I designed and bread boarded a preamp/compressor for bass guitar.
The design goal was to create a battery-operated preamp that is able to drive line level power amp directly from a passive bass.
I feel that properly preamped bass driven into proper amplification has too much dynamic range.
If the speaker's low end response is not sufficient then there is a good amount of "natural compression" but that's not the sound I want. That's why I wanted active compression.
The compressor design is initially inherited from "what compressor" but I reduced the part count a lot and connected the gain reduction before the compression detector.
This makes the compressor more of a limiter. It's easier to adjust that way and never goes to "negative ratio" which I don't like with bass.
Half-wave rectification instead of full-wave (like in what compressor) may introduce some extra asymmetric distortion but I haven't found it objectionable at least yet.
I initially had an option to add gain to the compression loop making it even more of a limiter. Works nice when slapping but sounds too much like typical VCA-compressor with finger style playing.
All the component values on the schematic may not be correct because I drew it from memory.
I have actually used NE5532 and TL082 so far but that OPA happened to be handy in the Eagle library when drawing.
There are two gain stages:
The first is a basic JFET gain stage followed by BJT buffer.
The second is non-inverting buffer driving inverting gain stage on a single opamp chip.
I previously compared the sounds of the JFET stage to an opamp and came to the conclusion the JFET sounds a bit "better".
The compressor part is an independent add-on.
There are controls for ratio (R5) "compression" (R10) and master volume (R15).
I initially had attack, release and pre-gain too but they are not that useful.
R10 is not real threshold as in this design the compression led is always on, so threshold is -infinity.
The R10 adjusts the compression sound in very useful way.
There is no "make up gain" as that would make a positive compression feedback loop.
I currently have just a normal LDR attached to a yellow led installed.
The vactrol is there just for easier drawing again. I may try a vactrol in there but then the R5 needs to be selected accordingly.
Please feel free to contribute and fix my mistakes.
The design goal was to create a battery-operated preamp that is able to drive line level power amp directly from a passive bass.
I feel that properly preamped bass driven into proper amplification has too much dynamic range.
If the speaker's low end response is not sufficient then there is a good amount of "natural compression" but that's not the sound I want. That's why I wanted active compression.
The compressor design is initially inherited from "what compressor" but I reduced the part count a lot and connected the gain reduction before the compression detector.
This makes the compressor more of a limiter. It's easier to adjust that way and never goes to "negative ratio" which I don't like with bass.
Half-wave rectification instead of full-wave (like in what compressor) may introduce some extra asymmetric distortion but I haven't found it objectionable at least yet.
I initially had an option to add gain to the compression loop making it even more of a limiter. Works nice when slapping but sounds too much like typical VCA-compressor with finger style playing.
All the component values on the schematic may not be correct because I drew it from memory.
I have actually used NE5532 and TL082 so far but that OPA happened to be handy in the Eagle library when drawing.
There are two gain stages:
The first is a basic JFET gain stage followed by BJT buffer.
The second is non-inverting buffer driving inverting gain stage on a single opamp chip.
I previously compared the sounds of the JFET stage to an opamp and came to the conclusion the JFET sounds a bit "better".
The compressor part is an independent add-on.
There are controls for ratio (R5) "compression" (R10) and master volume (R15).
I initially had attack, release and pre-gain too but they are not that useful.
R10 is not real threshold as in this design the compression led is always on, so threshold is -infinity.
The R10 adjusts the compression sound in very useful way.
There is no "make up gain" as that would make a positive compression feedback loop.
I currently have just a normal LDR attached to a yellow led installed.
The vactrol is there just for easier drawing again. I may try a vactrol in there but then the R5 needs to be selected accordingly.
Please feel free to contribute and fix my mistakes.