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Proco RAT Clone doesn't work
Proco RAT Clone doesn't work
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Old 17th March 2020, 05:33 PM   #21
Gnobuddy is offline Gnobuddy  Canada
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Join Date: Mar 2016
I suspect (but without proof at this point) that the problem is that your LM308 is unstable, and is oscillating at very high frequency (too high to hear). Fast op-amps often don't like bread-boards very much, and can be quite sensitive to layout.

Since all sorts of mysterious things keep happening with the Rat circuit, my suggestion is to start with a very basic circuit - an op-amp voltage follower - and once that's working, add on components one by one, until the whole circuit works as it should.

The attached screen capture shows the circuit I suggest starting with. It uses components you already have (they are all part of the Rat schematic you posted.)

If you have any pico-farad caps that are bigger than 30 pF (such as 47 pF, or 100 pF), I suggest using one of those for C1 instead of 30 pF; this will slow down the op-amp a little, and make it less likely to oscillate at radio frequencies.

Once you get it working, this circuit should behave as a clean buffer. No gain, no loss, whatever guitar signal goes in should come out virtually unchanged. And, as before, DC voltage on pins 2, 3, and 6 should all be half battery voltage, around 4.5 volts.

Get to that point first, then we can add on to it, step by step, until it becomes a Rat. Or one or both of us gives up in total frustration.


-Gnobuddy
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File Type: png Rat001.png (48.7 KB, 24 views)
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Old 17th March 2020, 06:14 PM   #22
Tsili is offline Tsili  Greece
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Join Date: Mar 2020
I have output!!!!!

But not clear. It has a lot of noise and I can here the input in the background. One more thing that I noticed is that when I move my hand closer to the circuit without touching anything the noise increases in volume.
Voltages at pins:
2->3.7V

3->4.3V
6->3.6V
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Old 17th March 2020, 10:29 PM   #23
Gnobuddy is offline Gnobuddy  Canada
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Join Date: Mar 2016
Quote:
Originally Posted by Tsili View Post
I have output!!!!!
Okay, that suggests the basic circuit is correct.
Quote:
Originally Posted by Tsili View Post
...a lot of noise...when I move my hand closer to the circuit without touching anything the noise increases in volume.
The circuit may be oscillating, or just picking up electrical interference, or both. (This is why guitar FX pedals are almost always built inside metal boxes, which are grounded, to reduce noise and interference reaching the circuitboard inside.)

There are a couple of things you can do that might help:

1) Is ground on the breadboard (negative end of the 9V battery) actually grounded to anything? If not, try running a wire from the breadboard ground to some grounded electrical appliance nearby, for example, the metal case of a PC.

2) Try placing the breadboard on some large grounded metal object. You can use a metal baking tray, or even a sheet of aluminium cooking foil. The baking tray or foil should be connected by a wire to the ground rail of the breadboard; a paperclip works well to make the connection if you use aluminium foil!

3) You can also invert a large metal object - a metal mixing-bowl, for example, or a large pot - over the entire breadboard, and ground it.

See if any of those things calm down your op-amp, and make most of the noise go away.
Quote:
Originally Posted by Tsili View Post
Voltages at pins:
2->3.7V
3->4.3V
6->3.6V
Hmm. Pin 2 & 6 should be identical (they're wired together!), and pin 3 should be within a couple of millivolts of the other two.

I'll buy 3.6V and 3.7V - they're close enough to be identical. But pin 3 being so very different raises questions. If the op-amp is working and stable, this cannot happen; so the op-amp is either not working, or not stable...and I suspect it is the second of these. I think your LM308 is not at all happy about being on a breadboard, out in the open, without a grounded metal box around it, and is oscillating like crazy.

This is where a good oscilloscope is worth it's weight in gold (it can show you the oscillations if they're occurring), but I'm gonna guess you probably don't have one; they are expensive tools.

Why do fast chips have problems with solderless breadboards? If you could look inside a breadboard, you would see a collection of short metal strips, spaced about a millimetre apart, each one about 25 mm long (see attached image.) This creates lots of small stray capacitances between adjacent metal strips (which correspond to adjacent rows of holes in the board.)

The net result is as though you had wired a small capacitor (a few pF) between every two pins of the op-amp. Some op-amps will tolerate this abuse (usually slower ones, which are fully internally compensated). Other op-amps will not, and will burst into oscillation instead. I think that's what you're dealing with here.

There is a chance that putting grounded metal under and above the board, as described earlier, will calm the LM308 down. There is also a chance that moving the LM308 off the breadboard, and onto a PCB, will calm it down, because there'll be a lot less stray capacitance.


