Muffsy MC Head Amp

The first prototype PCB is now in production, they usually take about a week to arrive. This is the final layout:

This layout uses dual op amps to keep the PCB size smaller, so the first test will be done using LM4562. With its 2.7 nV/sqrt(Hz) noise density, I expect the Signal-to-Noise Ratio to be somewhere close to 63 dB when referenced to a 0.5 mV input signal.

It's usually said that an SNR of at least 65 dB is needed for the preamp to be silent, but that does need testing to make sure. I will need to verify that by measuring, then hook this unit up to the Muffsy Phono Preamp to see if there's any noticeable hum.

If there is any hum, I will need to redo the layout with single op amps in the leftmost position and that is going to make the board larger. Not only that, the price of the kit will exceed the current $69 target.

The op amp in the socket to the right is the DC servo, so it will not be affected by these changes.

The input impedance is set with two 4-way DIP switches, one on each channel, with the following values:

The final schematic is not entirely ready (the current version is added at the bottom of this project log anyway), here's a teaser in the form of a PCB rendering:

The rendering is done using Copper App (http://www.copper-app.com). Although they have added quite a few through hole components, the four-way dip switches for selecting the input impedance were not yet available.

Here's the first attempt at a PCB routing, with component values:

This is the current circuit schematic, on which the PCB is based: