I have one of these boards (a clone of the TI example) running from a guitar preamp and it sounds great to my ears. BUT... it hums when I have the guitar volume turned up (magnetic pickups). No hum with guitar volume at ZERO. I was initially thinking it was my cables, but I noticed that when I unplug the supply (24VDC switch mode brick) the hum goes away for a second and the signal comes through just fine until the filter caps discharge.
So I'm thinking PSU issue.
But... is there something I could be overlooking? Do I need to just add more filtering?
So I'm thinking PSU issue.
But... is there something I could be overlooking? Do I need to just add more filtering?
So is there any connection to wall socket ground?
E-guitars usually need that. To the point that in the 1950s we "grounded" to the live wall wires, preferably through a capacitor which tended to fail.
E-guitars usually need that. To the point that in the 1950s we "grounded" to the live wall wires, preferably through a capacitor which tended to fail.
That's the problem. Wow. Thanks! I connected earth ground to the chassis via a wire run to an outlet '3rd prong' and that solved it.
This is the first time I've powered an amp via a DC power supply.
Is there any other solution than providing an earth ground to the chassis? For power, I have a 2.1mm barrel connector with the sleeve going to chassis ground and no trace of a mains power connection to the amp chassis.
Most 3-prong notebook supply adapters will do the trick. On the other hand, ALL 2-prong ac-wall warts are definitely a no-go for your guitar amp.
Good idea! There is a catch, though: every laptop/notebook adaptor I've seen puts out about 18 - 19 volts DC.
18 volts, along with today's typical bridge-mode class D power amplifier, will max out at somewhere around 15 watts RMS into an 8 ohm guitar speaker. (I've assumed only 1 volt lost to saturation in each output device, so you get a maximum of 32 volts peak-to-peak delivered to the speaker when powered from an 18 volt supply.)
Fifteen watts will not be enough for many guitar players. Particularly so from a class D amplifier - these tend to sound absolutely horrible if the output clips, so you usually want lots of power headroom, so that the power amplifier never runs into clipping at all.
If you want more than 15 watts, with a typical 8-ohm guitar speaker, you need more power supply voltage, and a laptop adaptor won't do the trick.
Some years ago I found out that most small "wall wart" SMPS not only did not ground their output, they actually connect output ground to the junction of a pair of small-value capacitors forming a voltage divider right across the incoming 120V AC line.
In other words, output ground is usually connected to a high-impedance AC voltage source of roughly 60 volts RMS at 60 Hz!
(This is in North America and other 120V AC countries. I assume you might have 120V RMS on your DC ground if you're using one of these SMPS in a country with 240V AC RMS mains electricity.)
I think this is done because (a) you don't want the output ground floating off to some enormous random voltage due to static electricity, and you have to do something to prevent that, and(b) you can't assume either of the two input prongs is a safe ground voltage.
So the cheesy, low-cost, ugly but practical solution, is to use the two caps to set the output ground voltage halfway between the two incoming AC prong voltages, and do this with a sufficiently high series impedance (the cap reactances) to limit ground currents low enough not to bother people who touch the output.
I note that Digikey stocks lots of "medical grade" power supplies (see attached image). I doubt hospitals will look favourably on patients being subjected to 60V rms AC at high impedance. Safety concerns aside, that would wreak havoc with ECGs and other medical electronics that reads skin voltages.
So I suspect these medical grade small SMPS have grounded ouputs. But I'm not sure of this. Can anyone confirm?
-Gnobuddy
Attachments
Lately I've been using a little Coolmusic BP40D as the amplifier for vocals and e-guitar at our jams. It's battery powered, so it's also great for playing guitar outdoors.
BP40D: https://www.amazon.com/Coolmusic-Acoustic-Amplifier-Portable-Bluetooth/dp/B09DYKHNML
BP40D review:
The BP40D is designed for acoustic-electric guitars, not pure electric guitars. My fix is simple, my electric guitar goes through a Flamma Preamp (https://www.amazon.com/FLAMMA-Digit...e/dp/B08LMLRQ44/ref=sr_1_5?crid=341APE5RXU9SW) and a few typical guitar FX pedals before being fed into the little powered speaker.
The Flamma has some very nice simulated tube guitar amp models, and includes its own cab sim; it's output is designed to be fed to a flat frequency response powered speaker, like the little BP40D.
The FX pedals aren't grounded (they're powered off a 1-Spot 9V DC adaptor with a 2-prong AC plug). And the BP40D isn't grounded, either. (It's usually running on its internal battery, but even when I plug in the power supply, it doesn't seem to ground it.)
And yeah, I tend to have some struggles with guitar noise, particularly when my hands are off the strings, and most particularly if I dial up one of the higher-gain simulations in the Flamma Preamp!
As an aside, audio quality from the BP40d has definite limitations, but the convenience factor is huge. I no longer have to lug around a bulky and heavy Acoustic AG30, a bulky Boss Katana 50, a little Mackie mixer, and a slew of cables.
-Gnobuddy
To avoid Class-D-clipping I always insert a soft-clipping stage between MasterVolPotentiometer and class-d-amp input. Works like a charm.