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In Reply to: RE: Phono Stage Motorboating posted by danlaudionut on April 05, 2009 at 13:06:05
With 5 stages being powered from the same capacitor bank, I'd say it's no surprise there is motorboating. More isolation/decoupling between stages will cure the runaway bass...interesting ckt...
Follow Ups:
IT
1) With cascoded DN2540s, I would think
the PSRR would be large enough to stop it.
2) Since the whole circuit is constant current,
there is no change in current then
how would motorboating occur?
I thought it was from changing current
setting up feedback oscillation that
created motorboating.
DanL
Me thinks adding decoupling caps to the B+ lines powering your plates may help break the oscillations.
FL
I added capacitance to my last stage of
the LCLC filter and all is well so far.
I hour and no motorboating yet.
Added plus is better bass.
I don't understand that but ...
DanL
It's simple and real, Dan. Every electronic device has impedance, capacitance, reactance, even resistive(very high resistance in your example) and inductive coupling is possible. Since all stages feed off the same B+ tap, the wiring and device reactances are "reacting" even though your preamp tubes are well regulated/constant current. Parasitic oscillations occur in the strangest places, often perplexing designers.
A tube power supply is charging and discharging the power supply cap storage bank at the speed of the power supply, which is 120 times per second with a fuul wave rectifier. Think of a car battery in winter. More storage capacity yields easier starting, more electrical oomph. In our tube gear, adding storage capacitance usually results in better bass impact, detail and control.
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