1. I have a feeling there is a reason I have not seen anyone else planning a 5-board build. What are the issues with this setup? I am especially thinking of the power supply; having one Sigma 22 driving three boards and one driving two seems imbalanced, unless I had one driving all four signal boards and a second driving just the active ground. Or would it be feasible to use just one Sigma 22 for everything?
Yes, the issue is that you cannot evenly divide 5 amp boards into 2 power supplies. The amp will "work" as you described. But the reason why this is an issue is not so much a matter of current draw imbalance. It's wordy to describe this so bear with me.
One of the key benefits of balanced operation as well as 3-channel active ground operation is
supply rail current cancellation. In a balanced amp with push-pull output topology, when there is no signal, the quiescent current on each of the hot and cold amp outputs flow from the V+ rail to V- rail through the output transistors, with no current going through the headphone transducer.
When you turn up the volume to play music, as long as the amp remains operating in class A, in additional to the abovementioned quiescent current, when the signal voltage swings in the positive direction, there is now current flowing from the hot amp's V+ through the top transistor into the transducer and then to the cold amp's bottom transistor into V-. Simultaneously, due to the differential nature of the output, current also flows from V- through the hot amp's bottom transistor into the transducer and then into the cold amp's top transistor into V+.
When the signal swings negative, the exact same thing happens except the current directions are all reversed. See illustration below. The dotted lines are when the amp is in no signal condition. The solid lines show when the amp is amplifying voltage:
pic2.png
In all cases the supply currents on the rails sum to zero, because the hot amp's current draw is exactly out of phase of the cold amp's. This results in completely quiescent supply rails at any output level (as long as the amp stays in class A operation). Also since none of the transducer's output current go to ground, the ground is also completely quiescent.
For a 3-channel active-ground configuration, the situation is similar, except that the ground amp handles the signal currents for both stereo channels. The total rail current still sums to zero, and the ground is also quiescent.
pic4.png
To maintain these benefits, there are some requirements on the power supply topology. In a balanced amp, you could put all four channels on the same supply, or have one supply for each of the left and right channels (two boards each), and the rail cancellation will work. In a active-ground 3-channel amp, you need to have all three amp boards running off the same rails to get that benefit.
Since you're sharing the balanced left hot and right hot amps with the unbalanced configuration, you would only feasibly get rail cancellation if all five amp boards run off the same power supply. The σ22 is designed to handle up to four β22 boards with the stock onboard heatsinks, and has four sets of output terminals. If you use one σ22 to run five boards, you'll need to use bigger heatsinks and provide plenty of ventilation. You may also need to reduce the bias settings on the β22 boards to lower their current draw. Depending on the impedance of the headphones you use, reducing the bias too much may cause the amp to go into class AB operation on signal peaks.
Adding a passive ground unbalanced output to the mix further complicates matters. To contrast with the above scenarios, let's complete the picture by seeing what happens in a conventional passive-ground output topology:
pic1.png
As you can see, when amplifying signal, during positive signal swings the current flows from V+ through the top output transistor into the transducer and then into the positive rail capacitor and power supply ground. When the signal swings negative, current flows from ground and negative rail capacitor into the transducer, and then through the bottom transistors into V-. There is no rail current cancellation, and there are currents flowing in and out of ground. Since ground is comprised of real-world wires/PCB traces, and add to that the rail capacitors' ESR, you have a non-zero ground resistance as seen by the amp. According to Ohm's Law, when there is current flowing through a non-zero resistance, there will be non-zero voltage. In other words, the ground is not always at 0V, but wiggling slightly above and below 0V with the signal.
2. Assuming I get two Sigma 22s, I am currently thinking of using two
Hammond rack-mountable cases, one 17"x13"x3.5" and one 17"x8"x3.5" stacked on top of each other, but their construction is aluminum which I have heard is worse at blocking EMI. Should I look for a steel option, or would the distances involved in two separate cases be sufficient to prevent humming? (I would presumably put the transformer coils as far away from the Beta 22 boards as possible)
Transformers emit magnetic fields and cause hum, the farther you put the transformer from the amp boards (and input wiring and volume pot/attenuator), the better. Just using steel cases may not be enough unless all panels are steel and without vents, and steel is hard to work with (drilling, etc).
3. In general, how do people get their front and back panels customized with the right openings? Do you order a premade case and have the panels machined? Order custom-made panels? This is the biggest part of the build that I am unsure of as of now.
Some people DIY the entire case, others buy pre-made cases and do drilling/machining of the panels on their own, and then there are also outfits like
Front Panel Express that would machine very professional-looking panels for you, either on your own material or theirs.
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