Setting gain on IC

[eightsevenzero]

I have a schematic I'm playing with and after substituting clipping diodes I'm getting less output volume than I need. In this particular schem how would I go about raising the gain of the IC? Is it just a matter of adjusting R5 & R6 values?

[ElectricDruid]

Changing the ratio of R6 to R5+P1 will change the gain (It's a non-inverting op-amp if you want to look it up).

Unfortunately that won't help you get any more volume (only more distortion) because the clipping diodes come *after* the gain. If you need a higher signal level, you need diodes with a higher forward voltage (LEDs for example) or a collection of diodes in series. This gives you a higher threshold for the clipping, and therefore a bigger signal.

[eightsevenzero]

Thanks for the quick reply and clarification. So I happen to be using a TL072CP for this circuit. The diodes I'm using sound really great in it, so is there a way to feed the clipped signal into the other half of the 072 to boost the output volume?

[GibsonGM]

Why, yes, there is, 870!    Down the bottom of this is some discussion of how that opamp can be configured as a simple boost for you to send your distortion into:

http://www.muzique.com/lab/boost.htm

[iainpunk]

Why, yes, there is, 870!    Down the bottom of this is some discussion of how that opamp can be configured as a simple boost for you to send your distortion into:

http://www.muzique.com/lab/boost.htm

the ones in this link have to much gain, i'd substitute the 14k with a 3.3k. resulting in a gain of 4.3 times, this should be enough with even the lowest Vf diodes you could find.

cheers

[eightsevenzero]

Thanks so much guys!

[antonis]

10k current limiting resistor together with 10k Volume pot isn't a wise choice..

Also, too many 1M resistors in bias configuration..

[eightsevenzero]

10k current limiting resistor together with 10k Volume pot isn't a wise choice..

Also, too many 1M resistors in bias configuration..

I'd love to understand the why behind these corrections! This is an old schematic for the old Dearmond Square Wave distortion unit. I found it incredibly shrill so I have been tweaking it to suit.

[Mark Hammer]

Later issues of the Distortion+ (which this is clearly a clone of) used a 50K or 100K volume pot.  That will resolve a lot of your issues.  Increasing the gain will not, since the diodes and volume-pot value will set a ceiling on maximum output level, regardless of how high the gain of the IC is set.

Changing to silicon diodes instead of germanium will also result in noticeably higher output level, although they do so by raising the clipping threshold, yielding a less intense maximum-distortion.  If you use silicon diodes but need more distortion, then THAT'S when you increase the gain, so as to be at or above the clipping threshold more of the time.

"Shrillness" is also easily addressed.  The Dist+ used a 1n cap i parallel with the diodes.  It wasn't enough.  This circuit ups that to 1n5, which still isn't enough.  Make that 3n3 or 3n9, and make C5 something closer to 47pf, and think you'll find the result something easier to live with that doesn't require you to turn the treble down on the amp or guitar.

[GibsonGM]

10k limiting R on the output, with the 10k vol. pot, creates a voltage divider.  Voltage out = Vin (R2/ R1+R2). 
In short, the 10k pot 'loads' the output more than a larger pot, like 100k.*

The 1M bias resistors 'starve' the opamp...lowering their value to something more like 10k, or 22k, can help improve the tone - but the original Distortion Plus used the 1M's as shown here....


The reasons 'why' are to be found in this, and documents like this.  https://mil.ufl.edu/4924/docs/TI_SingleSupply_OpAmp.pdf
Because it's hard to remember, and harder for ME to explain, I usually chaulk it up to 'stability' and things like that  



* at 50% rotation of 10k pot, you have 10k + 5k above the wiper (R1) at the output in a divider, and 5k (R2) below it...giving you an output .3 * the output voltage of the opamp, if we run that thru the formula above.

if the pot is a larger 100k, at 50% rotation you have 60K above the wiper, and 50k below... giving you output of .83 * opamp output...much better!! That is 'loading' and how it is avoided...

At full volume, you'll have 10k for R1, and 10k for R2 as drawn, yielding a max output of 50% no matter what....
With a 100k pot, that becomes 90%!  The R there, the pot...to ground, it is still part of the divider. Larger "R2" means more output, less loading.   I hope I'm explaining this right, I'm very tired and 'fumed out' from work, LOL! 

[antonis]

I hope I'm explaining this right, I'm very tired and 'fumed out' from work, LOL!

More right than many "teachers" would do, Sir..!!
(what counts is the notion to explain rather than scientifically true and correct apposition..)

[Mark Hammer]

Ignore D1, D2, and C7 for the moment.  R7 and P2 form what is essentially a 20k volume control that can never be turned up past halfway (i.e., 10k on one side of the wiper, and 10k on the other).  Right away, that reduces your possible output by half.  Now throw in the diodes and they limit what that "half" is half of.  If they are germanium diodes then that's more or less half of +/-300mv, which isn't much.  Throw in the extra cap and that reduces the output of high frequencies as well.

Using a 100k volume pot now makes the R7/P2 circuit behave like a 110k pot that can never be turned up past about 91% of the way, which isn't bad.