Aleph L with huge distortion at lower settings?

[panos29]

Dear all, its been a while since I have an aleph L clone designed so that the output potentiometer is half of the one found in aleph P(the L circuit is a single ended one). I used my PC spectrum analyzer to measure distortion.

So what I used was input sine of .8V rms, output was 4.5Vrms loaded with 20K. To my astonishment, I got at the minimum volume setting 1% THD and at the maximum setting 0,09% THD.

I pressume something is way wrong with the output pot, is it logical? I believe that at lower settings the output of the pre is loaded with with very low resistance causing distortion.If yes how can I improve this?

[Nelson Pass]

It is not logical, but you are not presenting enough data for
conclusive opinion. If your output pot is being used as a shunt
to ground and you are still demanding high output levels, you
can easily get that effect, but for constant input levels, the
distortion should not increase.

[panos29]

Dear Nelson,

Thank you for answering, you are correct, not enough info presented. So, I used the electronic pot of aleph P of one channel for both the L and R channels of my pre in the fashion of instead of + and - phase( in aleph P) I used L and R respectively. Just measured the actual resistances which are the the ones at the following schematic. Bviously when the pot is at lower setting the pre "sees" an impedance of 4K in parallel of whatever the load is, while at maximum setting sees only the load impedance. Do I have to exclude the electronic pot and replace it with a conventional one?

[Nelson Pass]

Perhaps if only to help isolate the problem, but again, you need
to remember that the L's output is a current source which is
variably shunted to ground. For an given input level, the
distortion will be a constant compared to output level, but if
you crank up the input level while lowering the volume to
achieve the same output level, at some point you may overload
the input.