DIY ProCo Rat Oscillation Problem

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DIY ProCo Rat Oscillation Problem

Postby spartanKH » 07 Apr 2014, 01:44

I just completed a Proco Rat on breadboard and upon testing it, it sounds fine until the gain knob is turned up about halfway. At half gain a high frequency oscillation is introduced that gets worse the higher the gain. I assembled it using the schematic from electrosmash.com and used all the same values for components besides the LM308 and the input resistors and input cap. I increased the two 1M resistors to 2M for better input impedance and changed the input cap to compensate for the resistors being changed to keep the same frequency response. I used a TL071 as seeing that it was the only op-amp I had that I read could be used as a replacement. The TL071 does not have pins for the compensation where the 30pf cap is placed on the rat. Now I know this affects the slew rate in the LM308 and reading the ProCo Rat analysis on electrosmash, the 30pf cap essentially causes the op amp to be unable to amplify high frequencies. So by using the TL071 op amp instead which has a much better slew rate, is there over amplification of the highs causing it to oscillate? Before the oscillation begins the sound is great though so I don't want to change the overall sound too much. Is there a cap I could place somewhere to reduce the oscillation without affecting the tone? Or is it just the fact that its on a breadboard and should be fine once enclosed? I am also using a battery so there shouldn't be any power supply noise but I did include the power supply filtering that the ProCo Rat uses. Im also playing an SG through a peavey 100W. Here is the schematic from electrosmash.com.


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Re: DIY ProCo Rat Oscillation Problem

Postby pedro.deus » 07 Apr 2014, 17:20

Try change the JFet, and use with 9V battery for testing. With battery the oscillation gone, the problem is you power supply is causing a ripple oscillation on Jfet for solve this problem usually works adding a 470uF electrolytic cap with 10nF ceramic cap in parallel in power supply jack on the pedal.
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Re: DIY ProCo Rat Oscillation Problem

Postby pedro.deus » 07 Apr 2014, 17:25

Don't use tL071/LM741 or other single opamp, have some differences between LM308/JRC308 and TL071. The TL071 have only gain stage with feedback resistor grid and the TL071 is more linear gain, lm308 has 2 gain stage one is the feedback resistor grid and the pins 8 and 1 like LM386.
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Re: DIY ProCo Rat Oscillation Problem

Postby jorri » 19 May 2014, 13:01

I have a similar issue. I think that the increased impedance you added may cause some problem. I'm not quite worked out on the solution but lowering the 1meg solved it.

http://www.freestompboxes.org/viewtopic.php?f=54&t=24496

my thread for reference hopefully some answers soon..
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Re: DIY ProCo Rat Oscillation Problem

Postby DrNomis » 19 Oct 2014, 07:40

I used to own a Proco Rat and a Proco Turbo Rat, from memory my Proco Rat always did have a tendency to go into self-oscillation when the Distortion control was turned up, but more to the point, I found I could change the frequency of oscillation by varying the Filter control too, I think self-oscillation is one of the quirks of the Proco Rat circuit design...... :thumbsup
Genius is not all about 99% perspiration, and 1% inspiration - sometimes the solution is staring you right in the face.-Frequencycentral.
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Re: DIY ProCo Rat Oscillation Problem

Postby nickthemen » 20 Oct 2014, 10:19

Its the lyout that is the problem, try the lyout of the original rat and your oscillation will be gone!
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