Next up: a Rat build

[thermionix]

So I'm gonna build a Rat, mostly because I got a good deal on some metal can LM308AHs (INB4 "mojo").  I'm gonna cheat on this one and use a "Runt" board from TheCrazyLegume.  Not doing a 1590A though, because I'm a battery guy.  Just gonna run wires to 16mm pots in a 125A.  Blue this time.

I'm expecting this to be a very straightforward build, but I've never built or even owned a Rat.  Is there anything I should look out for?  It's just an opamp and a FET, anything picky about bias?  Not really looking at mods or clipper switches at this point, just hoping to get optimal results doing the straight clone thing.  Probably won't socket the 308, looks like a PITA to get the legs into a DIP socket.  If I happen to solder in a noisy one, well it's useless anyway so I don't mind destroying it to facilitate removal.

Also, was there ever any difference between the earliest "Tone" Rats and the "Filter" ones, other than the CW-CCW thing?

[armillary]

Just a suggestion that might help with socketing your CAN-8 format LM308s. Those SIP IC socket pins that we all use to socket transistors will slip right in to DIP socket. So you can solder a standard 8 pin DIP socket on your project, then snip two rows of four of those SIP IC socket pins, stuff the correct LM308 lead in the correct socket pin, and slide the socket pins into the DIP socket. Easier to install, easier to remove and you can compare the LM308 with the OP07DP later.     

[Ben Lyman]

Check this out if you haven't already. I think it will give you the answer to the differences:
https://tornadoalleyfx.files.wordpress.com/2014/08/multi-rat.png
Also, my personal preference for Rat mods is Red clipping LEDs instead of 914/4148's

[thermionix]

Hey thanks Ben!  I already had that .png saved but I admit I hadn't yet studied it thoroughly.

I know it's hard to put into words, but how would you describe the difference between red LED clippers and 4148s?

[thermionix]

Oh yeah, and doesn't that make it a Turbo Rat?

[Ben Lyman]

I think that may be right, not sure though.
I would say LEDs give a more "open" and "crunchy" sound. Maybe even a bit "tube-like"
Definitely worth boarding it or just putting sockets in the build, or even a switch if you're into switches.
When I made mine, I had to work my brain pretty hard to decipher that .png but I gathered the info I
needed from it to make a pretty good Rat, it sounded just like my original 1981 "Big Box" at first, then
I added the red LEDs to the sockets and got a nice variant.

[GGBB]

Check this out if you haven't already. I think it will give you the answer to the differences:
https://tornadoalleyfx.files.wordpress.com/2014/08/multi-rat.png
Also, my personal preference for Rat mods is Red clipping LEDs instead of 914/4148's

Even better, check out the most recent version here:

http://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=45170&g2_serialNumber=28

[GGBB]

So I'm gonna build a Rat, mostly because I got a good deal on some metal can LM308AHs (INB4 "mojo").  I'm gonna cheat on this one and use a "Runt" board from TheCrazyLegume.  Not doing a 1590A though, because I'm a battery guy.  Just gonna run wires to 16mm pots in a 125A.  Blue this time.

I'm expecting this to be a very straightforward build, but I've never built or even owned a Rat.  Is there anything I should look out for?  It's just an opamp and a FET, anything picky about bias?  Not really looking at mods or clipper switches at this point, just hoping to get optimal results doing the straight clone thing.  Probably won't socket the 308, looks like a PITA to get the legs into a DIP socket.  If I happen to solder in a noisy one, well it's useless anyway so I don't mind destroying it to facilitate removal.

Also, was there ever any difference between the earliest "Tone" Rats and the "Filter" ones, other than the CW-CCW thing?

Socketing the can isn't bad at all:



It is a very straightforward build. You do need to watch out for the bias on the output buffer JFET. I'd recommend instead of 1M to ground (R10), use 1M to Vbias. Or else include R16 (same value as R10 - 1M or 2M2).

Of the stock RATs, I prefer the Turbo RAT (LEDs) but I'm not sure why - I think it seems a little more dynamic and responsive perhaps. But there's not a whole lot of difference - you just have to turn up the distortion knob a bit more with LEDs to make it sound close to 4148s. There are other clipping options worth trying. The circuit pictured above is what I've dubbed the OCR - Obsessive Compulsive RAT. It changes the clipping from between after the opamp output coupling cap and ground, to between before the coupling cap and Vbias - as is done in the OCD, and uses 2N7000 MOSFETS for clippers (3 variations). The responsiveness, clarity and dynamics are to my ears notably improved (although not vastly), but it still has that same RAT tone.

EDIT: And no - there 's no difference between production tone and filter RATs other than control rotation and PCB layout.

[ashcat_lt]

The difference between Si and LED is that the LEDs will be cleaner for a given gain setting, and the output will be louder overall.  You can turn up the gain to get more distortion, but unless you're putting in a 200K pot, you'll never quite get the same at max, and far more importantly the gain setting changes the frequency response so changing the gain for more distortion also means (proportionally) less bass (and treble really) hits the clippers.  Also, of course, the opamp itself clips more, and that slew rate/gain bandwidth product thing...  Since everything is dependent on that gain setting, you can't really make the two sound alike.

[thermionix]

Thanks for the replies everybody!  Since I will be using a pre-made (and very tiny!) Madbean PCB, there is less opportunity for experimentation and circuit variation.

Off to Youtube to listen to Turbo Rat clips...

