Onboard Active preamp for guitar?

[nopainkiller]

I'm looking for a good small preamp design I could stick in my control cavity of my rickenbacker 330 copy. It has humbuckers, so I've rewired it a buncha times differently and like how this sounds. I also built a little mini lm386 little gem amp and stuck it and a little speaker in my hollow body. I think there is a short circuit somewhere (i've checked time and again) because the chip gets hot, and battery drains faster than I expected. It works great tho.

But I really want to find a preamp to compensate for the capacitance of the cable, and the circuit. One that would preferably run on 2 3v calculator batteries or something similar. I don't know where the hell I'd fit a second 9v battery. Just something with enough power to make up for the load. Any ideas or suggestions?

Here is the badly/quickly drawn schematic/diagram, it might be a little off, but you can see what I've got.

[nopainkiller]

here a pic of the amp. I have pics of the guitar and other picks of the wiring and amp if anyone wants to see.

[nopainkiller]

anybody??

[jrfrond]

Don't know about the preamp, and my first impression is that you probably don't need it. BUT.....if your LM386 is running hot but working, there is no short. Firstly, you probably don't have adequate heat-sinking for the chip, and secondly, 9V batteries are not designed for that kind of heavy, extended current draw. You either need to fire that up with "C" cells or an outboard AC/DC adaptor that can supply the correct amount of current, or purchase stocks in Duracell or Eveready!

LM386's tend to run hot, and it will go into thermal protect if it goes over the rated temperature spec. However, if it were my project, I'd get a heat sink and epoxy it to the top of that chip.

If you followed the plans from the Craig Anderton book, which uses a lot of foil area on the PCB for heat-sink purposes, it will still run hot. An add-on heat sink is the only efficient way to keep the chip cool.

[nopainkiller]

Ah, thanks. a heat sink. I've heard this before, I thought it would be too big to put in. I can get one small for an IC chip like that?? Also, Ib wouldn't be able to fit a C battery in the hollow body.

In the other forum I post in (i just joined this one) some people said there could be a short. I looked and resoldered grounded, and even all the leads, and still it get's hot and battery runs out.

Here's the forum link, so you can see more pics, and stuff. The pic of the perf board was too big to post here.

http://homerecording.com/bbs/showthr...=1#post2986774

What kind of heatsink should I use? I'm totally new to this. Thatt's the best answer i've ever gotten. I should've started here. The other forum is a home recording one.

[Enzo]

I wouldn't expect a lot of battery life on something driving a little speaker. But a basic preamp based upon a couple op amps should get boo coo battery life.

[nopainkiller]

AH, ok, so basically low expectations of the battery life is how it's just gunna be, right? I can get the heatsink, but other than that, I'll be able to get a half hour to an hour of straight jammin with all that feedback. I bypassed the volume and gain pots on the amp so it's just an open chip, I also used a larger cap caping the out to the speaker, for more bass, so that uses battery I bet.

It's still cool tho, I can play a couple songs, it could run for an hour. and I just wanted something small to add to it.

But the preamp it really what I'm interested. Just something enough to boost a little bit. I;ve seen some designs but don't know which would work for what I want!! If a 9v really would last a long time on a small preamp for the signal I will use it, but I'd preffer smaller batteries.

Here are some designs I've come across. Some aren't op amps, because people say bad things about them. I haven't biult any so have no idea. But the ones without ic chips are said to be better accordging to some here. An FET preamp. I don't know which one of these would best do what i want

http://www.till.com/articles/GuitarPreamp/

a few here

http://www.harmony-central.com/Guita...uildPreamp.txt

these:

http://www.zen22142.zen.co.uk/Circui...o/lvpreamp.htm

http://www.uoguelph.ca/~antoon/circ/preamp.htm

also this is where I saw that you could use 3v lithium calc batteries. And joined this forum to ask about it. It's in the old archive, 1999.

"Although pickups don't have gain, and normal passive controls eat signal in order to shape it, most of the attendant problems can be surmounted by use of a simple onboard preamp, something with just enough gain to compensate for the effects of passive tone controls. If you use a discrete transistor pre-amp, these can run off of minimal current, and voltages lower than 9v, so you don't need to route anything or have a large control-cavity to take advantage of them. A transistor, couple of caps and resistors, couple of 3v lithium calculator batteries, and away you go. "

http://archive.ampage.org/threads/0/...control-2.html

[Enzo]

Well, I have nothing against op amps, the mixers and recording gear you listen to music through every single day are just full of them.

But I have nothing against discrete transistors either.

