Lentek Head Amplifier Upgrade
Designed by Peter Baxandall for use with the Lentek Entre moving coil cartridge, this little gem has a lot to offer when paired with other moving coil cartridges and a high quality phono pre-amp like the MX-VYNL. With +28dB of gain and a low nose figure the only missing functionality is its ability to provide the different cartridge loadings that are required to optimise moving coil cartridge performance. So I set about to remedy this shortcoming.
The head amplifiers design is simple by any standards, and cartridge loading is created by the input resistor value R1, see below.
One Channel Schematic
All that was required was a two pole switch with the number of loading options governed by the number of switch positions. Resistor values being selected to meet your own range of loading options while the type of resistor needed to be both high tolerance for exact channel loading match, typically 1%, and low noise, typically thin metal film or metal oxide. The switch selected could be anything from a two pole single throw to a two pole 6 way rotary. I had a three position two pole switch on hand so I elected to provide three loading options:
- 470 ohm for my Hana SL
- 220 ohm as an intermediate value
- 100 ohm for my Lentek Entre.
I had considered using a sub-minature two bank plastic film potentiometer (variable resistor) that would provide any loading from 10 to 1000 ohms but it would not easily fit in the available space, so it was ignored (for now).
Basic Variable Loading Technique
Calculating the fixed resistor values depends upon the method of switching selected. The simplest being a rotary switch where the resistance value simply equals that required, the more demanding is using a double pole double throw (DPDT) center off switch that aggregates resistor values to create the load value required, see below:
Three Cartridge Loading Values (mine)
Six Cartridge Loading Values
When using a switch that parallels two resistor values you need to use the formula:
1/R1+1/R2=1/Rrequired
OR
1/R1+1/R3=1/Rrequired
In my case this resulted in the following values that are the closest from those available in a 1% resistor range of low noise thin metal film resistors. In the above schematic:
R1=475ohm – Vishay Thin Metal Film
R2=422 ohm – Vishay Thin Metal Film
R3=127 ohm – Vishay Thin Metal Film
This results in three cartridge loading values:
- 100 ohm for the Lentek Entre
- 475 ohm for the Hana SL
- 223 ohm as an intermediate value
I modified the head amp to support the switch and then used 1/8″ vinyl lettering to label the three switch positions, see below:
So did the switching arrangement affect any noise levels or the sound quality? There was a very slight increase in 60Hz hum (inaudible at the MLP). Probably due to the internal unscreened cabling that I will fix during the next re-build. Otherwise it sounded exactly the same. What it does give me is a very versatile audiophile phono cartridge amplifier arrangement with great flexibility that can easily support any cartridge type and loading.
Coming soon:
Hardware has been ordered to create the following upgrades:
- A 2×6 rotary switch providing 100 ohm, 120ohm 150 ohm, 470 ohm, 560ohm and 680 ohm moving coil cartridge loading.
- A miniature 1K linear plastic film dual gang rotary potentiometer 3-1000 ohms. (experimental, as noise figures for these devices are not readily available)
- An external 12 volt analog toroidal power supply. (this is to replace the ‘less than stellar’ MX-VYNL switched mode supply)
UPDATE: Read more about the rotary switch and potentiometer upgrades here.
See my associated posts here:
