Turning a klon buffer into a boost?

[chumbox]

Hi all I've been tinkering with the klon buffer schematic

http://www.beavisaudio.com/techpages/Buffers/Buffer_clon.gif

and trying to turn it into a boost. In theory I thought simply running a pot between pin 1 and 2 (TL072) as a variable resistor would do the trick to get some gain out of it. However I can't seem to get any change in the signal. Granted I'm no op amp expert so is there something else I should be trying to get this from buffer to boost/light drive?

Thanks.

[slacker]

Have a read of R.G.s technology of the Tube screamer article, the clipping section part explains everything you need to know.

[chumbox]

Thanks heaps . I actually read that before I started to tinker but clearly I missed something take 2

update: I re read the geofx article and still don't quite see what I'm missing. They have a 51k with a 500k pot for gain and I used a 500k only. The main difference is the TS has the extra resistor and cap to ground (Zi) but this appears to only change the frequencies affected. Hmm. What am I missing?

Oh and I just rechecked my breadboard layout and it looks correct

[slacker]

"The gain of a noninverting opamp stage set up like this is [1+ Zf/Zi] where Zf is the equivalent impedance of the feedback network from the output pin to the (-) input, and Zi is the equivalent impedance from the (-) input to AC ground." This is the important bit.

In a buffer you have Zf = 0 Ohms and Zi = infinite Ohms because there is no Zi. so the gain is 1 + (0/infinity) = 1 if you just make Zf = some resistance and leave Zi = infinite resistance then the Zf/Zi part is still zero so the gain is still 1. To make any gain above 1 you need some Zi.

[chumbox]

Thank you so much slacker! Now that I reread it with the equation and your explanation it's actually very obvious. Alright back to the breadboard. Will report back in a day or so with some good news Id say.

Really, thankyou.

[Gus]

Here is a thread from the past.

http://www.diystompboxes.com/smfforum/index.php?topic=93699.0

Remove the parts left of the input they are for the guitar sim section

C8 is an optional high end roll off capacitor(lowpass filter)

R12 is the gain control

R10 can be a volume control

[chumbox]

Thanks Gus. Super helpful resource.

[pee-j]

I'm very happy with this topic, since I'm right into this, too... thinking about turning a "Plon" buffer into a booster...
I mean, why not?

so, although I'm no engineer, couldn't even explain how a transistor works...
that is, "before I talk, I should read a book" (Mesopotamia -- The B-52's)

but there's one thing I do know:
a buffer and a booster circuit (based on an op-amp) only differ in two components...
two resistors...,

one of them (Rf) is replacing the plain wire (short cut) between the op-amp's "out" and "negative in"... (pin 1 and pin 2 in the linked circuit picture of the OP), and the other (Rin)  going from pin 2 down to the ground...

like this:
http://www.learningaboutelectronics.com/images/Noninverting-op-amp.png


this is something which could have been shared with the OP,  I think...

and could also be mentioned that the gain is calculated / brought about choosing values of these two resistors...

could be calculated like this:
see the non-inverting op-amp calculator here:
http://www.learningaboutelectronics.com/Articles/Op-amp-gain-calculator.php

note that this gain value is not in decibels, though...


finally, it could be told our fellow forumer that with the Rin, we usually see an electrolytic capacitor going to the ground, too...
however, what makes some designers go that cap to Vr, not ground, takes some further learning :)

[GibsonGM]

I wish there were a bot that could comb forums and point out what might have been missed 7 yrs ago, don't you, Pee-J?

[ashcat_lt]

a buffer and a booster circuit (based on an op-amp) only differ in two components...
two resistors...,

uhh...

finally, it could be told our fellow forumer that with the Rin, we usually see an electrolytic capacitor going to the ground, too...
however, what makes some designers go that cap to Vr, not ground, takes some further learning

If there is a Vr, and Rin does NOT go there, then that cap is mandatory otherwise the VR gets the same gain as the audio signal, which will (usually) just jam the whole thing up against the positive rail and you'll get nothing audible out of it. 

I like to think of this negative feedback loop as like Bizarro World where everything is upside down.  This arrangement of two resistors is actually just a voltage divider with a ratio equal to Rin / (Rin + Rf).  The opamp applies exactly enough gain to make that back to 1, which is to say the inverse of that divider ratio, which when simplified down gets us the to the standard gain equation that you see everywhere.  If you already know how to calculate a voltage divider, you don't actually have to memorize the gain equation for a non-inverting opamp because you just do the divider calculation and then "flip the result over" by dividing it into 1.

[amptramp]

I would just add that the schematic on Reply#7 could use a small capacitor across the feedback resistor to add stability.  There is always going to be a capacitance to ground from the inverting input and this can lead to instability as the capacitance to ground will shunt the feedback to ground resulting in a rising response that will meet the falling response of the op amp open loop gain.  Where they meet, there is a phase shift that can cause oscillation or at least poor transient response.  This capacitor should be sized so that the feedback R-C time constant is above the audio range but drops the gain below the open loop gain.  With normal circuit values, these capacitors are usually on the order of 30 to 100 pF.

The result of oscillation is not always obvious.  The most common effect is that the circuit has reduced dynamic range because the op amp itself starts to clip the oscillation riding on the audio even though the audio signal voltage is nowhere near where it would clip without the oscillation.

[iainpunk]

I like to think of this negative feedback loop as like Bizarro World where everything is upside down.  This arrangement of two resistors is actually just a voltage divider with a ratio equal to Rin / (Rin + Rf).  The opamp applies exactly enough gain to make that back to 1, which is to say the inverse of that divider ratio, which when simplified down gets us the to the standard gain equation that you see everywhere.  If you already know how to calculate a voltage divider, you don't actually have to memorize the gain equation for a non-inverting opamp because you just do the divider calculation and then "flip the result over" by dividing it into 1.

sounds like something i would say, haha
great minds think alike

this rule of thumb also goes for filters and non-linearities. a mid scooping filter becomes a mid boost in a feedback loop, crossover distortion becomes soft clipping, and an amplifier with positive gain over 1 turns into a gain of less than '1'

cheers