Phono Cartridge Loading: Getting the Best Match between your Cartridge and Preamplifier

Phono preamplifiers are designed to accomplish two main tasks. First, they amplify the output of the turntable’s cartridge, taking the weak signal and raising it up to a level that is friendly with your amplifier’s input stage. Secondly, the circuit accounts for the RIAA curve—a mastering technique that allows vinyl to support longer playing times and approach high fidelity.

Cartridge Loading

One of the key characteristics of a phono preamplifier is the input impedance, i.e. the impedance that the preamplifier presents to the cartridge sitting at the end of your tone arm. Somewhere, back in the dusty recesses of history, somebody decided that the standard input impedance for a preamplifier should be 47K ohms.

This arrangement has worked well over the decades as moving-magnet cartridge manufacturers have attempted to design parts that are rated with an output impedance of 47K.

But this one size fits all approach is not ideal, and many DIY folks and general hi-fi enthusiasts have been experimenting with different input impedance values, sometimes with great results. In fact, a general consensus is that 47K is too high a value when you are dealing with different cartridges.

Input impedance in a phono preamplifier circuit is typically set by a resistor from the input jack of the preamplifier to ground. This resistance works in concert with any capacitance at the input (from the cable or from a capacitor in the signal path at the input) to form the input impedance. So a 47K ohm resistor wired from the input to ground would generally set an input impedance of roughly 47k ohms.

Around five years ago, gear whores started experimenting with the input impedance, trying to match it closer to either the manufacturer’s recommended value, or tuning the value by ear.

The general rule is that:

• a higher input impedance will shift the frequency response towards higher frequencies
• a lower input impedance will emphasize lower frequencies.

Given that there is a lot of variance in the cartridge impedance characteristics, it becomes clear that a fixed input impedance on our preamp prevents us from finding the ideal match between cartridge and preamp.

Some very high-end manufactures have implemented a variable input impedance scheme in their phono preamplifier products. For example, the Vacuum State SVP-1 and SVP-2 models include various RCA plugs that you can connect to the backplane to set impedance with a great deal of resolution. Unfortunately, the $6,000 price point of the SVP series puts it out of reach of anything but the most hardcore (and well-heeled) audio buyers.

Which is a shame. Because all that is needed to implement variable input impedance is a set of carefully matched resistors, and some type of switching mechanism.

Once you have decided that you want to experiment with adjustable cartridge loading, there are several practical considerations to take into account. You want to replace or augment the phono preamplifier's input resistor with additional values.

Multiple Resistors Plus a Switch

Let's look at a simple arrangement for configurable cartridge loading. We will use a standard passive preamplifier circuit with an operational amplifier. Note that only one channel is shown--for a true working circuit, you would duplicate the schematic for the other channel.

Figure 1 - A General Opamp-based Preamplifier with Fixed Cartridge Loading	In this generalized schematic, you can see a 47K ohm resistor from the input to ground. This value defines the input impedance of the circuit. The standard value of 47K ohms is what you will find in almost all commercial preamplifiers. It is designed to present the most common input impedance to the phono cartridge. As you can see, this simple implementation lends itself to easy modding. In other words, instead of just using a soldered-in 47K ohm part, you can add a switch and a bank of resistors.
Figure 2 - A General Opamp-based Preamplifier with Switchable Cartridge Loading	So to implement variable cartridge loading, all we need to do us come up with some type of switching arrangement and multiple resistors. In the modified schematic, we replace the single 47K ohm resistor with a multi-position selection switch, and three additional resistors. The values of R1, R2, R3 and R4 are chosen according to the cartridge loading values you want to use. Most likely, R1 will be the standard 47K ohm value with R2, R3, R4 values chosen according to taste.

Figure 3 - Using Parallel Resistors

By using a parallel switch arrangement, we can take advantage of parallel resistors for a more fine-grained cartridge loading control. To accomplish this, we use a series of SPST switches, or better yet, a DIP Switch. Resistors in parallel can be calculated using the following formula: 1 / RTOTAL = (1 / R1) + (1 / R2) + (1 / R3) + ... So we can now choose values for R1 through R4 that work well within our desired input impedance range. To simplify this task, check out the awesome calculators available online at www.electronics2000.co.uk. Here is the link for the parallel resistance calculator: http://www.electronics2000.co.uk/calc/series-parallel-resistor-calculator.php

Which Resistor Values Should I Choose?

For variable cartridge loading, you want values that go below and above the 47K ohm standard. This gives you the flexibility to raise or lower the cartridge loading. Assuming you are using four resistors per channel, a good choice for values is:

1. 150K ohm
2. 82K ohm
3. 47K ohm
4. 12K ohm

Of course, you can choose values that give you the range of cartridge loading you want.

Why Not Use Potentiometers?

If you have a good eye, you may have looked at the above schematics and all the switching arrangements and asked yourself: why not just use a potentiometer instead of fixed resistors and switches? After all, a potentiometer gives you an almost infinite range of resistance. The main problem is Channel Mismatch.

Because we are dealing with stereo, you would either have to use two separate potentiometers, or a dual-gang part. This pretty much guarantees that you won't get a good impedance match between channels except by sheer chance. If you use two pots, you will not be able to accurately dial in the pots so each channel matches. And with dual gang potentiometers, even high-end ones, you will end up with different resistance values on each of the gangs.

Protecting Against the DC Path

If you implement a switching design that opens the possibility for having no resistor in circuit, then you can have problems if you connect your turntable to the phono preamp and power it on. In this scenario, DC path issues can potentially damage your cartridge.

There are two ways to handle this:

• Always be absolutely sure to power off your phone preamplifier and disconnect it physically from your turntable before making any adjustments to input impedance.
• Install a large-value resistor in circuit so that DC path issues are handled regardless of configuration. For example, a 2.2 Mohm resistor will protect the input while it’s high value allows it to play well within a multiple resistor switching arrangement. The diagram below shows this type of installation:

Switch and Topology Choices

In the above schematic, we used a multi-position switch. The most common type of switch used in this arrangement is a rotary switch, as shown at the right.

A rotary switch allows you to switch between values by turning the knob. The wiring diagram for a rotary switch maps exactly to the "one resistor at a time" model shown in Figure 2..

This allows for one value to be chosen at a time. However, there are other ways to accomplish the same task but allow even greater flexibility. For example, instead of a rotary switch/one-value-at-a-time approach, how about individual SPST switches that can be turned on or off in any combination? This gives you a much greater range of values.

If you want to build a cartridge loading selector and have the switches  available on the outside of the enclosure, SPST miniature toggle switches are a good choice. You can easily drill the holes and mount four SPST switches for each channel. On the other hand, if you want the switches inside the enclosure for less frequent adjustment, a plastic DIP switch mounted to a PC board is easier to  realize. The DIP switch is familiar to anyone who has had to pop the case on older computers to set options. Each switch is simply a SPST switch that is on or off depending on the position of the small "nub" on the top.

Using a bank of SPST switches or a DIP switch, with the resistor values shown above, the following values are available for your tweaking pleasure:

Building Some Hardware

In an upcoming project, I'll illustrate several hardware approaches to implementing a variable impedance switcher.

Acknowledgments and References

Discussions about variable input impedance can be found in this fantastically long and involved thread at: http://www.vinylengine.com/phpBB2/viewtopic.php?t=6674

A particularly nice article on Cartridge Loading: http://www.hagtech.com/loading.html