Pro Co Rat: alternate approach in removing fizz?

[stonerbox]

Hai!

Is there any alternative way to get rid of the fizz this pedal makes except the use of a filtering cap? I'm not too keen on losing them "hi-ends". It is strange.. 'cause even when I turn down the distortion and get a semi clean sound a nasty fizz is present in the background when I'm playing.

Here is a clip: http://soundcloud.com/tropicalhippo/rat

I substituted the LM308 with a LM385N, so no comp-action going on here, and I used a BF246a at the output stage instead of the original 2N5458.



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

-Update May 09 -
I had the BF246a in backwards earlier but now with the MOSFET in the correct orientation I have updated voltage values down below.

MOSFET:
https://www.fairchildsemi.com/datasheets/BF/BF246A.pdf
G: 2.5V
S: 0V
D: 9.2V

[vigilante397]

I've built several RATs (my favorite distortion in the world) using the LM308 and I have never had any problem with noise in the circuit. What kind of power supply are you using?

Also, it should be noted that a filter cap on the power supply does not affect the tone of the circuit like a cap in the signal path will, and if a power supply cap is cutting the high end of the effect then you should probably double check your wiring.

[stonerbox]

I've built several RATs (my favorite distortion in the world) using the LM308 and I have never had any problem with noise in the circuit. What kind of power supply are you using?

Also, it should be noted that a filter cap on the power supply does not affect the tone of the circuit like a cap in the signal path will, and if a power supply cap is cutting the high end of the effect then you should probably double check your wiring.

It's not a constant background noise but a high frequency fizz that only occurs when playing, so I'm ruling out that the power supply would have anything to do with it.

Edit:
Go to the Soundcloud link and listen closely at the start of the clip, it's most prominent there. You'll probably have to turn up the volume to hear it clearly.

[vigilante397]

I listened again and I think I understand what you're talking about now. But I do have a couple questions:

1) What made you decide to use the BF246 instead of the 2N5458? I've tried transistors besides the 2N5458 for the output but never found one that I liked better.

2) What made you decide to use the LM358 instead of the LM308? That's one of the features that really makes a RAT a RAT.

3) What diodes are you using for clipping?

4) What layout are you using?

[stonerbox]

I listened again and I think I understand what you're talking about now. But I do have a couple questions:

1 & 2: 'cause that was what I had at home + the LM358n has a very similar slew rate (0.1) to the original LM308 (0.15).. and I like to experiment.

3: Switchable between 1n270 (symetrical) and MOSFETS (asymetrical). The issue remains the same with both.

4: http://tonepad.com/project.asp?id=45 merged with Synthroteks version of the Rat. Unfortunately, I can't get a hold of their schematic anywhere online any longer, but it goes something like this: The 1uF in the power filtering  (next to the 100uF in the tonepad layout) is swapped for a 47uF and the Ruetz mod is of course also implemented. They also added a 47nF cap to ground next to the 100uF electrolytic, but I did not add that one.

[aron]

The BF246A is not a MOSFET is it? In any case, is the gate supposed to have that much voltage on it? Maybe you have it wired in correctly.

[stonerbox]

The BF246A is not a MOSFET is it? In any case, is the gate supposed to have that much voltage on it? Maybe you have it wired in correctly.

From the info I gathered the BF246a is indeed a MOSFET and should work fine in this circuit. Maybe I stumbled upon some misleading information somewhere along the line? There sure is a lot of Know-it-All people out there so I would not be surprised if I did.

However, this is my first time around with MOSFETs and there for I suspect that I may have put it in backwards, gate where drain should be and vice versa. I better check it out.

[samhay]

The 2N5458 and BF246 are both N-channel JFETs.

[stonerbox]

So I've turned the BF246a around 180 degrees. I had put it in backwards after all. However.. the fizzing issue still persists just as before.

[Gus]

You used a dual opamp in place of a single.

How did you wire the other unused opamp?  
A link http://www.electronicproducts.com/Analog_Mixed_Signal_ICs/Amplifiers/Properly_terminating_an_unused_op_amp.aspx

[stonerbox]

You used a dual opamp in place of a single.

How did you wire the other unused opamp?  
A link http://www.electronicproducts.com/Analog_Mixed_Signal_ICs/Amplifiers/Properly_terminating_an_unused_op_amp.aspx

The second output and inputs are not connected to anything..  Big thanks for the link. I'll attend to that right away.

[duck_arse]

the bf246 has the gate to the centre pin, so, in this application, it will go either direction correctly.

