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Old 27th April 2008, 12:21 PM   #1
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Default lm3886 building as guitar amp

hi everyone..i wanna built a lm3886 amp that can be used as acoustic guitar amp..what do you think about this subject ? which design is more reliable for this application ? i will make a lm4562 preamp for this amp also..i dont wanna use electrolytcs caps on signal paths..i have arcatronics 10uf mkp capacitors..i will use them

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Old 27th April 2008, 02:14 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
use an AC coupled design topology.
Use a really big heatsink, about double the size that National recommend as the minimum for the respective load impedance and supply voltage.
For a guitar amp maybe 3times bigger might be better.
Any comments?

Aim for a frequency response down to F-3db of one full decade below your lowest open string. 10uF will easily achieve this. 1uF may be adequate for a high Zin style of input to the power amp.

Use an RF filter on both the preamp input and the power amp input.

Use a driver intended for PA work, robust and efficient.
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Old 27th April 2008, 02:36 PM   #3
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Many guitar amps use these. Marshall has a combo amp with two paralleled. SWR has a small bass combo the LA8 and it is used it many designs.
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Old 27th April 2008, 03:01 PM   #4
KSTR is offline KSTR  Germany
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Install a soft clipper ahead of the amp (soft clipping line/buffer stage). Acoustic guitar has sharp transients, and those are better soft-clipped before the amp than hard-clipped by the amp, just in case you run out of headroom (which easily can happen, even with very efficient speakers in the 100dB/1W/1m-category). And, as a matter of taste and depending on the speaker/cabinet used, make the output high-Z, ie as per Elliott Project #56:
http://sound.westhost.com/project56.htm

And you might probably worry more about the sonics of your pickup instead of worrying about electrolytic caps. Further, most probably you'll need a good parametric EQ to get a good sound.

- Klaus
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Old 27th April 2008, 03:08 PM   #5
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my friend will use wharfedale 8.2 speakers? it is 86db..i dont know if it is reliable..
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Old 27th April 2008, 03:34 PM   #6
AndrewT is offline AndrewT  Scotland
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I suggest you forget using "hifi" speakers for sound reinforcement duty.
You need drivers that can take sustained punishment AND are very efficient.
Try for >=96dB/W/m and can easily handle the peaks coming from the chipamp. Note Kstr mentions 100dB/W/m

Using 86dB/W/m speakers will require ten times as much power from the chipamp and that will make it run hot and probably clip on much of the signal content.
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Old 27th April 2008, 03:45 PM   #7
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its just an acoustic guitar amp..does it need much power ? i dont know really..my friend will use it in a flat at an apartment..and neighbours will be angry if there is too much sound
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Old 27th April 2008, 03:51 PM   #8
AndrewT is offline AndrewT  Scotland
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if he's playing in a flat or apartment then he doesn't need sound reinforcement.

Presumably he needs what he is asking for to play to a bigger audience.
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Old 27th April 2008, 04:24 PM   #9
Minion is offline Minion  Canada
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I have a few Guitar amps that i built useing the LM3886 and they work very well....For Acoustic Guitar you could just have a Buffer at the front end and then add an active Tone controll and then into the LM3886...On my amps I built a few Overdrive/Distorion stages that can be enabled or disabled so i can go from Clean to heavey and dirty.....

I use a 12in fender speaker with 96db ,and a PSU with 8000uF per rail and a 2X18v Toroid with gets regulated to +/-15 per the preamp and Overdrive stages and Unregulated 2x26v for the Power stage.....

Nice sounding amp but it really is the speaker that makes the most differance in sound , I tried many different HI Fi type speakers and none of them sounded any good till I broke down and spent the cash on a real Guitar speaker.....

Cheers
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Old 27th April 2008, 08:06 PM   #10
teemuk is offline teemuk  Finland
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I suggest you take a look at some acoustic guitar amplifiers to get some ideas and to get acquainted with their features and design practices. Most of them have:

- A flat frequency response when tone controls are set to middle. In oppose to ordinary guitar amplifiers, acoustic amplifiers preferably should not colour the sound in any way but preserve the tone of the instrument. Graphic EQ may be the best alternative for tone control.

- Full-range speaker set-up, often consisting of a woofer, mid-range element and horn/tweeter. This does not work well with conventional guitar amplifiers that are used for amplifying overdriven tones but is a good set-up for acoustic instruments. Do not use “guitar speakers”, often they are not flat enough and therefore will colour the sound. Do not use HiFi speakers either; they can’t handle the abuse.

- A lot of output power: Clean reproduction of the signal at moderate SPL levels can require a lot of power unless you have very efficient speakers. Have you considered using a bi-amp set-up where you would use a 20 – 30W amplifier for mid-range and high frequencies and the LM3886 just for bass frequencies?

- Soft limiter or compressor to limit the dynamic range of the instrument and to prevent harsh clipping of the output stage. This can compensate the lack of output power but will colour the sound. Excessive clipping sounds bad with most acoustic instruments, especially if it’s the harsh clipping of conventional solid-state output stages.

- Feedback suppressor to prevent acoustic feedback. This is typically a steep band-stop filter with controllable notch frequency. Preferably you would need two since acoustic feedback may often occur at two separate frequencies. Sometimes a switch that inverts the signal's phase may help a little as well. As far as I see it, feedback suppression is one of the most important features in an acoustic amplifier. I haven’t yet encountered an acoustic instrument that would have not suffered from acoustic feedback.

- Microphone inputs (with or without phantom power), in case one needs an input for vocals or an alternative way to plug in acoustic instruments.

- Decent set of “line level” output connectors and possibly a loop for outboard effects. These are small additions but can increase the versatility tremendously.

I have noticed that most of the guys that play acoustic instruments and require amplification also seem to value features like low weight, small size and ease of portability. ...Well, I suppose we all do but these guys don't give a damn about the rock and roll image that requires two Marshall full stacks just for the looks.

So, they generally don’t want to lug around a huge sound system, yet still wish the amplifier to provide a huge, loud sound if necessary. It's a difficult equation so it's no wonder that most acoustic amplifiers are nowadays utilizing switching circuits (either in PS, in PA or both) and use some special configurations that can increase the efficiency of moderately small speakers and cabinets. For example, the cabinets may have reflex structures or passive radiators, the woofers may be down firing / slot loaded to increase low frequency response etc. That´s something worth of considering...

Anyway, most of the mentioned stuff of course has nothing to do with building a basic power amplifier out of a LM3886 but as I see it, unless you’re trying to do something revolutionary, that subject is pretty straightforward anyway. Just choose/make a decent circuit/layout design and don’t skimp with the power supply and heatsinking. An acoustic instrument generally requires a power amplifier that is “sterile” and reproduces the input signal linearly but besides that it’s the other features (speaker set up, preamp etc.) that define how well the amplifier works in the concerned application.
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