Little Gem MKII not working on the BB

[Kipper4]

That don't sound right.
If you smell a whiff of burnt flesh it defo aint healthy.
I guess the input cap is not ok. Does it have an input cap?
I go look at the scheme.

Hmm NO input cap....

[Ben Lyman]

Ya Rich, no input cap shown so I put a 100nF there but to no avail.
I still want to work on this because it intrigues me, it's very loud and sounds good enough for a solid state practice amp.

BUT... does anybody have a better project that will give me one or more watts in a mini amp?
I have a TDA(somethingorother) chip that is a possibility, I'll go look for it. What simple amps can be made with that, or any other simple amps at all? thanks

[balkanizeyou]

if you want to keep it simple, just get yourself a laptop power supply and use it to power a straight-from-the-datasheet circuit for the TDA chip (you should be able to get a few watts into 4 ohms out of the TDA2040 in this configuration for example). Add your favourite preamp in front of it and you have a really loud and cheap amp. Just make sure to use appropriate power rails filtering, because a lot of laptop supplies are not filtered too well.

[Ben Lyman]

Update: my guitar ground was janky, got the shocker sorted. I think I should be able to ditch the gator clip direct guitar to board thing anyway and go back to my regular set up with the jacks.

Still working great, chips get a little warm when I play loud but not hot to the touch.

Does anyone think there is a problem if I only use one cap across pin 1&8 and leave the other chip clean? 
ROG says put a switch on each 10uF to lift them and go clean. I like lifting one cap for a gritty OD. Is that bad? One chip at gain of 20 and the other chip at gain of 200?

Thanks balkanize you, sounds like a fun project. For a preamp in front of that amp, can I just make a one transistor distortion like an Electra and install it permanently in the enclosure with the amp?

[anotherjim]

Personally, I'd use one of the many amp sim pedal designs out there to front end a chip amp. What amp would you want it to sound like?
Think of the power supply first. If using something you already have, then you can select an amp chip to suit. 12v DC is good if you can get it, because there are all the car audio chip amps to choose from. A single ended 12v chip amp into 4ohm would be close to a 5W tube amp volume wise. PSU current rating 2A minimum.
Run the amp sim circuit off a 9volt reg & it should do its tube amp-like clipping before the power amp itself, which isn't very nice.

[Ben Lyman]

okay, thanks Jim, I'll keep that in mind for my future amp experiment.
For now, what about my idea of lifting the 10uF from only one of the LM386 chips in this bridged amp?
It sounds really cool to be able to get 2 different OD sounds plus the clean sound as a 3rd option by pulling both 10uF caps but I don't know if it is bad for something.

Something like this, a volume knob and two switches for Hi boost, Low boost, both boost, or no boost.

[duck_arse]

I lashed this to the BB today, but I have "GL386" chips. dunno how difference they are, but the work in the standard circuit. EXCEPT, I hadda pull both 10uF caps, otherwise it was just 'broken', loud cracks and blurts and no good guitar noise coming though. running dead cold, they worked with single 220nF cap same as a pair of caps.

and to answer one of my questions, with separate caps, one chip input read 13mV, the other 11mV (and my meter was reading 5mV as 0V). outputs were 4V35 and 4V16 into 4R computer speaker.

[anotherjim]

Ben, I don't think there's a problem if one amp is "hotter" than the other. If one of them was silent, it's o/p is just sitting at V/2 and just behaving like an active virtual ground.

[Ben Lyman]

hadda pull both 10uF caps, otherwise it was just 'broken', loud cracks and blurts and no good guitar noise coming though.

Thank you so much for testing it Duck! I really appreciate that, sounds like you got the same result as me once I ironed out the wrinkles in my breadboarding. No good guitar sounds with the caps in, only crackles and splutters. Did you try putting a 10k volume pot between the 220n and inputs? or maybe just a 10k resister right there going to ground would work.

At any rate, it didn't perform for you as shown in the schemo, so I stand by my claim: BS!
Who's next? anyone? Bueller?

Ben, I don't think there's a problem if one amp is "hotter" than the other. If one of them was silent, it's o/p is just sitting at V/2 and just behaving like an active virtual ground.

Cool Jim, thanks. I don't think it's exactly silent but I don't know the theory behind it. Anyway, I've been playing through it with a 12" 8ohm and it's still working fine. Super clean and loud with both caps pulled, add a 1uF to one chip and get nice bright sparkly OD, replace it with a 100uF and get big fat OD, then put 100uF and 1uF at the same time for big crunchy distortion.