These are simple little basic amplifiers you are presenting. The parts add up to a dollar or two each, build them all. get some perf board or little experimenter circuit boards from radio shack and make a few types. See if one stands out over the others in some way. Then pick one and shape it up for your actual installation.

I don't have an op amp circuit on hand, but a simple one chip circuit could be found in many places. Beware when reading articles from 1992 or even 2002 about 1992 circuits. Parts change. I don't even know where I'd find a 741 these days. That old op amp is like three generations of op amps ago. That is like comparing mom's old table radio to the latest radio in a new car - the one that picks up stations from way far away.

[jrfrond]

In a dual opamp system (take your pick of any garden-variety dual opamp), and IF you were using stock tone controls, you would want to use one opamp for the pickup input, with the output of that feeding the tone controls, and the second opamp would be set up as a buffer. The high input impedance of the buffer would eliminate loading of the tone control, and the low output impedance could effectively drive a cable of a couple of hundred feet if necessary. You could also use a simple single-JFET source-follower amp to do the same thing, right at the guitar's output, since line loss seems to be one of your concerns. Then there's the Alembic system which uses a balanced instrumentation amp for each single-coil pickup to cancel hum and a state-variable filter section for tone-shaping.

Yeah, lotsa stuff. What I am getting at here is that there are seemingly limitless ways to add active electronics to a guitar, from the simple JFET amp all the way up to console-quality configurations like Alembic and everywhere in between.

Back to your onboard amp: your battery life sucks because (again) 9V batteries cannot meet your current demand. Small batteries = small current delivery. If you aren't going to use 4- "C" cells or an outboard PSU, I'd scrap that system. You might also try lithium 9V batteries, but they are pricey even at wholesale cost.

[nopainkiller]

I guess, I'm not doing the preamp for my guitar. But I'd still like to see if I can get more life and playing time out of the amp I got inside my guitar. The one driving an 8 ohm 1 watt speaker. The amp is 1/2. The chip gets hot and I think that's why it gets all scratchy, fuzzy, and high ended, then finally cuts out. But then I wait and hours later I can play again for as long as before before it cuts out again. Takes a while to figure out if the battery is dead or if it's just too hot.

Is there a heat sink available for something like the lm386??? The picture of the amp is at the top of this thread. So I'd need something I could install onto that. Is there anything I can do?


Another question, there is a passive mid cut on my guitar, a 500K linear pot, with a 220K resistor and a .039 cap in series, then a 1.5 milihenry inductor. I was wondering, how changing the resistor and cap value would change how this works. I'm thinkin it was designed for a 250k pot, so I was wondering, would I cut more lows with a larger or smaller resistor? Also would I cut more or less highs with a higher or lesser cap. I've experimented a little, but would like some experienced input before I do it some more. It's a pain the ass, and It's getting sloppier each time I redo it.

Sorry, i'm new to this. Thanks for all the input and any more in advance!!

[nopainkiller]

grrr I can't even get through one song sometimes!!

[nopainkiller]

maybe a different kind of battery? I tried lithium, i dunno if it was the brand or what, but it plays for 5 or 10 seconds then buzzez out faster than the dying duracells and energizers. The rayovacs take the longest. I just bought these special alkaline rayovacs. Rayovac ultra pro. wutever that means. I haven't played it till it cut out yet, but the chip is still getting hot, I can touch it. Maybe the rayovacs work better because the output is less on them? So maybe the lithium was putting too much out and heating it up too fast, same with the duracell. Duracell, seem to work the worst after the lithium. Energizer is fine. But the regular rayovacs last the longest. I don't really get cutt off, and the battery just dies....I looked up some things to try and figure out this stuff. I bought some rechargable 9vs, two diff types, so they'll have diff charges i guess. But then i read that they might last even less. hmmmmmm

I also tried cheap rayovac zincs. they lasted for about 10 seconds, just like the lithium. This is wierd. Could I have just bought a cheap one? "ULTRALIFE lithium battery cell".

Any ideas?? It's just the 1/2 watt little gem design from runoffgroove.com. But I left the gain open to 200 with a 100 uf cap. And no volume. Also 470 uf cap on the output for more bass. 8 ohm 1 watt speaker. Could it be faulty wiring? Or is this just how much the thing would draw. And then heating the lm386? A heatsink would help eh? If I can't find one, I could fabricate one, no? I have some thick metal from an old broken amp chassi. It was screwed to a tda2030, which someday i hope to turn into something.

