybe not. The thing about a feedback loop is they are best kept as short as possible to avoid RFI.