AMZ Overdrive Pro mods

[DaveTV]

The AMZ Overdrive Pro was the first pedal I built, so it has a special place in my heart. I was thinking of re-visiting it and making a few modifications and was wondering if anybody had any ideas for building upon this circuit.

Firstly, I was planning on adding a second op amp to increase the output volume, which seems to be the most obvious modification. To do this, I was thinking of using one of the op amps on a TL072 as a non-inverting amplifier. Based on the equations I've seen, in order to get a gain of 2, the two op amp resistors should be the same value. Is this correct thinking? Would two 10k resistors be appropriate, or is that value too high?

Secondly, I was hoping that I might be able to control the distortion level a little better in this circuit. I really like the over-the-top distortion this pedal gets, but it would be nice if I could also get a smoother, more mid-level distortion as well when the saturation and drive are turned down. I'd read that using LEDs for clipping might tone it down a bit. I was thinking it might be nice to have a switch set up to go between LEDs and regular diodes--maybe even some germanium diodes to mix it up a bit.

How about some other ways to control the character of the distortion? Would using FETs in the gain stages make a difference?

Any ideas would be appreciated.

[Mark Hammer]

The output of the Overdrive Pro is constrained by two factors: the clipping threshold of the diode pair (which sets a maximum output ceiling equal to the voltage drop of about a half volt), and the passive loss through the tone-stack.

Your gut reaction of incorporating a gain-recovery stage to compensate for these two factors is not unreasonable but there are other things to exploit first.

You can double the output level by simply use a 2+2 pair of the same types of diodes instead of the 1+1 pair in there now.  If you built it on the Tonepad board, that should be a piece of cake to retrofit.  Depending on the signal level you feed the box, it may have absolutely no impact on your ability to achieve VERY fuzzy sounds, or it may make it more difficult to get anything you'd classify as "ridiculous".  Your choice as to whether the diode mod has more tonal costs than benefits.  Certainly anything that permits you to push your amp much harder still contributes to overall distortion.

The tone-stack.  Some time way back in the annals of DIY history, either Jack Orman or Steve Morrison (though I'm pretty sure it was Steve) contributed a clever Fender amp mod to POLYPHONY magazine (which eventually became Electronic Musician).  Passive tone stacks of the type used in Fender/Marshall amps, and in the Overdrive Pro as well, provide selective signal bleed to ground, which is why they eat signal and require a recovery stage most of the time.  How do you stop bleed to ground?  Easy, by lifting the ground connection.  The amp mod consisted of simply lifting the midrange pot's connection to ground so that all signal went to the volume pot.  I did this to an old blackface Tremolux I had at the time and was impressed by the extra oomph it provided.  Easily a few more db.  Keep in mind that one loses any EQ settings in the absence of selective signal bleed.  That being said, simply lifting the ground connection of the tone-stack ought to goose the output level of the pedal and makes a dead simple bonehead "boost" function that you can implement with a simple SPST stompswitch, or implement with a status indicator using a DPDT switch.

Do both these mods and you'll notice a big hike in output level, probably enough that you'll rarely have the controls dimed afterwards.

[Ansil]

The tone-stack.  Some time way back in the annals of DIY history, either Jack Orman or Steve Morrison (though I'm pretty sure it was Steve) contributed a clever Fender amp mod to POLYPHONY magazine (which eventually became Electronic Musician).  Passive tone stacks of the type used in Fender/Marshall amps, and in the Overdrive Pro as well, provide selective signal bleed to ground, which is why they eat signal and require a recovery stage most of the time.  How do you stop bleed to ground?  Easy, by lifting the ground connection.  The amp mod consisted of simply lifting the midrange pot's connection to ground so that all signal went to the volume pot.  I did this to an old blackface Tremolux I had at the time and was impressed by the extra oomph it provided.  Easily a few more db.  Keep in mind that one loses any EQ settings in the absence of selective signal bleed.  That being said, simply lifting the ground connection of the tone-stack ought to goose the output level of the pedal and makes a dead simple bonehead "boost" function that you can implement with a simple SPST stompswitch, or implement with a status indicator using a DPDT switch.
.

hot Mama, i had thought about this but ididnt' think it would work for some reason..  i guess i should stop thinkin and do more tinkering.  maybe thats my problem.

is it possible to dial in similar eq shapings after you lift the ground.  or do you need to re think the eq from the ground up after you lift it.?

