Hey, I'm working on a tube amp design and wanted to reduce the signal a bit. I figured the simplest way was just throw in a resistive voltage divider. I know the ratio of resistors is how Vout/Vin gets calculated, but are there any rules of thumb for ball park of values to use? For example, what's the difference (if any) between using 2 470K resistors instead of 2 1K resistors if I wanted to cut the signal in half? Also, in some designs I've seen a capacitor in parallel with the resistor to ground. Is this now a voltage divider and low pass filter in one? Any other reason why the capacitor is necessary? Thanks.
why not start with a POT, adjust to where you want it..then measure the values and replace with fixed resistors. i would start with a 100K pot.
So I'm trying to keep the "maximum signal available" at a certain level. I know how large of a signal is being produced and I'd like to cut it in half, which can be done with 2 resistors of the same value. My question was more along the lines of "is there a best way of doing this?" Or, other things one should consider when doing this. Like ways to avoid stray capacitance or something along those lines. Just kinda thinking out loud.
An alternative. Rather than throwing away the signal you could reduce it and get something in return by using a transformer wired for a 2:1 stepdown. If it's at the input you would get freedom from ground loops and the ability to battery bias without an input cap. If it's between stages you would get more drive from the preceeding stage.
Of course, it would cost and weigh more than two resistors, though.
The 1k resistors would be less noisy and would be less sensitive to any following capacitance, but they would seriously load down the previous stage!
Yes, although I can't think of a common reason to do this in a hifi amp... You might see a capacitor across the upper resistor, to cancel out the effect of Miller capacitance and so get wider frequency response.
The value of the resistors is highly dependent on the output impedance of the driving stage. As a rule of thumb the total resistance of the divider should be 10 times the output impedance of the driving stage so as to not load it so heavily (you can go much lower but should analyze the effect of a such a load on the driver, ac load line). It is ideal to use the smallest resistors possible to reduce resistor noise to a minimum.
For a quick example if the stage before the divider is the usual 12AX7 the output impedance will be about 70k so you should use 700k in the voltage divider, don´t even think of using 1k resistors since the stage will distort. You can go as low as 300k total but it´s not worth it IMO.
You are right about the cap to ground, it forms a low pass filter whose 3db frequency can be calculated with the usual formula f=1/(2piRC) (R being the resistor in parallel) with reasonable accuracy. If you connect the cap in parallel with the upper resistor you get a high pass shelving filter (the widely known "bright" switch in guitar amps)
For a quick example if the stage before the divider is the usual 12AX7 the output impedance will be about 70k so you should use 700k in the voltage divider, don´t even think of using 1k resistors since the stage will distort. You can go as low as 300k total but it´s not worth it IMO.
You are right about the cap to ground, it forms a low pass filter whose 3db frequency can be calculated with the usual formula f=1/(2piRC) (R being the resistor in parallel) with reasonable accuracy. If you connect the cap in parallel with the upper resistor you get a high pass shelving filter (the widely known "bright" switch in guitar amps)
No, R being the parallel combination of both resistors (assuming negligibly small source resistance and negligibly large load resistance).marcelop said:
Yes, it makes a first order LPF.
The only places I've seen this is modulators that operate in close proximity to a transmitter to keep stray RF out of the first pre. It shouldn't be necessary for a HiFi amp.
The signal, that You want to decrease comes from some tube, or not?
If Yes, better choice is change this tube to another with 2 times less mu.
Or use in next stage, where signal goes, tube with 2 times less mu.
Use resistor divider is not good idea, that will decrease quality of sound, .....or You don't worry about quality?
If Yes, better choice is change this tube to another with 2 times less mu.
Or use in next stage, where signal goes, tube with 2 times less mu.
Use resistor divider is not good idea, that will decrease quality of sound, .....or You don't worry about quality?
Use appropriately, a resistive divider will not decrease sound quality - after all, that is what a volume pot is!
If You use high sensitivity speaker, You will sens signal, crossing big resistor /kohm/.
Every big potentiometer, with enough resistance, decrease quality, exclude inductive volume device.
/Cheep decision, for quality, always is using input potentiometer/.
Every big potentiometer, with enough resistance, decrease quality, exclude inductive volume device.
/Cheep decision, for quality, always is using input potentiometer/.
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Yes, inductors are well known for their linearity, whereas resistors are really awful.
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