Recommendation for a clean DIY amp

[xylix]

Hi,
I have built a few Rubys, Noisy Crickets etc. Next I want to make something a bit more powerful - perhaps around 10 ~20 watts. But I would use it strictly for amplifying an archtop guitar for solo jazz so I am looking for sparkling clean sounds without distortion. (I figure this is opposite to most people.) Can anyone recommend either an amp schematics or a good IC chip for this kind of purpose?

Craig

[teemuk]

A typical LM3886-based chip amplifier perhaps? I know, it's about 60 watts but, in my opinion, the effort to built one is about the same as for a 10-20W chip amp. Part cost is naturally a tad higher but you will appreciate that extra power in acoustic applications. Speakers are a big issue too: For nearly "sterile" clean tones (in a good sense) some fairly flat PA speakers could be a better choice than ordinary guitar speakers with lots of distortion and extremely non-linear frequency response. You might even consider adding a crossover and a HF speaker element to the system (think acoustic amps). That for the power amp and the speaker system...

For anything else... Well, the problem is - I likely could recommend some stuff but the information you give is quite scarce. You just state that it should be about 10 - 20 watts and pristine clean (not an easy equation by the way). That's not a very informative description. Do you only need a power amp or do you need a preamp as well? What about features? Tone controls (if yes, what kind), loops, limiters, feedback suppression etc? Overrall simplicity? Can you work from a schematic or do you need PCB layouts as well? Give us something to work on...

[xylix]

For anything else... Well, the problem is - I likely could recommend some stuff but the information you give is quite scarce. You just state that it should be about 10 - 20 watts and pristine clean (not an easy equation by the way). That's not a very informative description. Do you only need a power amp or do you need a preamp as well? What about features? Tone controls (if yes, what kind), loops, limiters, feedback suppression etc? Overrall simplicity? Can you work from a schematic or do you need PCB layouts as well? Give us something to work on...

Thanks for the response.
I was thinking of a fairly simple amp involving preamp, poweramp and passive tone controls. I don't think I need any loops or limiters and hadn't even thought of feedback supression. Overall something fairly simple. I can read schematics, and studies engineering 20 years ago, but am just a hobbiest. Yes, I can work from a schematic (at least the less complicated ones) and prefer that for simple point to point wiring.

The .5 and 1 watt amps I have built don't have enough power. On the other hand years ago I bought a big old 100 watt Fender (thinking I needed that to emulate some people's sound) and that was WAY too loud. I was thinking that sincle wattage is logarithmic something in the middle (~10 watts) might be suitable. I just want to avoid the fender situation where you are trying to find a nice volume level somewhere between 0 and 1!

Teemuk, you mentioned the LM3886. Are there any good fairly simple schematics out there for a preamp, poweramp, tone control amp using this chip?

Thanks,

Craig

[nashvillebill]

It's my belief that for a very good clean guitar sound, a tad too much power is far better than a tad too little.  It's easy to turn down a 60 watt amp, and still have power in reserve for those nice full chords that use lots of bass...headroom .  And if you ever want to play live, even solo jazz work requires plenty of power.

That's why I'd also recommend the LM3886 amp.  See www.chipamp.com  for a nice kit that's reasonably priced...I was really thinking about using these to upgrade the power in my Fender Princeton Chorus (rated 25 watts per channel or 50 watts total, it was a tad too quiet for live work, until I changed to more efficient speakers).   Get a toroidal transformer from www.antek.com and one of the chipamp kits, add a preamp and you're good to go without spending a lot of bread.  I also concur with teemuk that you may want a 2-way PA speaker, though I'm using Eminence Ragin' Cajun 10" guitar speakers in the Princeton Chorus and they're very smooth, whether with my semi-hollowbody, my Ovation acoustic, or the Ovation VXT hybrid.

