Add-verb 1.5 “crackling” with Bass (solved)

[bherren]

Hi Folks !

So I built an Add-Verb (1.5) for my kid.

It sounds great ! … with a guitar (squire strat). But the boy is first and foremost a bassist and there things seem to go a bit pear shaped. Not much, just a bit 🙂

When using the bass (we tried a passive Yamaha, he also has a couple of active ones including a Stingray but seeing other posts about input clipping we wanted to try “mild”), and while the reverb works, we do get some kind of “crackling” sound, ie, some “pops” like a dirty vinyl coming in (and reverberating) or frying some bacon 🙂 It doesn’t really sound like input clipping, it’s not fuzz, though it could be clipping of some transcients, I don’t know. It’s also not continuous. Play a note, and you might get 2 or 3 occurence of that “frizzling” sound.

I can try to use the oscilloscope see if I can spot anything … but it’s odd.  Lowering the volume on the bass doesn’t completely fix it though playing more softly kind of does. Funnily, hammering the guitar doesn’t seem to cause any of it.

It almost feels like the BTDR doesn’t like low frequencies… That or we *do* get some odd clipping that gets turned into something weird by the BTDR.

As usual I’ve doubled checked and measured the components, so all R’s are within 1%, all caps within 5% (electos within 20%) etc…  I replaced R2 (27K) with a 50K trimpot, but that didn’t make much of a difference (ie, I can hear the slightly reduced or increased gain but that doesn’t “fix” the odd noises).

All the voltages look close to what’s in your build doc, but I can re-check (I initially did a quick check before boxing it and so before testing).

I will dig more myself in the next few days, but any hint would be appreciated. Thanks !

[Barry]

Is the Bass guitar Active?

If you are hitting the front end of the Reverb with high output active pickups that will do it. Everything you have described including the crackling, only happens with the Bass, and the results of turning the Bass volume down sounds exactly like input clipping.

If the Bass is active I would suggest that the reverb is not going to work well with it and is not the fault of the Reverb. The Reverb is merely amplifying and extending the noise being caused by clipping the front end. Same as would happen on a mixing desk or recorder.

[bherren]

I was expecting something like this which is why we tried with the Yamaha passive. We didn’t even try the active stingray :-). I didn’t expect clipping at the input but who knows… I’ll probe & scope it and play with the input gain a bit more than my little trimmer allows for

[Barry]

Try changing R2 to 220k and C1 to 100n.

[bherren]

Thanks. I’ll try that.

For my education… R2 to 220K changes the gain of the input op amp from 7.5 to 0.9 which is … rather drastic 🙂 I have R2 as a 50K trimpot today, but that didn’t do much of a difference. I’ll replace it with a 220K resistor, we’ll see where that goes (or a 200K trimpot if I have one).

As for C1 to 100n, won’t that just change the cut off of the input high pass filter from about 7Hz down to about 1.6Hz ? Am I missing something ? (I suppose R2 does have some effect on that input..)

Thanks !

[Barry]

Yes, go big to see if you notice and then go from there.

A trimmer would be the best bet if it is not socketed.

[bherren]

Yes I didn’t socket. I’m still curious about why the C1 change 🙂

[Barry]

No life altering changes other than my OCD.

It is preventing any unwanted DC current flowing into the circuit.

I am much more interested in your resistance change.

 

So what are your results?

[bherren]

I’ve put a 200k trimmer instead of my 50k one. I didn’t touch C1 yet, it’s tricky enough to desolder with the BTDR in the way 🙂 I’ll try it when my son is back, probably tomorrow. Thanks !

[bherren]

Works a lot better now with the gain adjusted. Thanks ! If you ever respin that PCB it might be worth making R2 a trimmer from the get go !

 

Thanks !

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