How do I eliminate this popping noise?

[ToneRangerAudio]

Hey all,

Here's what I have going on. I built a SDD-3000 preamp pedal in a 1590BB enclosure with a footswitch to switch between 2 pot values for the volume (pre master). Basically like 2 presets. I have the gain pot wires going to the middle lugs of the 3PDT, and then the opposite sides of the 3PDT go to the two corresponding pots. I also have LEDS's hooked up on the remaining lugs so I know what pot is on.

The problem is I'm having is that when I press the 3PDT, there's a loud pop. Exactly like a circuit that doesn't have a pull down resistor. Does anyone have and idea how I can fix this? Can I add some resistors somewhere to cut down on the popping?

I've included the veroboard of the circuit I built.

Thanks!

[bluelagoon]

Here is a better image of the OP Veroboard Layout -




and web link to -

SDD-3000 Preamp

[idy]

Are we to guess you are calling volume on the layout "gain?" I think that is gain. That opamp only has a small cap and a pot in the feedback loop and the gain jumps when contact breaks, then comes back down.

When I have done this kind of thing I have used a couple of LEDs and LDRs and "cross faded:" When you hit the foot switch one LED turns off, the other turns on. The two parallel pots each have an LDR in series.

[ToneRangerAudio]

Are we to guess you are calling volume on the layout "gain?" I think that is gain. That opamp only has a small cap and a pot in the feedback loop and the gain jumps when contact breaks, then comes back down.

When I have done this kind of thing I have used a couple of LEDs and LDRs and "cross faded:" When you hit the foot switch one LED turns off, the other turns on. The two parallel pots each have an LDR in series.

Correct. Gain = volume. Just changed that in the OP.

Does doing the LED's that way eliminate the popping noise?

[FiveseveN]

Please attach a schematic of the circuit in question. You're much more likely to get pertinent help when we don't have to hunt down the necessary information in a PDF in a FSB thread referenced in a comment from a website someone else had to post. Schematics show how circuits work, layouts do not.

Does doing the LED's that way eliminate the popping noise?

He's talking about a circuit where LDRs do the switching, akin to what Mesa Boogie uses (used) in their amps. That way the op amp never goes open-loop and the transition is gradual.
A simpler solution could be to keep a resistor in the feedback loop and change the pots' and/or R5's value to preserve the same range. Or a make-before-break switch if you can find one.

[ElectricDruid]

Trouble is, the AnalogGuru links that it looks like the layout was derived from are all dead.

The best schematic I can find for the SDD-3000 preamp is one that Aion has done:

https://aionfx.com/app/files/docs/eclipse_legacy_documentation.pdf

This uses a bipolar supply though, like the original, whereas AnalogGuru had tweaked their version for a single 9V supply.

[duck_arse]

this thread is a winner so far. any chance of photos of what you built?

[ElectricDruid]

The problem seems to be that the OP is switching the feedback resistor in an inverting amp. Probably the amp goes for open-loop gain during the switching and *of course* the output flies towards one rail or the other. So, yeah, it'll pop like hell.

Solutions? Well, they've all been propsed:

Doing some "soft switching" thing with LDRs as suggested earlier would definitely work and be quiet. The On resistance would probably be <1K, which isn't much against a 100K pot value.

Keeping a resistor in the feedback loop at all imes, and then switching pots in parallel with it. This avoids the op-amp going to maximum gain and should avoid the worst of the popping.

Using a make-before-break switch. This also avoids the op-amp not having a feedback resistor briefly. Instead, it has two in parallel briefly, but that's not a problem since it *lowers* the gain instead of puts it through the ceiling.

Duck's right though. We could really use a decent schematic of what you've done and some photos of what you've built so that we know that the two are the same.

HTH

[PRR]

*of course* the output flies towards one rail or the other. So, yeah, it'll pop like hell.

-----------------
I hit post on that and it struck me: SERIES!!

[bluelagoon]

The series idea looks like a winner

[ElectricDruid]

The series idea looks like a winner

+1 agree, that is clever.

[amptramp]

Just as an aside, C2, C3 and C7 are electrolytics that spend half their time being reverse biased.  Once you go from a single-ended power supply to bipolar, you run into this problem.  The cure is to either to change the circuit so that these capacitors are always biased in one direction or use back-to-back capacitors of double the capacitance to replace them.  You can get bipolar electrolytics which are essentially back-to-back in values up to 10 µF and I have used them before.  You can get film caps up to 10 µF but they get rather large.

[PRR]

from a single-ended power supply to bipolar

You are correct on that; experience shows it gives no trouble for many years, essentially full cap life.

BTW: after a lifetime of wondering, I now think that the back-to-back caps should be "right size", not double. The back-bias of an e-cap is low-voltage but like 1,000X the nominal capacitance; the parasitic back capacitor is very thin. You would buy e-caps of 1.001X what you want to end up at; since tolerance and drift is far wider than 0.001, just get your value.

[MrStab]

Never trust break-before-make
Or else the pee pee you will take
Use fixed components, for goodness' sake
Or you will ruin your yummy cake

-William Blake

[stallik]

FWIW, Dirk Hendrik made a layout based on analogguru's circuit in 2008. The PDF includes the circuit diagram - from forum posts between them at the time, I think it should be close to, if not the original?

SDD3000_PDF

[antonis]

SDD3000_PDF

FWIW, C10 could (should) be omitted..
(or retained with a small value resistor right after buffer's output..)