Basic Q about Op Amp based buffers

[dan5150]

I am learning about buffers and am starting to build the one that Beavis documents here: (http://www.beavisaudio.com/techpages/Buffers/)

My question is about component quality. Since (as I understand it) a buffer is really all about managing the impedence of the signal, and should be unity gain, it is as important to use high quality parts? Meaning quietest Op Amp, metal film caps & resiters, etc?

[dan5150]

Follow on question. Why would I need more headroom out of a unity gain buffer? i.e. by feeding it 12v, as oposed to 9v. For a boost or OD, that makes sense. But for a buffer?

[R.G.]

Buffers are about improving the output drive capability of a signal by presenting a high impedance to the input signal and having a low(er) output impedance to drive other things.

In guitar effects terms, this is most important when buffering the high and variable impedance of a guitar output into a lower impedance without losing some frequencies disproportionately because the guitar's impedance is higher with treble frequencies than bass ones.

Every resistor and active device adds noise. Sometimes this does not matter much (as in a power amp buffering a preamp to drive speakers) and sometimes it's critical, like in front of a high gain distortion circuit. Distortion matters too, as if you're trying to NOT distort, you don't want your buffer hashing things up for you.

Buffers are often but do not need to be unity voltage gain. They are, by their nature, capable of high current gains though. Usually they're low voltage gain, and many times are unity voltage gain.

Whether you need to use lowest noise and distortion parts depends on what the buffer does. A buffer in front of a Super Grondz Megadistortion doesn't need to be low distortion - how would you tell? But it probably needs to be low noise because its noise gets Mega-gained. A buffer in front of an acoustic guitar amp needs to be both low noise and low distortion. You have to suit the parts to the problem.

You need headroom adequate to not make the buffer be the part that distorts. Presumably there is something about the abovementioned Super Grondz that you like the distortion of. A buffer might add unpleasant distortions of its own if it didn't have enough headroom. However, headroom only means anything if you have signals that use it. You really don't need much headroom for a raw guitar signal of 100mV to 1V. But if what you're trying to do is buffer +20dbm signals on a 600 ohm studio line, you may well need +/-24V power rails.

Again, suit the parts (and power) to the problem. There is no substitute for knowing the problem at hand: how big, how small, how quiet, how much power. 

[dan5150]

Thanks R.G. ! This really helps!

My application is pretty straightforward. My board has a combination of true-bypass and non true bypass pedals. I want to add a buffer to the front, and maybe end (going to do a lot of experienmentation here) of the chain in order to hopefully get back some of the high end frequencies I am loosing.

Based on this case, I am going to go with the lowest noise/distortion parts, but I don't think I need to worry about additional headroom.

-Dan-

[MikeH]

Super Grondz Megadistortion

Lol- got a schematic?

[R.G.]

Super Grondz Megadistortion

Lol- got a schematic?

I do, but it's a highly specialized tool for the musical professional. It's powered by a Chevy V-8 running a field modulated generator to get the necessary high gain and fully saturated, yet clear and angelic tone.

Probably it won't fit on most pedalboards.

[liquids]

Thanks R.G. ! This really helps!

My application is pretty straightforward. My board has a combination of true-bypass and non true bypass pedals. I want to add a buffer to the front, and maybe end (going to do a lot of experienmentation here) of the chain in order to hopefully get back some of the high end frequencies I am loosing.

Based on this case, I am going to go with the lowest noise/distortion parts, but I don't think I need to worry about additional headroom.

-Dan-

If you're like me, you'll realize that, in the end, you've become affectionate toward some of that high end frequency loss without a buffer early on in the chain.

So don't be afraid to add a tone control to the buffer - my post-guitar buffer is high input impedance buffer--> passive R-C network-->buffer--> out, and I tweaked it until the buffer adequately simulated the bypass tonality...similar high end, but with more 'oomph' from the superior drive of the signal chain and cables the op amp buffer provides (oomph - that's the technical term  ).

[Processaurus]

As a cliff notes type response to the very good responses above, for an opamp type buffer for audio in guitar pedals, a TL072 will be the solid choice almost all of the time, unless you have special needs, like rail to rail operation, or miserly power consumption.  It is high impedance (due to the JFET input stage, great for guitar), low noise, and cheap.  You don't need metal film resistors.  "Noiseless biasing" is good for those type of opamp circuits.

Ultra quiet operation is a kind of thankless thing to chase with guitar electronics, because the source, the instrument's pickups, are inherently quite noisy.  If this was a mic preamp with 80dB of gain it would be a different story, but guitar pedals are generally forgiving of not using top shelf components.