Hifi Preamps for your Record Player

Modify the Input Impedance of the Muffsy Phono Preamp

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

Other versions of this circuit has the following impedances, selectable with a four-way DIP-switch:

  • 18k, 33k, 47k and 62k ohms

These values are quite sound when looking at them one by one. Having the possibility of paralleling up to four resistors is bound to give a few more useful options, but the values above don't really do that. How about using these values instead?

  • 33k, 82k, 110k and 150k ohms

Let's see what we end up with:

Table of input impedances

(The value in bold is the industry standard impedance for MM  and HOMC 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:

Input impedance resistors

The big question is what to change the resistor values to. This is where the input impedance calculator comes in handy. Download it, play around with the values until you get it right, and substitute the resistor values with your own when you build the phono stage.


Here's a picture of the input impedance calculator, showing all the possible default values:

All default input impedance values

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.