Ruby amp build "farts"

[Tom Lauten]

Hi all,

I'm a relative newbie. I've built a Ruby amp based upon this board...

http://shop.pedalparts.co.uk/Ruby__Noisy_Cricket_Amp/p847124_12489371.aspx

And this circuit...

http://pedalparts.co.uk/docs/rubycricket.pdf

...generally it works but as soon as there is any real oomff the amp let's out a harsh burping, farting sound. It's tonal but harsh as hell...not really easing into any kind of over drive, just instant blapping.

Any ideas of what can be going on. It's frustrating as hell.

[Quackzed]

mine gets fart when the batterygets low, i kind of get the volume up till it farts then use it till it farts and lower and lower till i splurge on a good battery... what are you useing for power? try a fresh 9v bat, or a wart with more current maybee?

[Seljer]

I've had issues with motorboating on my LM386 builds. It goes away once I add a bigger capacitor on the power rail close to the chip.

On that circuit board it appears to be labelled as C1 and is 100uF. I'd maybe try another 100uF or 220uF soldered in parallel.

[wavley]

Let me use the search function for you...



Welcome to DIYstompboxes!

[Tom Lauten]

Interesting stuff.

I used a 12v dc wall wart (using a LM 386-3 chip), same sound.

Sorry for the dumb question...2 caps in parallel? How would that be done physically. Like legs on same solder pads? Would this knock down the volume?

Thanks guys!

[LucifersTrip]

Sorry for the dumb question...2 caps in parallel? How would that be done physically. Like legs on same solder pads?

Thanks guys!

yes...in parallel, you add the values...in the signal path, a larger val will generally give you more bass. In series, you need to make a calculation (and it's opposite for resistors)
http://www.electronics2000.co.uk/calc/series-parallel-capacitor-calculator.php

...and for your farting issue:

[Tom Lauten]

Sorry to be thick, but literally how would two caps be soldered in parallel?

[Luke51411]

Negative leg to negative leg, positive leg to positive leg.

[Digital Larry]

Motorboating aside, doesn't this circuit emit some nasty sounds when it clips?  Is that possibly what we're talking about?

[wavley]

Motorboating aside, doesn't this circuit emit some nasty sounds when it clips?  Is that possibly what we're talking about?

Yeah, in my experience it does.  We may be trying to fix a "feature" of the Ruby.

But the suggestions folks have been making should help minimize it.

Can you give us a sound clip?

[Tom Lauten]

I upped the C1 cap to 200 but no effect on that sudden "blatting" sound. I switched out the transistor and the chip. No luck. Seems pretty unforgiving and overly sensitive to me. Losing hope in getting a Ruby behaving so that I can hear it without it blowing out, let alone giving some OD type sounds.

[karbomusic]

LM386 can be finicky for this type circuit in my experience. I began with ruby, got a little frustrated with oscillations and other issues. Downloaded the LM386 and 2N7000 data sheets and designed my own, works like a charm. I say 2N7000 because I wanted an input buffer. Maybe I should dig that one up sometime and confirm the layout and post it here as a potential plan B since I know it works and I've seen a number of similar threads as this one. Btw, with proper power, mine doesn't fart even when overdriven into distortion. It does fart when underpowered or the PS can't keep up.

[anotherjim]

I don't know why it should make a difference, but LM386 data always show a 10uF cap in series with the gain control.
Another thing to try is putting separate power smoothing on the FET. That is a 100 or 220 ohm resistor from the +battery and then a 47 - 100uF cap to ground just before the FET. So the first pre-amp is isolated from power blips from the "power" amp.

What's the speaker impedance? If it's 8ohm, I think the 386-4 version is better suited (it's optimized for the higher voltage with high impedance load). If it's 4 ohm, I think you ought to lower the supply voltage. A 6volt lantern battery would be better -  it's only a poor little amp with no heatsinking.

[Tom Lauten]

Thanks Jim,

That all sounds hopeful but I must admit I am lost as to how/where to place the suggested components, I really am a newbie when it comes to the conceptual side of circuits and circuit diagrams. I can solder...at least I have that and a load of components! The speaker is 8 Ohm. I'll pop in a LM386 4 and see what happens. Could you be a little more "teaching the thickie twins" about where and how to place those smoothing caps? Are you talking about polarized caps?

I'd love to see your experiments Karbomusic!

[Luke51411]

Do you have a breadboard? IMO the best way to learn to understand schematics and where to place things in a circuit, parallel or series connections is to start with a simple schematic like a ruby, bass fuzz, fuzz face or something similar, and work your way through it on the breadboard. It did wonders for my ability to understand what connections are going where plus once you get a workable circuit you can tweak it to your liking much easier.

[Tom Lauten]

That's exactly what I was thinking last night Luke...too late for this, one though.

[anotherjim]

Hi Tom,

The power supply mod I'm suggesting is this kind of thing...

R12 and C6  are it.
What they do is smooth out any ripple and bumps in the power supply due to whatever the rest of the circuits are doing (whether that be flashing lights or fighting with an unruly loudspeaker.) They let the input amplifier do it's job without interference.

In the example above the power is at 24volt, so the extra resistor is quite high at 1K. For 9 or 12v operation, 270ohm (whatever you've got around that figure) for the resistor and 47uF for the polarized capacitor (or re-use the 100uF you added before).

Something like this...


You can also see a 10uF cap added in the gain control. This is something the chips datasheet always shows, but (to be fair) most small amp designs using the 386 leave out!
Not having it probably means a shift  of the chips internal bias when the control is at minimum resistance (which is max gain). This probably affects the distortion and may well be an intended feature - or it makes no difference at all.

This is what breadboarding is for - reinvent the wheel. But it's your wheel.

[PRR]

Does the '386 get HOT?

[karbomusic]

You can also see a 10uF cap added in the gain control. This is something the chips datasheet always shows, but (to be fair) most small amp designs using the 386 leave out!
Not having it probably means a shift  of the chips internal bias when the control is at minimum resistance (which is max gain). This probably affects the distortion and may well be an intended feature - or it makes no difference at all.

I'd use it, I'm a little ignorant but 386 is dying to oscillate in this type circuit and when I started from scratch things amazingly started working better when I started following the LM386 datasheet and ignoring schematics with shortcuts.  

For the cap strapped across the power rail, I suggest 100uF minimum or it will likely still scream/oscillate. Like I said, that IC is finicky in this type of configuration.

[Chaloney]

I recently built a fetzer-ruby and had this problem at first.  Then I breadboarded a standard rog schematic ruby an it worked no problem. I went back to the fetzer ruby and played around. The fetzer volume pot is 100k.  Switched that for thr ruby'so 10k, no difference.  Then I started playing with the trim pot. At 4.5v at the jfet drain, I got farts and squeals with or without a guitar plugged in.  As I decreased the drain voltage, it got better.  The further I decreased it, the higher I could turn the volume and gain pots before it made bad sounds. With the drain voltage at around 1.6 volts (iirc), I had the full range of controls without squealing and farting sounds.

To the op, I'd imagine you did this already but just to be sure, are you sure your battery is good?  Component values correct (I ask because at first I had all my caps off by a factor of 10--that didn't work at all--newbie problems).