embedded amp simulator/fx processor

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I've been seriously considering building a linux-based effects processor using a single board computer running linux/guitarix. I think I've decided on using an Udoo Quad since it hopefully has enough processing power, and with the onboard AC97 codec I won't have to deal with ADC/DAC's or USB sound which tends to have a higher buffer size than I'd like.

My idea is to have everything in a small console with a touch screen, and run JACK/guitarix with the html host for a simple display. There may be enough processing power to also run an audio recorder or do something with a looper or other fx software.

I'll use 1/4" inputs and outputs for guitar (or electric mandolin) in/line out, bring in mains power and have an onboard +12v power supply module, and have three switches: one for power, one for input gain selection (unity or gain of 2), and one for selecting whether to use a 1:1 transformer to isolate any ground loops on the output.

Opamps will be used for input and output to buffer (and optionally provide gain in the case of the input). Important since the guitar pickups have a high impedance, and may not be necessary on the output, but it couldn't hurt.

Anyway, I've attached a schematic of the circuitry (post 12v module). Anyone care to tell me if I've been careless somewhere? I can usually read a datasheet, but I'm still a bit of a n00b.


  • Amp Project Schematic 01.png
    Amp Project Schematic 01.png
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I wouldnt know how to respond on the linux processor - except to be impressed by it.

But Im not liking the input opamp gain switching. Its probably ok once set but depending on momentary conditions (make before break, or otherwise), there could be nasty milliseconds where there is no negative feedback while operating the switch, resulting in large transients?
That occurred to me last night. I'd either have to make it a make-before-break switch or omit it. Or just add another opamp and switch between the outputs. But I'm thinking a gain of 2 might work well for going between a guitar output and a mic input.

I chose those opamps because they have high input impedance and are fairly well reviewed, and also operate on a single supply.
I made a similar box, except that I used Windows 7 and Overloud TH2.

The prototype used an Intel DQ77KB motherboard with a Core I3 chip in it. Neither the chip or MB are currently available. I chose them because they ran from a 19 volt laptop brick, but I used a 24 volt SMPS and a 22 volt LIPO battery for a RC helicopter. The box also included a 7 inch touch screen made for a car computer, an SSD and a 1 TB laptop hard drive, a tiny wireless keyboard / mouse and Chinese 2 X 50 watt class D amp board designed for 24 volts.

I put the whole thing in a small box with 2 X 6 inch Parts Express speakers and it worked great. I could even run Ableton Live and plug my Push into it. It sounded good, and played quite loud...... for a while. The vibration from the speakers killed the hard disk within a few months, and after a year or so of abuse the motherboard became intermittent and now refuses to boot.

I plan to rebuild the whole thing using one of the Intel NUC's and a larger SSD, no spinning disk. I will also put the speakers in a separate box. Overloud has released TH3 which I use on my desk PC. My USB sound box has a guitar input.

I simply wired the input jack directly to the line input on the MB. There was enough gain, but more could have been used. I think that I will add a buffer stage using an opamp or vacuum tube in the next version. I would think a gain control rather than a bypass switch would be ideal.

The output of the motherboard has no problem driving just about any amp, just connect them together. Not sure about your particular board though.
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