So many questions, I'll just put it into one:

What would you guys do if you had put this in the guitar, got it to sound just right, and loved it so much, and played the guitar so much more for it. And really won't take it out, but have all these problems????!

thnk you!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

[6267]

Ok, well, I'm not a chip amp expert, but it sounds like your supply of current does not match your demand. Time to increase current supply! So, get four lantern batteries, 6v each. Series/parallel them for 12v double current. Replace the mono jack on your guitar with a stereo one. Build a little "stomp box" which has a stereo jack on one side and a mono jack on the other (in case you want to biamp ) the batteries will connect to the second (i.e. not tip or sleeve) wire in the stereo guitar cable as +, -/return being the shield of the guitar cable. If you were to plug a 'normal' guitar cable in, the chip will not work (the second channel connector gets shorted to shield), but with the special cable and supply box (easily concealed in a backpack), you could play for days (or at least an hour or two! hope it's not a short!). You will of course have to adapt your circuit to use the 12v supply. More headroom at least.
Good luck,
(disclaimer, I'm a tube kid and very amateur engineer that's never built a chip amp in my life!)

[Enzo]

If your output chip is getting too hot to touch, it certainly isn;t the battery's fault, and for that matter, all that energy to make the IC that hot has to come from somewhere - the battery - so short battery life is not surprising.

Without digging back through the thread, either the chip is bad, or the circuit around it. And possible problems are a lack of proper bias to the inputs or the thing might be oscillating at ultra high freq you cannot hear. If it is trying to output 50kHz as hard as it can, it will overheat and you wown't hear it. SCope it.

[Steve Conner]

Does it still get hot with the speaker disconnected?

Do you have the Zobel network on the output, as the LM386 datasheet (http://cache.national.com/ds/LM/LM386.pdf) recommends?

(The zobel network is the 0.05uF cap and 10 ohm resistor in series from output to ground. If you leave it out, the chip may oscillate, which causes, you guessed it, heating and excessive current draw. Yes, a 0.047 will do.)

[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???

[nopainkiller]

Oh jesus, I guess I can't read, either!

SO it's NOT the cap on the zobel network??? Or the 7th pin bypass??

How is this problem occuring. I don't understand! I'm sendning current from the battery straight to the speaker and not back???

[Robert M. Martinelli]

Ok - I guess we need to clarify some points.

First of all, measure if you have any DC voltage on the speaker's terminals.If so, go one step back and measure if you have any DC at the 386 output. The cap is there by design to emulate half of the supply voltage to allow the 386 to operate with a single voltage.

second - does your amp have a capacitor connected between pins 1 and 8 of the 386? if so, the gain is not 20, but 200, and, if so, pin 7 should be connected to GND via a bypass cap to improve stability.

Third - the 220-250 microFarad capacitor I was talking about is the one in series between the 386' s output and the speaker, not the bypass cap on pin 7.

Follow the link to the 386 datasheet Steve provided in one of the first posts, and read carefully the application notes for the 386. Check if you followed all the recommendations.

Hope this helps

Best regards

Bob

[nopainkiller]

ar gh I don't have a meter or anything. This was me dipping my feet in the waters! I was so happy it seemed so easy. Now I got this DC thingy problem I don't get. Will replacing the zobel network cap fix it? Or do I need to put a cap to ground from the 7th pin?? I do have a 10uf cap from pins 1 -8 for 200 gain. Is this my whole problem? Arrrgh! I've seen the circuit a billion times since, taken it out and changed a few things. There wasn't anything connecting that wasn't supposed to be. I don't see what this DC thing could be from! If I did get an ohmeter or whatever I'm not sure I'd know what to do with it argh! Putting a cap from 7 to ground won't decrease the amount of feedback and wailing I get, will it?

I just made a smokey external amp out of a lm386, two 47uf caps and a 1k linear pot and a pack of marlboro red shorts (small pack!). spdt on switch. No output jack. It was a bi*ch getting it all in there and getting it to fit, but I did it and it works great. only four parts compared to the little gem I got in my guitar! Just a 47uf cap from the output to speaker +. And a 47 uf cap from 7 to ground. 1k pot from 1 to 8 for gain. I guess this is actually my first succesfull fabrication!

[nopainkiller]

I rebiult that amp in my guitar, and it works perfectly now. I've buitl someother 386 things since, and have had some success. This time I built a bridged 386 design, and stuck it in a lunchbox. I'm getting the same dc across the speaker problem. I have a meter. And can test for the dc like you told me to, i just still don't understand how to solve the problem!! What do I do about the dc?? How do I fix it once I find it?! Please hrkp!!!