Onboard Active preamp for guitar?

[nopainkiller]

Yes, I have the zobel network on it. The schematic, and a pic are on the beggining of the thread. I ordered some heatsinks that are supposed to fit the lm386. I also heard of glueing pre 1982 pennies together with epoxy and using that as a heatsink.


The unheard oscillation is something I've never thought of. I alwyas just play for a while and then feel the chip. It doesn't burn me, it's just really hot. It shuts off before it gets TOO hot. This depends on which brand of bettery I'm using. Regular rayovacs seems to play non-stop, and get the chip warm. The rayovac plus play, but make the chip very hot. Duracell makes the chip so hot it cuts out after a minute or two. Energizer takes a lil longer. Lithium doesn't work .

[nopainkiller]

the little gem design has the zobel network, I used a .047 cap. But instead of a 220 uf cap where it says to put one, I did a 470 uf cap for more bass. Could this be the reason??

[nopainkiller]

I would like to incorporate LED lights into my amps. The one in my guitar, and the ones I make. Where would I place them?

[Enzo]

What do you want the LED to indicate Just power on? Blink on peaks? Flashes at low battery? That LED will be a few more ma from your battery though.

And you do understand that adding a heatsink to the chip will not reduce the current it draws or increase the battery life. It will just make the chip run cooler.

[Robert M. Martinelli]

Hi,
Just a thought.... it seems you like very much the way the guitar sounds, so, if you need more power, there' s a very simple way to prolong battery life - use a power supply!!

Build/buy a stabilized and well filtered 9 VDC power supply with a current rating matching your LM386 needs, then build a special signal cord for the guitar, this cable should carry your audio signal and the supply for your 386 amp, so there actually are two ways of building it : you could either use a 2 conductors + shield cable ( e.g. microphone cable ) and stereo plugs, this way you will have your audio signal ( plugs' tip ) from the guitar running between one of the conductors and the shield, and the power supply to the 386 ( plugs' ring ) running on the other conductor and the shield. The other way is even smarter, as you could use a normal mono guitar cord adding two decoupling caps in series to each end to separate the DC supply from the audio signal, and having both run on the same conductor. The only caution is to use big enough capacitors to allow the audio signal to pass through completely ( low Xc within the guitar bandwidth ), I guess anything around 100 nF will suffice.

So much for the "amp" question - As to the preamp question, I' ve designed and built many units in the past, using BJTs, FETs and opamps, I have found that an NE5534 or an OP27 yield good results, their noise figure is low and the preamp would also have a lower output impedance, eliminating your worries about losses across the cord.

Hope this helps

Best regards

Bob

[nopainkiller]

Thanks, And hellz yea VOX RULES! I'm gunna get the English Channel kit and biuld that. My first kit, since I was sposed to start there.

I'm not concerned with batteries, as much as I am the chip just cutting out after getting too hot. And I think the batteries might not have have benn draining too fast, just the chip went into shut down, aftern getting hot. Look at my shcematic. I did not use a bypass on the 7th pin and also used a 100uf to filter the power. Was I supposed to use a 100nf?????? Also, I used a 470uf instead of 220 after the zobel thingy for more bass. I wanted it to feedback jimmi style. Are any of these things just pushing the lm386 n1? Because this thing is rated fine for 9v.

With my freaking bass cut, parralell bridge turned a little down with the treble bleed mod, It sounds soo so o soo sweet. Like an acoustic with the mid cut, and I can do this cool thing when I wobble the mid cut from 0 to 3 back and forth. It's tremlo!! It's such a cool toy. I want it to run long enough for me to get bored of it!!!

[nopainkiller]

ACK! I just noticed from the pic I posted that the .047 uf cap is actually marked 472! for .0047!!!!! Could this be detrimental to everythignZ?/?! The one at the zobel netowrtkz!

[Robert M. Martinelli]

That RC network is supposed to block self-oscillation, by using a 4.7 nF instead of a 47 nF cap you shifted the cut frequency ten times up, the RC networks acts by shunting to GND all the frequencies higher than 1/2pi(R*C), being a "high pass towards GND" ( so, actually, it shunts frequencies over F0=1/2pi(R*C) to ground ).

As to the output cap i think that anything larger than 220 microFarad is useless,( unless you are willing to amplify the low B of a 5-string bass ), also, I don' t know exactly about the 386's capability to drive large capacitive loads, but there is a chance that with a 220 microFarad the 386's life will be easier...

