BAJA REACTOR 010119 [documentation]

[bajaman]

Hey Bajaman, nice work, as usual! Quick question, how would a signal chain look with the Reactor?

In an ampless rig should it be preamp/reactor/IR(cabsim) ?

Thanks!

yes

[temol]

try removing the 1uf capacitor before the depth control - just use a link instead

Partial success - it's hard to induce oscillations now but still possible

[bajaman]

try removing the 1uf capacitor before the depth control - just use a link instead

Partial success - it's hard to induce oscillations now but still possible

In my build - on the pcb I designed (which does not oscillate !) I did not use the 1uf capacitor - it was an afterthought
Perhaps the fault lies elsewhere - the power supply perhaps? I can assure you that at full rotation there is absolutely no sign of any instability in my build
I have also built this with +15v and -15v power supply rails and installed it between the preamp and power amp in a fully mosfet prototype amplifier (using LND150s and exicon lateral mosfets - no sign of any instability in the depth or presence controls
cheers
bajaman

[temol]

Perhaps the fault lies elsewhere - the power supply perhaps? I can assure you that at full rotation there is absolutely no sign of any instability in my build

I don't blame you
It looks like the circuit is quite sensitive to power supply - both main and reference voltage. But the breadboard doesn't help either. Sometimes some accidental bump into the wires causes excitation. I tried 2 different prototyping boards, two different layouts and each time I was able to induce oscillations.

[bajaman]

This is the layout I used for my pcb

[ding127063]

This is my PCB layout design, and it's not of high quality. I've used a lot of surface-mount resistors and capacitors. I think that there's no significant difference whether I keep or remove the 1uF capacitor.I'm currently working on the second version, attempting to implement a ±15V power supply.

[tuck]

May I ask for advice? My version is oscilating heavy.
For this dumb idea I put reactor, an omnicab and a headphone amp on one pcb.
The headphone amp works fine as well as the omnicab but the reactor sounds like a brachiosaurus.

- for drive I used a external pot
- CX is switchable to select two different caps
- the op amps are placed differently in my schematic to be able to place the chips conveniently for the traces
- I added 100nF caps to the op amps
- VBias is fine
- I removed the 1u as it was adviced before -> no change in oscillation
- the oscillation is noticable around IC1 but gets really loud after IC2a pin1

I attached my complete project.

[tuck]

This is what I see on the output, I'm new to oscilloscopes, I forgot everything I learned almost 25 years ago.

cx_330n.bmp (161.13 KiB) Viewed 1395 times

cx_82n.bmp (161.13 KiB) Viewed 1395 times

But in between the tinkering it worked. Maybe I should give it a shot and box it up? Ground all the pots, put it in an actual enclosure.
This was just hooked up with aligator clips.

[tuck]

Flicking the switch for Cx makes the oscillation stop.
I toggle it to the second cap and it works.
I flick it back and it oscillates but after two or three toggles it is completely fine also via the first cap.

[tuck]

Another observation: I added the 1uF back again and now it needs one toggle of Cx to stop the oscillation and all is fine.

[FaustoM]

Breadboard time...
on my breadboard it's a bit unstable. With Depth @ min it's ok. But combination of Depth and heavily muted low frequency notes causes oscillations. Frequency of the oscillation - around 94 Hz.

To address the instability and oscillation issue around 94 Hz in "Baja Reactor" circuit, particularly when the Depth control is set low and heavy low-frequency notes are played, you could try the following strategies:
1. Adjust the Depth Control Range
The Depth control might be too aggressive, allowing too much feedback at low frequencies. You could reduce the capacitor or resistor value in the Depth circuit to limit the boost at these low frequencies.
Try increasing the value of the capacitor in the Depth circuit (currently 4.7nF) to lower the low-frequency boost. For example, changing it to 10nF or more could shift the resonance to a less problematic frequency.
2. Add a High-Frequency Bypass Capacitor
A small capacitor (e.g., 100pF to 1nF) in parallel with the 250kΩ Depth potentiometer can reduce high-frequency oscillations and smooth out the resonance peak. This will help stabilize the circuit when Depth is set to minimum by damping oscillations in the low-frequency range.
3. Adjust Feedback Network
The feedback path involving the 220nF capacitor and 120kΩ resistor might be too sensitive to low-frequency feedback. Lowering the capacitor value (e.g., from 220nF to 100nF) will reduce the gain at low frequencies and may help reduce oscillation.
Alternatively, you could slightly reduce the value of the 120kΩ resistor to lower the feedback loop gain.
4. Increase Power Supply Bypassing
Add more decoupling capacitors (e.g., 100nF ceramic and 10µF electrolytic) to the 9V power rails near the op-amps. This can prevent power supply noise from feeding into the op-amp stages and causing instability.
5. Check for Op-Amp Instabilities
Ensure that the TL072 op-amps are stable in the feedback loop. You could add a small capacitor (e.g., 22pF) across the feedback resistor in the op-amp stages to limit high-frequency oscillations.
6. Reduce Gain in the Low-Frequency Region
To attenuate the low-frequency content causing oscillations, you can insert a high-pass filter (capacitor in series with resistor) at the input or between stages. For instance, increasing the value of the 22nF input capacitor slightly can limit low-end response, thus reducing feedback at low frequencies.
7. Lower the Presence Control Response
Similar to the Depth control, the Presence circuit could also be made less aggressive by adjusting the 100nF capacitors in that section. Increasing their value could smooth out the response and reduce feedback.
8. Damping Network
A damping network (small resistor and capacitor in series) could be placed across the output stage to provide additional stability. This would help attenuate frequencies around 94 Hz that are prone to oscillation.

