Digital "modelling amp" semi-DIY

[gpjt]

Searching for pre-built power amp boards; as you'd expect, there are no cheap mono ones out there, so it looks like it'll be easiest to get a stereo hifi one and just leave one channel unused. This looks like a solid option.

[Marcel]

Searching for pre-built power amp boards; as you'd expect, there are no cheap mono ones out there, so it looks like it'll be easiest to get a stereo hifi one and just leave one channel unused. This looks like a solid option.

Or you could leave it stereo and go the Roland Jazz Chorus amp path...

[Simon Barden]

This? https://www.amazon.co.uk/Nobsound-TP...08&sr=8-4&th=1

[gpjt]

Simon, your search skills are clearly better than mine! That's a really interesting point, though, Marcel -- I've been thinking of this as a way to run normal amp simulations, but as there's basically a guitar signal going into a computer and then getting fed to the headphones/speaker output, there's no reason not to run effects through it and have two speaker outputs -- say, a flanger that pans from side to side in time with its "whoosh" (there is undoubtedly a name for that and as a shoegaze fan I should know it, but I don't). Anyway, that sounds worth pursuing.

Had a bit of a brainwave about the power supply. There was something unsatisfying about the idea of splitting the mains two ways, and then having separate converters from the mains voltage, one for the power amp at 12V+ and one for USB at 5V. I was looking at my laptop's power brick and realised that it's 20V at 3.25A, so 65W. 20V is pretty good for most of the amp boards I've seen. Moving a little further along that line of thought, car cigarette lighters are normally 12V, but apparently trucks and buses can be 24V or more -- and it seems that most devices designed to plug in to a car dashboard can handle at least 24V -- at least, the good quality ones can. And good car USB chargers are easy enough to get.

So a 60W or so laptop brick sounds like an excellent option for powering the amp. Ideally, I want everything inside the amp box -- so, get a project box, have a hole in the side so that the kettle lead cloverleaf "socket" (which of course is actually a plug) is accessible from outside. Put the car USB adapter inside there too with the USB cables coming out; wire it across the 20V output of the power brick, and then in parallel have the 20V output wires to go out to the amp. Ground the metal enclosure with a wire to the 0V wire from the PSU, and we have a shielded metal enclosure which can be bolted inside the amp case.

But if that all doesn't work, and the power supply introduces buzz or hum, then at least the laptop power brick could be moved outside the case, leaving just the USB conversion in there.

Does that sound sane to those more experienced with the analogue side of amplification?

[edit: remembered that laptop bricks tend to have cloverleaf inputs rather than kettle plug]

[Marcel]

Using the brick supply is probably a good and quite safe way of doing things, however I'd be cautious on incorporating the brick inside my enclosure. The brick itself will most likely radiate quite a lot of RFI (Radio Frequency Interference) which if outside your modeling amp enclosure should not cause any issues. If it is inside your enclosure then it has the best chance possible to wreak havoc of all manner of types....

Of note is that VOX with their VT series of modeling amps chose to have their brick separate from the amp chassis, and have it supply power to the amp in typical laptop style via a suitable DC rated connector. It would be my assumption that those that have the "brick" inside the chassis have selected a device that is designed and built correctly for that important job.

Brick type power supplies are convenient and light, but as they are all switch-mode devices they do offer their own set of challenges, with RFI being the biggest problem on the analogue audio noise floor. RFI is not an issue on the digital side typically, though at high levels it can cause digital data corruption... Give it a try inside your enclosure though don't be surprised if you get better analogue noise floor results with the brick separated and away some distance from your main modeling chassis.

Also, the cheaper the "brick" the higher the RFI is likely to be. One way to make a good switch-mode supply cheaper is not fit the components that keep that nasty RFI under control.

[gpjt]

Hi Marcel -- thanks for the thoughts! As an experiment, I plugged my stock Squier Strat, which I'm pretty much sure has no cavity shielding, into the current version of the amp -- dialed down to a clean sound -- and then used the guitar as a poor man's RFI detector.

When the guitar was far away from any power sources, it was completely fine -- no audible buzz at all, even when not touching the strings.

Putting the guitar's input jack near a good-quality Dell laptop brick caused lots of buzz to come through the amp -- putting the selected pickup near the brick made it much louder. The effect with a mains to four-USB charger made by Anker was less but there was still some buzz. In both cases, touching the strings to add an extra earth through me had a minimal effect in reducing the buzz.

