5-10 watt coustic amp design

[pulaskite]

I want a small (I've got an Acoustisonic Jr)  super clean amp to play an archtop using  various combinations of magnetic pickup, acoustic transducer and an FET microphone.  The goal is to capture the sound of the unamplified guitar I hear playing in my easy chair w/ the f hole 12"  from my ear.  I plan to build a delay/reverb unit into the enclosure which will probably house an 8-10" speaker

Before I engage in a gratuitous design exercise I thought I'd ask here for suggestions/pointers to designs w/ similar goals.

[joecool85]

Is this going to be running on batteries or AC?  If AC, I would suggest using a LM1875 circuit, we have a guy on here that could set you up with the PCBs/components for cheap.  I have a LM1875 amp and it's very clean, very good sounding.  I haven't used it on an acoustic, but I used it for an electric guitar, and I've also used it to play music from my computer on.

[pulaskite]

AC primarily, but I'd like to be able to run off 12V DC.  LM1875 looks good but for the 16 V min on the data sheet.  I'm not much concerned w/ a PCB for a one off at audio frequencies.  (lots of RF experience  built point to point over a ground plane)

[joecool85]

Yeah, if you are interested in 12v DC, you are going to have a hard time finding a good sounding chip.  Some people have had luck with LM383 chips, but I did a kit with one and it sounded horrible running on 12 volts.  It could have been that it was a Veleman kit, I had never used one before, but I don't think so, I think it was the chip.  It had a VERY hard time staying clean, and it wasn't a good sounding distortion when it started to clip.

[teemuk]

Acoustic guitars tend to be quite cranky: You have to consider stuff like damping the feedback caused by large sound chamber. It is really annoying problem. Also acoustic guitars that have active pickups tend to have A LOT of drive signal and bass content. A miked signal does not neccessarily have a very high amplitude (depends on equipment) but it's still very bass oriented. My 12W Crate amp has so far crapped out with every acoustic guitar I've tried. You will need a lot of headroom to compensate those things.

5 to 10W amplifier really doesn't sound very sufficient if you want clean tone from an acoustic guitar but if you insist then at least use very sensitive speakers (to minimize power requirement) and put a soft limiter circuit before all gain stages that are likely to get overdriven. (Not clean sound but sounds more like that than a clipping SS amplifier.) At least the power amp will need one.

No doubt the frequncy response must be FLAT - acoustic guitar doesn't need those fancy tonestack circuits used to compensate odd frequency response of magnetic pickups. Basically, a powerful HiFi amplifier is good enough for the whole application unless you need a preamplifier for other purposes.

I have seen acoustic amps that use high-range speaker elements, I'm not quite sure why: typical guitar speakers reach to about 5 kHz and clean guitar sound will not likely contain much frequencies as high as this. You might want to use speaker that has a free air resonance below 80 Hz so that it will not "fart out" on low notes.

Oh yeh, and it's really worthwhile to browse through speaker datasheets and find a speaker with a FLAT response so that the speaker will not colour the tone - unless this is a wanted phenomenom, of course. Most speakers out there are horrible if you require flat reproduction so you might want to build somekind of an crossover or "frequency response linearization" circuit.

And last but not least, some food for thought: A great part of capturing that guitar sound you hear is an appropriate miking technique.

[pulaskite]

I've been browsing for chips and the TDA2040 looks on paper pretty good.   ~ 6 watts per channel @ 12 v and good distortion numbers (if they hold up at 12 V)  If anyone has experience to the contrary please let me know.

I'm NOT playing loudly, so feedback is not an issue.  Besides it's a highly damped archtop (Eastman 810CE)   I  need the high frequency response to replicate the scratchy sound of fingers on strings (I don't use picks or fingernails).  The Acoustisonic would be fine if it weren't so damned heavy.

It;s important to understand I'm not trying to get above the loudness of a normal speaking voice or a flattop guitar played moderately hard.  What I'm after is being able to control the dynamic range w/ my right hand rather than a pedal or knob and having the fine control that comes w/ playing softly.  Typically I set volume at 1-2 on the amp & midway on the guitar.

BTW if someone built what I'm looking for I'd buy it in a heartbeat.  I'm *very* out of practice at circuit design/construction which makes building it myself a very high dollar solution.  I only do this when I can't find what I want.