By adjusting one or a combination of these aspects, you can reduce the tendency of the circuit to oscillate, particularly when heavy muted low-frequency notes are played.
I'd like to have a feedback for above suggestions from circuit creator Mr. Baja

[FaustoM]

Here my contribution to this great project!
I built it so the board is VERIFIED.

It fits into a common 18x24 PCB.
I added 6 small 100n ceramic caps close to the TL ('cos I read about circuit oscillation)

I tested it with 9V supply and it sounds good, with 9V pay attention for distortion on E and A cords with neck pickup, while with bridge pickup all good.
With 9V I believe the isn't enough room for bass signal for such amplification, consider to slow down the guitar volume or replace some resistors values in order to cut the signal avoiding any clipping.

Have fun!

[tuck]

What else did you change in regard of your recommendation above? Except for the 100n caps, I also added them.

[FaustoM]

What else did you change in regard of your recommendation above? Except for the 100n caps, I also added them.

I used the spare 1/2 op-amp as buffer in output stage as per schematic of Rev.5.
Micro caps against pots as per my suggestions above.
All the rest is equal to Rev.4

[tuck]

To address the instability and oscillation issue around 94 Hz in "Baja Reactor" circuit, particularly when the Depth control is set low and heavy low-frequency notes are played, you could try the following strategies:
1. Adjust the Depth Control Range
The Depth control might be too aggressive, allowing too much feedback at low frequencies. You could reduce the capacitor or resistor value in the Depth circuit to limit the boost at these low frequencies.
Try increasing the value of the capacitor in the Depth circuit (currently 4.7nF) to lower the low-frequency boost. For example, changing it to 10nF or more could shift the resonance to a less problematic frequency.
2. Add a High-Frequency Bypass Capacitor
A small capacitor (e.g., 100pF to 1nF) in parallel with the 250kΩ Depth potentiometer can reduce high-frequency oscillations and smooth out the resonance peak. This will help stabilize the circuit when Depth is set to minimum by damping oscillations in the low-frequency range.
3. Adjust Feedback Network
The feedback path involving the 220nF capacitor and 120kΩ resistor might be too sensitive to low-frequency feedback. Lowering the capacitor value (e.g., from 220nF to 100nF) will reduce the gain at low frequencies and may help reduce oscillation.
Alternatively, you could slightly reduce the value of the 120kΩ resistor to lower the feedback loop gain.
4. Increase Power Supply Bypassing
Add more decoupling capacitors (e.g., 100nF ceramic and 10µF electrolytic) to the 9V power rails near the op-amps. This can prevent power supply noise from feeding into the op-amp stages and causing instability.
5. Check for Op-Amp Instabilities
Ensure that the TL072 op-amps are stable in the feedback loop. You could add a small capacitor (e.g., 22pF) across the feedback resistor in the op-amp stages to limit high-frequency oscillations.
6. Reduce Gain in the Low-Frequency Region
To attenuate the low-frequency content causing oscillations, you can insert a high-pass filter (capacitor in series with resistor) at the input or between stages. For instance, increasing the value of the 22nF input capacitor slightly can limit low-end response, thus reducing feedback at low frequencies.
7. Lower the Presence Control Response
Similar to the Depth control, the Presence circuit could also be made less aggressive by adjusting the 100nF capacitors in that section. Increasing their value could smooth out the response and reduce feedback.
8. Damping Network
A damping network (small resistor and capacitor in series) could be placed across the output stage to provide additional stability. This would help attenuate frequencies around 94 Hz that are prone to oscillation.

By adjusting one or a combination of these aspects, you can reduce the tendency of the circuit to oscillate, particularly when heavy muted low-frequency notes are played.
I'd like to have a feedback for above suggestions from circuit creator Mr. Baja

1. no change
2. no change
3. can't seem to find a 220n and 120k in the schematic
4. no change
5. only two resistors, 10k and 15k, 15k already has a 1nf capacitor in parallel. No change when adding a cap to the 10k.
6. no change
7. no change
8. didn't try yet

I know that my pedal is slightly different as I have the cabsim and headphone amp on the same power supply.
No change in oscillation and when I pull all the op amps and transistors of cabsim and headphone amp and get the signal from the reactor output.

[FaustoM]

not sure I get your reply on point 3
just in case you are wondering where are the components

[tuck]

Ok understood that you mean Rev4

[temol]

To address the instability and oscillation issue around 94 Hz in "Baja Reactor" circuit, particularly when the Depth control is set low and heavy low-frequency notes are played, you could try the following strategies:

Thanks for the suggestions, I will try them.

[tuck]

@FaustoM in regard of your #3 I added another 470K piggyback to the 470K in my Rev5 build and the oscillation stops approx. 2 seconds after the switch on.
Edit: As it turned out it only works sometimes.