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  #1  
Old 04-15-2012, 07:08 AM
sidewinder sidewinder is offline
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Default MPA as Guitar Effect Expression Pedal does not seem ideal

I tried to use the MPA as an expression pedal for my guitar effects, namely a Diamond Memory Lane 2 Analog Delay. An expression pedal is basically just a pot attached to a stereo jack (Tip=Wiper, Ring= A, Sleeve=B). Ultimately, I must say that I have not been able to get it to work as desired.

First it seems that the 100k MPA does not fully reproduce the delay's onboard 100k pot range. For example, if I'm controlling the delay feedback level, I can not get it to go to 1 repeat, the lowest I can get is 6 using a program change value of 001. Same with the mix, I should be able to get 0 audible repeats at the minimum value but It is still pretty audible. 001 only gets me to 9oclock compared to the knob I'm trying to control. However, setting the program change to 128 does seem to match the original pot's range when going completely clockwise.

Secondly: it doesn't seem to work well when using a power adaptor. It will work fine if the parameter you are controlling does not carry audio signal (Ie: Rate or depth of a chorus or tremolo) but when controlling the mix or the feedback level of the delay, I get a distortion on the echoes.

This does not happen when using a battery. The battery allows everything to sound pristine as it should. The only problem is that a normal 9v battery only seems to last an hour with the MPA. I installed a switch so that the battery would not drain when the MPA is not in use, and after 2 days, I've only gotten about an hour of use before the sound started to distort and the MPA shut down. As soon as I changed the battery, everything was good again, for another hour that is. The wierd thing is that the sound of the dying battery is the same as the clipped distortion effect I get when using the 500ma power adaptor...

Here are a few pictures of the completed project. Unless I can get it to work with a power adaptor and to properly reproduce the minimum range of my effects on-board pots, it will not be part of my setup.

Does anyone here have an idea on how to fix these issues?


Last edited by sidewinder; 04-16-2012 at 04:05 PM.
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  #2  
Old 04-16-2012, 05:15 AM
Jim McDougall Jim McDougall is offline
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I would treat the observation that it works fine when using fresh batteries, and when the batteries get low as a clue to the problem with the power supply. I sounds like what ever you are using for a power supply is not putting out a full 9V . The MPA like all of the HL boards has its own on-board regulator to provide a clean 5V to its circuitry. Because of inherent losses, a regulator needs at least a volt or 2 above the output voltage. That's why the specs call for a 9V power supply. Most wall warts can vary a volt or 2 from their spec so a 9V supply should supply at least 7V which will carry the regulator to output a full 5V. It sounds like your power supply is just not delivering the voltage is should under the load from the MPA.

Now this may be one or both of two problems:
a) weak power supply
b) there is a low resistance between the + and ground. This would have two impacts a) short battery life and b) voltage drop from the power supply.

As to the problem with the ranging, you are looking at the underlying problem of using midi. Remember midi only supports 128 steps for the range for a CC so if you map that to 100K you get about 781 ohms per step. At the top end you will get full resistance but at the bottom you may be out enough to miss mapping.

Is the original pot a 100K linear or log/audio taper pot? This can have a significant impact on response characteristics.


Does the switch disconnect the battery only or is it a general power switch? In other words, are the battery and power supply in parallel at any time? This should not be allowed. If they are, it would certainly account for the behaviour you are experiencing.
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Old 04-16-2012, 03:38 PM
sidewinder sidewinder is offline
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Hi Jim! thank you for the reply, it is very appreciated.

I can tell that the switch works to disconnect power from the MPA because when I flick it off and on, the MPA LED lights up for selfcheck. However, I may be missing something.

Here is my wiring (the switch is obviously a stock image and not the one in my box)



As for the power supply, I was using my Voodoolabs pedal power 2 plus (specs are found here: www.voodoolab.com/pedalpower_2.htm) at first and I was getting the same problem. I thought getting the MPA its own power supply would fix that but it didn't (although it did remove the hum I had without me having to add an additional ground)

Any ideas on what I'd have to do to get it to work properly? Do I need a 12v power supply ? I can try using one of the 12v outputs on the voodoolabs pedal power 2 plus.

