Trident Series 65 grounding: theory and practice

[JohnRoberts]

Do not waste brain cells on this distraction... if there was a valid reason for them being there it will reveal itself over time, if not it won't.

Well, if you're saying it's safe to ignore, then I believe you. I want to clean up the grounding in this board, but I don't want to yank something out and then regret it later, just because I didn't understand it at the time. In any case, now the output jacks look like this - that's pin 1 to chassis, and no more 12k resistors connecting pins 2 & 3 with pin 1.

PS: It may look like pin 1 is also going to signal ground via the blue and yellow cables, but those cable shields are not connected to anything on their other ends.

No I am saying it is not productive to spend time and effort on hypotheticals about why they might be there. It does not seem obvious to me either. Ideally we would ask the person who added the extra parts.

In lieu of that focus on the known unknowns, continue to clean up the noise floor and if the extra resistors were serving some useful purpose they will reveal themselves later in the quieter room.

JR

 

[leigh]

Well, while intending to let the 12k resistors go, I moved on to cleaning up the Aux section. And, right away, those 12k's came right back to bite me in the ass!

See this thread: http://groupdiy.com/index.php?topic=57244.0

In short: the 12k resistors are there to ensure stability of the pseudo-floating "EBOS" balanced outputs. Even if you hook the console up using poorly wired unbalanced cabling that leaves one output pin hanging, that leg of the output circuit will still have a 12k termination to ground to keep it stable. Otherwise, oscillation can occur (in the above case, it was a 12VAC squarish wave at 90 Hz).

 

[JohnRoberts]

Told you they would reveal their real purpose ..... or not.

JR

 

[leigh]

Getting back to the main topic of this thread (grounding), this issue with the instability of the quasi-floating output circuits (aka "EBOS") when their outputs aren't referenced to ground makes me question the feasibility of the pin 1 changes I had planned.

On the electronically balanced output jacks, I had planned to disconnect pin 1 from signal ground, and connect it only to the chassis. But, if I do that, then those 12k "stability" resistors to ground would be going to chassis ground, which (as I've mentioned elsewhere above), is actually a completely different ground on this console than the signal ground (chassis ground connects only to the 5V supply ground, which is an independent ground coming from the power supply). Sounds like a bad idea.

I could still run the 12k resistors to a signal ground, via a separate wire, and have pin 1 connect ONLY to the chassis, and not those resistors at all. Physically, that would be awkward – I'm imagining resistors hanging off pins 2 and 3, their other ends tied together but floating "in space", with a signal ground wire draped onto their common point.


Interesting note in this other thread, about MCI vs Soundcraft implementations of their EBOS circuits, mentioning that it "changed the grounding rules in that you had to carry a ground through somehow to give a reference to the electronic balancing": http://groupdiy.com/index.php?topic=34561.msg426304#msg426304


I guess what I'm getting at is that EBOS circuits and the AES48 standard to address the "pin 1 problem" don't make for easy bedfellows in this case. Disappointing (and confusing) that terminating the EBOS output across pins 2 and 3 with a 600ohm resistor does NOT solve circuit instability.

 

[radardoug]

I guess what I'm getting at is that EBOS circuits and the AES48 standard to address the "pin 1 problem" don't make for easy bedfellows in this case. Disappointing (and confusing) that terminating the EBOS output across pins 2 and 3 with a 600ohm resistor does NOT solve circuit instability.

If you do it properly there is no conflict. Put the 12K's on the pcb right by the drivers, and earth to local audio ground. Then earth pin1 independantly and correctly. You got into this, the least you can do now with your new knowledge is follow through correctly.

 

[leigh]

I guess what I'm getting at is that EBOS circuits and the AES48 standard to address the "pin 1 problem" don't make for easy bedfellows in this case. Disappointing (and confusing) that terminating the EBOS output across pins 2 and 3 with a 600ohm resistor does NOT solve circuit instability.

If you do it properly there is no conflict. Put the 12K's on the pcb right by the drivers, and earth to local audio ground. Then earth pin1 independantly and correctly. You got into this, the least you can do now with your new knowledge is follow through correctly.

Yeah, I didn't say it was impossible, it's just not an easy switchover. I looked at the PCB, but it's pretty stuffed around the drivers, so adding 16 new resistors onto that board would take a lot of fiddly tinkering.

The 12k's are currently wired directly onto the output XLR. It's probably easiest to keep them on the XLRs, but unsolder them from pin 1. Pin 1 gets a line direct to chassis; the now loose ends of the 12k's get a line to signal ground.

I'll have to see how much elbow room exists around those XLRs to see how gracefully that signal ground line can be physically constructed. I'd rather not leave the unanchored ends of the resistors just floating in space, so maybe I'll run a baby bus bar (12 ga solid wire) along the bottom of the XLRs, anchored to (but electrically isolated from) the chassis. The resistors' loose ends could just get flipped down and soldered onto that.

