Hi, I'm trying to mod the berocca pre amp for bass by robs mod for a friend by adding an option to select between 2 set of capacitors in the baxandall filter to have two centre frequencies for the treble control (one a 4.5kHz and other at 11kHz). When I tried that circuit it sounded like a very muddy and bad fuzz, saturating every thing and cutting off sometimes. Also noted that if de-soldered on combination of caps from the pcb and the switch the switch didn't cut off but had the same sound only that at a very low volume level. Can this be implemented as it shows in the photo or I can't put a parallel cap in the baxandall even with a switch?
Imgur: The magic of the Internet
Thanks!
Imgur: The magic of the Internet
Thanks!
An externally hosted image should be here but it was not working when we last tested it.
From that description I reckon the circuit's likely to be oscillating - did you shield the signals to/from the switch? Did you keep the wiring runs as short as possible? Its best to switch stuff like this on the pcb, not remotely, to reduce any unwanted capacitive loading or feedback paths.
Also if you switch something in a feedback path you should arrange that there's never an open circuit, as then the opamps will slew immediately to the rail creating a hell of a crack on the speakers. For this circuit you could have one of the caps permanently in circuit and the other switched in parallel to increase the capacitance - then you only need a single-throw switch too, reducing the number of wires running about.
Also if you switch something in a feedback path you should arrange that there's never an open circuit, as then the opamps will slew immediately to the rail creating a hell of a crack on the speakers. For this circuit you could have one of the caps permanently in circuit and the other switched in parallel to increase the capacitance - then you only need a single-throw switch too, reducing the number of wires running about.
Last edited:
I would use on-board audio relays, each located close to the parts being switched.
Even then, you will likely get some switching noise.
Even then, you will likely get some switching noise.
Last edited:
I want the switch to be made from the controls in the bass while playing, next to the pots. But I didn't shield much, was a quick prototype in a pcb.
Thanks for that, sometimes I over complicate stuff. And after a quick simulation in multisim I realized that I can achieve the same result but just putting one cap in parallel with c4 and using one single switch. Will update after the weekend if a build it and try it
It ended up like this Imgur: The magic of the Internet
Thanks!