Nw Clipper gain control

[black mariah]

Is there an easy way to add a gain control to a New Clipper?

[petemoore]

In schematics 2 [I think..or was it Munky page...] there's an Alfonso Hermide Dist+ Mod  With Active Gain Control.
 The NC and DIST+ are close relatives, t should be easy to graft from the Hermida mods

[Marcos - Munky]

It's in my site.

[Mark Hammer]

The New Clipper uses an op-amp in a non-inverting configuration.

Remember that for op-amps, the gain is determined by how much negative feedback from the output you DON'T provide back to the input.  The absolute highest gain will occur under what is called "open loop" conditions, namely no feedback whatsoever between output pin and input pin.

With non-inverting op-amps like this one, the resistors actually form a sort of volume pot between the output and input.  Volume pots work by dividing a voltage.  The voltage available at the wiper of the pot will depend on the ratio of the resistances on each leg of the pot.  If the leg to ground is very small compared to the leg between wiper and input then the voltage available at the wiper is drastically reduced.  If the leg between wiper and ground is very large compared o the other leg, then almost all the voltage available at the input of the pot is also available at the wiper.

So, back to the New Clipper....and similar devices.

The "pot" in this case, is formed by two resistors.  It is essentially a 150,150 ohm resistor with the "wiper" tapped so that there is 150R resistance to ground, and 150k resistance on the other side.  Under such conditions, as you can imagine, most of the negative feedback from the output to the input is pretty much killed.

Okay, your first formula.  Gain in a non-inverting amp like this is given by the ratio of the ENTIRE "pot" resistance (150K + 150R here) to the resistance of the leg going to ground (150R in this case), which turns out to be around 1000.  That's some kinda gain there, pal.

To reduce the gain, you can either make the resistor between op-amp output and "pot wiper" smaller, or make the resistance between "pot wiper" and ground larger.  With a 150R resistor to ground, and a 100k feedback resistor, the gain drops to 668.  With the original 150k feedback resistor, but a 1k resistor to ground (instead of 150R) the gain drops to 151.

A clever way of providing wide-range gain adjustment is by sticking in an actual pot, tying its wiper to the '-' pin (inverting) on the op-amp, and its two outside lugs to the existing fixed resistors.

Consider this arrangement.  The 150R resistor is still in place, but the 150k resistor is replaced with a 68k resistor.  A 100k pot (which will likely be something closer to 94k or so) is inserted as described.  At one extreme of rotation (max gain), all the resistance of the pot is sitting on one side of the wiper in series with the 68k fixed resistor.  That will likely be a total resistance somewhere around 155k in realistic terms, after tolerances are taken into account.  With 150R on the other side of the wiper, that's a bit more gain than we started out with.  As the pot starts to be rotated in the other direction, more and more of the resistance gets shifted to the other side of the wiper until at minimum rotation, we have the full value of the pot plus the 150R resistor as one leg, and 68k as the other.  This will yield a minimum gain of about  1.7.

You COULD just simply adjust one of the resistances, as is often done, but here we get to a second issue with non-inverting op-amps.

To keep DC out, you will see a 4.7uf cap in series with the 150R resistor to ground.  These two components, in sries to ground, form a lowpass filter whose rolloff is given by 1/(2*pi*C-in-uf*R-in-meg).  In this instance, that is around 226hz.  As the resistor in series with it goes up, the rolloff point goes down.  With a 1k resistor in there instead of 150R, the rolloff would now be around 34hz.

Principle: When the gain of a non-inverting op-amp is adjusted via the "pot" leg to ground, be sure to select a cap value that will yield a reasonable low-end rolloff at all gain settings.  IN the case of the MXR Distortion+ and the assorted DOD derivatives, the gain is set via this resistance, and at maximum gain (i.e., minimum resistance) the rolloff is around 700hz or so, which starts to get kinda buzzy and thin.  That may well be what you want at max distortion, but if it isn't, you may want to think about using a larger value cap.

Going the other way (i.e., the resistance between the op-amp output and inverting input pin) gain increases as resistance increases.  With most op-amps, once gain increases beyond a certain point, you start to see reductions in bandwidth at the upper end.  You won't see it when going from a gain of 3 to 20, but once you're up into gains of 500-1000, you start to see it, especially in cheaper op-amps.  That intrinsic high-end rolloff may well be suitable for your needs or what you want, but if it isn't enough (i.e., very fizzy tone), a cap in parallel with that feedback resistance will reduce high end, starting at a frequency set by, you guessed it, 1/(2*pi*C-in-uf*R-in-meg).  A 100pf cap in parallel with 150k resistance starts to roll off around 10.6khz.  A 150pf cap begins the rolloff around 7khz, which is enough for real midrange bite, but keeps the fizz away.  

If you increase the gain by increasing the feedback resistance, rather than by decreasing the resistance to ground, you can end up with the reverse problem.  That is, at either maximum or minimum settings you have either too much or too little treble for your liking.  Sometimes, it works out just right and is done deliberately.  Indeed, for the Tube Screamer, that feedback cap is part of what manages to yield a relatively smooth-sounding distortion at highest gain settings; because more treble is rolled off at higher gain than at lower gain.  

So, the bottom line is that you can use cap value selection, and the manner in which you decide to adjust/vary gain in this circuit (or any non-inverting op-amp circuit), in a strategic way to get different sorts of tonal changes accompanying gain changes.

One of the reasons why I kind of like the gain-adjust method where the pot wiper goes to the inverting pin is that it doesn't take huge changes in resistance to create big changes in gain.  If the pot value were 50k, the feedback resistor 120k, and the resistor to ground 220R, you could get a gain range of from x3.4 to x773, which is more than sufficient to go from pretty darn clean to pretty darn fuzzy.  At the same time, with a resistance change of from 220R to 50.22K (fixed resistor plus all the pot resistance on one leg of the pot) in the resistance to ground, the low-end rolloff would change from 154hz at max gain to just under 1hz at min gain.  The reduced change in high-end rolloff when using a feedback cap with a pot in the manner described is similar in terms of producing only minimal change in high end rolloff.


In contrast, the MXR Distortion+ accomplishes all the gain change on the resistance leg to ground, using a 1meg pot and 4k7 fixed resistor in series.  Here, the gain range goes from x1.9 to x426, but the low end rolloff changes from 3.4hz at min gain to 721hz at max gain.  That may be what you want but it may not.  

Hope this lengthy digression is helpful and gives you some ideas about what you can change on the New Clipper to get the tone right for you.

[Mark Hammer]

To keep DC out, you will see a 4.7uf cap in series with the 150R resistor to ground.  These two components, in sries to ground, form a lowpass filter whose rolloff is given by 1/(2*pi*C-in-uf*R-in-meg).  In this instance, that is around 226hz.  As the resistor in series with it goes up, the rolloff point goes down.  With a 1k resistor in there instead of 150R, the rolloff would now be around 34hz.

That should say "highpass" not "lowpass".  The cap to ground takes out DC (0 hz) and attenuates other frequencies higher than that, to a point set by the cap and series resistor.  Sorry about the confusion.