rodent rat - OA problem

[pop.adams]

my problem: with the audio probe i recognized that my lm308an distorts the signal like ****ing-hell. i use a sinewave to check and record simultaneously the original sine and the audio probe-tone with wavelab, so i can also check the phase, which is btw everything but correct. the distortion and the phase shift seem to be frequency dependent.
i changed R2 from 1M to 100K.
what the hell is going wrong? is there anyone who had the same or similar problem?

i use a wall wart with 13,3 V
OA voltages
1 - 12,55
2 - 6,61
3 - 5,56 (ups, the wrong number. it has to be 6,56)
6 - 6,61
7 - 13,22
8 - 6,66

thank you in advance. carsten

[petemoore]

Look for bad trace [non continuity] around the connections of the OA outputs. Also look for solder bridges.
 Using the DMM can find these problems sometimes, [I stare at board alot first]...depending on how much it's applied, using R Range, Voltage readings and Beep mode. In circuit resistance readings are moot without circuit dependant math equations being worked....otherwise you'll have to lift a R lead from one end to read it's actual value.
 One can check nearly every single in circuit component, [I avoid R math, 'till it's a last resort].
 Might as well put this in here...sometime...>somebody never asks me beforehand about tricks to make for easy circuit building/debugging so here it is now...[disregard if this 'bugs you], mainly applicable to perfbuilds.
 Getting the DMM probe to touch any point in the circuit is easier for me becuase I always:
 Leave a little lead above board for probe access
 Put some components above board [avoid long strings for better structural integrity] for build tightness/moddability/>EZ Circuit/Board reading<
 {Techniques for future build Debug preparation}^
 Could also be a misvalued resistor or miswire around the OA output[s.

[strungout]

I recently built one for a friend and the problem I had also created the same voltage as the source at the outputs of the OA (used a dual, one half unused). The problem was a defective 1M pot (replaced it with a 1M resistor and its fine now). Worth a look.

[strungout]

1M gain pot, I should specify.

[pop.adams]

first: thanks a lot for your answers.
i'm not really sure but i think that i found the problem which is more a misunderstanding on my side...the gain is set by the pot (R3) (or rather set by the relation between the pot and R4+R5). now, if i reach a point where the output is higher than the OA could "handle" (because of the limited supply) the output has to distort. i wonder why this pot has 100k when the limit is reached at roundabout 10k. i thought the main distortion is made by the diodes and not by the OA.
maybe this is the problem i had. any suggestions?

there is another thing i wonder about:
i read that if the diodes are not connected to ground the signal should stay cleaner. yes - it is cleaner AND definitely a lot louder. is this normal?

greetings. carsten

[Samuel]

i have definitely found that the RAT, both my rodent version and a Turbo Rat I borrowed tend to choke up with high output pickups. I can't put my gain knob past halfway before the signal chokes up.

lifting the diodes (basically taking them out of circuit) will definitely result in a cleaner louder rat, since you are removing a hard-clipping portion of the circuit.

[pop.adams]

thanks. sounds like i'm on the right way. should happen a bit more often to me  :?

[WGTP]

I think there are some misconsceptions about the Rat and other similiar type distortions.  The op amp DEFINITELY distorts by it's self, and the diodes add to it.  I don't remember the numbers, but the op amp gain is in the hundreds.  

Lifting the diodes does increase the output and MAY clean things up some, but the the JFet may start distorting.  Yours seems to be working normally.

You can try various diode combo's for more or less distortion/output.  Try an LED/SI combo.

Look for Tonda's mods in the original schematics section for some switching ideas.  Have fun.  

[pop.adams]

i will try these mods. thank you.

can anyone tell me what's the job of the jfet (2n5458 - not that one of the millenium-bypass)? is it a buffer?
i've got an ac-signal (sinewave) with 10mV, my OA is set by a gain of nearly 10. the signal at the output of the OA is 98mV. due to tonesucking components (pot, resistors, caps) the signal which reaches the gate is 96mV. but only 83mV comes out of the source. is that correct?

:oops:  sorry about my awful kraut-english

[WGTP]

It is my understanding that the Jfet is to buffer the tone control from the output impedance.  Not sure about the voltages.  I think an excellent mod would be to make it a gain stage like Aron's SB (in the original schematics section).  Also check out the Warp Control Artical at the AMZ website for diode mod's.