Hey - I have a little DC offset on the output of my recently poly build.
Currently, the signal is mixed down and then sent to an ms-20 style filter, then to a final volume control potentiometer, then to a series 1k resistor and a simple buffer.
I have a good quality 22nf film capacitor from an early amplifier project.
I was thinking about putting this in series between the potentiometer output and the series 1k resistor, which I think should remove the offset without introducing any filtering.
Is this right?
Is the value acceptable?
Would there be a better place to insert it?
The offset itself seems to vary a bit depending on waveform selection and other parameter choices, so I’d rather use a cap then mix in an offset adjustment voltage.
AC coupling is filtering, a high pass filter with a cutoff below audio frequency. It needs a series capacitance and a resistance to ground.
If you use a 22 nF before the 1k output resistor (and after any buffering), there’s “infinite” resistance to ground there, but if you connect a downstream module whose input resistance is a typical 100k then the cutoff frequency would be 1/(2πRC) ~ 72 Hz. That will have audible effects on your lowest frequency sounds, but maybe you can live with that.
If not, you’d need a larger capacitor value. 150 nF would get you down to about 10 Hz cutoff.
If what’s downstream is a line level mixer or something like that, whatever its input resistance is will determine the cutoff. If it’s going into something with “infinite” input resistance, the cutoff will be very low, but the time it takes to remove a DC offset might be unacceptably long. In that case, though, you could add for instance a 470k to ground after the capacitor to get about 15 Hz cutoff.
Buffer before the series resistor or after? There isn’t a lot of point to a series resistor before a buffer, but if that’s the case then, depending on the buffer details, it might present a higher, perhaps “infinite”, resistance to ground (or virtual ground) for a capacitor upstream of it, giving a different cutoff value.
Thanks, I will have a proper read through all of that and the link.
By the time it takes to remove dc offset… how is that experienced by the user? Does it change the sound or feel when playing the synth?
I can buy a bigger cap if that’s the easiest solution. It’s likely to be plugged into either a line level mixer or something like a guitar amplifier. Mostly a line level mixer.
The buffer is just a simple inverting buffer using a TL072.
Apologies for the crudeness. The 1k resistor was leftover from before I had the buffer. I can remove it if it complicates things.
I’ve put the cap before the buffer but happy for it to go anywhere, what ever would be best.
Before the buffer is a great place for it. It sees “infinite” resistance to ground there, so then just add a resistor to ground like maybe 470k between the cap and the op amp and that should be fine.
Thanks, installed the cap and resistor tonight and it seems fine. The buffer definitely stabilised everything, and hopefully now it’ll be all ready to send off.
I’m pretty sure I can’t hear a difference, which is great.
The article was a little over my head, but the bits I could follow were interesting!
