Does the output buffer in a Rat clip?

[Chugs]

Is the signal level high enough to overload the output buffer or, presumably, do the silicon diodes to ground clamp the signal down enough that it does not overload?

What about if LED's with their higher clipping threshold are used? Does the additional output level cause the buffer to unintentionally clip?

[GGBB]

Short answer - yes and no - even though the diodes do limit the signal voltage hitting the JFET.

The output JFET buffers in the RAT and Turbo RAT are slightly different (see R16 and R10 below):



In the RAT, the gate is biased to ground, In the Turbo RAT, the gate is biased to 1/2 Vin. The reason for that is what you are asking about - so that the higher clipping threshold of the Turbo RAT's LEDs doesn't overdrive the output JFET buffer.

In a regular RAT, even with switching diodes, the output buffer can "overload" depending on the specific JFET. It all comes down to its Vgs-off value which for a Fairchild 2N5458 can be anywhere from -1V to -7V. If it's down around -1V, there could be some pinching when the input swings negative. I suppose it shouldn't happen on paper/in theory because the diodes should clip around to +/- 0.7V or less, but JFETs are inconsistent beasts and diodes aren't perfectly consistent either.

[thermionix]

Yes, just to add to Gord's reply some, measure the voltage you get from source to ground on the output buffer.  I was getting too much fuzz/mud from my early Rat clone, and another thread led me to check that voltage.  I was getting about .6v, not good.  Second JFET gave me right around 1v.  Much better, but still not quite right.  Third JFET gave me 1.4v, and now it sounds correct.  You would think 1v would be enough to clear the clipping fv, but things happen in curves in the real world.  My recommendation is to mock up that part of the circuit on your breadboard, and go with the best JFET you have, rather than subbing them out on your PCB.  Unless you have a socket there.

[GGBB]

It might be easier to just do what the Turbo RAT does by adding a 1M resistor from V+ to gate (or 2.2M and change R10 to 2.2M as well). The earliest models of the Turbo RAT actually had that resistor tacked on the back side of the PCB because they were built using the existing RAT-2 PCB which had no place for R16. In the mid 90s they came out with a new PCB labeled "MULTI RAT" which had a proper home for R16. If you're modding a production RAT and it's from about 1995 or later, it should have an open position for R16.

Here's my 1990 Turbo RAT:

[Chugs]

Interesting. I thought as much.

My train of thought lead me here as I was thinking of what would happen if I replaced the jet buffer with an opamp buffer based to vbias. (4.5V) As in would there be any tonal difference? If there is some clipping/overloading then I guess the answer is yes.

[Ben Lyman]

Hey Therm, remember when we had that discussion about how important the FET is to getting a good sounding Rat? All I could add was, "I can hear a big difference when I sort through a pile of 5458's and try them one at a time in the socket." And I still say this to people all the time and get many answers ranging from, "weird, I wonder why" to "you're crazy, it's just a unity gain buffer!"

I still say, from the 50+ 2N5458's that I bought from Small Bear, some make my Rat big and loud, and others not so much.

Gord, this schematic is my favorite thing in the world, I carefully examined my 1981 Big Box (with the tail) and identified exactly which parts match the schematic. Now I can make clones of the single best Rat I have ever heard!  Thanks!

[Derringer]

if you set the gate at 1/2 V though, it negates needing the "goldilocks" fet, right?

[GGBB]

If there is some clipping/overloading then I guess the answer is yes.

But there isn't any output buffer clipping most of the time. And I think that's the intended behavior of the design. It's only edge cases where there is clipping. So of course you could play around with different JFETs (or even an op-amp) and invoke some extra distortion or other tonal impact, and maybe that might sound good to you, but I'd argue that isn't the key to getting a great classic RAT sound. That's chasing after mojo that doesn't exist IMO. But it could be a worthwhile learning experience.

[thermionix]

Hey Therm, remember when we had that discussion about how important the FET is to getting a good sounding Rat? All I could add was, "I can hear a big difference when I sort through a pile of 5458's and try them one at a time in the socket." And I still say this to people all the time and get many answers ranging from, "weird, I wonder why" to "you're crazy, it's just a unity gain buffer!"

I still say, from the 50+ 2N5458's that I bought from Small Bear, some make my Rat big and loud, and others not so much.

Yeah that was the thread that led me to check mine out.  I haven't tried anything like 50 jfets though, just three.  The third one was selected with a breadboard mockup, best out of maybe five I had.

[Chugs]

Overall, I find the biggest difference in tone from a Rat comes from the 2.2uf cap connected to the 47 resistor which sets the gain frequency at 1540z. As the cap is an electro with their wide tolerance range the difference in the frequency the gain is set at can be noticeable.

[thermionix]

Do you prefer it high?  Low?  Or right on the money?

[Chugs]

I like it right on the money.