Complete 100 watt guitar amplifier in1590b stomp Box

[rring]

Been long time since I have posted anything but I thought someone might might like my new amp design. There is more detailed info at my blog: www.circuitsalad.com .

I designed a 100 watt rms (4 ohm load) class D guitar amplifier to fit in a 1590b enclosure. It uses a 24 volt 5 amp or so switch-mode wall wart I got off of amazon for $18.00.

It's loud and sounds great but it doesn't distort so well when you push it to the output limit . The tone stack is basically a variable mid scoop.

I will post a audio clip in a week or so on my blog

[dschwartz]

Awesome project!!
You should build a preamp and some kind of limiter to avoid clipping on the power amp..

i´m curious about 100watts from that puny chip and heatsink..are you sure that heatsink is ok for 10W dissipation?..

also, a mixed mode feedback could be very useful here...
congratz!!

[GiovannyS10]

Hey dude, your project seems very good. The larger one i could built was a 10W LOL! Btw not think you need a cooler fan or a better heat sink or maybe you will be able to cook somethings in this 'pedal'. Hahaha - Great project.

[briandress]

id love to hear what a tube pre sounds like in front of that

[rring]

The heat sink is adequate because it dissipates through the PCB as well.  If I crank it up  it just gets warm to the touch. I played a jazz gig with a prototype and it got warm but not bad. In a situation like that I'm not cranking it up past 30 to 50 watts though. A continuous sine wave input with max output maybe a different story. The dynamics of the guitar signal certainly helps.

[stringsthings]

Any chance of sharing the PCB etching files?

[rring]

Yes I used express pcb, I can also generate gerbers. Its a double sided board...I made it using laser printer etching so the layout is geared toward this method. The amplifier is surprisingly small and has .65mm pitch pin spacing but is workable.

link to zip file containing linked expresspcb schematic/layout and gerber files:
https://www.adrive.com/public/QpRQMX/RED%20SCARE.zip

Just a FYI, there is a program: robot room  copper connection. It can read expresspcb files, print properly scaled and flipped images for laser printer etching and export gerber files. That is what I use

[stringsthings]

Thank you!   

[Marcos - Munky]

Wow, that is awesome! Wish I have the skills to work with SMD...

[printer2]

What do you have the FV-1 chip doing?

[rring]

The FV-1 is used for reverb. I used one of the internal programs. I didn't have room for an eeprom so I didn't write a program for it

[R.G.]

There are quite a number of people working on using Class D amps as the speaker driving "muscle" for small amps. Some of the better ones I've heard have used the old pre-clipping trick: so what that your amp clips in a nasty manner - only feed it a pre-clipped input that never lets it soft clip before the actual amp ever hits its own clipping. Built in as part of the power amp circuitry, you get good control of exactly where the power amp itself "clips".

That way it doesn't matter what the actual power amp clipping is like. It never happens.

[rring]

right... so your are saying set the gain distribution such that the driver stage(for the class D final) starts soft clipping before its output swing can become large enough to overdrive the final- correct?

In my design the gain distribution is such that you probably can't quite get to max output with a single coil pickup. This of course doesn't guarantee anything but it helps. I suppose a well designed discrete stage with an adjustable supply voltage - to set the max  pre-clipping level would work best as the driver.

[robthequiet]

Nice box design, too, sharp graphics. Thx for posting.

[dschwartz]

There are quite a number of people working on using Class D amps as the speaker driving "muscle" for small amps. Some of the better ones I've heard have used the old pre-clipping trick: so what that your amp clips in a nasty manner - only feed it a pre-clipped input that never lets it soft clip before the actual amp ever hits its own clipping. Built in as part of the power amp circuitry, you get good control of exactly where the power amp itself "clips".

That way it doesn't matter what the actual power amp clipping is like. It never happens.

I was thinking the same..
A limiter / clipper before the amplifier ..also i would include some kind of filter to emulate current feedback frequency response.
Btw the chip has an internal limiter..Plimit o something like that..

Also consider that @24v 4ohms, in btl mode the max power is about 60W @1% thd.  At 100W the thd is 10%...
As quick improvement, lower the bass going into the amp..the 1 uF cap will have a 20hz corner..try somewhere around 90hz to avoid low end clipping

[R.G.]

right... so your are saying set the gain distribution such that the driver stage(for the class D final) starts soft clipping before its output swing can become large enough to overdrive the final- correct?

Maybe, but I think not exactly. I didn't really have in mind something as fancy as "gain distribution".

Pick a place that's just the clean power amp, no funny stuff; just powers the speakers. Most hifi amps try to produce full output power with something like a 0dbm line signal, maybe 0.7 yo 1V rms. 100Wrms into 8 ohms is on the order of 28V rms, so the gain is set at about 28 for the whole power amp. That in turn means that the amp will start to clip at the input voltage which happens to drive the output into clipping. This is perhaps 1 to 1.4V peak.

The idea is to have a separate clipping circuit right in front of the power amp that prevents the input going into the power amp from ever getting to the voltage that clips the power amp. The power amp thinks it's being completely linear all the time, never clipping itself, and it really is. It's NEVER driven to clip.

The clipper in front of it does limit the signal, and the apparent sound of the clipping is that of the clipping/limiting circuit, not what the power amp would ever do on its own. The power amp clipping sound is hidden behind the clipping circuit and is never heard, so it doesn't matter how the power amp clips.

The "maybe" above reflects that it's possible to do this by distributing gain inside the power amp circuit, but overall feedback will do its best to hide the internal clipping. The pre-clipping/limiting needs not to be in the feedback loop from the speaker outjput back to the power amp feedback point.

[rring]

Sure that makes sense regarding the clipper idea...I guess you could just put some diodes across the feedback of the last op amp stage. As noted in an earlier post - the 100 watts is pushing it regarding distortion (10%) anyway . I suppose a general axiom of don't turn up the amp anymore after it starts sounding bad will work also

With regard to the low end roll off also mentioned earlier. I have some low end roll off in the first stage with the 1 uF cap in the feedback loop

Thanks for the compliment on the pedal etching. Half the reason I make the pedals is just to have an excuse to do the etching and make cool designs! I have a very reliable and straightforward technique using laser printer toner. I use melt-able fishing lure powder paint for the inlay. 

[printer2]

Could be in the NFB or just a couple of diodes in the back end of an opamp. As far as the internal limiter I'm pretty sure it will not do the same function without sounding any better than the chip running out of steam.

[rring]

you are correct about the P-limit it just reduces the maximum PWM duty cycle so it limits the maximum output but that just means you will clip that much sooner

[dschwartz]

Calculate the threshold of the limiter so you can get most level before entering power amp clipping..
The input sensitivity is +-3.9v..so around 3.5 per side ( blue leds? Or zeners maybe?)