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Author Topic: Finally finished my LM3886  (Read 4616 times)

mydementia

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Finally finished my LM3886
« on: June 22, 2007, 03:32:11 PM »
Hi guys.
Just wanted to send out a special thanks for all your help in getting my LM3886 chipamp going.  I spent this morning assembling/wiring it.  Worked great the first time I turned it on!!  I wired the level pot backwards (no biggie) but everything else was just fine.  Very neutral clean tone - really shows the speaker/cab/guitar 'sound'.  I was a little surprised that there wasn't more output volume available - but I didn't mess with the I/O resistors... just added the level pot (not like there's not enough volume...).  I'm sure it would be louder running into 4-ohms instead of my 8-ohm Swamp Thang...
I put together a little webpage documenting my components for future newbie builder reference.  Have a look.

http://www.freewebs.com/mydementia/LM3886/myLM3886amp.htm

Thanks again!!
Mike

teemuk

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Re: Finally finished my LM3886
« Reply #1 on: June 22, 2007, 05:30:11 PM »
I was a little surprised that there wasn't more output volume available

That's BrianGT kit, right? That is configured for gain of about 32. If you consider that nominal output amplitude of magnetic guitar pickup is at it's best only about few hundred millivolts that ain't much. For example, 200 mV * 32 = 6.4V - ain't going to result into high amount of output power.

You need a preamp that boosts the gain and makes an appropriate impedance match since guitars should be plugged into a device that has substantially high input impedance - i.e. at least over 200 kilo-ohms, preferably more than 1 megaohm and with most piezo pickups at least 10 megaohms. Anything else results into decrease in bandwidth and often kills the pickup's resonance that gives it its individual character. Those chip power amps by themselves really aren't optimal for guitar use.

Edit: Dear people, realize that 99% of those "gainclone" kits are designed for consumer HiFi systems not for guitar. Yes, they are good power amplifiers and there is nothing wrong with them in general. It's just that a guitar is far from being a signal source that is comparable with your average CD player. Due to this reason you need a preamp that acts as the interface between your guitar and the power amp.
« Last Edit: June 22, 2007, 05:40:25 PM by teemuk »

mydementia

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Re: Finally finished my LM3886
« Reply #2 on: June 22, 2007, 06:02:02 PM »
Yes, it is a BrianGT mono kit.

So is this circuit designed for a line level input (as opposed to an instrument/guitar level input)?  I read several posts over at diystompboxes.com from folks (including Joecool85) who built this chipamp for guitar use... it's interesting to hear tha we're all using it 'wrong'. 

Won't running the guitar into an effect essentially provide the power amp a signal with the appropriate impedance?  Most of my DIY stompboxes have a 1M pulldown resistor on the input (to avoid loading the signal).  Is this sufficient?  If not, could you point me to a good, clean/transparent DIY preamp (post a link?)?  The main goal of using a chipamp as a guitar power amp (in my case, anyway) is to amplify my guitar and effect pedals without coloration.  I could see using a clean MOSFET booster, like an ROG Omega, for the clean 'channel' and one of my SansAmp GT2's for the distorted sounds... or am I missing the point?

Thanks for the feedback... so much to learn!
Mike


teemuk

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Re: Finally finished my LM3886
« Reply #3 on: June 22, 2007, 06:42:25 PM »
Input sensitivity will naturally depends on the voltage of supply rails but this circuit (BrianGT) will likely have an input sensitivity of about 500mV. Most guitar amplifiers are designed for input sensitivity of about 20 - 50 mV. You see the difference. Note that we are talking about nominal levels here: Although output amplitude of magnetic pickups is generally quite small some of them can give even 2 V peak amplitudes at resonant frequencies.

Yes. Running guitar into an effect will in most parts provide the required interface - assuming the effect is properly designed of course.  In most cases an effect pedal is a proper interface considering the impedance match but not neccessarily when it comes to gain. Due to variety of different pedals its hard to say anything certain about this. Some pedals are essentially preamps and have pretty high output level that can drive the power amp accordingly.

If you need something clean and transparent try an opamp stage that has a gain of about 5 - 10 and an input impedance of 1Meg or greater. Run it from +-15V rails. It's hard to beat this configuration in simplicity when headroom and good distortion specs are required (THD 0,01% or less). It may sound crappy if overdriven but the point was transparency anyway. Input stages with low gain are pretty hard to overdrive anyway.

In typical designs the power amp won't color the signal nearly at all when it comes to signal sources that have reasonable low and linear output impedance. Typical Hifi equipment is good example of such signal sources and in most cases guitar pedals/preamps as well. That is why an ordinary guitar power amp may not be that much different from a Hifi power amp anyway.

Guitar, on the other hand IS NOT a linear and low impedance signal source: In fact, it has very high and non-linear impedance because the pickup forms a resonant circuit - usually in conjunction with the tone controls. If the pickup is hooked to low impedance device the resonance peak gets damped and there will be signal loss at higher frequencies. With piezo crystals the signal loss happens the opposite way around and impedance mismatch to amps with too low input impedance causes signal loss at lower frequencies.

See the following article if you are interested.

http://buildyourguitar.com/resources/lemme/

Despite some minor inaccuracies it explains the pickup's impedance/resonance theory rather well. The gain stage that hooks up to guitar is very important as it has a huge potential of coloring the tone in various ways. Guitar pickup is a delicate circuit and it's charcters are far too unknown to common DIYers. It's pretty easy to damp the pickup's resonance into oblivion (or to alter it) - even with the tone or volume controls inside the guitar. That's why people like to fit those treble bleeder capacitors to volume controls, keep their tone and volume constantly at 10 - or switch to active pickups.
« Last Edit: June 22, 2007, 06:44:14 PM by teemuk »

joecool85

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Re: Finally finished my LM3886
« Reply #4 on: June 23, 2007, 07:02:58 AM »
Yes, it is a BrianGT mono kit.

So is this circuit designed for a line level input (as opposed to an instrument/guitar level input)?  I read several posts over at diystompboxes.com from folks (including Joecool85) who built this chipamp for guitar use... it's interesting to hear tha we're all using it 'wrong'. 

No...we weren't/aren't using it wrong.  All of us that have used the LM3886 (at least as far as I know) have used something as a preamp.  I myself was using my Dean Markley K-20X as my preamp.  I made a lineout for it then hooked that to my LM3886 box, it gets painfully loud in a 14' x 20' bedroom with a 10" speaker cab.

Basically any preamp will do it, if you want to build something the world is your oyster.  If you want to buy something then prepare to spend big money if you want a real "guitar preamp."  Mic preamps work just as well and you can get a cheap one from musicians friend if you want, tube based even!

I'm personally working on building a LM386 based preamp (same chip as used in the little gem and other small 1/2w pocket amps.)  When I get my prototype done I'll post pics, schematic and sound clips.  But that might be a month or two.

**edit**
Oh, I forgot to say one more thing.  Your amps looks really professional, great job building it!
« Last Edit: June 23, 2007, 07:04:17 AM by joecool85 »
Life is what you make it.
Still rockin' the Dean Markley K-20X
"New" amp: Fender Frontman 25 DSP (FM25DSP)
thatraymond.com

 

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