Chopped Rat (FET-less), info needed

[PenPen]

Inspired by the "Technology of.." series at GEO, I set out to cut the Rat up into pieces to learn how it worked as individual stages, in the hopes of coming up with a new design.

So, first I gutted the main part of the circuit, the LM308 non-inverting stage. I drew up a schem, then perfed it out. Its not working out for me.

First, here is the schem I drew (poorly) in Paint:


Component values used:
C 
----------
1  - 22n
2  - 2.2u
3  - 1n
4  - 150p
5  - 10n
6  - 100p
7  - 1n & 1u, see text

R
---------
1  - 470k
2  - 100k
3  - 1.5k
4  - 100k
5  - 100k
6  - 1.5k
7  - 470

VR1 is 200k

For reference, here is a link to the Rat schem:
http://www.generalguitargadgets.com/diagrams/rodentsc.gif

I'm getting NO output at all in this arragement, so I changed C7 from 1u to 1n, still nothing. I tried omitting it, got low output with oscillation. Removed both R7 and C7 and the connection to ground, and I get sound, less than unity, but still some output.

I read an article discussing non-inverting amp arrangements with opamps. The Gain for this arrangement is Vout = ((R1+R2)/R1)*Vin. Ok, so in this design I have, R1 is VR1 and R2 is R7. This is similar to the original Rat. So, our gain is varied by VR1 (Dist pot).
At 180k value for R1, I should have just at unity gain (1.0026*Vin) for the input. I get nothing....


After typing this up, it occured to me that my VR1 is wired backwards, turning it clockwise increases the resistance therefore lowering the gain. I had mistakenly thought increasing to a higher value R1 would increase the gain. Still, a question I have is, why make R1 variable instead of R2? Doesn't it make more sense to vary that value instead?

Is it possible to NOT use the FET in the output stage like the original? Why exactly is that in there?

[PenPen]

Ok, rewired up the VR1 (wire had come off of the board, which is why it wouldn't work). I now have unity gain, sounds nice, but I'm looking for distortion. Now, I don't have clipping diodes in there, but there should still be some clipping from the opamp.

[PenPen]

After re-reading the schem for a normal non-inverting opamp distortion, I am stumped.

I have the values for R1 and R2 reversed. In the one article, R1 was the resistor to ground, and R2 was the resistor between the output and the invert pin. In the article from Design that Mark has up on his site, it was labelled the other way around, so I got them mixed up.

So, I'm back to my old problem. VR1 should be wired as a variable resistor where turning it clockwise increases the resistance. As resistance goes up, so does the gain. I'm not getting this at all, even with 200k of resistance for R2 and R1 = 470 in the equation Vout = ((R1+R2)/R1)*Vin. I theoretically should be getting tons of gain and a lot of clipping. But I'm not getting that at all, in fact, I'm not getting any audible change when I turn the pot, which even if I have it backwards it should be distorting at one end of the turn.

Can someone verify that increasing the resistance in VR1 should increase gain? I've read and reread the articles and that is how it reads to me.

[johngreene]

Yes, you have it right now. Increasing the VR1 resistance should cause the gain to go up. Since it doesn't, I would check C4 to be sure it isn't shorted somehow or the wrong value. In fact, remove it until you get it working. Remove C3 until you get it working also. It's there to reduce high frequency gain. Put the 1uF back in for C7, it will give you the most gain for a wider range of frequencies. Make sure that C7 and R7 are connected correctly and C7 is grounded. You have to take into account the impedance of C7 at a particular frequency and add it to R7 when computing the gain.

Taking voltage measurements around the opamp would be extremely helpful as well.

--john

[PenPen]

I haven't had a lot of time to work on this the past week, packing up and moving, but I am going to reevaluate my layout. I may have made a mistake going from layout to perfboard.

I'm getting some odd voltages on the pins, which I expect that they won't be the same as a stock Rat since I'm doing this differently.

Pin - V+ | Pin - V+
----------- |-------------
1 - 6.45  | 8 - 1.39 
2 - 4.34  | 7 - 8.8   
3 - 4.2    | 6 - 4.34 
4 - 0.0    | 5 - 6.45 
-------------------------

My battery is 8.8 Volts. What has me baffled is that Pins 1 and 5 are not connected to the circuit at all, that voltage must be coming internally. When I pull the chip from the socket, I only get voltages on pins 3 and 7. I'm thinking one possible problem is my biasing, pin 3 only has 4.13 instead of 4.4 like it should if the voltage divider is set up correctly, though I have 4.4V at the junction of R4/R2/R1. The bias voltage then is dropped more from going through R1, right? Cause I have 4.2 V before and after R3, so R1 must be reducing the bias voltage

Put the chip back in, I get 4.2 on pin 3 with no chip, 4.13 with chip. I don't think my LM318 is bad, since I'm getting clean output from it. Is it normal to lose some voltage with the chip in the socket? The voltage divider is now showing 4.35, chip in socket.

