Power supply sag in pedal or rack-based preamps

[cdemike]

I was talking to a friend the other day about tube-based preamp pedals, and we started discussing how it’s interesting that a lot of those pedals are based around Marshall, tweed Fender, and similar circuits. To me at least, the experience of hearing and playing that style of circuit necessarily involves power supply sag which those pedals can't do with no power amp to drop B+ under high signal levels. So if the power supplies won’t sag without a power amp dropping B+ upstream of the preamp nodes, I was thinking a rapidly responsive regulated power supply might be a way of getting a similar effect.

So my thought was to try to find a way to make B+ drop in proportion to signal magnitude to emulate the proportional drop in B+ that signal magnitude would effect in a complete amp’s output section:

Sagging B+ regulator draft 2.PNG

Has anyone tried something like this? I remembered seeing this thread trying to answer the same question a while ago, but I haven’t seen if anyone built something based on the ideas coming out of it: https://ampgarage.com/forum/viewtopic.php?t=35804

[Kagliostro]

About pedals I don't know (May be it can apply if they are tube effects on rack, or may be it can be adapted also to SS circuits ?) but about rack Preamp this same question was discussed years ago at EL34world and our friend Sluckey find a solution

See here the thread and give a look to the solution schematic proposed by Sluckey

https://el34world.com/Forum/index.php?t ... #msg124825



Franco

[cdemike]

About pedals I don't know (May be it can apply if they are tube effects on rack, or may be it can be adapted also to SS circuits ?) but about rack Preamp this same question was discussed years ago at EL34world and our friend Sluckey find a solution

See here the thread and give a look to the solution schematic proposed by Sluckey

https://el34world.com/Forum/index.php?t ... #msg124825



Franco

Very interesting thread! Very similar idea, and I think this same schematic would work in a standalone preamp or pedal if it were adapted so that the signal modulating the preamp B+ was not derived from B+ in a sagging power amp but instead directly from the magnitude of signal leaving the preamp. So rather than sagging the preamp B+ in proportion to a power amp's power section sag, it might be interesting to derive the "sag signal" directly from the magnitude of the output preamp signal. Does the BJT in Sluckey's schematic work as a source of negative feedback on the preamp sag?

I caught some mistakes in that first draft schematic (not least of which would be orienting the MOSFET backward...), which I don't think would work. This is a revised version:

Sagging B+ regulator draft 3.PNG