[teemuk]

There are tons of schematics for LM3886, just search for "chip amp" or "gainclone" and most importantly, read the datasheet. Personally, a chip amp design along these lines is my "favourite":

http://www.elektronika.lt/_sys/storage/2004/11/30/pic1.gif

Why:
- The input circuit is done properly with adequate ground referencing, DC coupling and RF filtering
- Feedback loop has some HF compensation and defines unity DC gain
- Has an output Zobel and an output inductor

It is possible to simplify the design even more but I don't see any reason to. In my opinion the design is already quite simple.

Power supply circuit is not that good, though: 1000uF is probably not enough rail capacitance. Since those big caps are usually within considerable distance of the power amp chip it is better idea to use local 2 x 100 μF caps that are closer to the chip and 2 x 100 nF capacitors located as close to the power pins you can humanly fit.

LM3886 chip is practically just an opamp with a higher voltage swing and output current delivering capacity, thus you can vary the basic design a lot. Datasheet will get you pretty far, so will a half an hour spent browsing through information concerning that chip and circuits using it. There's been a lot of discussion of LM3886 power amps, preamps and power supplies on this forum. Even more in places such as diyaudio.com. Heck, that forum even has a "chip amp" section.

Power supply will naturally be an important issue. Transformer should likely be about 120VA – 250 VA with proper secondary voltages. 4700uF per each rail should be good enough for filtering. There are various board designs around but if you have some clues of the design process (how currents should be routed, trace widths etc.) you can draw one yourself pretty quickly. That way you won't be tied to a specific design. (Most of them seem to have only one thing right).

That for the power amp. It will naturally need some kind of a volume control but I assume it will be in the preamp section. As for that, you can just derive ±15V rails from the ±30V power supply with regulators and use any OpAmp-based pre that looks nice. Since you are aiming for clean sounds simplicity and high linearity of OpAmps is actually a virtue. You only need to:

- Interface the pickup's impedance properly (usually only a question of defining input impedance that is higher than 1 Meg)
- Amplify the signal to level that matches the power amp's input sensitivity (very often about 1Vpeak)
- Limit the preamp's bandwidth between about 80 – 100 Hz (bass roll off) and 5 kHz – 10 kHz (HF roll off).
- Fit a volume and tone controls to a proper place

All this will likely require two or three simple OpAmp stages and the concerned passive volume/tone controls somewhere in between them. I don't know whether you like "scooped mid-range" tone or flat response but whatever it is you can choose between a typical Fender/Marshall/Vox tone control (scooping) and "Baxandall" (flat). Passive circuit may require an additional gain stage for signal "recovery".

---

About the loudness issue: Hearing is logarithmic all right, that means a 100W amp is only two times louder than a 10W amp if all else is equal. In essence, that 10W is still extremely loud but unfortunately it also distorts much more which makes it less versatile. This is especially a concern if you want to build a clean amp. (Look at power ratings of acoustic amps – they're in the range of 100W – 400W). If you want lower sound pressure levels then the better option is not to reduce power but to use speakers that are less efficient. Also, at some point amp manufacturers used linear volume potentiometers instead of logarithmic ones (some still do). This practice made an amp seem "louder" at lower volume settings (which I guess boosted sales) but naturally that is all due to poor tracking of logarithmic hearing (the pot is extra "sensitive" about 3/10 of its range, after that it doesn't seem to do pretty much anything). Consider this: Your average home stereo set may have 100W of power per channel, yet you hardly have issues with dialling in proper volume levels.

[tarahall]

There are tons of schematics for LM3886, just search for "chip amp" or "gainclone" and most importantly, read the datasheet. Personally, a chip amp design along these lines is my "favourite":

http://www.elektronika.lt/_sys/storage/2004/11/30/pic1.gif

Why:
- The input circuit is done properly with adequate ground referencing, DC coupling and RF filtering
- Feedback loop has some HF compensation and defines unity DC gain
- Has an output Zobel and an output inductor

Hi Teemuk,

Would you apply the same recommendation to virtually the same circuit using a TDA7293 instead of an LM3886?