[oneplustwo]

Hey folks,

Quick question. On the attached image from AMB's site, what are the 4 little rectangle things that the "In's" point to and why are they connected to ground? I'm looking to execute this option when I convert my 3 channel to a 4 channel and wanted to make sure I did all my homework.

Thanks.

 

[oneplustwo]

Nevermind! I think I just figured it out while looking at the pre-amp option section. On that diagram, those rectangles are labeled volume so I assume that's the answer to my question as well.

 

[digger945]

Are you wiring the amp for just balanced output only?

 

[oneplustwo]

no, for both balanced and single ended as described on AMB's "options" page.

 

[digger945]

The one with the DT4P switch? I think the diagram is on Headwise which is down at the moment. I do have a printout of it.

EDIT: Ahh I see now. Your using the diagram on AMB's website. The one I mentioned above is for the switch that grounds the negative amps when listening to a SE source with balanced cans.

Are you using Neutrik combo jacks?

 

[amb]

You might want to use this scheme I posted sometime back instead:
http://www.head-fi.org/forums/4665889-post704.html

This is how the krmathis amp was wired and is more flexible than the one shown on the β22 website. It allows a balanced and unbalanced source to be connected simultaneously and a switch selects between them. It also allows both balanced and unbalanced headphones to be used regardless of whether the selected source is balanced or unbalanced. I need to add the new diagram to the website.

Note that this diagram only shows the input side of things. At the output side, the balanced wiring is as you would expect. The unbalanced TRS jack's sleeve should be wired directly to the σ22 G terminal (passive ground).

 

[digger945]

^That's exactly what I did. I really wanted to seperate the grounding and keep each channel seperate in balanced mode, all the way to each sigma22, but it just doesn't work. Any way I connect it just keep getting noise and hum 'till the left and right grounds are connected. Connecting the balanced input signal ground helped more than anything to make things quiet.

 

[oneplustwo]

digger945 - Yes, I was planning on using the Neutrik combo jacks that were suggested on Amb's site.

Amb - would you mind answering some follow up questions?

1. since I'm putting the σ22 in a separate case, would I just go ahead and wire up the TRS jack sleeve to the corresponding ground pin of the XLR jack (like you have shown in the 2 channel wiring diagram)?
2. I presume the 'R+' output goes to the R TRS jack position, the 'L+' output to the L TRS jack position?
3. Finally, I assume I would do the same for both combo jacks? Or should I just pick one? My guess is I can do either with the understanding that the volume matching will depend on what headphones are plugged in.

Also, two semi-related questions:
1. Any concern with running balanced with just one σ22 with a 100VA toroid? Just more heat?
2. Also, any suggestions for gain with my Sennheiser HD650's? I was thinking a gain of 2x in balanced mode would be fine given the multiplier effect of being balanced plus my experience with my 3 channel amp running 8x but maybe 5x would be safer.

 

[amb]

oneplustwo,

1. The TRS jack sleeves go to the σ22 case in separate wires (I used two 4P Amphenol connectors for the power umbilical, 3 of which are used for each side's V+/G/V-, and the fourth is used for the TRS ground return. The reason for using a separate wire for the TRS jack's ground is so that the headphone return current does not ride on the signal ground wire to the β22 boards.

2. Yes.
3. Yes. You can use just one or use them both.

One σ22 can support four boards, but yes it would run hot, and does not give you the elegance of having a dual-mono scheme you see in the krmathis amp setup.

The appropriate gain is also highly dependent on what source you'll be using. Balanced sources vary all over the map on this issue.

 

[johnwmclean]

oneplustwo, I believe with a 4 channel build single ended listening runs only in 2-channel passive-ground configuration. If I was building a single ended and balanced β22, I would probably go five channels, if you’ve got the wallet.

 

[oneplustwo]

Ah yes, the wallet. I'm actually thinking about going the other way and just building it for balanced headphones. i'm not sure i'll be taking advantage of the single ended feature anyway and it seems like it would be a cleaner build just to stay focused on both balanced inputs and outputs.

While I'm thinking out loud, I'm also considering making a 4 channel fresh and then listening to both for a while. That way, I can have a way to compare the 2, 3, and 4 channel varieties all at the same time. Whereas if I convert my 3 channel into a 4, then I'll only be able to compare the 2 and 4 channel iterations simultaneously. That will be a significant outlay of cash, however, than I'm not sure I'm willing to foot for the sake of comparison alone. Maybe someone will offer to buy my uber cookie tin.

Then there's the question of one or two sigma 22's... another money sucking option.

And of course, finally, there's the question of when the Buffalo DAC kits will be available again and when I can talk myself into spending the money on that.

Decisions, decisions! All fun stuff though. Besides, for me, the journey is often just as fun as the destination.

 

[digger945]

Once you listen balanced the others wont matter much(with a good source/cans of course).
It ain't cheap no.

 

[oneplustwo]

Yah, exactly my thought! I know there's a lot of debate over balanced, SE, blah blah blah so I wanted to compare for myself. But for my wallets' sake, I might as well just convert what I have into a 4 channel (maybe with single ended features) and be done with it.

 

[oneplustwo]

Amb,

On the alternate schematic, it looks like the signal for L- and R- go to the volume pot (in my case stepped attenuator) first before going to the switch. Is that necessary or can I go to the switch first and then to the attenuator. The only difference I can determine is that you wouldn't be "attenuating" the ground channel when switched to single ended mode but does that matter?

 

[amb]

oneplustwo, there is a reason why the L- and R- inputs are wired that way. The switch is after the pot because we don't need a volume control there. If we wired the pot after the switch, it will add resistance that vary with the volume control pot position needlessly, and resistance causes noise (hint: google "resistor noise").