UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Capacitor Selection Pi-Section Smoother.

kalee20 · 3rd Mar 2021, 12:50 pm

Since microfarads are cheaper than henries, I'd spend the money there.

Craig's comments are bang on, but for the low-current demand (which moreover is constant anyway), there's no point in having a choke-input filter.

Another comment: the EZ80 does have a maximum reservoir capacitance of 50μF, but, rules are made to be broken... If you add sufficient resistance in series with the transformer secondary to limit current to 90mA, the EZ80 can never be over-run, so you can use as many μF's as you like for the reservoir, and the small 10-15mA of load current can still be supplied. And then follow with a couple of RC smoothing stages. You'll soon end up with hum levels at the microvolt level.

The purist in me would indeed keep semiconductors out of an all-valve circuit...

3rd Mar 2021, 12:50 pm	#12
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,088	Re: Capacitor Selection Pi-Section Smoother. Since microfarads are cheaper than henries, I'd spend the money there. Craig's comments are bang on, but for the low-current demand (which moreover is constant anyway), there's no point in having a choke-input filter. Another comment: the EZ80 does have a maximum reservoir capacitance of 50μF, but, rules are made to be broken... If you add sufficient resistance in series with the transformer secondary to limit current to 90mA, the EZ80 can never be over-run, so you can use as many μF's as you like for the reservoir, and the small 10-15mA of load current can still be supplied. And then follow with a couple of RC smoothing stages. You'll soon end up with hum levels at the microvolt level. The purist in me would indeed keep semiconductors out of an all-valve circuit...