Pro Co Rat: alternate approach in removing fizz?

[Gus]

Use 3a for wiring the other half of the dual opamp

The wiring of the circuit is important and more so with high gain circuits.

I think you have your gate and source measured voltages reversed

[GGBB]

It sounds like the "fizz" goes away as the note decays, suggesting some sort of high frequency clipping. This is definitely not something you normally hear in a RAT. Sometimes the output FET can clip.  If Gus is right about the voltages - the FET could be hovering around cutoff.  Try biasing the FET gate at 1/2 Vin to stabilize it and give it more headroom (that's what's done in the Turbo RAT). Add a 1M resistor between drain and gate. Not sure this will do anything but it's easy enough to try.

[Gus]

If you wire it like 3a, try a different dual opamp if you still have the fizz

How is this built on a breadboard or a PCB or?
Is it in a metal enclosure?

What resistor values are you using for the voltage divider? If they are two 10K you could use the same Vref for the 3a wiring AND connect the 1meg gate resistor to this node instead of ground, works the same as what GGBB posted about biasing the jfet.

[stonerbox]

I finally managed to once again get a hold of the Synthrotek schematic. Not that much of a difference from the original other than a couple of caps and the R-mod: http://www.synthrotek.com/products/effect-pedal-circuits/rat-clone/rat-schematic/

It sounds like the "fizz" goes away as the note decays, suggesting some sort of high frequency clipping. This is definitely not something you normally hear in a RAT. Sometimes the output FET can clip.  If Gus is right about the voltages - the FET could be hovering around cutoff.  Try biasing the FET gate at 1/2 Vin to stabilize it and give it more headroom (that's what's done in the Turbo RAT). Add a 1M resistor between drain and gate. Not sure this will do anything but it's easy enough to try.

Ok, I see what your talking about on your (?) multi-rat schematic. I guess your referring to the R10 and R16 to the far right? Will definitely try that.
http://music-electronics-forum.com/attachments/26656-multi-rat.png

Use 3a for wiring the other half of the dual opamp

The wiring of the circuit is important and more so with high gain circuits.

I think you have your gate and source measured voltages reversed

................................................

If you wire it like 3a, try a different dual opamp if you still have the fizz

How is this built on a breadboard or a PCB or?
Is it in a metal enclosure?

What resistor values are you using for the voltage divider? If they are two 10K you could use the same Vref for the 3a wiring AND connect the 1meg gate resistor to this node instead of ground, works the same as what GGBB posted about biasing the jfet.

I tried to wire the BF246a in all possible ways and I get the same sound no matter what. It's strange 'cause the signal should cut out in at least one of the wirings? Then again, perhaps I haven't yet tried the supposed drain pin as a "source pin". I can't remember any longer.  

Let me get this straight, so you suggest that I also 3a the unused second input pin on the LM358n?

The pedal is right now set up on both a breadboard and on PCB in a metal enclosure. I'll give the biasing a shot now. The voltage divider, at the start of the schematic, consists of two 100ks.

[duck_arse]

the gate is wherever you want it. depends on the manufacturer, it seems ......

http://pdf.datasheetcatalog.com/datasheet/siemens/Q62702-F219.pdf

[stonerbox]

the gate is wherever you want it. depends on the manufacturer, it seems ......

http://pdf.datasheetcatalog.com/datasheet/siemens/Q62702-F219.pdf

I'm such an idiot. I was certain I had old Fairchild MOSFETS but upon inspecting them again I realized they are Motorola's! It's hard to tell when the text is a bit worn down.

You where right all along Duck!
http://www.datasheets360.com/pdf/2448457400732652043

[aron]

Wait, but they are not MOSFETs, they are N-Channel JFETs. And the fizz (to me) sounds like a misbiased component.

[stonerbox]

Wait, but they are not MOSFETs, they are N-Channel JFETs. And the fizz (to me) sounds like a misbiased component.

C'mon man, you gotto be kidding me?! 

