rat output buffer

[innovine]

I didnt have a 2n5458 so I used a TL071 opamp for a unity gain output buffer. Can anyone give me some suggestions on how the tone may be different? I read somewhere that the op amp buffer might be a bit more cold and lifeless. Do you think that is obvious in a rat? The circuit sounds quite bright, fizzy and not really doing it for me at the moment. If the 2n5458 would make a big difference i might hold off on putting this to stripboard, but if its not gonna make much of a difference then I'll go ahead right now and get this done this weekend. What do you think?

[Kipper4]

Have you auditioned differant op amps in the output buffer 4558 for example ?

[ashcat_lt]

There's nothing in the buffer that will help you with a fizzy sound.  As long as you've actually got that 071 wired properly it should work just fine and switching to the transistor shouldn't make a significant difference.

What opamp did you use as the main gain stage?  Slew rate and GBP will have a noticeable effect.  Increasing the value of either the Tone control cap, the 1.5K resistor before it, or the 100pf cap in the feedback loop might also help.  The buffer is just a buffer.

[GibsonGM]

There's nothing in the buffer that will help you with a fizzy sound.  As long as you've actually got that 071 wired properly it should work just fine and switching to the transistor shouldn't make a significant difference.

What opamp did you use as the main gain stage?  Slew rate and GBP will have a noticeable effect.  Increasing the value of either the Tone control cap, the 1.5K resistor before it, or the 100pf cap in the feedback loop might also help.  The buffer is just a buffer.

I have to agree.  You have places before now to shape the tone, so you should be able to "offset", if you will, the presence of the chip.  Sometimes, we sort of "blame" a chip, or device, when what we are really hearing is "reality", ha ha.  As in - our idea of the ideal sound often conflicts with what we get at the end!   However - the 'dissatisfaction' is what keeps us working towards that ultimate goal we never seem to be able to achieve....  *cue music*

[innovine]

Thanks. I'm using an lm308 for the gain stage. I don't have any other op amps available to test with at the moment so I'll just go with this one. Tweaking the feedback resistor has been useful, thanks  

[GGBB]

As the others have already said, the op-amp shouldn't be a problem - it's just a buffer.  But if you're looking to get as close as possible to the 2N5458, try another JFET if you have any - 2N5457, MPF102, J201, ... 

RATs are normally bright and fizzy.  I always have my filter knob up fairly high, as I think most people do.

[ashcat_lt]

Well, the 308 is THE opamp for this circuit.  With the specified comp cap it should be about the best (worst?) you're going to get.  You could try increasing that comp cap, but I think you quickly reach a point of diminishing returns.  Does it get better if you turn down the T control on your guitar?  Increasing the value of the 0.001uF cap significantly will put less treble into the clipping section, which should mean less fizzy high harmonics out, except that this thing clips so hard and rudely that I don't know how much it can help.  

There's only one resistor in the feedback path, and that's the gain pot.  You were talking about the cap there, right?  I think that's a good place to mess around because its effect is dependent on the gain pot.  Removing treble elsewhere in the circuit could make the lower gain settings too dark.  With this one, the rolloff is kinda proportional to the amount of clipping that happens.  If you mess with it, you might find a point where the rollof is balanced better with the new harmonics.

Edit to add - I don't have problems with fizz from mine (TL080, 180pF comp cap), but I run it into relatively dark amps which are adding their own overdrive on top of it.  It's probably not appropriate as the "gain" channel of a really clean, bright amp.  Not the worst I could think of, but not the best either.

[MrStab]

i had a wee look for any specs which could be drastically changed by an opamp output buffer, but i couldn't really find anything significant. this link: http://ratdistortion.com/products/specifications/ suggests the output impedance is already 1k, which probably wouldn't be much improved upon by an opamp (if at all). there are more variables than that, though - not to mention psychoacoustics, as others have mentioned.

it's quite a contentious term round these parts, but many claim that opamps are more "transparent" than single-transistor buffers (that's why i prefer em, just that thought puts my mind at ease!). "sterile" is another adjective brought up a lot. as Ashcat said, if anything you should just be hearing a more real version of reality! lol. i don't know to what extent the original buffer and its load-driving/frequency response characteristics make up the Rat sound people have come to know and love, but those would just be the results of a happy accident, and probably irrelevant if people use it in the middle of a pedalboard anyway.

maybe something else you've changed along the way, such as the output cap, pulldown resistor etc. could be to blame? the TL0** series is pretty much the de-facto "no BS" opamp.

also, i usually don't like to jump to this conclusion, but have you tried another chip, even if of the same variety? http://geofex.com/circuits/when_good_opamps_go_bad.htm