Any idea how many MAs the MPA required to function at it's best?

As for the pots I'm replacing, Diamond pedals (the manufacturer of the pedals I'm trying to control) had confirmed that 100k was the best value to use for an expression pedal for all their effects. Is there any way I can fix this by adding an extra resistor?

Thank you for your help!

Last edited by sidewinder; 04-16-2012 at 04:29 PM.
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  #4  
Old 04-16-2012, 04:02 PM
sidewinder sidewinder is offline
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And in case you were wondering, this is how I'm wiring my Dc jack, which is the same that's shown in the picture:

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  #5  
Old 04-17-2012, 12:35 PM
sidewinder sidewinder is offline
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I tried 12v and nothing changed, tis quite perplexing.
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  #6  
Old 04-18-2012, 02:36 AM
Jim McDougall Jim McDougall is offline
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Ok First a couple of dumb questions:

1) When you use the power supply - is the battery still in the circuit as well ?

2) When you are using the power supply - is it only plugged into the MPA and suppling power to nothing else ?

Now onward !

you should be able to get the pot functional by doing the following -- if you don;t have one already pick up a cheap multimeter from Radio Shack or equivalent. Based on the values you are seeing, a CC value of 1 still produces too much voltage for your effect- so we need to trim it down. Pick up a 5K trimmer resistor -- connect the output of the MPA to one side of the trimmer and then connect from the wiper to the effect. Do not connect the other side of the trimmer to ground. We want to create a variable resistor not an addional impedance load. Since you are getting the full output at cc value of 128, then stepping is such that there is some signal being passed at cc value 1 -- so we need to drop the overall output. There is going to be a bit of a balancing act. You want to send cc values of 1 and 128 alternately and watch the effect on your equipment as you slowly adjust the trimmer --- at some point you will hit the best compromise between the reaction to the lowest limit and the highest limit .

According to the specs -- MPA draws about 300+ ma plus another 25- 50 ma per load. If all four chips are installed then I would expect another 100+ ma so that will bring you up to a wall wart that is rated for 500ma or higher just for the MPA alone. If you are driving the MPA and another effect from the same supply , then that supply probably has to be in the 1000 -1200 ma range. 12 V is too high and the key here is enough current delivery capability.

The Voodoolabs supply is way too small on any of it outputs to support the MPA.

If what you have been using to power the MPA is less than 350 ma then I would expect to see the same effect as low voltage battery. As the power supply tries to meet the current requirements , if it cannot deliver, then the voltage will sag to compensate and you will drop below the input min for the circuit to operate correctly.
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Old 04-18-2012, 02:00 PM
sidewinder sidewinder is offline
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Hi Jim, thank you so mch for the reply.

to answer question #1, no I am either using the battery as shown in my first diagram, or the dc jack as shown in the second, not both at the same time.

As for #2, when using the 500ma supply, only the MPA is connected. Does this mean that the MPA draws more than 500ma ? This would explain why the MPA only lasts an hour on a 9v since a typical Duracell is 590mah. Should I try exchanging the 500ma power supply fo a 1a?

As for the resistance, I wrote to Diamond pedals and this is what their head technician wrote back:

"Hi Vince, I'm guessing it is one of two things- either the pot tolerance on the Memory Lane is coming in to play and the pot is actually 105k, 110k, etc. Or, the digital potentiometer is not covering the full range- do you have a multimeter that you can use to check the range of the digital potentiometer?"

So if I were to use the 5k trimmer, how exactly would I wire it up? (sorry, I'm very visual so I'd need a diagram).

Does this seem right?




Ps: what would I use the Multimeter for? And is it possible that I'd need a value greater than 5K?

Thanks Jim!!!

