Trident Series 65 grounding: theory and practice

[leigh]

Okay, thank you Leigh. 

My question about the insert switches in bypass is that even though it's being shunted internally, the insert send/return jacks on the back of the board are still plugged out to the patch bay (which has a normal).  The way I've wired the switches, the send signal is always connected to the send jack, I just tapped off of the send's molex pin for the input of the switch.  There is electrical continuity from the send all the way to the return via the patch bay.  I am concerned about noise from the return line as per this electrical continuity.  Are you saying that the internal shunt overrides the snake to the patch bay because it is the shortest path and therefore I needn't worry about the dummy patch cords?
 
I have one more simple question right now.  At the beginning of the forum you say you swapped the summing amps for LME49710's.  Are these just IC20 and IC22 on the master section?

So your insert switch bypass isn't a "true bypass"? I'm still not clear on the wiring of the switch, maybe you could sketch a diagram. As I understand it, your insert send is always "split", to both the insert send jack (and thereby to cabling and to your patchbay) and to one lug of the bypass switch. But it's only the return that is actually switched, between either the insert return jack, or the internal molex pin from the send?

About the summing amp chips - yes, IC20 and IC22 are the summing amps. For the purposes of reducing noise, that is the most important pair of opamps to upgrade. However, there are other sonic benefits of better opamps besides just reducing noise, and so I have replaced all the chips in that section.

For example, for the left channel, all these are now LME49710's:

IC20    summing amp
IC19    insert send driver
IC24    fader buffer
IC16    remix output driver, - leg
IC15    remix output driver, + leg

Haven't seen any oscillations from the upgraded chips.

As an aside, as mentioned elsewhere in the Trident threads, IC15 and IC16 together work as a "cross-coupled output driver" aka EBOS (Electronically Balanced Output Stage). For various reasons, the circuit is kind of crappy (for one, it's unstable, which is why they added those 12k resistors on the output jacks between each output leg and ground), and ideally it would be replaced with a better output driver circuit.

 

[leigh]

Also, just to clarify, it is advantageous to chassis ground all the jacks on the rear of the console whether balanced or unbalance, input or output, XLR or 1/4"?  Or are there some that this is not a good idea?

Chassis grounding is for cable shields. Not for conductors carrying signal OR conductors carrying "audio ground" (which you might think of as a 0v signal that you don't want disturbed). So it depends on what kind of cables you are using, not necessarily whether their signals are balanced or unbalanced.

To wit: Take another look at that AES48 spec. It shows balanced signals, but you could also send an unbalanced signal with that wiring scheme, if you send the unbalanced signal on a 2-conductor-plus-shield cable. Pin 1 is cable shield, it goes direct to chassis. Pin 2 is signal. Pin 3 is "audio ground", or 0v. Although the signal is unbalanced, you are still separating "clean 0v reference ground" from "dirty shield/chassis ground".

That's my take on it, anyways...

 

[blake1234]

So your insert switch bypass isn't a "true bypass"? I'm still not clear on the wiring of the switch, maybe you could sketch a diagram. As I understand it, your insert send is always "split", to both the insert send jack (and thereby to cabling and to your patchbay) and to one lug of the bypass switch. But it's only the return that is actually switched, between either the insert return jack, or the internal molex pin from the send?


[/quote]


Yeah, your description of my insert switch is spot on. 

 

[blake1234]

Chassis grounding is for cable shields. Not for conductors carrying signal OR conductors carrying "audio ground" (which you might think of as a 0v signal that you don't want disturbed). So it depends on what kind of cables you are using, not necessarily whether their signals are balanced or unbalanced.

To wit: Take another look at that AES48 spec. It shows balanced signals, but you could also send an unbalanced signal with that wiring scheme, if you send the unbalanced signal on a 2-conductor-plus-shield cable. Pin 1 is cable shield, it goes direct to chassis. Pin 2 is signal. Pin 3 is "audio ground", or 0v. Although the signal is unbalanced, you are still separating "clean 0v reference ground" from "dirty shield/chassis ground".

That's my take on it, anyways...
[/quote]

I guess my question would be; without switching to TRS, is it advantageous to chassis ground the insert returns and leave the internal cable shield hanging?  Also, would it be advantageous to chassis ground the Group outputs and Main outputs the same way you did with the Aux's (pin 1 to chassis and audio ground to a common point with the two resistors)?

 

[leigh]

I guess my question would be; without switching to TRS, is it advantageous to chassis ground the insert returns and leave the internal cable shield hanging?

Ah, there's the crux of it: no, unless you change those jacks to TRS, you can't follow the AES48 scheme. What your next-best option would be depends on individual circumstance, I suppose, including whether you have problems picking up RF interference in your location.