What about if I build this thing with a power toggle switch and LED indicator? Should be okay right?
Also, it doesn't work if I run the +9v through a 1n5817 plus 47r before the circuit?!?! So, should I put a different kind of polarity protection on it?

[PRR]

> it doesn't work if I run the +9v through a 1n5817 plus 47r

A fully loaded 8 Ohm amplifier looks like about 48 Ohm to the power source.

Bridge-loaded makes that close to 12 Ohms.

A 47+12 divider puts about 1.8 Volts on the LM386s. Even they can't eat so little.

[Ben Lyman]

... Even they can't eat so little...

Thanks but do you mean "this won't work" or "it should work fine" unless I boogered something?

also, is there a better way to "polarity protect" and filter power. I'm not planning on building this with a battery snap, it's only going to run off a One-Spot 9v adapter
Thanks again!

[anotherjim]

A relatively high resistance in the supply is the reason you aren't cooking the 386's. Usually power amps have little or no series resistance, because it absorbs power you want to go in the speaker. If there is a resistor, it's low R with high power rating.

[Ben Lyman]

Ok, thanks.
So I'll not go with any resistor for power filtering, just the 100uF placed close to the IC's

Have you ever heard of placing the ground wire next to the ICpin4 and doing a "star ground" from there to all the other parts?

[anotherjim]

Correct. As direct as possible to chip-amp power pins and filter cap close to them. You "star" connect other parts (pre-amp especially) from the amp supply points via a filter R & C network as you do in a pedal. That way, any increase in supply ripple when the amp is working hard has little effect on the pre-amp. If ripple does reach the pre-amp, you end up amplifying the ripple (op-amps are not as fussy about this as discrete pre-amps) .

[Ben Lyman]

Cool, thanks again Jim. Just tried it on the BB and this amp is working great, even after I pulled my 10K volume pot... what was that I said about BS?
ummm... I'll take a big helping of crow with a side order of my foot, thank you! I guess I owe ROG an apology, sorry ROG.

It still sounds strange when I turn my guitar volume down and/or pick softly. It's like this fuzzy/static distortion that goes away when I turn up/play louder.
I'm hoping it's just some weird breadboard side effect that will go away with a proper layout.

I think for now I am going to go back to the simplest possible plan as a test run on the build, no volume no switches, just plug'n play.

There's one more issue that is baffling me... how can I install a 1/4" speaker jack so I can use this with my 2x12" cab?
Do they make plastic jacks with isolated rings or sleeves or whatever it's called?
or is there another way besides having a hole with two wires dangling from it?

[Kipper4]

"There's one more issue that is baffling me... how can I install a 1/4" speaker jack so I can use this with my 2x12" cab?
Do they make plastic jacks with isolated rings or sleeves or whatever it's called?
or is there another way besides having a hole with two wires dangling from it?"

Sure they do Ben




I did the same with my noisy cricket and used the switched kind so when the cab cable is plugged in the amps speaker is bypassed.

Bosh

[Ben Lyman]

Thanks Rich. Dammitalltohell!!! I just ordered a bunch of jacks from BLMS and didn't know I would be needing one of those 

[PRR]

> Dammitalltohell

So get an all metal chassis-grounding jack, solder wires, but DON'T mount it in chassis. Wrap with tape or mount in a plastic pill-bottle. Let it dangle outside the case.

By the time you get tired of the "ugly", you'll probably be placing another order somewhere.

[Kipper4]

> Dammitalltohell

So get an all metal chassis-grounding jack, solder wires, but DON'T mount it in chassis. Wrap with tape or mount in a plastic pill-bottle. Let it dangle outside the case.

By the time you get tired of the "ugly", you'll probably be placing another order somewhere.

I like them onions

[Ben Lyman]

> Dammitalltohell

So get an all metal chassis-grounding jack, solder wires, but DON'T mount it in chassis. Wrap with tape or mount in a plastic pill-bottle. Let it dangle outside the case.

By the time you get tired of the "ugly", you'll probably be placing another order somewhere.

If this idea came from anyone else, I'd be shaking my head and groaning. Here's a funny true story:

Every time PRR has given advice in one of my threads, I have either taken the advice and put it to good use...
or, not taken the advice and tried another way, then a few days later, sometimes a few months later, I have ended up, one way or another, doing it just as PRR advised me in the first place!

So, for now, I am trying to think of another way to do this but I know, in the end, I will be busting out a freakin' pill bottle and dangling it from the back of my amp!

So go ahead and shoot this idea down: I have a big plastic box to put the amp in...  What? Why not? RF? Electro-static from the neighbor's ferris wheel? What? Something else?