Boss FA1-ish becomes gated fuzz at max treble only?

[Rob Strand]

This example is the same as the previous V1.1 example however I've added a 4.7pF cap across the gain control (keeping the tone control 47pF).   The response is when the gain control is full.  The amount of roll-off rises when the gain control is backed off but the absolute HF gain should stay pretty constant.

You can see below 20kHz the effect is minimal but the amount of attenuation outside of the audio band is fairly significant.  Very good for ensuring circuits don't oscillate.

[amptramp]

I have sometimes seen the Baxandall treble control implemented with a capacitor in place of R9, the resistor going to the pot slider and capacitors connected to each side of the treble control in place of C3 and C6.  It has one benefit in that you can match resistors a lot easier than capacitors and should be cheaper to implement - but it might be worse for oscillation.

Pots have a considerable interelectrode capacitance and capacitance to ground and depending on how the wiring goes, if there is a lot of capacitance from the max treble end of the pot to ground, you may get tone control stage oscillation which can be mitigated somewhat by adding a small capacitor from op amp output to inverting input which forms a voltage divider from output to input to ground and reduces the rising gain characteristic with frequency you get with capacitance to ground from the inverting input.  Something in the 47 pF to 100 pF range should work.  You will get a bit of rolloff but you are not looking for high fidelity treble in a guitar amplifier.

Also, the first stage should have some capacitance across R6 / VOLUME 1 which will definitely be prone to oscillation with the capacitance to ground of the volume control.

One stunt you could try is moving the C3 connection from the junction of C8 and R7 to the other side of C8 at the first stage op amp output.  This may reduce the effect of capacitance to ground (or make it worse with the capacitance to ground of the volume control) but you never know unless you try.

[drdn0]

Finally got around to making some changes! Capacitance across R6 made absolutely no difference, but 100pf across in/out of IC2 worked perfectly. Weirdly enough it -did- appear to increase susceptibility to RF noise slightly (?) but it's only noticeable with everything dimed.

Thanks everybody for their input!

[Rob Strand]

Finally got around to making some changes! Capacitance across R6 made absolutely no difference, but 100pf across in/out of IC2 worked perfectly. Weirdly enough it -did- appear to increase susceptibility to RF noise slightly (?) but it's only noticeable with everything dimed.

Thanks everybody for their input!

I wouldn't expect adding the cap to the tone control to increase RF problems.   Weird interactions are possible but the fault may lie in the wiring/layout/enclosure.  Is your enclosure metal?  If not, are the metal casings of the pot connected to ground? - that can be a bad source of RF.

If you look at your schematic and bluelagoon's schematic yours is missing the 10k on the gate of the input JFET.   The resistor can help prevent RF.

However, there's many ways RF can get in.   Not so easy to diagnose and it can depend on the build/layout specifics. 

What are you calling RF?  Buzz?  Radio Station signal break-through?

[drdn0]

Finally got around to making some changes! Capacitance across R6 made absolutely no difference, but 100pf across in/out of IC2 worked perfectly. Weirdly enough it -did- appear to increase susceptibility to RF noise slightly (?) but it's only noticeable with everything dimed.

Thanks everybody for their input!

I wouldn't expect adding the cap to the tone control to increase RF problems.   Weird interactions are possible but the fault may lie in the wiring/layout/enclosure.  Is your enclosure metal?  If not, are the metal casings of the pot connected to ground? - that can be a bad source of RF.

If you look at your schematic and bluelagoon's schematic yours is missing the 10k on the gate of the input JFET.   The resistor can help prevent RF.

However, there's many ways RF can get in.   Not so easy to diagnose and it can depend on the build/layout specifics. 

What are you calling RF?  Buzz?  Radio Station signal break-through?

Aluminium enclosure, single ground point to the enclosure through a pogo pin right on the input jack ground. Jacks are isolated, but pot/switch bodies are not.

No radio breakthrough, but a bit of buzz/faint strange noises getting modulated in (turning various things off and moving them around, my PC is definitely one source along with my phone). It's not unmanageable as I can't see anybody having everything dimed especially when the spicy amount of treble on tap, but it is annoying.

I've got some new switching PCB's coming with massively OP power filtering and a RF filter at the PCB input, so I'll see if they make a difference.

[Rob Strand]

I don't think any of those small oscillation prevention caps are going to help much.   The noise is in the audio band.

It might take some experimenting to narrow it down.  Gain + treble boost is going to make small problems show up for sure.

You could try increasing the 100nF VREF cap, maybe 10uF like the original.   Wouldn't hurt to try.