idea: mo betta volume control

[Marc.yo]

this is for fuzzes and distortions mostly but it gets the job done anywhere. the normal 100kA pot to ground doesn't do it for me...here's why:


cap -> resistor going to ground (Rload) -> out


that's a high pass filter, if you want to destroy all the goodness your sound - i'd use it. however i'm not a fan of treble. i would guess that people think that if you choose your pot/capacitor value correctly...it should be fine but i disagree. here's why: the upper harmonics will not be attenuated (the ones that are the notes you aren't hearing directly)....but everything else will be.

here's my idea:

take an inverting op-amp and set it up so that the resistor in the feedback loop is a pot and the resistor before the input is equal to the maximum value of the pot in the negative feedback loop here's a pic from the natnl semiconductors ap note
here's where i'm coming from on that:
as the equation for gain in an inverting op-amp is
vin/vout = -(Rfeedback / R in)

so if we set the Rf as equal to Rin, 1k for an arbitrary value...it can only lower the volume as you turn the knob, but the maximum value would have a gain of one.


so the idea is, this doesn't create a high pass filter but works on the same principle of a simple voltage divider volume control. not to mention its easy to fit in if you have an extra op-amp left on that TL072 or TL074...you get the idea
some of you more experienced DIYers out there can possible correct me?

[Marc.yo]

here's another little chunk on that inverting op-amp thingy...give the positive input a reference voltage of half the power supply, 4.5V in most cases. do that via a voltage divider between Vcc and ground and throw a 100uf electro cap running in parallel with the resistor running from 4.5V to gnd in there for good measure.

[earthtonesaudio]

But if you make the output cap large enough it doesn't matter, right?  A value of 0.1uF means yes, it's a high pass, but it only attenuates signals below 15.9Hz...

[DougH]

But if you make the output cap large enough it doesn't matter, right? 

Correct.

The real issue with volume controls is to ensure the impedance is low enough to properly drive whatever is hanging off the output of the pedal. Otherwise, the tone will vary as you vary the volume level. Either pick an appropriate sized pot, low enough to provide the drive but not too low to load whatever is inside the pedal. Or use a JFET/BJT/Opamp buffer on the output.

[Marc.yo]

earth's tone audio: you are correct. and i considered that as i was writing that. i realized that the other nice part about this volume control: it can act as a boost to the overall volume if you make the pot higher than the Rin value

[earthtonesaudio]

I think your idea is a good one if you have an extra op-amp just sitting unused.  Mainly because you can use lower value resistors than with a standard volume control, which might mean lower noise if you have a quiet op-amp.  Those CMOS inverter projects tend to have a few "spares" that could use this method.

[Marc.yo]

word, i didn't think of that. i was just working a pedal design of my own and i decided to implement a better tone control