Stage Center Reverb Question - Dry signal distortion (Not reverb distortion)

[jasonsguild]

Hi,

I've bread-boarded a reverb driver using the GGG Stage Center Reverb schematic (http://www.generalguitargadgets.com/projects/25-reverb/43-stage-center-reverb).  

I've changed R7 (470K) to 300K and the reverb portion is nice and relatively distortion free, but the dry signal seems to be getting overdriven very easily.  If I turn off the dwell, and leave the mix pot all the way at the dry extreme, the distortion is at its worst.  I changed R5 to 10k to keep the output at unity, which helped a bit, but it's still getting distorted.  I'm not really sure where the distortion is coming from if there's no major amplification (that I see) to the dry signal.  

Is this common with this reverb driver?  I guess I could attenuate the dry signal at some point in the circuit...any better suggestions for cleaning up this distortion?

Thanks

[Mark Hammer]

How are you powering it?

[jasonsguild]

Standard 9V one spot to a Max1044 to get bipolar +9 -9V.

[Mark Hammer]

You certainly have enough current in the supply, but ther MAX1044 is limited in how much current it can provide.  Reverb tanks usually need to be fed a fair amount of current to do what they do.  I'm just wondering if the means of powering is providing insufficient current to run the thing.  True, the dry seems to be a much bigger problem than the wet side, but then reverb tanks tend not to have much bandwidth, wo distortion will show up more readily in the dry signal.

Just speculating.  This is NOT a particularly complicated circuit, nor one where there are critical thresholds for anything that might result in clipping.  Which is why my attention turns to the power source.  If you can, try powering it with a real bipolar supply, rather than a charge-pump derived one.  If it still clips under those conditions then we'll try another strategy.  BUt for now, if it were my project, I'd want evidence that current is NOT the problem.

[jasonsguild]

Thanks for the help Mark.  I hunted through all my stuff, but unfortunately I don't seem to have a real bipolar power supply accessible.  I know you mentioned ruling out a charge pump based power supply, but I found a LT1054CP chip in my stash.  Would it be worth it to replace the Max1044 with the 1054 and try things at 12V instead of 9? 

[Mark Hammer]

Well, the first thing is to verify for yourself (and for us too) what the current requirements of the SCR are.  My question arises from the knowledge that charge pumps are restricted in how much current they can supply.  Personally, I don't know if the Max 1044 OR the LT 1054 are capable of providing it or not.  They might, or they might fall just a little shy.

Perhaps one of the other folks here might be able to chime in regarding what those two chips can offer.  They ARE good enough for a lot of projects, but driving a low-impedance load like a reverb pan often starts to move into the sort of current requirements more typical of a digital pedal; i.e., noticeably greater than the few analog chips might suggest.

[PRR]

> get bipolar +9 -9V.

Use VoltMeter. Do you actually GET +/-9V (at least 8V)?

Poke all opamp IN and OUT pins. In this scheme they should all be near zero Volts DC (+/-0.1V).

Is it possible you've made a 10K/100K or 33K/330K stripe-color mistake? (That's one of my favorite screw-ups.)

[jasonsguild]

Resistors double checked, voltage at pins 4 & 11 appear good, and the ins and outs aren't reading high.  I switched from my Humbucker guitar to a single coil for testing and the distortion isn't as frequent.  Seems like I'm just overdriving the input circuit?

[PRR]

> overdriving the input circuit?

But as you say, there's no major amplification (that I see) to the dry signal.

IC1A is unity-gain, eats + and - 9V. Should take nearly 18V peak-to-peak or over 4V RMS. That's way more than any passive guitar. R10 reduces any signal. IC1C has gain of 3.3.

So even with R10 maxed-up, the input can be 4V/3.3 or over 1V, still more than any normal guitar, and at that point the output could be 4V which will over-drive the snot out of any normal guitar-amp's input.

> I changed R5 to 10k to keep the output at unity

Oh, OK. Then same argument except no gain at all, headroom far higher than guitar level, it "can't overload".

You could instead just not max-up R10, leave it at 1/3rd for all normal playing. 1/3 * 3.3 = unity gain. With an Audio taper pot, 1/3rd will be near "7", so you still have 7/10ths of the knob for "less" and 3/10 for "free boost".