[DaveTV]

Thanks, Mark. Those sound like some excellent ideas. This will be my first time tweaking a circuit, so I'm looking forward to trying lots of things out. I'm planning to start from scratch and finally using my breadboard on this one.

I'd still like to figure out a way to make the saturation and drive controls a little more interactive. They both seem to do sort of the same thing--make lots of distortion. It would be nice to give each control a little more character from the other.

[DaveTV]

Thanks, Mark. Those sound like some excellent ideas. This will be my first time tweaking a circuit, so I'm looking forward to trying lots of things out. I'm planning to start from scratch and finally using my breadboard on this one.

I'd still like to figure out a way to make the saturation and drive controls a little more interactive. They both seem to do sort of the same thing--make lots of distortion. It would be nice to give each control a little more character from the other.

[Mark Hammer]

Dave,

Each one (drive, saturation) fines tunes what the other one does, so there is really no need to make them interactive unless you have some starved battery experiments in mind, or unless you increased the value of C5 so that upper frequency bound would be set by the interaction of C5 and R7, making an accessible R7 desirable.  For my money, dime R20 and make R7 a chassis-mount pot.

Ansil,

Lifting ground on the tone stack means you lose about 95% of any EQ-ing, which is then restored as soon as the ground connection is established.  This has an effect on the output level (and by consequence an impact on the amplifier clipping) but has no effect on the amount of distortion generated internally.  If you rotated each tone pot appropriately, so that the lowest resistance path to the volume pot input was created for either the treble, or the bass, you'd retain *some* tone-shaping, but not tons of it. On fender amps, the effect of the tone-stack ground lift is to make the sound more "round"; a bit like making any pickup a Les Paul neck pickup.  Given that the sound is already shaped by the amount of distortion generated, that's not a huge loss, and may well be what some folks want for a "lead" tone.

[samuelk]

sry to hi-jack your thread DaveTV, but i´ve a question about the amz od pro mods. are you supposed to ues 3mm or 5mm leds for the clipping mod? are there any tonal differences?

[petemoore]

  Yeah, the 4 knob tone stack, the 4rth knob being between the standard tonestack and it's ground connection, would let the tonestack 'amount' [how much shaping overall] be adjusted.
  I can't see why it wouldn't work, at this point it's just a suggest, I don't know it's been tried, might change the eq as it's rolled down, you'd have to decide on a pot value, 500k would surely lift output...but maybe overvalued than necessary.

[WGTP]

The different size LED's I have measured are all between 1.5v and 1.8v.  Yes they will sound different than the SI's, but not as different from each other.  As many advise, socket and try them.  Radio Shack used to have a multi pack of various size and color LED's.   

[stm]

  Yeah, the 4 knob tone stack, the 4rth knob being between the standard tonestack and it's ground connection, would let the tonestack 'amount' [how much shaping overall] be adjusted.
  I can't see why it wouldn't work, at this point it's just a suggest, I don't know it's been tried, might change the eq as it's rolled down, you'd have to decide on a pot value, 500k would surely lift output...but maybe overvalued than necessary.

In the particular case of the Fender tonestack you don't need a fourth tone control; just make the MID pot 100kA instead of 10kB. Thus, first half rotation will serve as the usual MID pot, while the second half will reduce the mid scoop and lift overall level more and more.  If you compare the all-pots-at-50% with the mids dimed in this way you get around a 12 dB boost and a nearly flat frequency response--not bad!

In the case of the Marshall it is not so strong--you can gain just 4 to 6 dB's if you change the mid pot from 25kB to 250kA and dime it.  It doesn't get any better if you further increase the pot's value or just lift it--at most 1 dB more.

I checked the above with Duncan's tonestack if you want to experiment.

[dv8]

I went for red LEDs in place of the 2 diodes and was very happy.  I didn't try Mark's suggestion of the 2+2.  With the LEDs, I was very happy with this pedal.  Not much of a smooth overdrive pedal, mucho distortion though.
-brian