Pro Co Rat: alternate approach in removing fizz?

[GGBB]

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? 

Yup. Something very wrong somewhere.

[mth5044]

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

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

This was terminated incorrectly. Check out that link that was given to you about terminating opamps (specifically Figure 3a). Positive input should be at Vref and the negative input should tie to the output of the opamp. Incorrectly terminated opamps can be susceptible to noise. Not saying that's your problem, but it's not helping. 

[Scruffie]

Pin 3 has a 1M bias resistor, low impedance meter may be struggling to read it correctly.

[stonerbox]

YESSS!

Here's a major culprit and a damn hard one to spot too but the meter gave me a hint of what was really going on: a broken off connection.
At first glance it looked like the wire/pin of the 100pF cap that had been soldered to pin 3 of the op amp, but that was of course not the case.

After I had resoldered the connection the fizz went down substantially. It is still present but way further off in the background.  The debugging continues...



Here are the updated voltages. Pin 3 got a tiny boost of about 0.5V, not much but always something.

LM358N:
http://www.ti.com/product/lm358-n
1. 4.3V [Output 1]
2. 4.3V [-1]
3. 0.9V [+1]
4. 0V    [Vcc-]
5. 0V    [+2]
6. 0V    [-1] (corrected)
7. 0V    [Output 2]
8. 9.2V [Vcc+]

[duck_arse]

^ that photo looks like the ones my dentist takes of my teeth.

if your pin1 volts is good, it means the pin 3 volts are good enough. but, I can't see how you can have 4V3 on one of the second opamp inputs, if the output is at 0V.

[stonerbox]

Fixed the wiring of the unused op amp now. According to the Fig 3b over here: http://www.electronicproducts.com/Analog_Mixed_Signal_ICs/Amplifiers/Properly_terminating_an_unused_op_amp.aspx

Could anyone confirm if this is correct?
Positive in 2 > Ground  
Negative in 2 > Output 2


I threw in a RC4558P, just for the hell of it, and the fizzyness was no longer there on the lower distortion settings but the chip did not sound that pleasing. Cheap and kind of vintage (middy in the bad kind of way). A big cut in low end and treble in compared to the LM385N and that is probably why the fizz was gone, the RC4558P may behave a lesser. When this amp was connected I noticed that Output 2 and Negative input 2 both read around 8V.

I also tested the hi fi Burr-Brown amp OPA2134  and oh man the cleans on this thing rocks! No fizz, super detailed and very sweet. This op amp also have the most overdriven and probably harshest sound of all the one's I tested, but if you roll off some of the highs with the filter the distortion sounds pretty OK. Somewhat too compressed for me though and not as dynamic as some of the others on higher gain settings.

I recorded all of the amps to get some perspective. First lowest gain, then half and finally maxed out. Got a terrible headache today so don't you dare to comment on my shitty playing.

OP2134:
https://soundcloud.com/tropicalhippo/pro-co-rat-opa2134-bb?in=tropicalhippo/sets/pro-co-rat-op-amp-test
RC4558P:
https://soundcloud.com/tropicalhippo/pro-co-rat-rc4558p?in=tropicalhippo/sets/pro-co-rat-op-amp-test
LM358N:
https://soundcloud.com/tropicalhippo/3-pro-co-rat-op-amp-test-pro-lm358n?in=tropicalhippo/sets/pro-co-rat-op-amp-test

Noticed that the 1k after the coupling cap 4.7uF following the op amp is for some reason after the cap instead of before on the Tonepad layout. Any reason for this or just a layout error?

^ that photo looks like the ones my dentist takes of my teeth.

if your pin1 volts is good, it means the pin 3 volts are good enough. but, I can't see how you can have 4V3 on one of the second opamp inputs, if the output is at 0V.

Hahaha! Oh snap! You better get some work done then.  

I hooked the -2 input to Output 1 (active amp), like a good noob. Now I have it correctly connected (?) to Ouput 2 (of the inactive one).

[GGBB]

Noticed that the 1k after the coupling cap 4.7uF following the op amp is for some reason after the cap instead of before on the Tonepad layout. Any reason for this or just a layout error?

There are a couple of RAT schematics floating around with the same variation. Not sure where it originally comes from but my schematic is right for the four RATs I've owned (at least as far as I can remember) and agrees with most of the web schematics. It makes no difference to the circuit, AFAIK. If I were doing a layout and that kind of change would help, I wouldn't hesitate.

What are your voltages now that you've corrected the unused op-amp termination?

[Cozybuilder]

R8 and C9 on the schematic are in series between IC pin6 and the filter pot. The tonepad layout also has them in series, which is fine. However the tonepad layout has the wiper going to the other side of the filter pot compared to the schematic. The lugs are not numbered on the schematic, nor is the turn direction given.

[bloxstompboxes]

It makes no difference in a series circuit where the cap or resistor are placed. The output is the same. What will change is the point between the two. But that doesn't matter in this situation.

[stonerbox]

There are a couple of RAT schematics floating around with the same variation. Not sure where it originally comes from but my schematic is right for the four RATs I've owned (at least as far as I can remember) and agrees with most of the web schematics. It makes no difference to the circuit, AFAIK. If I were doing a layout and that kind of change would help, I wouldn't hesitate.

What are your voltages now that you've corrected the unused op-amp termination?

OK, that's good to know. I'll probably "fix" that either way.

I wrote about this in the post up above but I'll repeat it just in case anyone missed it the first time: I found that the Output 2 and Negative Input 2 read somewhere around 8V on the RC4558P, while the LM358N has zero volt on the same pins (7 and 6). Strange? I think so.

