Input Impedance

A project log for Muffsy Phono Preamp PP-3

DIY Hifi Phono Stage

skrodahlskrodahl 06/10/2015 at 14:020 Comments

Although the default input impedance for most cartridges is 47k ohm, many are reporting that other input impedances are giving better results.

The standard CNC circuit has the following impedances, selectable with a four-way DIP-switch:

These values are quite sound when looking at them one by one. Having the possibility of paralleling up to four resistors should give a few more useful options. The four values above don't give many other options though.

How about using these values instead?

Let's see what we end up with:

(The value in bold is the industry standard impedance for MM cartridges)

There are a few options that are too close together to be useful (which I haven't listed in the table above), but we get quite a range to choose from nonetheless. Look at this next bit for a complete list of impedance choices.

Modifying the Input Impedance

Part of the fun of building something yourself is that you can adapt the project to suit your needs. Here's how you can modify the input impedance.

It's all a matter of changing the resistors shown in orange below:

These are the input impedance choices you get with the default resistors:

To help you decide the best set of resistors, use this Excel impedance calculator. Simply fill in different resistor values and see the impedances change.

NOTE: The industry standard input impedance is 47 kOhm. I would recommend that at least one of your settings matches that, or has a value that's reasonably close to 47 kOhm.

You may notice that there are a few more values in this table (which uses the default Muffsy PP-3 values) than what I have presented earlier. That's because I chose to ignore some of the impedances that are almost equal.