Valve pre-amp question ?

[Kent Kassler.]

Are there "real" benefits to using lots of chokes and less capacitance in the power supply of a pre amp?What are the reasons that some designs prefer this route and is it a lot more costly and completely unnecessary?

[handsome]

chokes have a very high AC resistance but a low DC resistance - so for AC - ripple, spikes/nasties on the power line, etc - they filter very effectively whilst for DC - the power your amp wants - has a relatively unimpeded flow. Sort of like having two resistors in your power supply; a small one for DC and a very large one for AC. In the old days large capacitors were expensive and chokes cheaper so one saw lots of chokes and small value capacitors. chokes unfortunately are reactive and in combination with the capacitance will have a resonance that usually pops up in the audio band. the larger the capacitor the lower the resonance, so a small amount of capacitance is not actually ideal with a choke. chokes are very effective filter components but they are large, moderately expensive these days and susceptible to hum. electronic regulation can be far more effective, smaller and suffers no hum issues but can be a pain to construct and is more complex. in audio you can never have a PSU that is too clean.......

[Steerpike]

Choke input filters are also very gentle on rectifier valves. A large capacitor straight after the rectifier is not good for a valve. Solid state rectifiers, they aren't stressed.

[Kent Kassler.]

Thank-you gents!

[Ampdog]

In a valve pre-amp one could get away with resistors instead of chokes, because the h.t. current drawn should be constant and low. There will be a higher voltage drop but which can be compensated for by a higher voltage power transformer.

In such applications one must also not under-estimate the use of a series transistor in place of a choke. High-voltage models are available, and filtering at the transistor base amplifies the capacitor value by the amount of the transistor gain. Although heat is dissipated, again for pre-amp use it need not be that much. (This is for use as a filter only. One can go one step further and make a rudimentary regulator with a few extra transistors. The regulation need not be perfect, one uses it more for ripple and noise rejection.)