Rat Circuit Theory

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sammo303

Member
Hi folks,

I just finished a Rat build, first one I've done and I decided to test it with my signal generator and scope which led me down the path of some modifications. These modifications were only to make it more usable and I'm left a bit confused by the standard circuit.

First off let me clarify that I am using a TL081CP opamp at the moment, MMBF5457 JFET and I have 1N914 and LED clipping diode options on a switch.

Probing the output of the opamp, it was clear that the signal was hitting the power rails hard, even at a relatively low distortion pot setting. I believe this is normal behavior and the clipping diodes chop this down even further.

What I found, however, was that changes in the waveform shape occurred only at low distortion settings. After about 30% of the travel, things seemed constant. I checked this with FFT and indeed the harmonics up to 25k were fully formed by this point and no visual change happened with further increase in distortion setting.

I removed and measured the pot and got a reading of 7k. From an A100k pot! I read the Electrosmash article to better understand things and the large opamp gain is mentioned. Is that much really necessary? I saw the relationship between the 47R and 560R resistors, pot value and amplification factor and used this with my 7k measurement to calculate new resistor values that would result in the same level of gain but at the full travel of the distortion pot. I ended up with 8.2k and 560R which are a world away from stock but the scope showed the full pot travel was now usable and a playing test showed that classic Rat tone.

I guess I have a few queries about this.

1. Are the stock values really associated with opamp gain that makes very little of the distortion pot travel useful?
2. How can the opamp impact the sound when the diodes chop off more than the opamp does? Is the impact of slew rate a different phenomenon?

Final point was my use of a MMBF5457 output JFET. This is all I had to hand and it made for a useful half wave rectifier! I ended up biasing the gate by connecting a 2M resistor from the 9V rail. Given there is a 1M resistor from here to ground, this gave enough positive voltage to avoid chopping off the negative portion of the signal even with the higher 1.7V forward voltage of the LEDs in the clipping position.

Learning is fun.
 
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A 081 isn't going to behave anything like a 308 in this circuit. If you want to sub, try an OP07. A 13Vus slew isn't a rat and the circuit isn't going to behave the same.
 
I find the opamp clipping makes the sound more bassy and sludgy, even when followed by diode clipping at a lower threshold. This can be compared by running at 18v instead of 9v, so the op amp rails don’t clip.
 
Thanks for the replies.

I am definitely thinking of trying an opamp such as OP07 where I can use the compensation capacitor but it's a little expensive to buy on its own from a reputable source so I think I'll wait until I need a few other things.

Interesting what you say about bass response with opamp clipping. I only viewed things at 1khz with the scope so quite a limited view of things. I guess frequency response of the opamp would definitely influence the sound.

I'm running standard 9v Vpp, nothing unusual there. Maybe the modifications are due to the use of a TL081, though I did try the other single opamps I had to hand (NE5534 and LM741) and saw the same behavior.

I am thinking about further modification to the diode setup. I think I can leave the LEDs always connected given their high forward voltage and simply switch in some additional diodes. Doing this with an On-Off-On switch wins me 3 combinations. I'm thinking I'd use the standard dual 1N914s in one position and maybe an assymetric arrangement in the other, maybe a schottky / germ combo. I guess I could just experiment and see what I like.
 
Your sine input is 9Vp-p?
Should be more like 250-500mVp-p at most.
Comp cap won't work with the Op07 as it's internally compensated. It's just the cheapest closest chip currently produced vs a 308. The low slew rounds off the high end at high gain. Higher slew chips can sound harsher and more brittle. Check out the electrosmash analysisif you haven't already.
Chuck has a good writeup here in the boneyard as well iirc.
 
Thanks for the reply.

Haha, no, I must have misread, what an idiot! I meant 9v supply voltage, the sine wave was 150mV.

Are you sure about the OP07? The datasheet shows pins 1 and 8 as offset voltage pins rather than NC like most single package opamps. I thought I'd read about the compensation cap selection having the ability to slow the slew rate to LM308 levels but maybe I'm wrong as this doesn't seem to be the function of pins 1 and 8 as described in the datasheet.
 
Yea, I'm sure. Pins 1 and 8 are offset adjustment, not compensation. One would wire a trimmer to rebalance the output around any DC offset. Not needed for our uses.
 
sammo303 said:
Or found a thread on Reddit.:LOL:
Click to expand...
I've seen a lot of misinformation about the OP07 and the compensation cap--I think it's partly because Proco left it in the circuit even when production switched to it, and largely that people don't read the datasheets. I think they left the cap there to make it easy to swap in a lm308.
 
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