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Old 03-27-2011, 08:52 AM
phill.cann phill.cann is offline
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Default MD24 switching channels on a guitar amp.

Hi.

Recently got one of the MD24 decoders. Absolutely great, should do exactly what I want it to. That being said, I am having a little difficulty working out some of the finer points of how I do what I want and was wondering if anyone might be able to point me in the right direction. Not sure how much background to give on my project so I shall start at the beginning.

I am trying to build a Midi switching unit for my guitar amp. Marshall for some reason did not build this amp with anything other than their foot switch in mind to control the channels and features.
not a big problem, I have disassembled the foot switch and derived the following diagram as a result.



It's worth noting that the pins (11,8,2,1,10 and 15) run at 5v when you switch the amp on and to switch "on" a channel / feature, you ground that specific pin to enable the feature. The amp channels operate like a latch, just ground that channel its on till you ground a different one even if the voltage returns to 5v. The Reverb and Solo channels work slightly differently in that they must remain grounded for the duration of their use.

Now then, the following diagram is pretty much what I thought I needed to implement to allow the MD24 to trigger a transistor to switch my amp channel.



I believe I have worked out correctly that R1 needs to be approximatly 175 ohms if I am taking a 5v, 25ma signal from the MD24 and dropping it to 0.65v to trigger the transistor.
R2 i believe would be used to prevent the connected pin having any signal path unless the transistor's base/emitter are on.

Just an aside, I am pretty sure this circuit will do the exact opposite of what I want at the moment as my understanding is that when there is no signal passing through the base/emitter then the collector/emitter will be the path followed. I have solved this just using NoteOff and NoteOn signals the other way around for the MD24 for the time being. This isn't a permanent solution because I believe it may mean when I first switch my Guitar amp on, all 4 of my channels will be grounded, therefore switched on and I am not sure quite how much noise / destruction that would produce. And quite frankly, do not care to find out. :-)

Basically, long story slightly less long, the problem i've got is that I cant seem to make the MD24 "switch" the transistor on and off. I am not sure if I am missing something like a diode where I have got Pin 0 from the MD24 and Pin 14 (0v) from my amp connected?, or if I've worked out my resistor values wrong? or indeed something else all together.
Sadly my fundamental understanding of electronics isn't quite good enough to work out my problem un-assisted so any help would indeed be greatly appreciated.

The supply voltage I'm feeding the MD24 is 12v @ 400ma, not sure if that affects the Midi output current. As I saw 25ma mentioned in one of the PDF's so used that value to work out my resistor value.

Thanks in advance.
Phill Cann.
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Old 03-30-2011, 08:41 PM
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John John is offline
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Hi Phill, welcome.

Based on your post, I suspect that you might benefit from some reading about basic electronics concepts. Specifically--the relationship between voltage and current, ohm's law, etc. It's not hard to learn and it will pay off with time savings in this and other projects.

Here are some links to the info:

http://forum.highlyliquid.com/showthread.php?t=14

Something to keep in mind--voltage is the cause and current is the effect. Current specs on power supplies and other components are generally maximum ratings.

I wrote a detailed post about power supply selection here:

http://forum.highlyliquid.com/showthread.php?t=447

...regarding your application:

Using a switching transistor can be useful, but it's may not be necessary in this case. The transistor approach helps to control currents larger than what the MD24 can directly sink/source.

For example:

http://forum.highlyliquid.com/showthread.php?t=3

However, the transistor may not be necessary if there is not much current flow when the function pins are grounded. If that's the case, you might get away with something like this:

(MD24 output) --|<|-- (function pin)

That's a diode in the middle. This will allow the MD24 output to effectively "ground" the function pin.

I'm not too knowledgeable about guitar amps...but you might find some of the guys discussing effect switchers (look in the MSA support forum) and ask for some help. They might have some good suggestions for you.

Also, the MD24 has "inverted" output modes, so that will give you an inverted signal, if it helps. (as in: 5V to 0V to 5V, instead of 0V to 5V to 0V)

Hope this helps...
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Old 04-01-2011, 07:14 AM
phill.cann phill.cann is offline
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John,

First of all, thanks very much for getting back to me. I shall have a look through those links a little later on when I am back from work.

The inverted output modes from the MD24 that you brought up would be the solution to my problem as I have actually had the MD24 switching all of the channels on the amp properly when I have configured my Midi Foot switch to send a NoteOff signal to turn on a channel, and a NoteOn signal when I switch to a different output on the MD24.
The only reason I haven't used that as the final solution is because when I first switch the MD24 on, all of the outputs were at zero volts. This had the effect of switching on all of the channels on the amp.
If I could have all of the outputs (or at least the 6 connected to my amp) from the MD24 at 5 volts by default when the board gets power, that would completely solve my problem.

I have had a quick look into the .pdf files attached to MD24 page and can I gather from that the way the inverted output mode is implemented is to create a SysEx message to reconfigure the specific ports I want to?
I have never used SysEx messages before but I am assuming that some sort of midi sequence software would be able to send the message chain to the MD24 to configure the output modes?
If this is the case, when the configuration has been set, will it stay set like that? or do I need to run it everytime I power the board down?

Again, thanks very much for your help so far.
Phill.

Last edited by phill.cann; 04-01-2011 at 07:24 AM. Reason: Updated : Found documention
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Old 04-04-2011, 02:41 PM
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Quote:
Originally Posted by phill.cann View Post
I have had a quick look into the .pdf files attached to MD24 page and can I gather from that the way the inverted output mode is implemented is to create a SysEx message to reconfigure the specific ports I want to?
I have never used SysEx messages before but I am assuming that some sort of midi sequence software would be able to send the message chain to the MD24 to configure the output modes?
Yes. There is detailed info regarding sending sysex messages here:

http://forum.highlyliquid.com/showthread.php?t=280

...the thread is written for the MIDI CPU but applies to the MD24 as well.

Quote:
If this is the case, when the configuration has been set, will it stay set like that?
Correct--once you get the configuration the way you want, you don't have to use sysex anymore. The configuration will "stick" even when the unit has been disconnected from a power supply.
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Old 04-04-2011, 09:56 PM
phill.cann phill.cann is offline
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Hi John.

All of this information has been most useful and I have successfully completed the project I was working on. :-)

Thanks for all your help.
Cheers.
Phill.
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