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Dave Garretson

I recently implemented a circuit that allows continuously variable cartridge loading using Silonex light dependent resistors(LDR). The range is adjustable from 35R-250K via a control circuit comprised of coarse, medium, and fine mini-potentiometers, in conjunction with a 5V Burson Superregulator. The control circuit works outside of the audio signal path to govern LED brightness and set the resistance value for the LDR photo-resistive element in the audio signal path. The signal path is straight from PCB through soldered-in LDR. There are no switch points or wires or carbon pots in signal path to degrade sound. To set resistance I made a patch cord from input of phono stage out to an ohmmeter.

In my fully balanced ARC PH-2 phono stage, the LDR works as a shunt between the XLR signal phases, in parallel with Caddock TF020 high-value fixed resistors from each signal phase to ground. In a single-ended phono stage, an LDR could entirely replace a 47K load resistor. The LDR circuit could optionally be built externally and placed in series with interconnects.

My initial interest was to evaluate the sound of Silonex LDRs relative to other top resistors. The topic has been explored in a DIY forum thread in the context of the Lightspeed passive attenuator. But of course one wants to hear this for oneself. In short, a Silonex LDR in parallel with TF020 sounds better than TF020 alone. The LDR sound is resolving, clean and extended at the frequency extremes in the manner of TX2575, with an enveloping yet articulate LF that blends the best attributes of detail of TX2575 and smoothness of Audio Note tant.

Why would anyone want to do this? It is particularly interesting to have precise loading across a wide range for MM/MI cartridges. Many MM/MI cartridges sound best when loaded well above the nominal 47K spec. For example, my Empire 888/999, Astatic MF-100, and Andante P-76 all sound best in the 75K-100K range. In addition, variable loading of MC cartridges can sometimes be meaningful +/- 10-20R. The only downside is that optocouplers drift with temperature. However, they drift predictably, so as the chassis warms up, the LDR returns to its preset value. To stabilize them a SS phono stage may be left on constantly. They can be potted in beeswax.

Here are some pictures of the project. This PH-2 includes many other mods such as Schottky rectifiers & replacement of stock coupling and filtering caps.

Vinyl Asylum

Continuously Variable Cartridge Loading Using LDRs

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