[Gus]

I wonder why people don't build this type distortion with a emitter follower(EF) output instead of a source follower.  Something like the Brat EF stage
Maybe use a high beta transistor like a MPSA18 with something like 1.8meg +9VDC to base and 2.2meg base to ground with a 10K emitter resistor

Measure the source voltage to ground.

The Ruetz mod changes the sound.
FWIW the cap in the 47 ohm leg is one place the type matters, IMO a tant or film or ceramic should be used there and should be measured for value.

[stonerbox]

Ok.. so I connected the 2+ to ground and the 2- to the output (1) of the IC. No change what so ever. I'm giving up. @#$% it.

Thank you guys very much for trying to help me in this. I've learned some new important stuff and that sure is a win at least.

the bf246 has the gate to the centre pin, so, in this application, it will go either direction correctly.

Nope, sorry Duck but believe me I check this a "couple" of times.
https://www.fairchildsemi.com/datasheets/BF/BF246A.pdf

I wonder why people don't build this type distortion with a emitter follower(EF) output instead of a source follower.  Something like the Brat EF stage
Maybe use a high beta transistor like a MPSA18 with something like 1.8meg +9VDC to base and 2.2meg base to ground with a 10K emitter resistor

Measure the source voltage to ground.

The Ruetz mod changes the sound.
FWIW the cap in the 47 ohm leg is one place the type matters, IMO a tant or film or ceramic should be used there and should be measured for value.

The Source reads nothing.. dead 0V. The drain reads 9.2V and the gate got 2.2v.

Are you referring to the 0.01 cap right under the op amp on the tonepad layout? On the Synthroteks version it's a .047 cap so I went with that. All caps are film.

[anotherjim]

Maybe squeeze in a big muff type 1 knob tone control - possibly with a fixed pot. This circuit has a mid range notch "scoop" and you can tweak values to move it up or down. Idea is to have notch cutting the fizzy frequency.
Do you have a graphic eq so you can find out what frequency notch best controls the fizz?

That said, the LM358 isn't really compatible with the 308. The 358 has asymmetric voltage swing  - it isn't equal around 1/2 vcc, so you will get a lot of second harmonic distortion not present in the Rat (that is, it will clip at the positive rail long before it clips at negative).
You'd probably be closer to the 308 with a 1458. If you've socketed the 358, try a 1458 before anything else.

[stonerbox]

Maybe squeeze in a big muff type 1 knob tone control - possibly with a fixed pot. This circuit has a mid range notch "scoop" and you can tweak values to move it up or down. Idea is to have notch cutting the fizzy frequency.
Do you have a graphic eq so you can find out what frequency notch best controls the fizz?

That said, the LM358 isn't really compatible with the 308. The 358 has asymmetric voltage swing  - it isn't equal around 1/2 vcc, so you will get a lot of second harmonic distortion not present in the Rat (that is, it will clip at the positive rail long before it clips at negative).
You'd probably be closer to the 308 with a 1458. If you've socketed the 358, try a 1458 before anything else.

The center fizz is at 8 Khz but really spans from about 7 to 9Khz, where a lot of other important frequencies roams. Perhaps getting a 1458 or a 308 would solve the issue. Probably.


If somebody out there got too much time on their hands and a Rat in your belonging, would you do me a favor?
Test your rat with the distortion all the way down and leave the filter inactive (no high end cutting) and check if you get the same result as me.

[vigilante397]

If somebody out there got too much time on their hands and a Rat in your belonging, would you do me a favor?
Test your rat with the distortion all the way down and leave the filter inactive (no high end cutting) and check if you get the same result as me.

I'll do it today at work and let you know what I find.

[stonerbox]

I'll do it today at work and let you know what I find.

I really appreciate it man, thank you!

[vigilante397]

If somebody out there got too much time on their hands and a Rat in your belonging, would you do me a favor?
Test your rat with the distortion all the way down and leave the filter inactive (no high end cutting) and check if you get the same result as me.

Just tried my RAT with no filter and distortion all the way down, and it was very clean, no noise, and with the volume cranked all the way it was at unity gain. Switching back and forth between bypass and this setting gave no noticeable change in tone.

This was a RAT built "stock" based on the Tonepad layout using LM308 and 2N5458 with 1N914's for clipping.

[stonerbox]

Just tried my RAT with no filter and distortion all the way down, and it was very clean, no noise, and with the volume cranked all the way it was at unity gain. Switching back and forth between bypass and this setting gave no noticeable change in tone.

This was a RAT built "stock" based on the Tonepad layout using LM308 and 2N5458 with 1N914's for clipping.

Alright, cool. Thank you for taking time checking this for me. You rock Nathan!