Lentek Head Amplifier Adjustable Loading

The original three loading positions that I added to this head amplifier worked just fine, but I wanted to see if I could make it even more versatile and flexible.

I initially tried using a linear 1000 ohm dual gang 20% & 10% tolerance plastic film rotary potentiometers from Bourns and Vishay. This would have provided a “infinitely” variable loading option from 0 to 1000 ohms, the ideal solution.

Bourns

Vishay

While this approach worked and did not increase any noise levels there was a major drawback that I thought would be an issue. The track resistance matching and tracking was not good enough to ensure that both the left and right channels were accurately loaded throughout the pots rotation. This resulted in both a slightly varying output level and a slightly different frequency response from each cartridge channel as the pot was rotated. So while the technique worked it was not practically useable in a high end audiophile system. Going beyond this simple passive approach requires a more complex active design that shall not be discussed here.

I therefore opted to use a two pole six way rotary switch with Vishay 1% 500mW thin metal film resistors from the MF50 range to provide the required ‘variable’ matched channel loadings. The 2×6 rotary switch selected was made by Grayhill – model 56D30-01-2-AJN.

MF-50 Current Noise

Grayhill DP6T Rotary Switch

I selected the loading values based upon the recommended values for each of my moving coil cartridges with two additional values increasing the preffered load values by approximately 20% and 50%.

The six cartridge loading values selected:

1. 100 ohm – recommended value for the Entre 1
2. 120 ohm
3. 150 ohm
4. 470 ohm – >400 ohms is the recommended value for the Hana SL
5. 560 ohm
6. 680 ohm

See here for the connection points A & B

I modified the head amp to support the rotary switch and again used 1/8″ vinyl lettering to label the six switch positions, see below:

This time I used screened cabling and grounded the rotary switched metal case. So did the switching arrangement affect any noise levels or the sound quality? No, there was no increase in either hum level or background white noise. The battery test LED is still present just below the rotary knob and I am looking for a smaller knob size. What I now have is a very versatile audiophile phono cartridge amplifier arrangement with great flexability that can easily support any cartridge type and loading and can easily be updated if required for new loading values.

Update:

Since the upgrade I have had chance to try the additional two loadings with the Hana. In my system I definitely prefer the 560 ohm load as it provides notably increased low bass output with no change in tone or coloration. This lower loading also slightly increased HF detail, transient impact and output level, with no obvious audible distractions. Listening to Sheffield Lab 20, The Sheffield Track Record  was a revelation!

Reduced damping on moving coil cartridges can give rise to increased electrical overshoot, ringing and a rising HF response. I have to assume that the lower damping on the moving coil structure is allowing slightly greater and easier movement of this assembly. As that is all I can think of that, in particular, would explain the improvement in LF performance. The changes in HF detail and transient performance may be as a result of a small increase in overshoot and a rising HF performance. (the 680 ohm loading made the cartridge sound too bright).

For now, 560 ohms is my loading of choice.

Coming soon:

Hardware has been ordered to create the following upgrade:

1. An external 12 volt analog toroidal DC power supply. (this is to replace the “less than stellar” MX-VYNL switched mode supply and Lentek PP9 battery)

See below for related posts:

• Lentek Original Rebuild.
• Original Adjustable Loading Upgrade
• A Little Lentek History
• MX-VYNL Review
• Toroidal Power Supply For The MX-VYNL
• Low Noise MX-VYNL Power Supply – DIY
• Lentek Rechargeable Battery Supply – DIY