Standard circuit done all the time in nosebleed high speed digital IC design: supply bypass caps with Nchannel MOSFET switches in series with the ground end.
Generalize that by putting an analog voltage on the gate such that the mosfet is biased (0 < Rds < infinity) and presto, we have your idea. 50 amp MOSFETs are cheap; 50 amp potentiometers are not.
Generalize that by putting an analog voltage on the gate such that the mosfet is biased (0 < Rds < infinity) and presto, we have your idea. 50 amp MOSFETs are cheap; 50 amp potentiometers are not.
Is filtering reduced if a resistor is placed in series with the neg of a filter cap and ground? If so, could a correctly spec'ed pot be placed there to make it controllable for an intended swing ??
This may be another dumb question, but I'm just gonna ask it anyway
If you really want to do this. Simply use several small caps in parallel and wire a switch in series with each one. If you want very fine control make each cape twice the size of the next. Then by using binary combinations you can have 16 effective cap values with only four switches or 8 with three switches.
As was said above you can use mosfets in place of toggle switches. You then control the mosfets with anything you like.
I'd start the experiment with just plain old toggle switches and see what the effect sounds like.
Webber sells capacitor boxes that let you dial in an cap you like. Try one of these first.
I have read this thread all the way through and am intrigued as to the possible use of the proposed scenario... purely out of curiosity. Most of what I do applies to either valve guitar amps or the complete opposite end of the spectrum in digital telecommunications (go figure)... so I don't mind asking a dumbass question now and again if I'm going to learn something
dumbass question now and again if I'm going to learn something Yes,
Use a Capacitance Multiplier Circuit with a potentiometer in place of the resistor
Description at Elliot Sound Practices
Capacitance Multiplier Power Supply Filter
And Shoog's experience
http://www.diyaudio.com/forums/pass-labs/9193-my-experience-capacitance-multiplier.html
Cheers,
Ian
Use a Capacitance Multiplier Circuit with a potentiometer in place of the resistor
Description at Elliot Sound Practices
Capacitance Multiplier Power Supply Filter
And Shoog's experience
http://www.diyaudio.com/forums/pass-labs/9193-my-experience-capacitance-multiplier.html
Cheers,
Ian
now, i have to go digging through some old books (and i think the one i'm looking for is from 1965) but i remember seeing a variable capacitance multiplier using two bipolar transistors, a cap and a potentiometer. i think the particular novelty of the circuit was that neither end of the "capacitor" needed to be grounded....
Yeah - this place full of wild and crazy guys (and gals
?)...That's why I asked my question... I was trying to figure out why you might want to vary the capacitance of - presumably, given the forum area - a filter stage in an instrument amplifier... and presumably - a valve or tube amplifier....
I thought what could be gained and I suppose if you varied the first stage filter caps you would get tighter or flabbier bass response....
Again I am forced to ask... am I barking up the right tree with this???
Effective Capacitance will not radically change the sound of a gain stage in an MI Amplifier as those stages all run in Class A (that is, with a constant current draw from its power supply node).
Where it may be useful is in the very first power supply stage which powers a Class AB Push Pull Power Amplifier where the current draw varies with loudness, lower effective capacitance will allow power supply sag imparting some compression to the sound.
Cheers,
Ian
Where it may be useful is in the very first power supply stage which powers a Class AB Push Pull Power Amplifier where the current draw varies with loudness, lower effective capacitance will allow power supply sag imparting some compression to the sound.
Cheers,
Ian
Thank you for concurring GT.... that's what I'm getting at... I can't think of any other purpose for wanting to alter the amount of capacitance in an amplifier PS... and it would be exactly where and why you've stated...
Having said that I personally wouldn't bother... but I prefer to run things as tight and in your face as poss but not to the point that the thing loses it's personality - there's a point where an amp starts sounding too tight and sterile if you have too much smoothing but truly exciting and musical if it's not too saggy... that's where I'd personally prefer to have my amp set up...
Having said that I personally wouldn't bother... but I prefer to run things as tight and in your face as poss but not to the point that the thing loses it's personality - there's a point where an amp starts sounding too tight and sterile if you have too much smoothing but truly exciting and musical if it's not too saggy... that's where I'd personally prefer to have my amp set up...
making subtle changes to the way an amp behaves in order to custom tailor the sound of an amp is perfect for instrument amps, as opposed to hifi amps where the goal is a "straight wire with gain". a more effective way to tailor the sound, however would be to have an amp with variable output impedance since such a control would allow you to select how much the speaker and cabinet resonances interact with the amplifier. just as long as you avoid a negative output impedance (which could become an oscillator).
It was just a thought I had, but this is exactly the idea. Not intended for a huge difference in sound, but just a subtle control in the "backbone" of the amp, so to speak. If it's not practical to implement then oh well...
Correct me if I'm wrong but, I would think the goal of this to be able to create a sag in the supply voltage which would cause the amp to distort at lower volume levels so you don't have to crank your amp to get the sound you desire. If it works it seems reasonable for a tube amp, but I think it will not sound so good on a solid state amp.
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