Noisy Cricket lo-fi mini combo

Starting a new thread for this, since it's really a separate project to the foot drum. But inspired by needing something to plug the foot drum/stomp box into, I have decided to build a super cheap, lo-fi combo amp based on the Noisy Cricket circuit.

First up, I found this old and ugly Sanyo speaker, made a long time ago in a universe far far away. Or at least from before this sort of cheap home stereo gear was made from moulded plastic.
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The 6" 5W speaker is perfect for now (a standard Noisy Cricket puts out 0.5W). If I want a better sound later I can swap to a car speaker, or even a 6" guitar speaker (but only if I can get one dirt cheap).

I did an initial test using an old headphone practice amp I built some years back. It sounded surprisingly sweet although the volume was very low. Even though the amp is based on the same chip (LM386), it is designed only for headphones and has a much lower output.
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I am thinking of top-mounting the controls, input jack at bottom right, and DC jack at the back, with a fake control plate made out of metal that I can screw to the speaker enclosure. I am not sure what to do about the ugly box. I can embrace the brown and leave it as is (apart from a clean), I could paint, or cover in something else if I can find some vinyl on the cheap. Open to suggestions.

The circuit I am building has a switchable bass cap as well as a passive low-pass tone control. If that's not enough EQ I will adjust later. I don't think the cheap little speaker will deal with too much bass anyway.

DC

I'm actually super into that box - it's the kinds of ugly that's so bad it's good

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Originally Posted by JohnH

I'm actually super into that box - it's the kinds of ugly that's so bad it's good

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I am leaning towards keeping it as is too. The knobs I got on the $5 broken Kustom 30W chassis are exactly the right sort of brown as well, so long as I can find chamfered shaft pots (all my current stash have knurled shafts).

DC

I know you're looking to do this on the cheap, but if the amp intended for just the foot drum/stompbox, you could go with a 4"subwoofer/driver LIKE THIS

20 bucks and it would have more thump than that Sanyo will give you. My (budget) studio monitors have 4" drivers and they really kick.

Good idea. Which reminds me of something important. Out in the shed I have a crate of e-waste that I need to drop in at the recycling centre. One item in there is an old PC gaming subwoofer. I thought of using it but it only has SPDIF digital inputs. However I might be able to open up the case and liberate the driver itself.

Of course, if all that fails, I will just get one of the drivers you found for me. It's not that I can't afford to buy a speaker, it's more the challenge of building a mini amp from mostly reclaimed parts.

But the amp won't be just for the stompbox, so now I am wondering if could run the sub driver in parallel with the other speaker, or if I need to build a crossover. Or maybe I could just put in a switch to select bass or normal mode.

Forgot to mention the thing I learnt tonight: always score the copper side of a stripboard before snapping it to size. I ruined the trace on the edge of this one. I can still use the rest for other things, but I had to cut a new board for the amp.
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Tore down the old gaming subwoofer just now. There are a few nice components I can salvage for projects, but unfortunately the driver is not so nice. It's using some sort of flexible foam/plastic around the edge of the cone to provide flex, but it has perished and the slightest touch makes it crumble. I hooked it for a test and it sounds, well, like a torn speaker. But not a nice fuzz, more like a flatulent frog. Definitely sticking with the original Sanyo speaker for now.
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Board done. The two resistors end to end is where I soldered 2 values in series to replace a value I don't have. Apart from that, it was plain sailing. Didn't start the off-board wiring since that needs decisions about how to wire up the cabinet.

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Looks great - have been thinking about making a small practice amp and looking at the diyguitarpedals chime amp - my attempts previously with stripboard have been.... well.... awesome - if the success criteria is tuning in the local ABC - not so if the success criteria is a fuzz pedal.

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Originally Posted by Brendan

Looks great - have been thinking about making a small practice amp and looking at the diyguitarpedals chime amp - my attempts previously with stripboard have been.... well.... awesome - if the success criteria is tuning in the local ABC - not so if the success criteria is a fuzz pedal.

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Radio interference is a common problem, particularly in fuzzes. The op amp circuits are much less prone to it. I get non-specific noise (ie not a radio station) on breadboard circuits but haven't had a problem with stripboard at all. According to AnalogMike, a small capacitor from the signal input to ground can help filter out these very high frequencies.

The chime amp would be a good option if you want to avoid stripboard.

My next step on this build is to cut a hole in the enclosure top. I have some thin MDF that I will cut to size and paint to use as a control panel. The cabinet walls are too thick but I don't want to put everything on the back panel. If electrical interference becomes an issue I might have to do something creative with metal foil, glue, and tape to shield inside the box. I am hoping the low-gain, low power circuit wont be too prone to noise.

