How much MORE gain do you need for an on-board boost?

[Bill Mountain]

You could always run at 18 or 24 volts and lower the voltage to the first stage or put 2.1 volt to 5.1 volts zeners in the feedback path of the first stage to mimic rail clipping.

[Bill Mountain]

Here is one of the bass distortions I have been working on:



It's a distortion plus with a bass boost after it.

No amount of math in the world could keep the 2nd stage from clipping on 9 volts.

I have tried it up to 30 volts and it becomes really clear wth a nice solid boost but I missed some of the 1st stage clipping that I got at 9 volts.  This is on the back burner for now but I have 1 PCB left so I plan to sort it out eventually.

Good luck with your design!


EDIT:  And before someone says "of course that's not going to work at 9 volts"; those values are just place holders.  I've tried a hundred variations of diodes and amount of available gain with the basic schematic.  Some were better than others and some were unusable.

[Bill Mountain]

I seemed to have killed this thread.  I was hoping for a nice discussion on 9 volt boosts.

Mark,  do you want to show us what you're working on?

[Mark Hammer]

I will once I get home.

In the meantime, the verbal description is that it's essentially an MXR Distortion+ "engine", with a 2+1 GE diode complement for clipping, some tweaks to the toneshaping, a little more gain (304x istead of 213x), and a three-position toggle with a few tonal options.  Middle gives the stock less-bass-as-gain-increases sound of the Dist+.  A 2nd position gives more bottom at all gain settings but does nothing to the top, and a 3rd option "thickens" the sound by adding both an Si pair across the feedback loop and an extra feedback cap.  Essentially Q&D double clipping by means of one set of diodes that have a high forward voltage than the subsequent set of diodes.  Since the double clipping will introduce more harmonic content, the extra feedback cap tames the result with a lower rolloff.  All 3 are musically useful.

That goes into a simple non-inverting op-amp set for a default gain of 2.5.   That gain stage has a 50k pot in series with a 15k fixed resistor in the feedback loop, and a 10k on the ground leg.  Max gain is achieved by the full pot resistance+15k (gain = 75k/10k = 7.5x), and min gain is achieved by simply shunting the pot in the feedback loop with a footswitch so that you go back to 15k feedback resistance.  Unshunt the pot and whatever gain is set via the pot now comes into effect.  I made a point of using a feedback cap value in that stage such that there was full bandwidth when unboosted, and restricted bandwidth when boosted.  I forget the specific value, but I think it may have been 1nf.  at full boost that would roll off around 2.5khz, which is helpful for extracting more pleasing "vocal" distortion from the amp.  Keep in mind that at a certain point in level one is relying disproportionately less on the sound of the pedal itself, and more on the joint product of the pedal and amp.  In this case, I made absolutely no attempt to be able to produce a clean sound with full bandwidth.  I have other pedals for that.

One could do the boost preset in a variety of ways.  It could be done via a discrete transistor stage using a cap/pot pair from the emitter to ground.  It could be done via a fixed gain stage and a variable resistance in series with the output volume pot.  It could be done via the same fixed+variable resistance thing I did, only along the ground leg of the op-amp.  I elected to use the feedback loop because I believe the impact on bandwidth to be useful in the context of pushing amps.  YMMV.

I couldn't be bothered with using a charge pump this time out.  Maybe next time.  What I have sounds pretty good, and the boost amount is more than enough to punish my amp's input stage.

[Mark Hammer]

As promised.   The "name" is a curling-related joke.  "Hurry hard" is what you yell at your team-mates to sweep more furiously, so as to add a little more velocity to the rock sliding across the ice, or maintain whatever momentum it already has.  I really like that ruby-red metal-flake paint.  When I coat it with a matte clear finish, it acquires an iteresting lustre.  The yellow rub-on lettering was purchased at a model airplane shop.  I have a feeling the on-off-on toggle is pooched on one side because the thick/normal works great, but no matter how much additional feedback capacitance I add for the "warm" setting, it doesn't get any duller.
Schem: http://img.photobucket.com/albums/v474/mhammer/schem.png  The feedback diodes are 1N4148.  The ones going to Vref are assorted Ge.

[Bill Mountain]

This looks great!

I love everything about it.

I have tried stuff like this in the past but I always used a buffer before the distortion stage for consistency but I do prefer the sound with a passive instrument and no buffer.

I did a stage like this with Ross Distortion values (4.7k, 47nf, 500kC, 1M) and it sounded great but I couldn't get over the fact that I had to run it first in my chain or the highs would be annoying.