Distortion from mlcc capacitors??

[dschwartz]

Hi there!
I'm designing a cabinet simulator and used 0603 smd capacitors for signal coupling (input and output dc block)..
It works fine, but at gain higher than 5x, i get a nasty clipping..the stages are unity gain sallen key filters, with film caps..
I'm analyzing the causes, it seems weird that the opamps would clip at such low levels, do you think the mlcc caps are the culprit?
I know that mlcc introduces distortion in form of capacitance variations against voltage across them..but does this sound like clipping??

[Bill Mountain]

How big is your input signal?  How big is your supply voltage?  How close can your opamps swing to the rails?

What is the maximum gain available before hitting the rails?  Is it less that 5x?

[dschwartz]

How big is your input signal?  How big is your supply voltage?  How close can your opamps swing to the rails?

What is the maximum gain available before hitting the rails?  Is it less that 5x?

the iput signal is a humbucker loaded guitar, so i assume is around 100mV average with 800mV peaks.. the power supply is 8 volts after filtering, so i have a theoretical headroom of +-4V, the opamps are tl074, so lets, say, +-3V headroom. 800mv x 5= 4v..so, yup. maybe that´s the issue (although 800mv transient sould be instantaneous)

i have:
input buffer--> gain stage (max gain is 11x)-->high pass SK filter-->twin-t mid control-->LP SK filter-->LP SK filter

the main issue is that at gains lower than 5X i dont get clipping, but the output has lower volume than the input

i tried reducing the gain stage and compensate making the last LP filter an amplifier via feedback resistors, but that only yielded oscilation..

i think ill maybe move the gain stage for the last stage, and put a divider in its place to avoid clipping by large signals...

[Bill Mountain]

Cutting mids with the twin T filter can suck the life out of the signal but leave plenty lows and highs that can clip the IC's when you boost your signal to compensate.  I've shredded my fair share of speakers this way.  Your best bet is to probe it to see where the clipping is.

[dschwartz]

Yes , the low pass filteris actually a chebyshev, so it has a resonant peak there..aroud 10db at 75hz..sounds pretty good actually (its for bass)

[Bill Mountain]

If it's for bass then I'm all about it!