[johngreene]

The voltages look relatively normal. Voltage on pin 1 is ok as it's connected internally to allow for compensation, however pin 5 is supposed to be a 'no connect'. Anyway. The amp appears to be biased close enough to work. The only other thing I can think of since you are getting clean output and no change in gain with VR1 is to check the C7 - R7 connection to ground. If there isn't a path to ground through these devices, you are only going to get unity gain (R1 = infinity).

And you are using an LM308 and not a LM318, correct? They are very different opamps and you would not want to connect a capacitor to ground from pin 8 on a LM318. Compensation on a LM318 is between pins 1 & 5 and on a LM308 it's between pins 1 & 8.

--john

[PenPen]

The voltages look relatively normal. Voltage on pin 1 is ok as it's connected internally to allow for compensation, however pin 5 is supposed to be a 'no connect'. Anyway. The amp appears to be biased close enough to work. The only other thing I can think of since you are getting clean output and no change in gain with VR1 is to check the C7 - R7 connection to ground. If there isn't a path to ground through these devices, you are only going to get unity gain (R1 = infinity).

And you are using an LM308 and not a LM318, correct? They are very different opamps and you would not want to connect a capacitor to ground from pin 8 on a LM318. Compensation on a LM318 is between pins 1 & 5 and on a LM308 it's between pins 1 & 8.

--john

Oh geez. I feel like such a moron now. I thought I'd stuck the 308 in there, but I grabbed a 318 and didn't notice. I'm really sorry for wasting your time. Once I get my parts unpacked again I'll throw the 308 in there and see if that works ok.

I remember thinking a long time ago that the 318 could sub for the 308, thats probably the reason why I didn't notice I had the wrong damn chip this time. To me, the pinouts and specs look almost identical, aside from the extra comp pin 5 in the 318. Now that I look at the datasheet again, I see you are right, the 318 has very different comp schems. After I finish with this 308 circuit I'll try adapting the idea for the 318, with the proper compensation scheme.

Again, I am really sorry, I feel like a fool.

[PenPen]

And now I'm even more mad at myself. I decided to pull out my parts bin and toss the 308 in the socket. I fired it up, and no sound at all. Then I smelled the bad smell.....

I think I fried my only 308. Damn thing was $4 and I cooked it right off the bat. After I pulled the battery I touched the chip, it was hot to the touch, and the battery was hot too. After letting it cool a bit I hooked it back up for 10 secs at a time, probing the pins with my DMM. I read full voltage on all pins, except for pins 4 and 5, which are at 0. I also noticed that the voltage divider was still ok, but after R1 and R3 voltage was at full. Oddly, the voltages are still right if I pull the chip, but if I have it in there the voltages are off right there.

So, I decided to trace my own layout to make sure I'd put it together right. And thats where I spotted my mistake. I had accidentally wired R7 in front of C4/VR1 instead of after. Everything else was right, but I guess the 308 is far less tolerant than the 318 I had in there. Same circuit that I got unity gain from with the 318, but it totally cooked the 308. Least I think its dead. I can't imagine it surviving. I am going to fix my layout once I get my iron unpacked and try the 308 one last time to see if its dead. If so, I'll have to be dropping more money ordering a few more.

Thank you again for responding.

[MartyMart]

Don't be too hard on your self, we've all done it once !!
I burned my finger in the process too 
Be sure to check the original for power connections etc ....
I'll never do that again !

Marty.

[PenPen]

OOkay. I got an order with a new LM308, so I pulled my iron out and fixed my perf, and mounted a nice real 100k pot to the board (I had previously used a trimpot for VR1). After fixing the wiring, I stuck the LM318 in first and hooked up power to check the pin voltages, in case I still had something wrong that would burn my nice expensive 308. Worked just fine. I didn't get distortion with the pot maxed, but you can definitely hear an increase in volume (gain). So, I put in the 308, and it worked. Well, actually I got the exact same results as with the 318. So it appears that despite the different compensation scheme shown in the datasheet for the LM318, it does appear to work just fine with a compensation cap from pin 8 to ground just like the LM308. Handy info for later. Now, the questions:

1) Why am I not getting distortion? I have a gain of: (100k+470)/470 == 100470/470 == 213. Is this not enough gain to distort the signal? Why does it work in the stock Rat then, I understand it uses 100k and 607, for a gain of 165. Am I leaving something out of the equation?? Even without the diodes I should still be hitting the ceiling on the OA.