Hope this helps

Best regards

Bob

[nopainkiller]

I just performed quick style er surgury on the my ricky cop amp and didn't ike the way it sounded with the .047. So I put the .0047 back in the zobel network. Also replaced the .047 cap on my assignable tone knob with a .039. I couldn't find the .022 that I wanted and the .039 seems to work alrite. I was getting way too much cut with that .047, more than I did with the normal linear tone pots I had before I switched to the 500k push pulls. But I like being able to choose which pick-up gets the mid cut and/or tone. Bass cut is master. I love this thing! I'm considering phase on top of the bass cut, but don't wanna mess with it. And since it's a ric copy don't wanna mess with the sexy. A little dpdt switch wouldn't look too bad. I already considered where to put when i was thinking about the strawberry ice passive distortion. But I tried it and my pick-ups aren't hot enough for it or something. nothing like the sound clips.

hmmm phase or no phase. Anyway, after I decide, then I will shield evrything to the pickgaurd instead of the bridge. This will be a pain in the ass considereing and I had to drill out bigger holes in my knobs and super glue some to the weired push/pulls I got at smallbearelec. Maybe I shoulda got them somewhere else.

[nopainkiller]

put the slide on heatsink. the dinky liittle thing gets hot! Maybe it is trying to put out 50hz as hard as it can, or somethin. how can I check this?

[nopainkiller]

You were right about the oscilating thing I thnk. I just had to take the whole thing out to diagnose why it cut out. I was playing it and the speaker lead came undone somehow. they are barely there, the speaker is really delicate. I fixed it, who knows how long it will last. It's the only 8 ohm 1 watt I have, the other one broke the same way. Could be the vibrations.

The heatsink gets really hot. Dunno if it's really doing it's job. It slides under and over the lm386 and then folds in a few directions with flaps. It looks cool, but I don't think it's meant to cool the kinda heating I'm getting.

Anyway, when I was looking at the speaker, when I just turn it on, the speaker pops out. Like it's putting out a signal all the time. but I can't hear nothing. Could u take a look at the circuit?? The error in the schematic I drew is that the .047 cap on the zobel is actually a .0047.

[Robert M. Martinelli]

Hi,
well, if the heatsink is getting hot this means heat is being transferred from the 386 to it, either the heatsink's too small or you have too much current flowing....check the 386's current drain under working conditions with a DMM in series to the PS.

As to the speaker, the 386 can give a continuous 1W over a 8 Ohm load, so I wouldn't mind having a speaker capable of handling at least twice the power....The loud "pop" you hear is most certainly due to the output electrolytic charging during powering up, so I wouldn't be particularly concerned about that.( I recommend to lower that capacitor's value, though, as you know the speaker's impedance gets lower at very low frequencies, so there is the chance too much current flows through the 386 output stage, and this could also be the reason for the chip to overheat, I would definitely stick to the 220-250 microfarad the National datasheet recommends, after all you are playing a guitar through it, not a 5-string bass ).

As to your question about how to check for oscillation, an o-scope is practically the only option, if you' re going to keep on building and experimenting with electronics you will face many situations where only an o-scope will give you the answers you are searching for.

Hope this helps

Best regards

Bob

[Enzo]

Aha.

If it were oscillating, it would be doing so at a high freq above your hearing. The speaker couldn't move that fast and would seem to just sit there.

When a speaker moves out - or in - and stays there, that is not oscillation, that is DC on the output. And if your little amp chip is putting out DC into the speaker, it sure as hell WILL get hot.

Either the 386 is bad and putting out DC or there is an open in the feedback loop or something similar causing it. or even DC sitting on an input pin, and the 386 is just trying its little heart out trying to "amplify" that DC signal.

You want to check for high freq output without a scope? tack this little circuit together:

from the output run a diode to a small cap and return the other end of the cap to ground. Use something that will handle the circuit voltage - and at 9v that should be easy. Now measure DC voltage across that cap. What you did was rectify any RF and then filter it with teh cap. Any RF present then woul dbecome DC in this little rectifier circuit.

BUT

This assumes there is no DC already on the output. This "RF detector" cannot ignore DC coming through.

Get rid of your DC problem first.

[Robert M. Martinelli]

Oops...
Sorry Enzo, I misunderstood nopainkiller's statement about the speaker, I thought it was only "popping" when powering up, reading your answer and going back to nopainkiller's last post now I understand the speaker moves out and stays there....you are right, there must be some DC on the speaker, either the 386 or the output cap is bad.....

....And I thought my English was improving.....I was wrong, I can't even read!

As to the probe, I have built some RF probes with coils, caps and diodes, and they' re good if you don' t have a scope, but, OTOH, I think that anyone willing to get seriously involved in electronics will, sooner or later, have to get himself an o-scope.

Bob

[nopainkiller]

thanks. I guess I will try changing it back to the correct value. Altho, I already did before I realized it was the problem, and thought the sound was a lil darker. Oh well. If it will fix the problem. But there is also suppsoed to be a cap on the 7th pin, that I've left out. I don't even ground the 7th pin. I think the value for it is sposed to be 100nf. What's the purpose for this???