[aron]

The circuit calls for a JFET anyway, so you are actually using the correct type of transistor. I would build an audio probe and listen to the circuit before it hits the gate of the FET.
http://www.diystompboxes.com/wiki/index.php?title=Debugging

Listen to the circuit right after the tone control. See if the fizz is there. If it is, then your problem is with the op amp. If not, then your problem is with the FET stage.

[stonerbox]

The circuit calls for a JFET anyway, so you are actually using the correct type of transistor. I would build an audio probe and listen to the circuit before it hits the gate of the FET.
http://www.diystompboxes.com/wiki/index.php?title=Debugging

Listen to the circuit right after the tone control. See if the fizz is there. If it is, then your problem is with the op amp. If not, then your problem is with the FET stage.

Just made a probe and tested, the fizz is coming from the LM358N. Could perhaps a modification of the voltage divider (the two 100k) help solve the problem? If so, what values should I start out with?

[GGBB]

LM358N:
http://www.ti.com/product/lm358-n
1. 4.8V
2. 4.8V
3. 0.9V
4. 0V
5. 0V
6. 0V
7. 8V
8. 9.2V

Pin 3 seems way too low - shouldn't be a lot less than 1/2Vcc. Maybe that has something to do with the unused op-amp. Can you remeasure and verify?

[ashcat_lt]

Isn't pin 3 the non-inverting input?  That just can't happen!  How the heck are the other two pins not also almost 0, since they should be following pin 3?  Are we sure that's not a typo or measurement mistake?

[stonerbox]

Pin 3 seems way too low - shouldn't be a lot less than 1/2Vcc. Maybe that has something to do with the unused op-amp. Can you remeasure and verify?

Ok, here's the updated voltages, still however very low voltage on Pin 3. The second op amp is not floating anymore and I connected it in the following way:

Pos Input 2 >> Ground,  
Neg Input 2 >> Output 1
Output 2  >> Floating

LM358N:
http://www.ti.com/product/lm358-n
1. 4.3V [Output 1]
2. 4.3V [-1]
3. 0.4V [+1] - There is a faint hum when I measure this one
4. 0V    [Vcc-]
5. 0V    [+2]
6. 4.3V [-1]
7. 0V    [Output 2]
8. 9.2V [Vcc+]

Edits: Spelling and missing words, was in a bit of a hurry.

[stonerbox]

Isn't pin 3 the non-inverting input?  That just can't happen!  How the heck are the other two pins not also almost 0, since they should be following pin 3?  Are we sure that's not a typo or measurement mistake?

Those values are from way back and long before I ever hooked up the second op amp.

[GGBB]

LM358N:
http://www.ti.com/product/lm358-n
1. 4.3V [Output 1]
2. 4.3V [-1]
3. 0.4V [+1] - There is a faint hum when I measure this one
4. 0V    [Vcc-]
5. 0V    [+2]
6. 4.3V [-1]
7. 0V    [Output 2]
8. 9.2V [Vcc+]

Pin 3 is way too low. What are the voltages with the op-amp removed?

[stonerbox]

LM358N:
http://www.ti.com/product/lm358-n
1. 4.3V [Output 1]
2. 4.3V [-1]
3. 0.4V [+1] - There is a faint hum when I measure this one
4. 0V    [Vcc-]
5. 0V    [+2]
6. 4.3V [-1]
7. 0V    [Output 2]
8. 9.2V [Vcc+]

Pin 3 is way too low. What are the voltages with the op-amp removed?

I'll check right away and edit this message when I'm done. I guess you mean op amp 2, right?

Edit/Update: Exactly the same. 0.4. Weird...

[GGBB]

I meant with the entire dual op-amp chip removed.

[stonerbox]

I meant with the entire dual op-amp chip removed.

Just checked that too and it's the same. 0.4.

[GGBB]

It should be around 1/2Vcc - so something is wrong in the power supply section between power in (Vcc) and pin 3. Check components values and make sure the caps are good, check wiring/soldering, and make sure you actually implemented the circuit properly.

[ashcat_lt]

But pin 1 and 2 can't be at Vref if pin 3 is near ground unless the is another problem.  Is there something's screwey with one or both of your coupling caps?