2) There are many versions of the Rat schem out there. Some show a FET on the input. Others, like the one in AMZ's ebook that I have, show only a FET at the end, I'm presuming to be a gain recovery stage? If I put a Darlington (two bipolars cascaded) on the input will that help the signal clip? I wanted to avoid using a FET in this entirely, but I have a ton of bipolars I could throw down in a darlington config. Opinions?


I am now sketching out a new layout, a bit tighter so I can add a (BMP) tonestack and gain recovery in case its needed. I also have designed in a switchable jumper to use either comp scheme, either the 30pf from 1 to 8, or 100pf from 8 to gnd.

[octafish]

My understanding of the FET in the rats tail is that it just a buffer between the tone control and the vol control. It stops them interacting with each other. Check out Mark's Stupidly Wonderful Tone Control for a FETless alternative. (Its a thread from the forum and it's instigated by Mark Hammer, you know it'll be good.).
I thought the RAT was just a hard clipper, no first hand experience of one though. Increase to 1M and kick that opamp in the pants.

[PenPen]

My understanding of the FET in the rats tail is that it just a buffer between the tone control and the vol control. It stops them interacting with each other. Check out Mark's Stupidly Wonderful Tone Control for a FETless alternative. (Its a thread from the forum and it's instigated by Mark Hammer, you know it'll be good.).
I thought the RAT was just a hard clipper, no first hand experience of one though. Increase to 1M and kick that opamp in the pants.

Thanks for that idea! I just searched and found that tone control, very clever. If I stick with a lowpass like the Rat has, I'll likely use this config. I'm leaning toward a tweaked BMP control, I modelled it in the Duncan simulator to be somewhat similar to a Vox tone curve (though not exactly, but as close as I could with just one knob), so I may go with that. I'll stick it in and see how it works out, if not I can always fall back on Mark's great idea.

I'm still curious if I'll need some kind of transistor in front of the OA, though. Again, I've seen some with and some without. I'm kind of itching to try out a Darlington built from two bipolars, ever since I saw that on FolkUrban. I need to do a bit of reading on how emitter-followers work out, I've never really built one before. And it may help push the OA plus giving a bit of clean gain with the drive all the way down too if I'm lucky.

I can just bump up the pot value to get clipping, but I'm more just curious as to why it doesn't distort in the current config, given that it has more gain than a stock Rat, and those definately clip even without the diodes IIRC. I will probably stick a diode pair in just to see if that is the problem.

[spinoza]

*Bumpy*

IMHO you could almost call it a "rat face"! I'm looking for something similar. Does this work? If not is there another option for a "mini-rat","minimalist-rat-like newbie pedal"?

[PenPen]

Whew. Never expected this to get bumped. Yes, I did build one of these as drawn, and after I fixed my problem with an incorrect solder trace, it worked correctly. I need to verify the component values, but they should be correct. I would recommend lowering R7 from what is there. However, it didn't really distort at all. The lack of diodes makes it more of an overdrive or volume booster. Also, lack of a output buffer gave me less output than expected. I meant this really as a learning project to learn the basics of opamps, my current Rat build has added clipping diodes, even less resistance for R7, and a small bipolar (no FETs, those are more expensive and precious to me) gain stage a la BMP for an output buffer.

Since there's interest, when I get a bit of time later I'll post up a redrawn version of this here with verified component values. Since my current build is on breadboard right now, I can pretty easily pull off the extras I put on to test out this here. Thanks for the interest! I'll post back later this evening.

[WGTP]

Make R7 47 ohm and C7 2.2uF - 4.7uf and see if that doesn't give it a kick in the but.  It should distort plenty even without the diodes.   

[PenPen]

Yes, that was going to be in the reposted version. I'm at work now so I don't have access to my notes from my other build, but I recall using something like 33 or 47 Ohm for R7. The cap is really a matter of taste. It will provide SOME distortion, but a far cry from what it would with diodes. I'll mess with it when I get off work and post up.

[WGTP]

You could also use a 500K pot.   

[PenPen]

Oh of course, the pot is the primary tweak when you are dealing with diode-less distortion. Pure OA distortion at that point. As I've gotten older I've come to appreciate the less is more approach to distortion, so I stuck with 100k. I'll note that when I redraw.