Spent a while tonight playing around with Inkscape, slowly getting more productive with it as I learn more features. Made a 1:1 scale mock of the control panel that is going to go on the top of the speaker enclosure (for the wood texture I just took a photo of the speaker and loaded it into Inkscape):
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Once I have the layout looking good, I create a drilling template in another layer. This template gives me the location and size of the hole to cut in the speaker enclosure, the size of the MDF sheet that I am using to make the control panel (no metal sheets on hand), and the drilling template for the MDF:
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This is the same method I have been using for pedal templates lately. Mock layout leading to the drilling template and the decals.

Two gains? Confusing without more explanation. If you can, I'd move the power switch to the other end so that the gains canthen line up with each of the jack inputs (if that's how it works).

Yeah, I can move everything around.

I could also leave power where it is and move the guitar input to the other end.

The 2 gains come from the two circuits I am building in. The Noisy cricket is the group of 5 controls on the right + the guitar input. The piezo preamp circuit is the preamp input and preamp gain. When the preamp jack is connected it runs the preamp into the main amp.

I can also rename the preamp level to something else, like "level". Maybe also draw boxes around the two control groups to make things a bit more clear.

I'd certainty label the piezo input with a Piezo tag. And boxes around things always helps IMO.

Revised :)
Since it's MDF, I have mocked up 3 colour options based on paints I have around. I can't print white decals so can't go too dark. Leaning towards the brown myself.
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Yeah, I'd vote for brown too. New layout looks good

Whilst the piezo control may strictly be preamp gain, as it feed straight into the power amp then in reality it is affecting the channel volume. So maybe change that control to just say 'Volume'? It's what you'd get on a non-master volume amp. And it stops anyone thinking that the 'volume' knob on the other channel affects the piezo channel volume.

I like the yellow myself. Too many acoustic amps have awful brown fascias.

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Originally Posted by Simon Barden

I like the yellow myself. Too many acoustic amps have awful brown fascias.

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It's funny. Since posting I was thinking gold actually, which is sort of in between the yellow and brown. And since I am out of waterslide decal sheets, I am thinking of doing the control panel with decoupage followed by satin poly.

And yes, I have found it a bit frustrating getting the names for the controls right. As you identify, the preamp has a volume or level control. Gain is the wrong term, since the op amp is set with a fixed gain of 3.2. This is well within the range of the TL071 so there should be no clipping at all. The pot is a standard output volume control.

For the noisy cricket circuit, the gain pot is true gain, controlling the amount of op amp gain while the volume is again a standard volume on the output.

Now I am wondering if I need the preamp volume at all. Since it is feeding direct into the power amp, I should have enough control over the levels through adjusting the power amp gain and volume. If the piezo input is a bit high, reducing the power amp gain should compensate just fine. Unless it is too high with the power amp gain all the way down, in which case the output is high enough that it could simply plug straight into the guitar input.

I will experiment with this. If I do end up omitting the preamp volume I will need yet more shuffling of the control panel.

Part of me is thinking that I am completely overthinking a $20 DIY amp, but the other part of me is thinking that the detailed planning, design, and tinkering is the best part of this hobby. No prizes for guessing which part is winning!

You want/need to get it right! So that when you put it all together you can rest at night and not worry about what you should have done differently.

True Simon, true.

I am putting the control layout on hold for now. I need to finish building both circuits so I can test whether I can skip the preamp volume control or not. Got a bit distracted tonight starting to tear down and reverse engineer the Joyo pedal for BDs new pedal.

The coping saw I bought for the tele headstock worked nicely for this job too. This is the cutout for the controls. It will be hidden by the MDF control plate which I also cut today. I didn't drill it though since I haven't tested whether I can omit the preamp volume or not.Attachment 35535

Made a bit of progress wiring up the off-board stuff. Still haven't drilled the panel, but this is complete enough that with some alligator clips I could test.
The amp sounds OK. A bit disappointing but OK. At higher gain settings it starts to clip in a solid-state not-so-pleasant way but that is avoidable by winding the gain or guitar volume back a bit. The weakest point is the speaker. It sounds like playing a guitar through a hifi system, which I am sure we have all tried at least once. I tested all three spare speakers (none of which are actual guitar speakers) and it made a big difference. Oddly, the best sounding speaker of the three is the old torn subwoofer. It fits into the cabinet so I will start with that but keep my eyes open for something better.

I am not sure it's going to work that well for the foot drum though, so I have decided to build the piezo preamp into a pedal enclosure instead. That way I can use it with any amp.