C5 and C6 will see 20 volts, so 25 volt rating will be fine. For C4 and C24 I would go with 35 volt, but If I had good quality 25 volt rated caps, it wouldn't bother me for this design. If you had a regulator fail shorted you would be over voltage, but you would have other problems at that point?
Two capacitors to consider, C9 across the 100k and the gate-to-source on the output MOSFET.
If you reduce R16 to 10K from 100k you will reduce the gain by 20dB.
At the same time, you move the frequency of the pole. C9 is there for stability
You will raise the frequency of the pole by 100k/70k (neglecting the effect of the MOSFET capacitor). I don't think it will have a major effect upon stability.
ThanksThe capacitor in conjunction with the resistor R13 100K form a -3dB high pass filter. With the specified values the cut-off frequency is 8 Hz. Changing the capacitance to 1uF lowers the cut-off to 1.6 Hz. I don't think there is any benefit to such a low cut-off frequency. Note that unwanted low frequencies such as turntable rumble, warp wow, etc. could be issues if too low of a high pass frequency is used.
I'm just starting another Pearl II. I've checked Mouser, DigiKey, Newark and RS and they are out of stock on ZTX450. I prefer not to purchase from eBay and risk counterfeit devices. I've read that BC550 or MPSA18 is a working substitute, but not necessarily in the Pearl II. Any suggestions for sources or substitutes will be appreciated, thanks.
Octopart shows some ZTX450 in stock at Arrow, Newark, Future Electronics, etc.
https://octopart.com/ztx450-diodes+inc.-13152700#PriceAndStock