 

[leigh]

However, the Trident's summing amps don't have "isolated" ground references at all. In fact, pins 3 (the + input) of IC20 and IC22 are each within a 1/4" of PCB trace from the grounded end of C22 and C69, respectively. Those caps (not shown in the schematic attached) are the big local power bypass caps (originally spec'd as 100µF, now bumped to 470µF).

Anything other than a perfect ground on pin 3 of the summing amps gets amplified by something like 30dB. So, question #1, was placing the grounded end of power bypass caps right by those + inputs a poor/compromised decision, or is there some deeper wisdom to it?

Having now gone full circle on this, I do wonder if in fact there was a "deeper wisdom" to the PCB layout.

I went in and did some surgical cuts to the PCB traces, completely isolating those 0v reference pins of the summing amps. I tried running their own longish wires to the ground pin of the master module, and then alternately running shorter wires to the far end of the "local bus bar" (the 12 ga wire I installed on master module). At best (the short jumpers), this added another 4 to 6 dB of 60Hz and harmonics peaks. At worst (running the long wires), I got radio pickup...and that was inside a shielded chassis.

When I ran the short jumpers to the 0v reference pins, I tacked them onto the far end of the local bus bar, and had all the other local ground connections (like the ground sides of power bypass caps) tacked onto that bar closer to the module's ground connection. This was an attempt to implement the better grounding topology described in Figure 9 of Analog Devices AN-202. The basic idea, as I understand it, is to keep power current return loops "smaller" than the 0v reference span, so that power currents run back to their source without trampling over your 0v node. My thinking on this point is admittedly fuzzy, however. And in practice, as I mentioned, this only brought the power hum harmonics up a bit.

 

[leigh]

OK, here's a concise question: For the shielded cable runs within the console, should those shields connect to signal ground or to chassis ground?

I could see it going either way... I'm trying to wean off ever using signal ground for shielding purposes. However, if you use chassis ground for internal shielding, then you are potentially running RF-induced currents that should remain in the outer chassis shell right through the middle of your circuits... not to mention running them in parallel with the very (inner) signal wire you are attempting to shield!

 

[PRR]

> For the shielded cable runs within the console

Un-balanced?

Use shielded twisted pair. Red is Hot. Black connects the two signal commons. Shield to garbage grounding (chassis).

 

[leigh]

Un-balanced?

Use shielded twisted pair. Red is Hot. Black connects the two signal commons. Shield to garbage grounding (chassis).

For both unbalanced or balanced lines, the Trident has most (if not all) the internal cabling as shielded cables. (The unbalanced ones have just a single conductor wire inside.)

The shield for those cables, in their stock wiring scheme, connects to signal commons, and from there also to pin 1. I am in the process of "solving the pin 1 problem", by connecting pin 1 only to the chassis, and I didn't know whether those internal cable shields should also get switched over to be chassis-connected or not. You're saying they should indeed be chassis-connected.

As I said, that makes sense following the dictum to keep the signal commons as clean as possible, and therefore not to relegate it to the dirty work of shielding.

However, there is the competing principle to terminate pin 1 with the shortest possible wire to the chassis. Ideally, pin 1 would be terminated outside the chassis, so that stray EMF don't enter inside the shielding plane of the chassis (from the Rane note on grounding and shielding, "For optimum balanced performance connect the shield to chassis ground at the point of entry. This means that the shield conductor, to avoid spraying any induced RF energy into the box, never passes the chassis' outer plane."). So, if I'm shielding internal cabling with that same "garbage grounding", then aren't I encouraging that EMF to get picked up internally, by any of the unshielded circuit boards or the big, module-interconnecting motherboard?

 

[leigh]

Selected Bibliography / Helpful References

Balanced Line Technology - by Douglas Self
http://www.douglas-self.com/ampins/balanced/balanced.htm
- every I/O combination examined

Dealing with legacy pin 1 problems - by Hypex Electronics
http://www.hypex.nl/docs/appnotes/pin1_appnote.pdf
- a brief historical look, and how to update your gear now to comply with AES48
- "All that time invested trying to convince suppliers to make special parts with insulated screw holes and it just doesn’t bloody work."

Grounding and Shielding Audio Devices - by Rane
http://www.rane.com/note151.html

Sound System Interconnection - by Rane
http://www.rane.com/note110.html

 

[JohnRoberts]

Un-balanced?

Use shielded twisted pair. Red is Hot. Black connects the two signal commons. Shield to garbage grounding (chassis).

For both unbalanced or balanced lines, the Trident has most (if not all) the internal cabling as shielded cables. (The unbalanced ones have just a single conductor wire inside.)

