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.
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:
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.
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:
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:
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
Load the Magnets!!! from http://www.tnt-audio.com/sorgenti/load_the_magnets_e.html
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