-Gnobuddy
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File Type: jpg breadboard-backing.jpg (34.6 KB, 18 views)
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Old 18th March 2020, 08:35 PM   #24
Tsili is offline Tsili  Greece
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Quote:
Originally Posted by Gnobuddy View Post
See if any of those things calm down your op-amp, and make most of the noise go away.
Indeed they do. I used aluminum foil. The signal is very low. And there are some frequencies that are about the same volume (or greater) than the input signal. There is a really high frequency noise and on really low. The output signal is lacking most of the low frequencies.



Quote:
Originally Posted by Gnobuddy View Post
Hmm. Pin 2 & 6 should be identical (they're wired together!), and pin 3 should be within a couple of millivolts of the other two.

I swear. I didnt change anything at all at the circuit and I dont have voltage at pin 2 and 6.



Quote:
Originally Posted by Gnobuddy View Post
This is where a good oscilloscope is worth it's weight in gold (it can show you the oscillations if they're occurring), but I'm gonna guess you probably don't have one; they are expensive tools.

No I don't. I am willin in investing in this hobby. But I made a try a long time ago coping the Big muff pi and it tottaly failed. Now I am seeing that this project is a disaster too. I can't understand why other people copy effect pedals so easy and I am always facing problems.



Quote:
Originally Posted by Gnobuddy View Post
There is a chance that putting grounded metal under and above the board, as described earlier, will calm the LM308 down. There is also a chance that moving the LM308 off the breadboard, and onto a PCB, will calm it down, because there'll be a lot less stray capacitance.
Shall I solder the lm308 at a pcb and join it to the bradboard with cables?
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Old 19th March 2020, 05:30 AM   #25
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by Tsili View Post
...signal is very low.
When working properly, the circuit you just built has unity gain. The output should be exactly as strong as the input.

Frequency response should be flat, with no reduction in bass or treble.

Voltage gain is only unity (1), so there really shouldn't be lots of trouble with noise, hum, buzz, etcetera.
Quote:
Originally Posted by Tsili View Post
some frequencies...same volume (or greater) than the input signal...high frequency noise...one really low. The output signal is lacking most of the low frequencies.
What are you using to provide the 9V power? Have you tried using an ordinary flat 9V battery? If all this noise you're describing comes from the power supply, using a battery instead might cure it.

One more thing: is the breadboard reasonably new, and/or have you tried building the same circuit on a different area of the same breadboard? Sometimes an old and well-used breadboard will have one or two "holes" that don't make good contact.
Quote:
Originally Posted by Tsili View Post
...Big muff pi...totaly failed.
That must have been very frustrating.

The Pi is a relatively complex circuit, with a lot of parts in it, and multiple gain stages. A small mistake in any one gain stage ruins the whole circuit. IMO, it really isn't suitable for a beginner to electronics.

It is possible for an adult beginner to build one, but you would have to know how to fault-find and fix your mistakes as you go.

There are almost always some mistakes when you build a circuit with that many components on a proto-board or breadboard. I'm in the middle of designing and building a project with about the same complexity as the Big Muff. Even though I have built electronics circuits as a hobby since I was 8 or 9 years old, and that was a long time ago, I still made one mistake while building my circuit, accidentally leaving out one resistor.

Of course the circuit didn't work when I tested it - but some DC voltage checks, followed by a close physical inspection, quickly revealed my mistake. I soldered in the missing resistor, and the circuit now works.

I wish I could help you more. If I lived near you, I would invite you to bring your circuit and come over, and we'd quickly get it working over a cup of coffee. But from this distance, it's hard for me to do more than guess at what the problem is. And the problems you're describing are quite bizarre ones, nothing typical that I can easily put my finger on.
Quote:
Originally Posted by Tsili View Post
Shall I solder the lm308 at a pcb and join it to the breadboard with cables?
No, that won't help - it's the breadboard itself that might be the problem, in combination with the old, obsolete, externally-compensated LM308 op-amp. If the op-amp is unstable and oscillating, adding longer wires will only make everything worse.

That's why I asked earlier if you had any different op-amps to try. Chances are that a cheap and easily available op-amp like a TL072 would be more stable, and it doesn't cost much to find out (unless you have to pay lots of postage to have it shipped to you.)

Incidentally, the TL072 contains *two* op-amps on one chip, and you will only need to use one of the two for this circuit.


-Gnobuddy
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Old 19th March 2020, 11:58 AM   #26
Tsili is offline Tsili  Greece
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Join Date: Mar 2020
Thanks everyone for your time. I abandoned this project and I already build (its at breadboard state) a Big Muff Pi that is working. So after I solder the circuit to the pcb I will try again with the RAT. That was a well used carantine time.
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