The shield for those cables, in their stock wiring scheme, connects to signal commons, and from there also to pin 1. I am in the process of "solving the pin 1 problem", by connecting pin 1 only to the chassis, and I didn't know whether those internal cable shields should also get switched over to be chassis-connected or not. You're saying they should indeed be chassis-connected.

Shield connections for internal point to point wiring is a different situation than shield connection for input/output wiring. Input and output shields are expected to carry RF and and even chassis to chassis ground currents.  Shields inside a console should be in a much more benign environment wrt noise.  Inside a console the shield is more likely used not to reduce noise pickup as much as control crosstalk pick up or transmission of crosstalk into other circuits. 

As I said, that makes sense following the dictum to keep the signal commons as clean as possible, and therefore not to relegate it to the dirty work of shielding.

You don't want to dump switched LED currents and the like into audio 0V, but an internal audio signal shield which will mainly be some weak capacitive coupling is not likely to corrupt 0V as much as the signal current flowing in few send pots.

However, there is the competing principle to terminate pin 1 with the shortest possible wire to the chassis. Ideally, pin 1 would be terminated outside the chassis, so that stray EMF don't enter inside the shielding plane of the chassis (from the Rane note on grounding and shielding, "For optimum balanced performance connect the shield to chassis ground at the point of entry. This means that the shield conductor, to avoid spraying any induced RF energy into the box, never passes the chassis' outer plane."). So, if I'm shielding internal cabling with that same "garbage grounding", then aren't I encouraging that EMF to get picked up internally, by any of the unshielded circuit boards or the big, module-interconnecting motherboard?

Yup, pin one as a sewer line, but once you dump the sewage to the chassis at inputs and outputs, it should no be around inside the console to corrupt internal shields.

Of course this ASSumes the console is inside a well grounded metal chassis, so the inside is already shielded. .

JR

 

[leigh]

You don't want to dump switched LED currents and the like into audio 0V, but an internal audio signal shield which will mainly be some weak capacitive coupling is not likely to corrupt 0V as much as the signal current flowing in few send pots.

Thanks for weighing in on this, John. So, you're saying that it would not be an awful violation of the 0V (signal ground) line to utilize it for a little internal shielding?

Yup, pin one as a sewer line, but once you dump the sewage to the chassis at inputs and outputs, it should no be around inside the console to corrupt internal shields.

Of course this ASSumes the console is inside a well grounded metal chassis, so the inside is already shielded. .

Pin 1 as a sewer line when it leaves the console's chassis, ok. But my understanding is also that, even if you connect pin 1 to chassis right at the point of entry, if you simultaneously extend a pin 1 wire inside the console, there's no guarantee that wayward currents will all be sucked out right at the chassis wall. They may just as well continue into the heart of the console, on a pin 1/chassis connected wire. And hence, my thinking that internal shielding should NOT be pin 1 connected.

For the record, Trident's metal chassis seems up to the task of shielding. I held my 40W Weller iron, which spews out EMF hum like crazy, right under the console, and could not hear it show up on the output at all.

 

[JohnRoberts]

You don't want to dump switched LED currents and the like into audio 0V, but an internal audio signal shield which will mainly be some weak capacitive coupling is not likely to corrupt 0V as much as the signal current flowing in few send pots.

Thanks for weighing in on this, John. So, you're saying that it would not be an awful violation of the 0V (signal ground) line to utilize it for a little internal shielding?

Yup, pin one as a sewer line, but once you dump the sewage to the chassis at inputs and outputs, it should no be around inside the console to corrupt internal shields.

Of course this ASSumes the console is inside a well grounded metal chassis, so the inside is already shielded. .

Pin 1 as a sewer line when it leaves the console's chassis, ok. But my understanding is also that, even if you connect pin 1 to chassis right at the point of entry, if you simultaneously extend a pin 1 wire inside the console, there's no guarantee that wayward currents will all be sucked out right at the chassis wall. They may just as well continue into the heart of the console, on a pin 1/chassis connected wire. And hence, my thinking that internal shielding should NOT be pin 1 connected.

I use the sewer analogy on purpose. Current flows like water following the path of least resistance. The path to ground for noise coming into pin 1 should be lowest resistance through the chassis which should have it's own low impedance connection to ground. If that noise current has to travel through  audio circuits to find it's way to ground, the audio can get corrupted.

[edit] if the audio 0V is connected to power ground through 10-100 ohm resistance that will make little difference, but would steer current from say a common connection to pin 1 to follow the low impedance path through chassis metal to ground. [/edit]

For the record, Trident's metal chassis seems up to the task of shielding. I held my 40W Weller iron, which spews out EMF hum like crazy, right under the console, and could not hear it show up on the output at all.

