Digital "modelling amp" semi-DIY

[gpjt]

Hi all -- for a while I've been planning to build something roughly equivalent to a modelling amp head. The idea is to use a Raspberry Pi (credit-card sized barebones computer), some kind of USB sound card, and -- at least initially -- the Guitarix amp modelling software as a base. It would have an LED touchscreen to allow configuration and adjustment of the modelled stack. As all of the amp modelling would be happening in software, the power amp stage could just be a class D or "class T" amp attached to the line out of the sound card. Should be easy enough to build a decent case to keep it all tidy.

I see that pretty much all of the threads here are to do with making classic analogue amps, so maybe this isn't the right place to post build notes/thoughts on this... on the other hand a lot of the posts elsewhere on the forums suggest that people are using digital amps when playing, so perhaps there might be some interest?

I've done some initial tests of the stack apart from the power stage, and have something roughly working -- will post a bit about that next, but if the consensus of the forum is that this isn't the right place to post about the build, just let me know and I'll stop

[gpjt]

First step: a Raspberry Pi 3 screwed on to a 7" touchscreen. Using a version 3 here rather than the new version 4 because the 4 has micro-HDMI output, and I don't have a micro-HDMI cable to hand.



Next, plug everything together. A total rat's nest. Note that the sound card (top right) is at a bit of a funny angle. It works fine despite that, but it's not an acceptable long-term solution:



Power on and boot test. Works fine, no magic smoke escaping from the hardware... touch screen works out of the box, so I can unplug the mouse (which means that the jammed-together USB stuff will no longer be an issue):



Next, install and fire up Guitarix. Plugged in guitar and headphones. Getting the software working well was quite a lot of work; the normal operating system setup does not work at all well for real-time audio processing and initially I was getting about 100ms latency with any non-trivial modelling going on. That's 1/10s between hitting the string and hearing the note, which makes it really hard to play at all -- I don't even want to imagine how hard it would be to keep in time with the rest of the band with that.

However, with some tweaks to the OS setup, that came down to 5ms, which is -- at least for me -- perfectly playable. If there is interest in this thread, then I'll post about that, because there's some interesting stuff in there about how digital audio processing works.



Anyway, what I've got here is a horrendous mess that actually works pretty well as a simple modelling headphone amp -- kind of like Amplitube, but no need to pay lots for branded models. I think the whole thing could be shrunk down quite nicely -- the limiting factors are the size of the screen, and the power supplies that will be needed (USB for the computing stuff, and probably 12v or so for the power amp stage).

Only problem so far is that there's a lot of hum when I'm not touching the strings or bridge on the guitar -- I suspect that there's an earthing problem, which sounds plausible with this hacked-together mess.

[dave.king1]

Keep it coming, not only is this the right place for it it's different and interesting

[PJSprog]

Absolutely interesting.

[Simon Barden]

Hum from non-string touching normally just indicates a lot of electrical noise around. The guitar strings give your body a proper ground connection so stop it acting like an aerial, plus any signals it does intercept get taken to ground, so can't pass on to the guitar.

Shielding the Strat (if not already done) will help cut down on the noise significantly, but it's still got single coil pickups so you can't totally kill the hum.

A grounded metal enclosure for the Pi would probably help, as would moving it as far away as possible from the guitar.

[RocknRolf]

Definitely interesting @gpjt!
Keep it coming!

As I get a bit older, still young though, but able to sit in one place long enough to build stuff, I have entertained the thought of doing something like this in the future, tough you are bringing it to the next level

Very interested to see where this is going

[Marcel]

Definitely the right place for this project. Modeling amps are as accepted in the guitar community as tube amps.... so post away

Plenty of interest from this corner of the woods...

[Andy40]

I got my popcorn....go

[gpjt]

Thanks for the encouragement, everyone

Super-busy day today with work, will post more tomorrow.

[gpjt]

Spent the day noodling with the setup as it is right now, trying to work out if it's the right way to do things. It works really well as a headphone amp; there's no need for a keyboard, and it's just about usable without a mouse. Setting things up after power-on is a bit of a pig, because it forgets how the internal connections are set up and you need to essentially draw lines on the screen between a thing that represents the audio in to a thing that represents the input to the emulated amp stack, then another from a thing that represents the stack output to the audio out, and that needs more precision than is easy with my sausagelike fingertips. It needs to start up with all of the virtual connections pre-set and Guitarix already running. That feels like a Linux software configuration thing, which is a big chunk of my day job, so I'll work that out.

Bill of materials so far:

* Raspberry Pi 3 B+
* 7" IPS touch-screen with HDMI in for video, and USB out for the touch-sensitivity. No case.
* El cheapo USB sound dongle
* 6.3mm to 3.5mm audio jack adapter
* HDMI cable to link the monitor to the screen
* USB cable for the touch-sensitivity bit.
* USB cable to charge the screen
* USB cable to charge the Raspberry Pi

Plus a pair of headphones, a guitar (Squier Bullet Strat, which is a lovely little thing for the money but undoubtedly does *not* have shielded cavities) and a lead, of course.

The Pi3 is a bit underpowered for running all of this; although I could get latency down to just over 5ms, as soon as you start using the more complicated amps with a few effects (bass boost, reverb, presence) it glitches. But the Pi4 is much more powerful; the only reason I'm using a Pi3 at the moment is that I don't have a micro-HDMI cable to hand. Amazon Prime to the rescue!

Plugging all of that stuff together is pretty simple and obvious, I won't bore you with it. Things will get a little more interesting later, because I want to reduce the trailing cables by making my own short-run cables -- but that can wait for a bit. And once I have a software setup I'm happy with I'll give the details, but I suspect it'll be fiddling with that for a little while before I'm happy with it.

Time to move on to the rest of the amp!

Having thought about this a bit, what I'm building here is basically a combination of a PC with a decent audio I/O system, and an amplifier. It seems to make sense to split it into three separate modules:

* The digital stuff -- compute board, monitor, DAC/ADC -- which will need at least two high-quality USB power inputs, alongside an audio input and output. The audio output should be routable to a headphone jack or to the power amp. It would also be nice to design it so that an effects loop could be put in between those, but maybe that's something for the longer run... Oh, and USB ports so that a mouse and/or keyboard can easily be plugged in as needed. This should not be over-shielded, as I suspect that the Pi's WiFi could be useful.
* The power amp. This will need a volume control, audio in, audio out, and power in -- looks like 12v is likely to be needed for this.
* The power supply. This will need to provide the USB power out, and also 12v. I'm thinking that building this kind of like a desktop PC PSU would be the way to go -- a self-contained unit holding all of the dangerous-voltage stuff. So I'll get a metal project box, whack a kettle plug connector into it, earth the box as a whole (just solder yellow-and-green to the box) and then split the input to two separate units inside the box, one USB adapter (maybe a stripped-down Anker unit, I like their stuff) and one 12v adapter. Run the cables out through holes in the box. As all of the high-voltage stuff will be in an earthed metal enclosure, it should be safe and also well-shielded in RF terms.

For the box as a whole, I think I'll put something together out of pine and thick plywood, aluminium on the front and back, box corners. Basically the classic amp head look.