Audio and Chassis ground in an 8 channel preamp

Where we discuss new analog design ideas for Pro Audio and modern spins on vintage ones.
rp3703
Posts: 10
Joined: Tue May 31, 2016 2:55 pm

Audio and Chassis ground in an 8 channel preamp

Post by rp3703 »

I am trying to take a Shure AMS-8000 and turn it into an 8 channel API 312. I have designed new preamp boards as well as a new PSU but I am still stuck at one issue with regards to the ground path for phantom power. My PSU +/-V rails as well as +48V all run off the same transformer, so they share 0V. The existing channel cards are designed with the input(XLR) and output(TRS) mounted directly to the preamp channel PCB. There is also a metal plate that mounts to the channel card PCB that when installed in the unit, creates the back panel of the chassis. Kind of like if a 500 series preamp slid into the back of a lunchbox instead of the front. With the existing design, this metal plate makes contact through an unmasked area of the ground plane to make a chassis connection to audio ground. All the channel cards are tied to the PSU through a ribbon cable.
My issue is related to the audio chassis ground connection. From everything I’ve read, the chassis and audio ground should only connect at one place in the entire unit but at the same time pin 1 should always be connected to chassis. From what Douglas Self writes in his books, pin 1 is the best place for the audio and chassis ground to connect. How do I resolve this? One option is to just combine audio and chassis grounds at pin1 on each channel card but if I do, should I also make a chassis to ground connection at the PSU? Will this not create ground loops? Another option is to not connect pin 1 to the chassis at the preamp channel card and run a separate chassis ground through the ribbon cable back to the PSU where it will connect to the chassis at only one point. This would create a separate plane for audio ground as well a chassis ground on the same PCB. Another option would be to connect audio ground and pin1 on the preamp card but not connect them to the chassis and then just run all 0V back to the PSU where it ties to the chassis at only one place.
User avatar
JR.
Posts: 3700
Joined: Sat May 24, 2008 7:21 pm
Contact:

Re: Audio and Chassis ground in an 8 channel preamp

Post by JR. »

rp3703 wrote:I am trying to take a Shure AMS-8000 and turn it into an 8 channel API 312. I have designed new preamp boards as well as a new PSU but I am still stuck at one issue with regards to the ground path for phantom power. My PSU +/-V rails as well as +48V all run off the same transformer, so they share 0V.
Should not be a problem.
The existing channel cards are designed with the input(XLR) and output(TRS) mounted directly to the preamp channel PCB. There is also a metal plate that mounts to the channel card PCB that when installed in the unit, creates the back panel of the chassis. Kind of like if a 500 series preamp slid into the back of a lunchbox instead of the front. With the existing design, this metal plate makes contact through an unmasked area of the ground plane to make a chassis connection to audio ground. All the channel cards are tied to the PSU through a ribbon cable.
My issue is related to the audio chassis ground connection.
Not aware of an "audio chassis ground". Chassis ground is a robust low impedance ground for shields, et al.
From everything I’ve read, the chassis and audio ground should only connect at one place in the entire unit
I really do not like the term "audio ground". I prefer audio + and audio - for audio signal pairs. Any equipment can have multiple local audio low reference nodes, all with slightly different voltages on them (like on each card). Since the audio is received into and sent away from the local audio low nodes differentially, their actual voltage doesn't matter (within reason). These sundry audio low nodes will ultimately connect to PS ground through some finite impedance.
but at the same time pin 1 should always be connected to chassis.
yes, pin 1 should always be a low impedance connection to chassis ground (which connects to PS ground, or PS 0V).
From what Douglas Self writes in his books, pin 1 is the best place for the audio and chassis ground to connect. How do I resolve this?
No problem just forget about audio grounds, or ask Douglas Self.
One option is to just combine audio and chassis grounds at pin1 on each channel card but if I do, should I also make a chassis to ground connection at the PSU?
Still unclear about this "audio ground" you talk of.

I am too lazy to google a schematic of an API 312 (preamp?). Presumably it has a balanced input so input pin 1 goes hard to chassis ground, pins 2 and 3 connect through some input impedances. However the input differential converts the balanced input to single legged, that will be wrt a local 0V "audio low" node. The output hopefiully is also balanced or differential wrt this internal audio low node also. This local node can connect to PS ground through some finite impedance, but doesn't have to be a dead short.
Will this not create ground loops? Another option is to not connect pin 1 to the chassis at the preamp channel card and run a separate chassis ground through the ribbon cable back to the PSU where it will connect to the chassis at only one point.
There can be multiple connections between chassis ground and PS ground, who cares, it mainly wants to be low impedance. I would never hard connect any audio signal (Audio + or Audio -, to chassis or PS ground).
This would create a separate plane for audio ground as well a chassis ground on the same PCB. Another option would be to connect audio ground and pin1 on the preamp card but not connect them to the chassis and then just run all 0V back to the PSU where it ties to the chassis at only one place.
Do not look for some magical audio ground node that will be the same 0V AC everywhere inside a product. It can't and doesn't exist.

Chassis ground is low impedance, the better to absorb shield noise, and chassis ground noise from other chassis. Audio will always be balanced and have impedance between audio lines and chassis/power ground. That's why we use differential inputs and outputs. Local audio low errors are common mode so ignored.

Sorry if this isn't immediately helpful... BTW even if there was a ground loop (an effective winding that converts magnetic field energy into current/voltage flowing in that loop). The pin 1 voltage should be common mode to the audio on 2 and 3 so cancel out.