Some of the jacks on the board (the input channels' line in jacks, for one) are already TRS, you just need to get in there and snip the "pin 1" connection to audio ground, and replace the plastic isolating jack washers with toothed metal washers that will electrically connect pin 1 right to the chassis.

Also, would it be advantageous to chassis ground the Group outputs and Main outputs the same way you did with the Aux's (pin 1 to chassis and audio ground to a common point with the two resistors)?

Group outputs and Main outputs are already balanced signals on XLR outs, so yes I chassis-grounded their pin 1's. Those two 12k resistors, one off each balanced leg (as mentioned above, these were added to the design to stabilize the EBOS circuits, and they do not appear on the schematic), then need to go somewhere else. It becomes a bit of a pain in the ass, since to do the wiring by the book, you then need to run an audio ground wire out to that point. But you might try just leaving those 12k resistors running to pin 1 (which has been newly made a chassis ground point, of course), and see if it's not all that terrible.

Or, if you're really trying to finesse improving this circuit, you would look into a replacement output driver. Something stable, that wouldn't need that 12k resistors hack. I think we started talking about other options earlier in this thread, or perhaps it was in another thread... I'd have to go back and look around.

 

[blake1234]

Okay you said about chassis grounding XLR outputs that in order to do it by the books you would need to run an audio ground out to the resistors, and this is a pain.  Isn't the original pin 1 connection the audio ground?  Therefore you already have it running out to the jack and could tie it with the two resistors?  Or you just don't like the idea of having the resistors floating in space?

 

[leigh]

hey blake,

Right you are that there is an audio ground wire running to the output jacks (formerly connected to pin 1). I believe what I meant was, yes, I don't like resistors floating in space.

It can be made reasonably mechanically sound, by using a triangle shape. One end of each resistor is anchored because it's soldered into the XLR soldering cups for pins 2 and 3. Then twist the free ends of the resistors around each other once, and solder that connection. Now you've got a little triangle shape, and you can wrap the ground wire once around the apex of the triangle and solder that in place too.

It's all just a little more fidgety than I like. Not at all bad to do to a single pair of outputs in this manner, but it makes for slower going if you're doing that across a bank of 16 XLR outs...

Leigh

Okay you said about chassis grounding XLR outputs that in order to do it by the books you would need to run an audio ground out to the resistors, and this is a pain.  Isn't the original pin 1 connection the audio ground?  Therefore you already have it running out to the jack and could tie it with the two resistors?  Or you just don't like the idea of having the resistors floating in space?

 

[MrMesh]

Hey leigh,

Just wanted to put out a thank you for taking the time to create this thread and go through the painstaking work that you did to find workable solutions.

I have a few questions to throw into the mix:

i. Would it also be beneficial for the sockets of the 'direct out' jacks to be connected to the chassis via a metal lock washer?
ii. Do you have any pictures of your wiring for your insert send/return jacks which were converted from TS to TRS? Would it be beneficial to perform this task on all insert sends/returns?

-Christian

 

[leigh]

i. Would it also be beneficial for the sockets of the 'direct out' jacks to be connected to the chassis via a metal lock washer?

ii. Do you have any pictures of your wiring for your insert send/return jacks which were converted from TS to TRS? Would it be beneficial to perform this task on all insert sends/returns?

hi Christian,

i. No, as mentioned earlier in the thread – unless you change those jacks to TRS, you can't follow the AES48 scheme. The Trident channel strips only provide an unbalanced output for the direct out jacks (which I don't use anyways) so they're currently TS jacks, and I didn't do anything with modding them.

ii. Which send/return jacks are you asking about? It's been a little while, but looks like I did the 8 group send/returns and the 2-bus send/returns. Never did the send/returns on the individual channel strips though.

 

[MrMesh]

i. No, as mentioned earlier in the thread – unless you change those jacks to TRS, you can't follow the AES48 scheme. The Trident channel strips only provide an unbalanced output for the direct out jacks (which I don't use anyways) so they're currently TS jacks, and I didn't do anything with modding them.

Ok, thanks. I might eventually convert all Direct Outs to TRS, so that's worth keeping in mind. The Tape Returns are on TRS jacks, so  I'm guessing those are definitely worth connecting to chassis, too?

ii. Which send/return jacks are you asking about? It's been a little while, but looks like I did the 8 group send/returns and the 2-bus send/returns. Never did the send/returns on the individual channel strips though.

I guess any of them, but if you converted the group insert send/returns to TRS I'd be interested in seeing how you converted those. Has the signal at those insert points also been converted to a balanced signal or was the jack conversion simply for grounding purposes?