Questions about switching in balanced M^3

[tennisets]

I probably won't do a very good job asking this question (or questions), but I will do my best.

I'm planning on building a balanced M^3 with both unbalanced and balanced inputs and outputs, as well as speaker terminals. I've seen amb's diagram for balanced/unbalanced switching in the b22, and could certainly implement that (it implements a passive ground). However, what would be better would be to use the active ground channel on one of the M^3 boards when the unbalanced input is selected. This is a similar situation to a 5 board b22, I think. Of course, a balanced M^3 uses two M^3 boards, using the Right and Left channel on each for + and -. I would also like to use the G channel on one of the boards when unbalanced input is selected.

Another question: am I ok just using a 4PDT switch to switch between headphone and speaker output? Additionally, Zobel networks are recommended on the speaker terminals for the b22, but no mention is made of Zobel networks in the M^3 literature. Are they needed or unnecessary?

Edit: I think this would work, but I might have missed something. Sorry for the poor diagram, I just did it in Paint since I don't have any real drawing programs on my home PC. The diagram shows 2 4PDT switches, one for switching input between balanced/unbalanced, one for switching output between speakers/headphones. When I say "L Board G" input/output, I am referencing the fact that I want to use the ground channel on one board, the + channel on that board and the + channel on the other board (+ instead of R/L since it's a balanced configuration). Any input/output labeled with "Board" is obviously on the circuit boards, anything labeled "Balanced" or "Unbalanced" refers to the input jacks on the rear of the chassis. I didn't include the single ended headphone out in this diagram, but that part is straightforward. Amb's original b22 input switching diagram is still attached at the bottom.

I don't care if I can use the balanced input with unbalanced out; it is not necessary. On the other hand, being able to select an unbalanced input even when using a balanced output has obvious advantages from a convenience standpoint.

Please offer any criticism. I may be an EE, and I may know my way around a soldering iron, but this is not my area of expertise (I'm a signal processing guy).

 

[tennisets]

No one has any input?

 

[guzzler]

Assuming you're using a sigma22 for power, I wouldn't bother with an active ground channel - the impedance of the sigma22 is already low and it's a "real" ground. Have a browse through this thread, might give you some thoughts.

 

[tennisets]

I was planning on using a sigma11. I just looked into using a sigma22, since I hadn't really considered it, and I think it will be easier to just do the switching I described above than have to worry about inserting power into the board in non-standard places. The M^3 board is built with a single rail power supply in mind, so I think it would be easier to just stick with that.

Plus, I've seen a post by amb stating that with the b22 switching arrangement in the diagram above, the sleeve of the TRS output actually has to be run all the way back to the power supply board through a separate wire in order to avoid contaminating the balance - channel with TRS ground, or something like that. That is not a direct quote and is probably a little off technically, since I don't want to go find the post, but the result is what's important anyway. It shouldn't be an issue with the active ground config though. I also feel like since I don't need any extra boards or anything to implement active ground, I might as well use it.

Anyway, I've thought about this quite a bit over the last couple of days and the switching arrangement I posted above should work. The only thing that occurred to me today is that I'll only have the full power of balanced output for speakers with the balanced input. That's what I'll be using 99% of the time anyway though, so it's not a big deal. If it ever became an issue in the future I could implement a balancing circuit on the unbalanced inputs.

Sorry for the long, rambling post. I think I'm good as far as the switching now (feel free to correct me if that's not true though).

 

[amb]

Since the M³ already has 3-channels per board, it's just a matter of populating the ground channel(s) on the board to implement an active ground. The input switching scheme would be unchanged from my β22 diagram (except that M³ has the volume pot onboard so you'll need to hack the board to do what the diagram does). The unbalanced TRS jack sleeve would be wired to the ground channel's output.

 

[tennisets]

amb,

Are R3 and R1 irrelevant when the input to, for example, the L channel on the M^3 board is switched to Sg? In other words, in the ground channel R3 and R1 are not present. Does their presence affect the operation of the L channel as a ground channel, or does L with grounded input operate effectively the same as G? Hope that makes sense, and I hope it's not a stupid question.

I guess the same question could be phrased this way: What effect is there (if any) of having active ground done separately for the left and right headphone drivers, since it would be done by the - channels of each amplifier respectively for balanced headphones? Basically, should there be any functional difference between 1) Running through the balanced output jacks with the - channels' inputs grounded and 2) Running through the single ended output jack with a balanced to single ended adapter cable?

I thought better of my idea of switching the output since I'd like to use a rotary switch, but there are no inexpensive rotary switches with a high enough contact current rating.

Thanks for your help.

 

[amb]

tennisets, R3 is missing in the ground channel because we need no voltage gain.
R1 is missing because the ground channel needs no input.

If you look at my switched balanced/unbalanced input diagram above, you'll notice that it doesn't do anything to R3 so when the L/R channels act as the ground channel, it will still have voltage gain. The ground amp(s) will have a bit higher noise floor this way, but in practice, unless you're using very efficient headphones it probably isn't audible.

The diagram also shows that the "-" channels' amp inputs are switched to ground in unbalanced mode (and disconnecting it from the input), which effectively does the same thing as not having R1. Note that this is done after the volume pot.