JR

Note: perhaps google "pin 1 problem" for discourse on what happens if the audio from pins 2 and 3 get corrupted by noise on pin 1... there will always be noise on pin 1 so make sure that does not corrupt the local audio low node after the input differential.
Cancel the "cancel culture", do not support mob hatred.
rp3703
Posts: 10
Joined: Tue May 31, 2016 2:55 pm

Re: Audio and Chassis ground in an 8 channel preamp

Post by rp3703 »

"There can be multiple connections between chassis ground and PS ground, who cares, it mainly wants to be low impedance. I would never hard connect any audio signal (Audio + or Audio -, to chassis or PS ground). "

I don't understand this bit. I've attached the schematic for the API 312. It shows COM. connected to audio-.
API 312 mic amp.jpg
API 312 mic amp.jpg (227.35 KiB) Viewed 18393 times
User avatar
JR.
Posts: 3700
Joined: Sat May 24, 2008 7:21 pm
Contact:

Re: Audio and Chassis ground in an 8 channel preamp

Post by JR. »

My eyes... :lol:

OK what they call com is PS ground.

Since they are transformer balanced in and out, this local audio 0V "can" be connected to PS Ground with little concern. In fact since they are driving some decent AC current into that local audio 0V. Depending on gain setting as much as input V /200 ohm through C3, and whatever that transformer draws.

So In that case making that 0V node a low impedance node to PS ground will keep AC voltage on that node modest. Note any voltage there is common mode so not a big problem but why ask for trouble?

I ASSume this schematic is incomplete if you use 48V phantom power supplies. For phantom you will need 6.8K series resistors and pin 1 will need a return path to PS ground (com).

I don't see any loop from tying COM to pin 1 and chassis. The input and output transformers will keep outside world audio signals (pins 2/3) isolated from chassis and power ground (com). That said for good audio integrity all of the local audio 0V node connections around the gain stage, need to be direct connections, and this node then connects through a single path to COM. The circulating audio currents inside the gain stage need to be kept local. Looking at the schematic you can see how everything connects to each other at pin 4 of the input transformer, and then a straight shot to the PS COM.

Transformers make this all pretty easy and harder to mess up, but keep the layout clean and follow the schematic's general suggestion for how to connect everything together.

JR
Cancel the "cancel culture", do not support mob hatred.
rp3703
Posts: 10
Joined: Tue May 31, 2016 2:55 pm

Re: Audio and Chassis ground in an 8 channel preamp

Post by rp3703 »

Thanks for pointing out that pin 4 connection. I did not catch that. So here is the schematic I drew. I took it a bit further and added the DOA directly to the PCB instead of socketing it like API does. I figured since I will have to put together the DOA as well, why try fitting all that stuff onto a 1-1/4" square when I can spread it out and make life easier on myself. Hopefully that is not a mistake. I did not draw the switches for phantom, pad and phase. Kicad would only get confused so I left them off.
Attachments
AMS-312 SCH.gif
AMS-312 SCH.gif (92.09 KiB) Viewed 18392 times
rp3703
Posts: 10
Joined: Tue May 31, 2016 2:55 pm

Re: Audio and Chassis ground in an 8 channel preamp

Post by rp3703 »

So I have already laid out one version of the channel card PCB. There are some changes I need to make. The weird exposed ground plane area by the input XLR and the output TRS are to connect the ground plane to the metal plate that I described in my original post. My original plan was to keep them separated from the rest of the ground plane but it sounds like I should just tie it all together. My issue with doing that is with the switch for phantom. It runs all the way to the opposite end of the PCB from the in’s and out’s. My worry is that the phantom return path could somehow get into the audio circuit below and cause problems. Should I keep the separate traces for phantom while it’s near the audio circuit and then combine them near the power connector.
Attachments
AMS-312-Channel Card.gif
AMS-312-Channel Card.gif (233.43 KiB) Viewed 18372 times
User avatar
JR.
Posts: 3700
Joined: Sat May 24, 2008 7:21 pm
Contact:

Re: Audio and Chassis ground in an 8 channel preamp

Post by JR. »

I don't see an output transformer?

JR
Cancel the "cancel culture", do not support mob hatred.
rp3703
Posts: 10
Joined: Tue May 31, 2016 2:55 pm

Re: Audio and Chassis ground in an 8 channel preamp

Post by rp3703 »

The output transformer has wires to coming in and out, so all I did was put solder pads to connect. The input connections are made at the phase switch, so the two solder pads labeled pad go to a DPDT switch and straight into the output transformer. The secondary of the output transformer goes to the solder pads located near the TRS jack. I was trying to reduce the distance the signal has to travel.
JohnP
Posts: 6
Joined: Sun Jun 22, 2014 12:19 pm

Re: Audio and Chassis ground in an 8 channel preamp

Post by JohnP »

JR has been advising you about "grounding", but before you etch any boards, your schemo for the 2520 is incorrect.
The bases of the output transistors (Q7-8) are not connected properly.

JP
User avatar
JR.
Posts: 3700
Joined: Sat May 24, 2008 7:21 pm
Contact:

Re: Audio and Chassis ground in an 8 channel preamp

Post by JR. »

JohnP wrote:JR has been advising you about "grounding", but before you etch any boards, your schemo for the 2520 is incorrect.
The bases of the output transistors (Q7-8) are not connected properly.

JP
yup base of q8 probably goes to collector q5 but check the whole schematic...

JR
Cancel the "cancel culture", do not support mob hatred.
Post Reply