Powering the Phono Stage

A project log for Muffsy Phono Preamp PP-2

An Improved AudioKarma CNC Phono Stage

skrodahlskrodahl 03/22/2015 at 22:130 Comments

An Ultra-low Ripple Hifi Power Supply

The power supply has its own project pages:

Opamp based circuits require a dual power supply to have optimal performance. Usually +/- 15 volts.

Getting there usually requires transformers, which hooks up to mains electricity, which is an electrocution hazard first and a fire hazard second. So... What if we could circumvent that by using an ordinary wall-wart?

Well, I built one of those, and it's a perfect match for our little phono stage. :)

Above is a 15V AC-to-AC adapter, and it hooks up to the PSU which powers the CNC phono stage like this:

I now have the phono stage, an enclosure and a power supply. This brings me much closer to a fully functional phono preamplifier, and I'll keep you up to date on my build.

I could stop here, but there is something else I'd like to share. That's how the PSU performs. You see, I hooked my multimeter to the power output of the supply, to see how much ripple it had. The meter said a nice 2-4 millivolts.

But if you remember from the gain stages project log, there are some bypass capacitors. They prevent opamp oscillation and smooth out the power.

So, I connected the multimeter on the opamp power input pins and measured again. The meter said 0.000 volts. It just won't measure anything below 1 mV.

Inspired, I brought out the oscilloscope and measured the PSU output again. It has less than 2 mV of ripple:

The effect of the opamp bypass capacitors is really good, as the ripple was lowered to a measly 0.1 mV:

To be fair, the scope varied from 0.07 mV up to 0.14 mV. I'd call that low ripple any day, and that this is decidedly a good Sunday to be building a power supply.