A good thing...

JR

 

[leigh]

And those connections to chassis from the XLR case and p1 need to be AS SHORT AS POSSIBLE.  For RFI protection 2" is way beyond too much.

One more post before I gotta run - do you have a recommendation for how to best make the connections to chassis?

Revisiting this issue, and with fresh eyes, the answer is obvious of how to best make the pin 1 connections to chassis for the input TRS jacks: remove those plastic isolation washers! 

Problem solved. Will replace with metal washers, and for some of them, use the Dremel to scrape off a little paint around the chassis holes.

 

[Andy Peters]

Problem solved. Will replace with metal washers, and for some of them, use the Dremel to scrape off a little paint around the chassis holes.

Metal lock washers with the teeth are your friend!

-a

 

[ricardo]

And those connections to chassis from the XLR case and p1 need to be AS SHORT AS POSSIBLE.  For RFI protection 2" is way beyond too much.

One more post before I gotta run - do you have a recommendation for how to best make the connections to chassis?

Revisiting this issue, and with fresh eyes, the answer is obvious of how to best make the pin 1 connections to chassis for the input TRS jacks: remove those plastic isolation washers! 

Problem solved. Will replace with metal washers, and for some of them, use the Dremel to scrape off a little paint around the chassis holes.

The ideal is for the connection from shield to chassis to be made OUTSIDE the box.  There's some good stuff about this on the web describing the special Neutrik XLRs which have built in ferrites & coaxial ceramic caps.

ANY length of connector inside the box is an aerial, the longer the connector, the better the aerial.

So letting TRS sockets connect to chassis is excellent.

 

[leigh]

Problem solved. Will replace with metal washers, and for some of them, use the Dremel to scrape off a little paint around the chassis holes.

Metal lock washers with the teeth are your friend!

Indeed! I didn't know if those would be sufficient to cut through the paint on the chassis, but in testing over the weekend, they seem to be doing the job. Thanks for the tip!

 

[stitch-o]

I just finished cleaning up a Trident series 65 desk and used some of the grounding ideas in
this thread, specifically the line input-back panel mods to pin 1.
Thanks to Leigh for the many PMs he answered.

End result: the desk is ridiculously quiet.
My client's jaw was on the floor when we powered it up.

I wanted to chime in on the side of practical experience in regards
to these ideas Leigh has sussed out. Very valuable.

Cheers and thanks again Leigh!

http://www.google.com/search?q=pin+1+problem

http://www.rane.com/pdf/ranenotes/Pin_1_Revisited.pdf

Good stuff, I had read those again recently. Muncy coined this "Pin 1 Problem" in 1995, and the Trident Series 65 pre-dates that by over ten years. A little redesign is in the cards, I suppose.

So, is this your preferred "Right" connection, or do you favor connecting the shield via a small cap (so RF still goes to chassis but low freqs do not), ferrite bead, or some other more complicated setup?

 

[leigh]

Been working on rewiring a bunch of the i/o jacks on the Trident, to make them in line with the AES48 standard. As stitch-o noted, it really does make a difference to the noise floor! I just got the 8 Aux outs rewired at last, which are on XLR's. That made for a tricky operation, because of the 12k terminating resistors that were soldered onto the jacks between signal ground and pins 2 and 3. I'll explain that mess later, and share some photos of how I found a solution of how to keep the terminating resistors but provide a pin 1 to chassis ground connection.

Tonight, however, I have been taking a look at impedance balancing some of the 1/4" outputs that are currently simply unbalanced. I understand in principle how this is supposed to work, but in my testing, adding a terminating resistor between pin 3 (well, technically, the "ring" of the TRS output jack) and ground has only hurt the noise floor.

An example: on the Trident's group modules, the insert send signal is sent by IC 13, out through 100Ω buildout resistor R81. So, in theory, if I add a 100Ω resistor between the output jack's pin 3 and ground, that should (roughly) "impedance balance" what was formerly a simple unbalanced signal. In testing, however, adding any resistance only increases the level of background hum. It's only by a couple dB (between 0Ω and 100Ω termination), but it's definitely not helping things. And so this practical example is not conforming to the theory of what I thought "impedance balancing" was.

Any ideas?

Two technical addendums:
1. Stock, the insert sends are TS jacks. I have replaced those with TRS jacks (allowing separation of signal ground and chassis ground, which drops noise floor in testing).
2. When comparing 0Ω and 100Ω pin 3 termination, the same jumper wires are in place, running to the same trimpot. In other words, I am comparing different settings of a trimpot, not comparing the presence vs the absence of extra jumper wires (which could be picking up extra noise).


PS: Attached is a schematic of an impedance-balanced output, courtesy this Douglas Self article.