Audiosector LM3875 - Volume Control / Preamp options


2009-10-30 10:43 pm
Hi all,

I have built this kit successfully and have used an ALPS 50k log pot, however the music sounds "restrained" on playback.

But, if I use the preamp section of my receiver (Yamaha RXV1900) and turn the pot all the way up on my LM3875's I get the sound I am wanting. My source is a Gigawork DAC.

So, what is the best way to move forward? Will changing the ALPS out for something else be OK, or do I need a preamp? Is my ALPS at 50K to high? How do I calculate the correct value? If the preamp route is the way to go, can anyone point me in the direction of a suitable kit?

Here is the schematic of my passive attenuator. It is a dual input/dual output. It also contains a single ended class A MOSFET headphone amplifier. I use a CDP direct into the pre as my primary source. I have a tuner as my second source, and although the output voltage is less than the CDP, it still sounds very good (imo).

I tried 20k and 100k before I settled on the 50k/47k shunt option. In my opinion, it gives the most natural presentation.


  • Passive Schematic.jpg
    Passive Schematic.jpg
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Zebra - so adding a 47K resistor on my inputs is all I would need to try?
Adding resistors at the entry is right if you dont forget to also modify the configuration of the pot as per the Zebra100 schematic : the pot output is not at the variable pin (pot wiring detail)

I tried this setting on my own Audiosector LM3875 amp and found the sound to be a little bit clearer than with standard alps pot, particularly at low volumes.

Now, there is a negative side : to work fine, pot and resistance must have about the same value, so even at full level this is a divider where you lose half of the sensitivity of the amplifier...