DIYstompboxes.com

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 :icon_biggrin:. The tone stack is basically a variable mid scoop.



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

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!!

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.

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

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.

Any chance of sharing the PCB etching files?

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

Thank you!   :) :)

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

What do you have the FV-1 chip doing?

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

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.

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.

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

Quote from: R.G. on June 17, 2017, 09:11:26 AM
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

Quote from: rring on June 17, 2017, 10:39:59 AM
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.

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 :icon_mrgreen:

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. 

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.

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

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?)

right... you can also change the gain setting on the final amp such that a couple of back to back silicon diode pairs will be in the correct range. Maybe I am full of it..but never liked the sound of LED's clipping.

Quote from: rring on June 18, 2017, 03:16:51 PM
right... you can also change the gain setting on the final amp such that a couple of back to back silicon diode pairs will be in the correct range. Maybe I am full of it..but never liked the sound of LED's clipping.

Well the function will not be high gain clipping, but limiting instead..
I would combine leds with a resistor and Si diode in series ( like dynatouch clipping stage)
This way, the signal is reduced if higher than the led Vf, and clipped if higher than led+ Si Vf.
Then you'll have a nice transition between clean, compressed and clipped

Or...a cmos inverter stage with V supply of 7 volts..in my opinion they sound very like tube power amps

yes makes sense... the  limiting will be clipping so it will  end up acting like an overdrive of a sort...so don't like LEDs so much... Love the inverter idea. The beef stew 4049 overdrive concept with a limited supply...that would probably work great...need to look and see if there is a soic 8 or sot23 type dual or single inverter package that will work(not much room).

Quote from: dschwartz on June 17, 2017, 04:07:19 PM
Also consider that @24v 4ohms, in btl mode the max power is about 60W @1% thd.  At 100W the thd is 10%...

I think this kind of thing is pretty much an expectation of even most commercial amps...especially when this Class D chip is what you will find in many amps claiming 100W output.

For home stereo stuff I use a Class D amp rated to 250 Watts, but rarely push it past 50W peak output power (if that).  In fact, my power supplies won't give much more than 120 Watts, but I rigged them up with huge capacitor banks to handle transients at more than 100W -- based on a typical audio signal at even 40 Hz could do more than 300 W short term.  Since it's built up as an "all-in-one" with no external inputs, I have coordinated the output limiting of the USB audio device on the raspberry pi with the total preamp gain so that it's all clean and the PSU can keep up when my volume knob is cranked and the Rpi output is limiting.  As-is if it's cranked it's too loud to listen comfortably anywhere in the house.

The point is the unit sounds really good at normal listening levels even though the rated capacity would be insane for it's intended use.

That said, a "100 Watt" guitar amp will blow you out of your bedroom and keep your guitar in the mix in most moderate sized venues without having to max it out.

My conclusion is this thing is just about perfect for a general purpose guitar amp.  Nice work!

As an aside, from a psychoacoustics point of view I believe it has come to be understood that an SPL increase of 10 dB will be perceived as being twice as loud.

This means it takes 10x the power to sound twice as loud, so a 100W amp cranked is perceived as only 4x as loud as when the power output is 1W.  On a good audio taper pot, the drive from 50W to 100W is in the last ~1/8 turn of the volume pot.  Another way to think of it is 1/2 way up is only about 10W output.  This gives some understanding for why the little Zvex 1W tube amp really cranks when plugged into a 4x12 stack.

Of course, speaker efficiency plays a big role in perceived loudness.

Yes you pretty much nailed it. I just said it was 100 watts for hype! We all know its BS :icon_mrgreen: To your point I am generally using a 2 10" cabinet with celestion 30 watt speakers. They are very efficient (cheap also). In tandem with the amp its just plain loud and pushing it to the max starts to sound like an exploding garbage can. Its more than sufficient for club/bar setting. I do like the idea of the inverter stage as a limiter and perhaps over-drive...need to make room for a new knob! Perhaps I can use a few inverters and drive the power amp differentially for cheap thrills.

Rring, did you put big caps at the power​ rails?

yes they are 220uf low ESR types and they are right next to the two sets of supply pins on the power amp IC.

Try putting a 2200uF for extra power

Mine uses 4700uF but that's just because I got free samples.

Energy in a certain amount of discharge = 1/2 C(V1^2 - V2^2)

Energy = Power * Time

well I added a blue led clipper for the hell of it! Works really well...starts to clip at about 2.4 volts...which is just about perfect

(https://s4.postimg.org/x1comrozd/Stomp_Amp_100_mini.jpg) (https://postimg.org/image/x1comrozd/)


Here is a little video recorded on my phone at full volume...you can't tell but it was painfully loud..My phone was overloading so it actually sounds much better in person.. but you will get the idea...it actually sounds good as it overdrives...go figure...you can see the LED's driven to conduction and flash

https://youtu.be/2YG01f64nNw (https://youtu.be/2YG01f64nNw)

Heck, you could get a light dependent resistor and almost have a compressor. Just kidding, glad it turned out. I may have to add it to my amp, if I ever get to building it.

That sounded pretty nice, RRING. Good job!

It occurred to me upon reflection that what we really need are 100Wrms SPEAKERS in a 1590.

I still have very clear memories of carrying a 4x12 up stairs to a gig.

In many ways, it would make sense to have the power amps always inside the speakers, and only put a preamp on the pedalboard.

Or to never go more than one speaker per box, with its own amplifier in it.

Yes true...most the burden of schlepping equipment is the big cabinets...and I have had requests for small light amplifiers from other players but you still have to lug around the cabinets. Should look into the lightest speakers you can get in 12" and 10"(maybe 8") and come up with a really rugged but light weight structure for the cabinet...such that you actually end up with a loud, nice sounding lightweight rig in totality. If you could make a significant reduction in weight(and to some extent size)...it would have great value to journeymen musicians.

... or a diy crane that climbs stairs and fetches food ...

A music shop sales guy tried to sell me an Ibanez amp on the basis that it contained a Tube-Screamer.

Now I can go back and tell him that my Tube-Screamer contains an amp. Brilliant!

I always wandered  about small "tuned" cabs like bass reflex and stuff..
Once i heard a small bose box with very small speakers but it had an amazing bass response for the size

Not to beat a dead horse with this thread...but here is a sound clip of me playing Jazz in a small club using the amp at 70% of max