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Author Topic: Pre-amp / compressor for bass guitar  (Read 9227 times)

jokunen

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Re: Pre-amp / compressor for bass guitar
« Reply #15 on: July 28, 2013, 04:56:12 PM »
I guess my 5532 was broken because it induced a 100mv offset in the buffer side of the chip and that ate the headroom big time.
Maybe a coupling cap would be a good idea between the stages there.
Another headroom eater was my LDR. It seem to go really high resistance when the led is off so the buffer stage lost it's "4.5V" bias. I fixed that by adding a 1M resistor in parallel with the LDR.
I worked fine with a TL082 and now I put a OPA2134 in there to be on the "hifi" side.
The 2134 also has quite low standby current draw. I guess a good amp for the side chain would be the TL062 because of the low current draw.
The 9V rails are not an issue anymore. It seems to drive line level input into clipping before audible clipping from the preamp.
Of course adding a little compression makes it even easier not to clip.

I changed the LED driver to 2N7000 enhancement mode MOSFET and took out the red led from the source bias circuit. That seems to work better.

The first JFET bias changing with lower battery seems to be only remaining problem but I think I can live with that if the other option is adding voltage regulation for it. That would also waste battery power.
I still need to check the total current draw and maybe adjust some parts if it draws too much.

From usability perspective there are many styles of compression available. One special is saturating the compression led with "too low" threshold so the led is on all the time.
For actual live use there are too many interacting controls and the adjustment range is not optimal.
The threshold goes too low and too high with the MOSFET. That should be an easy fix.
The controls are too interactive. Gain, Threshold and Ratio all control the level of compression in the other 2 are in somewhat normal positions. I'll remove at least one of them when I find which settings are actually good.
It's nice to have all those extreme settings available but it makes practical adjustments more difficult.
It seems like fixing the gain at 25K and threshold at around 4.5V would make a nice straight-forward one-knob compressor but  there are other tempting options too.

jokunen

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Re: Pre-amp / compressor for bass guitar
« Reply #16 on: October 15, 2013, 05:00:30 PM »
I now created version 2 of the compressor.
This time I dropped the JFET input as this kind of pedal is used for live rather than recording and the sound difference is marginal.
Instead I upgraded the virtual ground to be able to sink/source some current and the envelope detector to use a full wave rectifier.
The gain reduction is now moved to the feedback loop of the gain op amp although I think it introduces some distortion. Theoretically it should have the same effect as shunting the signal to ground but somehow I feel that in practice it's not...
The LDR LED is not current controlled instead of voltage controlled which was made possible by the enhanced virtual ground.
It's bit more difficult to maximize the attainable gain with 9V rails with this device but I think I can get pretty close to line level.
I also tried using the 7660 voltage doubler but had trouble getting rid of the noise it generated on a tight vero layout. Possibly chokes on the rails would have helped but I didn't have any at hand.