Help with an op amp booster

[Heemis]

Alright, so I also posted this yesterday and got no replies, so maybe second time is a charm!

I'm working on Colin from EA's analog bit crusher... schem here:  http://experimentalistsanonymous.com/diy/Schematics/Fuzz%20and%20Fuzzy%20Noisemakers/Analog%20Bit%20Crusher.gif

I've breadboarded it out and it sounds great, but I'm trying to convert the input buffer to a variable gain boost.  I'm close, but I keep having crazy noise problems.  First, I tried the microamp/distortion plus non-inverting op amp, which works, but only at the end of the pot rotation, and brings in a lot of noise. 

I also tried the very last schematic on this page:  http://www.muzique.com/lab/boost.htm (variable gain op amp) only I used a 100k pot and a 22k resistor for a max gain of roughly 5.5.  This method also works, but it results in tons of noise.

I'm wondering if there's something I'm missing because of the rest of the circuit.  I don't know much about proper circuit design, but I'm trying to learn.  At the moment I'm sort of in the trial and error stage.  If anyone can offer any tips or insight, I would appreciate it!!

[brett]

Hi

I also tried the very last schematic on this page:  http://www.muzique.com/lab/boost.htm (variable gain op amp) only I used a 100k pot and a 22k resistor for a max gain of roughly 5.5. 

If you use this type of circuit, you probably should use a capacitor (2.2uF) to ground where it shows 4.5V.  That will give DC isolation and let the circuit "float" at the bias voltage.  It's generally not a good idea to try to use 2 bias voltages (4.5V) in the one circuit.  Any difference gets magnified by the gain of the circuit.

I would also suggest that you check your pot.  They are sometimes crackly and noisey.
Good luck

[Heemis]

Hey thanks for the reply... I'll definitely give that a shot and see if I can make it work. 

One more quick question:  Should I use a small value cap across the inverting input and output of the op amp a la the distortion plus/micro amp schematics to prevent oscillation, or is this a bad idea with the bit crusher circuit?

[brett]

Hi
concerning the capacitors out and in question, the answer depends on which op-amp you are using.
With a high input impedance opamp such as the TL072, a small cap *probably* won't make a difference.
With an NE5332, it will probably upset the applecart.
Maybe best not to include a cap and just do DC switching with the JFET.
cheers