02-06-2021, 02:47 PM
Diabolical Artificer Wrote:As I have it we want a PSU that has a low OP Z and not resonant at VLF
No, it's a simple-looking circuit which is difficult to analyse. But:
You have a capacitor-input filter, driven by a bridge rectifier. The input (reservoir) capacitor can't swing down very much, resonantly, because it'll be continually topped-up by the diodes pumping charge in at 100 pulses a second.
But you can look at the 5H choke and your 350μF smoothing capacitor. These have a resonant frequency of 3.8Hz. And the characteristic impedance (reactance of either L or C at resonance) is 120Ω.
So, if the circuit does silly things at 3.8Hz, the HT rail won't look low-Z, it'll actually look really high-Z. And, this isn't massively below the lowest audio frequency, especially at maximum bass boost.
What can you do? Well, the characteristic impedance, being 120Ω, means that it'll have to be damped by this order of resistance to kill the resonance. It'll suck DC, but a trick here is to put this 120Ω in series with a capacitor. The disadvantage is, you'd need a capacitor significantly bigger than 350μF. Adding 10μF in parallel with 350μF won't make itself felt.
Why not 'scope the HT rail though, and see if it is bouncing?