Do you think that putting the brick inside a grounded metal enclosure would reduce the RFI?

[Simon Barden]

Do you think that putting the brick inside a grounded metal enclosure would reduce the RFI?

As long as it has sufficient cooling available, it isn't going to do any harm and may obviously do some good.

[Marcel]

When the guitar was far away from any power sources, it was completely fine -- no audible buzz at all, even when not touching the strings.

I think that is your clue..... Separation.

Keeping the brick itself generally away from the amp (and guitar) will be your best long term bet. I know in my case with my VOX VT40x the amp sits on a stand or on on another cab to get it off the ground for better listening, and its associated brick inevitably sits on the floor some feet away. I'd probably put most of my 'best practice' shielding efforts into the modeler amp enclosure, and try and have the brick as far away from things as practical.

Putting the brick in its own shielded ventilated enclosure will reduce the RFI by some small amount, but is probs also just making things more difficult for yourself for not much improvement. Much of the RFI is radiated by the brick itself, and often a sizable portion is radiated by the leads to and from the brick.

[gpjt]

Thanks, guys -- makes sense. I do like the tidiness you get with a traditional amp, where there are no extra boxes littering the place and causing trip hazards... but this is probably the wrong thing to obsess on right now. I normally play through a Orange Micro Dark, which also has an external power brick, and by putting the amp on top of the cab and the brick on the floor behind it, everything works OK with no noticeable RFI. I assume that the signal to from amp to speaker is high enough power that any RFI is just drowned out, because the brick is not far from the speaker input.

(As an aside, would a transformer-based power supply produce less RFI than a switching one? Intuitively I would guess it would be much better, as there would be no high-frequency stuff going on -- just the normal 60Hz mains cycle. Not planning on using one in this build, just interested.)

[Marcel]

(As an aside, would a transformer-based power supply produce less RFI than a switching one? Intuitively I would guess it would be much better, as there would be no high-frequency stuff going on -- just the normal 60Hz mains cycle. Not planning on using one in this build, just interested.)

You are very correct there.... Transformer (or analogue) based supplies produce virtually nil RFI.. ever!!..... On my ham radio rig for 7MHz SSB work I resorted to a very old Yeasu 12VDC regulated 20A transformer supply because every suitably sized commercial switch-mode supply created noise hash across all frequencies from broadcast medium wave AM radio right up past 27MHz CB frequencies and beyond. And in certain situations even VHF and UHF were unusable depending on the switch-mode supply used.... They (switch-mode supplies) often produce far more RFI than they ever should, and sadly most people would never notice. Many ham radio operators resort to incorporating batteries for their 12VDC HF/short wave gear simply because of the RFI issue with switch-mode supplies and the increasing unavailability of good regulated 12VDC transformer based power supplies of useful amperage size.

What most people of the general public do notice is those switch-mode brick supplies are compact and light weight and convenient with many voltage options, whereas a transformer supply is typically fixed voltage, large and heavy and cumbersome. And the bigger the output current the more the size and weight become an issue. Under 1A and things are pretty even size and weight wise... get to 10A and the the switch-mode might crack 1kg and the transformer version could easily hit 10kg and still be comparable in size. Step up to 50A and weights shift to 5kg and 50kg respectively.... I have a Regulated 12VDC only 60A (800W) rack mounted 8RU high brute of a transformer based power supply that easily weighs in at over 50kg, and a variable switch-mode 0V to 60VDC 70A 2RU high rack mount unit that would be hard pressed to reach 10kg..... Weight and convenience is an issue to many if not all, and often at the expense and detriment of RFI.

You will notice that any RFI problems will be most apparent in and around the higher impedance and small signal areas of the project. RFI in of itself is only a tiny signal which appear to be big when beside other small signals on high impedance circuits such as those coming from a guitar. Once you get to line level the impedance's of the circuit are often lower and the wanted signal is often higher so RFI still being the same size will have far less overall effect. On output stages that have high currents on lovely low impedance 4, 8 or 16 ohm values then the tiny RFI signals will be near impossible to detect. The net result is having a buffer/small boost pedal on a relatively short lead to the guitar often defeats many RFI problems in most situations due the slightly increased signal size and a change to typically a lower instrument line impedance from the pedal to the amp..