Low noise grounding tips in mic preamp

Where we discuss new analog design ideas for Pro Audio and modern spins on vintage ones.
ilya
Posts: 88
Joined: Tue Dec 29, 2009 2:10 pm
Location: Russia
Contact:

Low noise grounding tips in mic preamp

Post by ilya »

I've been building quite a bit of different mic preamps. They are reasonably quiet, but most of them have a very small amount of hum to them. The kind of hum that I hear if I really crank the volume of my headphones AND apply a gain of around 20-30 dB. I'm talking about the filter cap ripple as this hum is often at 100-150Hz and appears even with an external PSU (thus it's not power transformer induced).

I was wondering if it's possible to make a hum-less preamp on the linear regulated supply. When I lay out the board, I try to trace the ground from lower level parts of the circuit up to higher-level parts. The output stage gets connected to the PSU output ground point which is the main filtering cap right after the diode bridge. I also take this point to the chassis.

Am I doing the grounding correctly or are there any other tips and tricks to lower the level of hum?
User avatar
JR.
Posts: 3700
Joined: Sat May 24, 2008 7:21 pm
Contact:

Re: Low noise grounding tips in mic preamp

Post by JR. »

There isn't a single simple answer for this because there are multiple vectors for hum to get in.

Like the lump of dirt under the carpet you have to stomp each one down.

Back in the 80s I actually rolled my own regulated PS to get a cleaner PS for a phono preamp but that was arguably over engineered.

Find the hum, and stomp it.

JR
Cancel the "cancel culture", do not support mob hatred.
User avatar
mediatechnology
Posts: 5444
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: Low noise grounding tips in mic preamp

Post by mediatechnology »

I agree with JR: You need to find the vector first.
Electrostatic, magnetic or conducted?
ilya
Posts: 88
Joined: Tue Dec 29, 2009 2:10 pm
Location: Russia
Contact:

Re: Low noise grounding tips in mic preamp

Post by ilya »

mediatechnology wrote: Fri Jan 19, 2018 4:39 am I agree with JR: You need to find the vector first.
Electrostatic, magnetic or conducted?
I think that I've found the vector, which is "conducted". It's not magnetic because the source of magnetic field is removed (external PSU, no power coils in the circuit). It's not electrostatic - it's hum "footprint" doesn't change if I move the device. So it has to be the ripple from PSU conducted into the audio GND. Whan I don't understand is WHERE it gets into AGND. I'm tapping the "clean" ground at one point and it's generally at the filter cap OR at the vreg filter cap. I don't see any loops in the layout. The ugliest part of this is - if I want to check out an idea I have to cut traces which quickly becomse a mess (I often have a lot of ideas and many of them are not quite right :) )
User avatar
JR.
Posts: 3700
Joined: Sat May 24, 2008 7:21 pm
Contact:

Re: Low noise grounding tips in mic preamp

Post by JR. »

ilya wrote: Fri Jan 19, 2018 4:51 am
mediatechnology wrote: Fri Jan 19, 2018 4:39 am I agree with JR: You need to find the vector first.
Electrostatic, magnetic or conducted?
I think that I've found the vector, which is "conducted". It's not magnetic because the source of magnetic field is removed (external PSU, no power coils in the circuit). It's not electrostatic - it's hum "footprint" doesn't change if I move the device. So it has to be the ripple from PSU conducted into the audio GND. Whan I don't understand is WHERE it gets into AGND. I'm tapping the "clean" ground at one point and it's generally at the filter cap OR at the vreg filter cap. I don't see any loops in the layout. The ugliest part of this is - if I want to check out an idea I have to cut traces which quickly becomse a mess (I often have a lot of ideas and many of them are not quite right :) )
You haven't mentioned anything about the topology but I will ASSume differential input so audio input should only care about pin 2 vs pin 3. Hum on pin 1 should be harmlessly ignored, as well as common mode hum on phantom supply.

Somewhere between the differential gain stage and output the differential signal should get single ended then referenced to the output ground.

