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.)