Daim Ntawv Qhia Tus Neeg Siv

What if we, just for fun, reduce C3 to 10µF ? It still works, only that THD increases slightly from 0.75%
to 1% (for 20V input 10Hz into 4R).
We have mentioned above that the optocoupler has been changed to one with a lower CTR (4N26
instead of 4N35). What if we now put back in the 4N35, as specified ? You can see how it looks like
in Fig. 4. Here, the input is only 20V into 1.5R, compared with 24V in Fig.3. So for Class B operation
at least, a high CTR is not an advantage.
Fig. 4 Optocoupler with Higher CTR
But what is the disadvantage of a low CTR ? Let’s assume a CTR of 0.5 (e.g. 4N26) instead of 1 (e.g.
4N35). At DC, 2x the LED current is required to generate the same current at the photo-transistor for
proper biasing. That means the voltage drop across R10,11,12 increases from 162mV to 325mV.
That in turn means that the voltage across R13-R14 increases from 1.26V to 1.43V. Or, the DC bias
goes up from 1.34A to 1.52A -- too much dissipation per FET with 25V rails.
Note also that there is quite a bit of difference in forward voltage between different optocouplers.
Having looked at the likes of CNY17, SFH608, MOC20x, etc., the 4Nxx series seems to have the
lowest Vf. One might still have to increase R13, R14 to say 0R56 in real life circuits to keep bias
below 1.3A.
Oh, one more important point. R13, R14 are specified at 3W in the original Pass schematics. This
means max Class B sinusoidal current is +/-3.5A, or +/-14V into 4R. Going to MPC74 (5W) will get
you up to 4.6A, or 18.5V into 4R. To go even higher on voltage and/or lower in load impedance, you
need to replace them with Caddock MP915s or the like.
To sum up, we made the following changes :
Q5 4N26
R10,12 150R