[GGBB]

The difference between Si and LED is that the LEDs will be cleaner for a given gain setting, and the output will be louder overall.  You can turn up the gain to get more distortion, but unless you're putting in a 200K pot, you'll never quite get the same at max, and far more importantly the gain setting changes the frequency response so changing the gain for more distortion also means (proportionally) less bass (and treble really) hits the clippers.  Also, of course, the opamp itself clips more, and that slew rate/gain bandwidth product thing...  Since everything is dependent on that gain setting, you can't really make the two sound alike.

Higher distortion setting means less bass hitting clippers? Are you sure it's not the other way around? In any case, no they aren't identical, but they are close (adjusting for the obvious volume/gain differences) - IME using both extensively. But not exactly the same, which is why as stated I have a preference for LEDs. Often overlooked, and probably a factor in my preference, is the difference in the JFET biasing between regular and Turbo RATs. Turbo RATs move the gate-source voltage farther from pinch, apparently to accommodate the higher output of the LED clippers.

[ashcat_lt]

Higher distortion setting means less bass hitting clippers? Are you sure it's not the other way around?

Yep I'm sure.  Lower gain is closer to flat.  The gain knob is basically a treble (or maybe high mid) boost.

[GGBB]

Higher distortion setting means less bass hitting clippers? Are you sure it's not the other way around?

Yep I'm sure.  Lower gain is closer to flat.  The gain knob is basically a treble (or maybe high mid) boost.

Do you know how exactly the knee and slope are calculated then? Always been curious about this - others have led me to lean the opposite way in the past. I have never been able to find the math behind it.

[robthequiet]

I was under the impression that led's basically give you more clean signal before clipping and at a higher level, so you get more full-wave signal. So your average silicon diode clips at a lower level thus edging up the high frequency as the signal becomes more square wave than sine/triangle. My question is what happens when you have multiple diodes in series, does it set the signal clip level by adding the voltage drops or is there some mystical formula? Better yet, will two diodes in series clip at a higher voltage than a single giving more headroom, or do they hit the clip threshold at the same time and round off the waveform more?

[thermionix]

^Yep with diodes in series you basically add their forward voltages, so more clean headroom before clipping.

I did watch Youtube clips this afternoon, there's a pretty good one where the dude is alternating between a Rat 2, a You Dirty Rat, and Turbo Rat.  They all have their strengths and weaknesses, all sound fairly similar, so I'll probably split the difference and do a Si clipper version.  Maybe not 4148s though.

So, the obvious next question is, anybody ever try asymmetrical in a Rat?

[ashcat_lt]

Do you know how exactly the knee and slope are calculated then? Always been curious about this - others have led me to lean the opposite way in the past. I have never been able to find the math behind it.

Though there's two of these that overlap in the Rat.  Note that this says the cutoff should get lower with bigger gain pot values, but the difference in level between the lower freqs and highs gets bigger, which is what I meant when I said "proportionally" above.

So, the obvious next question is, anybody ever try asymmetrical in a Rat?

Yes.  Well, I tried...

An asymmetrically clipped wave looks a lot like a square wave with a DC offset, but the diodes in the Rat sit between two caps which won't let that DC offset exist.  They will charge or discharge to the point where the wave is centered between the diode drops, and it will become less asymmetrical with time.  That time in this case is on the order of a few cycles of the lowest frequency in the signal.  Basically, it doesn't stay asymmetrical for very long at all.

[GGBB]

Though there's two of these that overlap in the Rat.  Note that this says the cutoff should get lower with bigger gain pot values, but the difference in level between the lower freqs and highs gets bigger, which is what I meant when I said "proportionally" above.

Thanks so much. That really fills in the gaps for me. It explains the bigger gain pot / lower cutoff aspect. I had always thought and heard this but didn't know the math behind it, and the partial math I did know told me lower. This is like a light turning on  .

[GGBB]

So, the obvious next question is, anybody ever try asymmetrical in a Rat?

Asymmetrical can be done if you move the clippers like I did for the OCR. At least I think so. It doesn't sound much different to be honest - just the headroom factor changes (so maybe it's not really asymmetrical?). But I haven't spent a huge amount of time comparing - just flipping the switch back and forth a couple of times on occasion.

[ashcat_lt]

So, the obvious next question is, anybody ever try asymmetrical in a Rat?

Asymmetrical can be done if you move the clippers like I did for the OCR. At least I think so. It doesn't sound much different to be honest - just the headroom factor changes (so maybe it's not really asymmetrical?). But I haven't spent a huge amount of time comparing - just flipping the switch back and forth a couple of times on occasion.

I'm not sure about that either.  You might think that opamp has enough grunt to push the things wherever it think they should be, but it is hampered by that 1K (1.5?) in between, and there's still that cap at the end floating toward the average.

I think you really need to remove both coupling caps so that it's DC coupled all the way to the buffer.  I haven't actually looked at what you'd have to do to adapt the buffer for that.

But the truth is that assymettry is only really interesting when it's pretty severe - when one side is clipping off a significantly greater percentage of the swing than the other.  With Si x LED assymettry, it might be noticeable when the signal is just big enough for LED to start clipping because then it's something like 50%:100%.  But if the signal is ten times as big...well the ratio gets close to 1.  And this thing has a lot of gain even in the middle of its sweep and an absurd amount of gain at the top.  It's definitely worth playing with, but it's not going to be a night and day difference in most cases.

[thermionix]

Hey folks, started populating this Madbean Runt board, and I had a question about something.  Maybe it's nothing.  Here's the Runt schematic:



As a secondary guide I've also been referring to this Keen/Orman schematic from the web, I guess it's an actual Rat:



Noticed that R6 and C7 are reversed in order.  Does that make any difference?  Is one an error?  Same-same either way?

Thanks!