is the AMZ Overdrive Pro Rev. A too quiet?

[bigjonny]

I am considering putting together the AMZ Overdrive Pro (rev. A), and am hoping to clear up some confusion before I build...

I have read some of the build reports on Tonepad, and around the web... A lot of folks seem to think the AMZ Overdrive Pro's output is too quiet, and suggest adding another buffer (after the tone stack), or omitting the output buffer altogether.

Is this issue resolved, nowadays (thus, the "rev A" text on the schematic), or does Q2 actually REDUCE the signal level?

I am no electronics whiz, but it seems silly to me to add another buffer into the signal chain to amplify a signal, just to attenuate it with the next buffer.

Is there a best approach to this?  I can see that Q1 receives sends the guitar signal from base to collector, whereas the Q2 sends from base to emitter.  Does this mean Q2 actually attenuates the signal?  As I've said, I am no electronics whiz.  Can I just sub in a higher (or lower?) gain transistor for Q2 and call it a day? Or, move everything from R12->OUTPUT to the collector side of Q2?  If Q2 DOES attenuate the signal, why would this be done?

If (for some reason) the best or most practical solution is indeed to add a buffer after the tone stack, can this be done easily on Tonepad's PCB?

thanks,
-jOn-

[DaveTV]

The Overdrive Pro was my very first build, and I eventually built a second one to make up for the shoddy work I did on the original.

Indeed, the output level of the Overdrive Pro is pretty low. With the volume knob all the way up, the level is maybe just above the bypassed level. You lose a lot of signal through the tone section and the two clipping diodes. From what I remember, the output transistor is set up as a buffer for impedance matching, but you don't get much of a boost there. You might try replacing the output transistor with another op amp. Connect the op amp's input to the wiper of the treble control, with a 0.1uF capacitor in between them. Then configure the op amp so it has a boost of maybe 10 or less using a volume control pot similar to how the other op amp is set up. I'd highly recommend testing all of this out on a breadboard before soldering a circuit together. There aren't a lot of parts in the Overdrive Pro, and if you haven't used a breadboard before this would be an excellent exercise.

[RDV]

If you used a Dual OA, the unused 2nd stage would make a great volume recovery circuit.

RDV

[bigjonny]

Hmmm.  These all seem like good ideas, but seem to require some pretty extensive mods to the circuit layout.  While the ideas don't seem too intimidating to me, it does seem like a lot more effort and hassle, if I want to use Tonepad's PCB (which I do).  Testing and playing with circuits is great, to an extent, but thus far in my tone quest, i much prefer the tidiness of a PCB compared to perf.  I would want a solution that could be easily applied to Tonepad's PCB.  Cutting traces: sure; Adding jumpers; ok.  It seems to me, the easiest "tweak", therefore, if Q2 does provide gain, is to use a higher gain transistor, asuming such a thing exists.  Or, am I missing something?  Can I somehow use a dual opamp on Tonepad's PCB?

One more transistor question I am hoping to have explained: what is the difference when sending signal from base->collector vs. base->emitter?

Thanks...

[bigjonny]

Oh, I see.  Well, i am just looking at Tonepad's layout document, and realizing the solution is fairly simple, isn't it?

Tonepad's doc actually calls for the TL072 (dual opamp) IC, so adding in the second opamp is reasonably straightforward...

From the document's layout perspective, the IC is "upside down", compared to a TL072 datasheet I found online.  In other words, Pin 1 is the bottom right pin; Pin 4 is top right; Pin 8 is bottom left; Pin 5 is top left, right?

Tonepad's document also shows that the TL072's 1st Opamp is amplifying ground (no signal), in the schematic at the bottom of the page.

Easy enough to fix, I suspect.  Here's my solution...  Any input is appreciated (oops -- no pun intended):
Cut the traces on the PCB between Pin 3&4 and between Pins 1&2.  Hopefully we can shove the IC in, along w/ hookup wire in the holes for Pins 1,2,3&4 (Perhaps a little drilling to widen these holes, or soldering the wire copper-side?).  From the Treble pot wiper (incorrectly labeled "Mid" in Tonepad's schematic, but not layout), run a jumper wire to Pin 3, then run a jumper from Pin 1 to the Volume pot.

At this point, I am stuck: how do I adjust the gain?  I don't know enough about Opamps and transistors.  Do I simply tap part of the run from Pin 2 to Pin 1 to ground?  I realize I am short on space and might need a trim pot (probably in series w/ fixed resitors to keep clipping to a minimum)...  I assume we could put some perf on "stilts", by soldering leads to the perf, and shoving them into say, Pins 1,2,4&7's holes (4 is to get Ground to the perf; 7 is for physical stability- no electrical connection from Pin 7 will be made) around the IC, and have it float above the IC.  Suggestions are much appreciated...