Mistakes you made with your β22 or σ22

1. Don't mount the power supply transformer in the β22 chassis, you'll end wasting weeks (and $$$) trying to resolve transformer hum and then end up moving it to an external chassis (and end up with lots of extra perforations in the bottom of the amp chassis).

2. Don't put a power switch on the output of the σ22, the surge from powering on the β22 will eventually blow the output transistors and resistors. It won't do it right away, but if a curious coworker decides to quickly toggle it off and on,

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i believe Amb mentions in his website not to do this...

Refarding sigma22, don't switch the mofsets, you'll get a high voltage reading (I wanted 30, it gave me 54 )

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That would lead to point #3:
3. Read AMB's instructions carefully (although I don't recall seeing the warning). Then again, I had already built the power supply and it was working for months before I went back and did the separation.

On my first build, I didn't put the insulator on the heatsinks, the MOSFETs got blown when the probe touched the exposed part of the heatsinks when I was poking around.

Luckily, AMB came to the rescue

Additionally, inspect the boards carefully before building to reduce the chances of having shorted traces.

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Ti had written a very good article on scenario #2 a few years back. I don't know where to find it anymore.

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I disagree. If you use a fully-shielded transformer from someone like SumR or Plitron, transformer hum shouldn't be an issue in a one-chassis solution. The extra cost of the transformer is saved several times over by not having to buy two chassis (and umbilical hardware)

For me, a 100VA 30V encapsulated was only about $40 more than an unshielded one.

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x2. No hum from my SumR encapsulated transformer.

Now back to the OP:

- Measure carefully all your boards and wiring layout before drilling, punching and cutting your enclosure. I work hard to make my volume pot wiring as short as possible by using an extension shaft and bring the pot at the rear near the RCA inputs. But I finally had to install the IEC module somewhere I didn't expect initially, about a inch or two from the pot... But luckily, the IEC module very close distance didn't induce any hum in my precious TKD pot.

- Order initially the parts need for the Zobel network even if you plan to use your B22 exclusively with headphones. My B22 oscillate with my DT-880 when the volume is fully turn counter clockwise.

- For a few bucks extra, order at the beginning all the parts needs for all the gain settings. You never know what gain you really need. I want to reduce my gain to 5x and had to pay shipping again for a few caps and resistors...

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From amb site:

“Zobel network: if the amplifier is intended to drive speakers, the Zobel networks as shown (22Ω 2W metal oxide power resistor and 0.047µF 100V film capacitor in series) should be connected across the output binding posts of each channel to compensate for the inductive speaker load. Without the Zobel network, the amplifier may become unstable under certain conditions. If your β22 will only serve headphone duty, then the Zobel network could be omitted....”

so if your not using “outputs” or “speakers” you don’t need the zobel network for purely headphone listening?

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If pretty certain if your read the B22 support thread on HW, you could find some posts where AMB recommend to install a Zobel network where some headphone reactive loads could make the B22 oscillate.

I ordered the parts recently but I didn't install the Zobel yet. I don't use very frequently my DT-880 since I got my RS-1... But I'm pretty confident the Zobel will resolve the issue I have with my DT-880.

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This has me curious since I hope to start my second build soon. How do you measure each solder joint? It sounds like a great thing to do, but I don't know exactly what to do.

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Look at the schematic and see what each lead of each part is supposed to connect to. After soldering, use your DMM to check the resistance between every lead on that "net" to make sure you have ~0 ohms (or whatever your DMM would actually read when the two probes are touched together).

This exercise also makes you associate the PCB layout to the schematic, so you're aware of the portion of the circuit you're populating (in contrast to just blindly stuffing the board with parts). Hopefully you'll take this opportunity to try to understand the circuit a bit. Despite the complexity, the β22 topology is made up of many common basic building blocks (differential amplifier, cascode, Vbe multiplier, source follower, etc). Look these up on the web and see if you could identify them in the circuit.

One recommendation: If you're using Vishay-Dale RN55 or CMF55 series resistors (or any that marks the resistance in numeric code rather than color bands), install them with the resistance marking facing up so you can see at a glance what they are. (e.g., a 2.21K ohm resistor will show as "2211"). This is what I do in any project that use these resistors, not just the β22/σ22.

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Is there an added danger when doing this that a stray probe could accidentally short a component? I imagine so long as you follow the circuit traces carefully between components that have continuity you’ll be ok. Still worries me though.

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In general, you should only take resistance measurements when there is no power applied to the circuit.

That's right. You should never measure the resistance of anything in a live circuit. Always do so with the power off, and wait for all the capacitors to discharge completely.

I think that if you do it as described, as you solder each component, you won't have power anyway.

Thanks for the description. It sounds like an ideal way to do things.