Phono Stage problem

I have a problem which sounds like the bass is out of phase, I've looked at the cartridge 301, Hadcock arm and stepups also wires etc.
Are there any suspects in the schem I could tweak or could it be a component not up to spec.
Thanks Will.

Sorry

[James D]

Will,

Has it always been like this or just develop recently? If it developed recently then have there been any system changes in the same timescale?

On the design I have a couple of observations that concern bass performance.

1) The output impedance of the stage is high - about 60K so it doesn't have much drive. As a consequence it expects to see an input impedance of at least 250K and 500K would be more like it. Then its fine provided the interconnect isn't high capacitance. The output capacitor is sized with this in mind and is fine for a 500K load but marginal for a 100k or less load - it would certainly have an effect on bass phase below about 200Hz. Making it much bigger won't help unless the output stage has more drive too.

2) The decoupling capacitors are 1uF after 1K resistors - that is a 160Hz F3 so these caps are not good as decopling and are part of the output current loop for the SE stages... these will introduce poor bass performance and again affect the phase from below about 300Hz.

You may be hearing the combined effect of these if it has always been there.

ciao

James

[NickG]

Something else I noticed, the low frequency corner of the RIAA is almost entirly dependent on the Ro of the first stage, so as the valve ages, you will start to develop more and more bass emphasis. I have tried to replicate this in spice, and you can see the effect as I reduce the emmisivity of the valves down to 50% or their nominal value.

http://www.lurcher.org/nick/audio/willphono.jpg

Thanks James, Nick, Ive pulled the 470k as I have a 100k pot on the preamp input my idea was to tx couple to the preamp but I ended up using it as a stepup, it sounds better than the mic tx's. I replaced the 1uf caps with 4uf input and 16uf output. Gain is about +6db, bass has improved, more drive (power) and the old perforated eardrum has stoped rattling not perfect but a big improvement.
Nick does that mean its Drum'n'Bass for me init..
will keep Steve S in beer money.
Cheers Will.