One obvious question, does the hum increase with preamp gain? If yes the hum has corrupted the input signal. If no it is corrupted in the differential to single ended and output driver circuitry.

Divide and conquer.

JR
Cancel the "cancel culture", do not support mob hatred.
ilya
Posts: 88
Joined: Tue Dec 29, 2009 2:10 pm
Location: Russia
Contact:

Re: Low noise grounding tips in mic preamp

Post by ilya »

JR. wrote: Fri Jan 19, 2018 11:46 am One obvious question, does the hum increase with preamp gain? If yes the hum has corrupted the input signal. If no it is corrupted in the differential to single ended and output driver circuitry.
This particular preamp I'm working on is class-A, transformer in and transformer out. 3 stages. When I engage the last stage (on the very high gain settings) I can hear hum injected in the audio path.
Yes, it increases with gain setting, so it's obviously signal corruption. The ground trace of this stage ends up right at the star point near the vreg filter cap. I'm puzzled...
User avatar
JR.
Posts: 3700
Joined: Sat May 24, 2008 7:21 pm
Contact:

Re: Low noise grounding tips in mic preamp

Post by JR. »

Well a transformer input should deliver excellent differential action and CM rejection.

While still guessing about the gain stage, a typical non-inverting op amp or even discrete gain stage will have an effective - input in series with the gain leg ground... so that gain leg ground needs to be the same ground as the input transformer secondary ground or 0V.

(this assumes there is no hum leaking into the transformer which is a magnetic device).

JR
Cancel the "cancel culture", do not support mob hatred.
ilya
Posts: 88
Joined: Tue Dec 29, 2009 2:10 pm
Location: Russia
Contact:

Re: Low noise grounding tips in mic preamp

Post by ilya »

JR. wrote: Fri Jan 19, 2018 2:36 pm While still guessing about the gain stage, a typical non-inverting op amp or even discrete gain stage will have an effective - input in series with the gain leg ground... so that gain leg ground needs to be the same ground as the input transformer secondary ground or 0V.
Thanks for that info! This was exactly the problem on the other preamp. While it had very faint traces of hum (not audible under normal conditions), this last step killed it completely! Will apply this to the first preamp and see if I can get the same results...
User avatar
mediatechnology
Posts: 5444
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: Low noise grounding tips in mic preamp

Post by mediatechnology »

The output stage gets connected to the PSU output ground point which is the main filtering cap right after the diode bridge.
This statement raises a lot of questions.

What do you mean by the output stage gets connected to the PSU output ground? Where does the preamp's common mode rejection stage reference?
which is the main filtering cap right after the diode bridge
Oh really? There's a huge peak current flowing through that node between filter caps.

A schematic with pics would be an excellent start.
ilya
Posts: 88
Joined: Tue Dec 29, 2009 2:10 pm
Location: Russia
Contact:

Re: Low noise grounding tips in mic preamp

Post by ilya »

mediatechnology wrote: Sat Jan 20, 2018 11:14 am
The output stage gets connected to the PSU output ground point which is the main filtering cap right after the diode bridge.
This statement raises a lot of questions.

What do you mean by the output stage gets connected to the PSU output ground? Where does the preamp's common mode rejection stage reference?
Sorry, I was mistaken. The PSU ground output is at the filtering cap after VREG. I treat it as a star ground point, sort of an absolute reference. Maybe I shouldn't. What do you mean by preamp's common mode rejection stage reference?
mediatechnology wrote: Sat Jan 20, 2018 11:14 am A schematic with pics would be an excellent start.
Schematic is simple. It's an API312 type pre, basically that is an op-amp between the transformers. As I mentioned earlier, bringing the "-" input of an op-amp to the gnd @ the input transformer secondary killed the hum completely.

The second preamp is a bit more complicated. It's a Neve type discrete 3-stage circuit. It's the one that has a faint hum when the 3rd stage is engaged. I don't have an access to it right now, but I need to check some ideas based on what JR said earlier when I can get my hands on it again.
Post Reply