No zener

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In the pass zen circuit a 10V or 20V zener diode (depending on what version) is shown across the gate terminal of the output fet. I beleive this is for over-voltage protection of the gate terminal. Why is it not necessary/desirable to have the same sought of protection on the CCS fet. I ask this question after I lost a CCS fet. I'm not sure what happened, maybe I wasn't careful enough with static precautions while I was modifying the circuit.

Also would it be advantageous to include a fuse in each positive supply rail before the CCS to give some form of short circuit protection? Would I size the fuse by tonging the supply rail under normal operating conditions? Any other comments/suggestions?


I left my crystal ball at home today, but I would guess that your surmise about the CCS device blowing during modification is a good one. The Zeners are there to protect the gates from all the wild & wooly things that can happen in the outside world--unpredictable things like winter static electricity charges jumping in the input jacks. The parts further inside live in a (usually) more predictable world.
Note that the Zeners are not, strictly speaking, necessary for the circuit to function. They're just there as insurance. The stock SOZ circuit doesn't show them, for instance, although I believe Nelson does mention them in the text.
If you want short protection, put the fuse in the speaker line. Yes, purists argue that it's audible (and I noted whilst reading Slone or Self [not sure which] that the numbers crowd have--once again--found that the people who use their ears were right...again, no credit given, natch), but sometimes safety is more comfortable than the last degree of sonic purity. At the moment, I've got fuses in the lines of my midranges since the Aleph 2s are sitting naked in the floor at the moment (I'm getting ready to try some film caps in the power supply and to bypass the 220s in the front end w/styrene caps), and I might get clumsy and twiggle something with a toe as I walk around.

For fusing, I believe you'll get the best results by:

1. Put a fuse in each power-supply rail. If one power supply provides power for multiple channels, each channel needs separate fuses.

2. The fuses should be of the slow-blow variety. The current rating should allow the amp to run into 3 ohms (or less) without blowing.

3. The output transistor bank should be able to conduct AT LEAST twice the fuse rating (Self recomends more like 5 times).

4. A few small (50-200uF) reservoir capacitors should be located in the area of the output transistor bank. These should be bypassed with 0.1uF capacitors.

5. The current limit circuitry should limit the peak output current to, at most, 80% of the transistor continuous peak (Ic).

I feel that the combination of the above will give a high probability that the rail fuses will blow before the transistors fry. The capacitors in the output section will also help greatly with transients.

Good luck.
Note that some circuits (e.g. Aleph) will promptly and enthusiastically deliver the full current to the load if one rail fuse blows--not quite the result that one might hope for. Better to fuse the AC line and (optionally) the speaker line.
Off the top of my head, I'd say it was probably safe to fuse the rail on the Zen...but I'd recommend taking a close look at the circuit to verify that it would be okay. (It does just have the one rail, right? I don't have the schematic handy.)

Sorry Grey. My reply about fusing was in general, and wasn't for any specific amp.

The Zen amp is a Single-Ended Class-A amp with a single power rail. Fusing this in the power rail should work fine.

If an amp dumps one rail to the output when the fuse on the other rail dies, then there's a problem with the DC monitoring circuitry.

DC output protection is in the same class as turn-on delays and overtemp protection. Most DIY amp plans/projects don't include it. This usually has to be added as an external item.
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