Here are the current voltages:

LM358N
http://www.ti.com/product/lm358-n
1. 4.3V [Output 1]
2. 4.3V [-1]
3. 0.9V [+1]
4. 0V    [Vcc-]
5. 0V    [+2]
6. 0V    [-1] (corrected)
7. 0V    [Output 2]
8. 9.2V [Vcc+]

[mth5044]

Fixed the wiring of the unused op amp now. According to the Fig 3b over here: http://www.electronicproducts.com/Analog_Mixed_Signal_ICs/Amplifiers/Properly_terminating_an_unused_op_amp.aspx

Could anyone confirm if this is correct?
Positive in 2 > Ground  
Negative in 2 > Output 2

This was terminated incorrectly. Check out that link that was given to you about terminating opamps (specifically Figure 3a). Positive input should be at Vref and the negative input should tie to the output of the opamp. Incorrectly terminated opamps can be susceptible to noise. Not saying that's your problem, but it's not helping. 

[stonerbox]

This was terminated incorrectly. Check out that link that was given to you about terminating opamps (specifically Figure 3a). Positive input should be at Vref and the negative input should tie to the output of the opamp. Incorrectly terminated opamps can be susceptible to noise. Not saying that's your problem, but it's not helping.  

Fine then! I'll do the bloody voltage divider, BlaaaarArgh!

[mth5044]

You've already got a voltage divider. In the schematic that you posted of the multi-rat a few posts up, R2 and R3 form a voltage divider. Take the ~4.5V from them for the positive input and then tie the negative and output together. Easy-peasy.

[stonerbox]

You've already got a voltage divider. In the schematic that you posted of the multi-rat a few posts up, R2 and R3 form a voltage divider. Take the ~4.5V from them for the positive input and then tie the negative and output together. Easy-peasy.

What he said. Most brilliant thing I heard in weeks. I'll get on it.


But in all honesty, I thought I had to make a new pair and experiment with values until it sounded good or looked right. Really don't know why I would think that though...

[ashcat_lt]

Does this opamp bias itself the way an LM356 does?  The datasheet doesn't seem to indicate such a thing, but it's the only scenario that I can think of where these numbers make sense. 

Either way, though, for that input to be so close to ground, either your biasing resistor is open or your the coupling cap at the input is shorted.  I'm not sure that's gonna cause the problems you're experiencing, but it can't help!

[mth5044]

You've already got a voltage divider. In the schematic that you posted of the multi-rat a few posts up, R2 and R3 form a voltage divider. Take the ~4.5V from them for the positive input and then tie the negative and output together. Easy-peasy.

What he said. Most brilliant thing I heard in weeks. I'll get on it.


But in all honesty, I thought I had to make a new pair and experiment with values until it sounded good or looked right. Really don't know why I would think that though...

Nope, have a read of the link - it says you can use a voltage somewhere between the power supply rails. Luckily you have a divider already giving you a voltage somewhere between your power supply. You used figure 3b, which is for a bipolar supply. Your supply is 9V and ground, so when you tied the + input to ground, you were plugging it into a power rail, not inbetween a power rail. If your circuit was +/-9V, then you could put the + input to ground, but that's not how your circuit is powered.

[stonerbox]

Nope, have a read of the link - it says you can use a voltage somewhere between the power supply rails. Luckily you have a divider already giving you a voltage somewhere between your power supply. You used figure 3b, which is for a bipolar supply. Your supply is 9V and ground, so when you tied the + input to ground, you were plugging it into a power rail, not inbetween a power rail. If your circuit was +/-9V, then you could put the + input to ground, but that's not how your circuit is powered.

I guess my English skills was lacking 'cause I never fully understood what "between power supply rails" meant in the article. Thanks for clearing that up Matt!
________________________________
The pedal sounds f-cking amazing now! No more fizz from the LM358N and a night and day difference in the quality of the cleans and distorted fuzzy sounds you get out of this monstrous thing. I really digg how the LM358N somehow maintains a dynamic and "living" quality to it even when it's maxed out. It's just sex-making perfect! (I gotta tone down my language)

I added two trim pots on the back. One (white) to adjust the amount of low cut, either to boost mids (original style) or just open it up for a more fuzzy fat tone. The other one (yellow) goes to a AMZ Mosfet Booster I implemented before the RAT for a over the top saturation/distortion. It adds some noise when fully boosted but sounds crazy, the way I like it.

When I first heard about the pedal some years ago it did not impress me at all with its very middy, edgy and just plain harsh sound but a couple of mods makes all the difference. Nothing new under the sun but a "wow" for me though.

It's a keeper on my board and in the studio. Probably til the day I die. 'Fuzzheads' should give this one a try.


Finally, I would like to thank every single one of you who have helped me and contributed with your thoughts in this thread. I couldn't have done a successful debug without you. I'm now in deep debt to you all. Thanks!

[stonerbox]

Does this opamp bias itself the way an LM356 does?  The datasheet doesn't seem to indicate such a thing, but it's the only scenario that I can think of where these numbers make sense. 

I don't think so either.. Didn't even know there where such a thing as auto-biasing amplifiers, cool!

[duck_arse]

nice big knobs. and now that it's working, can we have the good voltages?

[stonerbox]

nice big knobs. and now that it's working, can we have the good voltages?

Thanks! That's some worn down (got them like that) Tayda knobs right there. Everybody likes some nice big boo.. knobs!

LM358N
http://www.ti.com/product/lm358-n
1. 4.3V [Output 1]
2. 4.3V [-1]
3. 0.9V [+1]
4. 0V    [Vcc-]
5. 3.9V    [+2] - First up to 4.3v then slowly goes down to a stop at 3.9v
6. 4.3V    [-1]
7. 4.3V    [Output 2]
8. 9.2V [Vcc+]