Yet another pedalboard variation

[WickedBlade]

Hello,

I'm new here and I've started designing another take on the pedalboard design.

You can find the current state of my project (along with a short introduction of yours truly) on the page I've started on the wiki: here.

It's still an early draft, but all comments and thoughts are welcome.

[lucem]

Hi,

you've put a lot of thought into it beforehand, that's good :)

Some minor corrections:

9V power supply)

Using a 9V PSU at 1A, you will have about 1A of current at 5V too. The MBHP uses linear voltage regulators,

those do not transform power, but create a series load of variable Z in series to the 5V circuits.

Current draw)

You calculated 1.12A as maximum continuous current draw. That's already a huge value, but you're not done here yet.

You need to credit for parts tolearances and the fact that spike current may be much higher - so expecting 2A pike current

as a rule of thumb is a good value. There are 9V wallwarts available that do provide that much current, although those are

a bit more expensive.

DIN module count)

Skimming over your specifications, I find there are lots of additional digital signals needing attention that you did not calculate in, like the break switches in the TRS jacks that notify you if something is plugged in.

Dimensioning for 2 DINx4 modules should be safer.

One thing I noticed is you're going to have the analog guitar signals and the digital circuitry in one case.

Now that's perfectly fine, but to avoid noise pollution of your small signals, you should consider taking some preparations:

- use impedance converter circuits at main input and output

- use shielded cable for audio signals

- keep digital and analog signals spatially and electrically separated

- use dedicated power supplies for analog, relays and digital circuitry

- if that is needed at all, only connect the ground planes of the circuits in just one place

- keep audio ground and case ground separate and do not connect those drectly, but only through a sufficient LP filter

Now for that, I'm curious, do you have any case drawings to visualize your ideas?

I'd love to see that :)

I had been planning to do something quite similar once, but later gave up on it

because my gear changed into another direction, so I could do everything with midi now

and had no more stompboxes to account for in the signal chain :)

Good luck!

[WickedBlade]

Thanks for your input. Here are a few answers and more thoughts, in reaction to your post.

9V power supply and current draw)

Ah, that explains why the wiki states that the higher the difference in voltage between the input to the 7805 and 5V, the more it's heating...

I have a silly idea: if I keep my 1A 9V power supply but equip my box with another 7805, wired in parallel to the CORE one, and dedicated to powering the relays, won't I somewhat be able to get a bit more current than 1A at 5V from my 9V 1A power supply?

Having 2 separate power supplies that plug into the same box would seem awkward to me...

DIN module count)

If I understood well, one DINX4 module can handle 32 digital inputs. If we take back the 10 (or 12) footswitches and a few more for the edition buttons and rotary encoders, that still leaves more than 10 inputs free, if I ever want to use TRS jacks.

But frankly, I'm thinking that having something actually plugged into a loop if the loop is activated is something for the user to ensure. I don't think I really need to check that with the PIC. I'm keeping the idea in mind, though.

other stuff)

- the impedance converters yo uare talking about are, I think, what I labeled input and output buffers. I plan to use opamp buffers, with high quality opamps (Burr Brown)

- I'm not sure if you're talking about shielded cables inside or outside the box

- The digital and audio signal path are totally separated, that's the nice thing about using relays. Or did I get it wrong?

- the stompboxes will use a separate "power brick", so no worries here

- I've read stuff about star grounding and all, but I'm not very well informed on that matter, I'm kind of hoping (against hope) that it will all go well

- separate audio and case ground, noted

[lucem]

No, you cannot get more current going out than is going in. Kirchhoff's rule.

Impedance converters = buffers, yes.

By signal separation I mean not only electrically, but also physically (as far away as possible).

Shielded cables outside and inside.

In some cases it might be worth checking TRS connects with the PIC, just think about it for a minute.

I don't mean the audio path, but i.e. expression pedals, channel switches etc.

I didn't mean the stompboxes with separate power supply, but having separate regulators for the relays, the core and the audio path. This way ripples and noise coming from the high load and digital sections will not spread easily to the audio section.

[WickedBlade]

No, you cannot get more current going out than is going in. Kirchhoff's rule.

Mmmh, I'm not convinced, but I'm definitely not the expert, here. I guess I'll have to find either a 2A PSU or buy two 1A PSU. Bugger!

By signal separation I mean not only electrically, but also physically (as far away as possible).

Shielded cables outside and inside.

Physically speaking, I was thinking of having the various signal jacks on the left side of the unit, while the MBHP modules (and related LCD and 7-segment digits) would be to the right. I'm not quite sure where I should put the relays, though; I'm thinking with the signal paths, to the left, but this is the part of the design where both worlds connects...

In some cases it might be worth checking TRS connects with the PIC, just think about it for a minute.

I don't mean the audio path, but i.e. expression pedals, channel switches etc.

I was thinking of connecting the loop outputs to ground when they have nothing plugged in (I'm not sure this is feasible with only TRS, though). This way, no need to warn the PIC about it. Or I could wire the relays ground to the TRS, so that it would have no ground if nothing is plugged, which means that it won't activate, and the signal will bypass this particular loop. The only caveat is that this means that the audio signal and the relays power supply share the same ground. Any thoughts?

The expression pedal inputs definitely need to be grounded if nothing's plugged, that hadn't escaped my attention :)

As for channel switching, I don't think this is actually a problem, since this isn't something we hear, and having nothing plugged won't bother anyone. I was planning on using transistors for this, by the way, because they're cheaper than relays and this is not in the audio signal path.

II didn't mean the stompboxes with separate power supply, but having separate regulators for the relays, the core and the audio path. This way ripples and noise coming from the high load and digital sections will not spread easily to the audio section.

The CORE has its own regulator, so that's covered. I'll definitely add a regulator for the relays, now that you mention it.

But I'm not sure what you mean about the audio path. There is nothing in the box that changes the audio signal, so there is no component (aside from the relays) that needs any kind of power supply.

[WickedBlade]

Guys, I think I need help or pointers to some good papers. I've searched the internet (and the forum and wiki) to no avail.

Basically, I want to separate analog ground from the MBHP ground, and I don't know how to go about that. Would a simple low-pass RC filter do, between the two grounds? If so, what R and C values should I use?

Also, if I want to use a separate 5V supply for the relays, does that mix well with the fact that the ground for the relays will be output by the ULN2803, which means, I think, that this is the same ground as the MBHP?

I was thinking of using a separate voltage regulator, too (7805 or 78S05), but I don't know how to wire it up properly. Should I copy the schematics from the CORE module, with the rectifier diodes and caps (C3 to C6 and X1 on the PDF)? Please note that I'll feed either 5VDC or 9VDC, so the diodes shouldn't be necessary.

Finally, I'm wondering how to hookup the opamps that I'll be using to make impedance buffers for my audio signal. It can be fed with 5VDC, but which one should I choose, since, I guess, this means that this signal will somehow be mixed with the audio signal that goes through the opamp... Should I use yet another regulator?

I'm very very confused, and would welcome any help!