Last edited by sidewinder; 04-18-2012 at 02:26 PM.
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  #8  
Old 04-19-2012, 02:05 AM
Jim McDougall Jim McDougall is offline
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Blue wire from jack to outside of trim pot where you have the red wire now. Center trimpot to input as shown. In other words, remove red connection to trimpot. Blue -- from jack to edge trimpot. Different color from center trimpot to MPA output - where blue wire continues on your diagram.

To my way of thinking a 500ma 9v power supply should be sufficient and a 5k linear pot should be sufficient.

A multimeter would provide the tool to check several things. We could measure the voltage from the power supply before it is plugged into the MPA and then the voltage when plugged into the MPA. This would tell if there is too much of a voltage drop for the MPA to work properly. We could also measure the actual current draw to see what the MPA is trying to pull from the power supply. It would also allow checking the original pot's resistance.
IF you are going to be an experimenter, it is a necessary tool that's cheap !

It is quite possible that there is something wrong on the board that is causing the MPA to draw more power than it should and is causing the power problem.

John needs to jump into this discussion because I would believe that the MPA should last more than an hour on battery. The other boards draw about the same power and last for hours and hours on battery.
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  #9  
Old 04-19-2012, 02:24 PM
sidewinder sidewinder is offline
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Hi Jim, it seems to me that there could indeed be something wrong with the MPA if it is drawing so much...

I'll PM John and ask hm to look at this thread,

As for the order of the wiring, do you think you could draw it for me? It seems like you're having me switch the way my TRS jacks are hooked up the the Pot which I can't do if i want it to function properly. The Tip must connect to the Wiper, the Sleev must connect to the B and the ring must connect to the A if I want 001 to be my minmum value and 128 to be my max.
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  #10  
Old 04-19-2012, 08:52 PM
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John John is offline
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Hi Sidewinder,

About the issue of the MPA not producing the same output range as a regular pot--the digital pots on the MPA have a pretty loose tolerance. If you take a look at the "DC Characteristics" in the datasheet:

http://ww1.microchip.com/downloads/e...Doc/11195c.pdf

...you'll see that the expected resistance is between 70kohms and 130kohms for a part with a nominal value of 100kohms.

I'm guessing that if you disconnect the pots from the effect circuits and measure their resistance, the measured range is somewhat less than 100k, which would explain the difference in the performance of the effect circuit.

You can try using a fixed resistor in series with one of the digipot terminals--perhaps this may change things to your liking. If using the MPA outputs as a rheostat, it's also possible to connect two MPA outputs in series and give them identical configurations, effectively creating a variable resistor with a doubled resistance value.

Regarding the signal noise--I'm guessing that you have a "ground loop" problem. This is a very common issue in audio electronics and you will find many related discussions on the web. Unfortunately, my ability to diagnose the root cause is limited both by my lack of experience with guitar equipment and also by my remote location which prevents me from attempting a trial-and-error approach with your circuit.

My best advice is to continue with battery operation if you can, and continue to experiment with the wiring to solve the grounding issue. Perhaps if you ask some of the guitar guys here (check the threads on the MSA forum) they will have some ideas.

Thanks for posting the project photos!

Last edited by John; 04-19-2012 at 08:55 PM.
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  #11  
Old 04-20-2012, 11:09 AM
sidewinder sidewinder is offline
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Hi John, because the MPA only lasts an hour with a battery it is not realistic for me to keep on using batteries. The distortion doesn't sound like ground loop hum it sounds the same as when the battery starts getting weak after an hour. If I measure the output of the MPA pots and they are only 80k for example. Does this mean that I would need to use a 20k resistor inline with the MPA to cover the full range? Would I wire it between the Tip of the jack and the Wiper of the pot?
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  #12  
Old 04-20-2012, 06:42 PM
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John John is offline
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Quote:
Originally Posted by sidewinder View Post
Hi John, because the MPA only lasts an hour with a battery it is not realistic for me to keep on using batteries. The distortion doesn't sound like ground loop hum it sounds the same as when the battery starts getting weak after an hour.
Have you measured the voltage at the MPA DC IN terminals with the DC power supply connected and the MPA in operation? DC power supplies (the "wall wart" kind, especially) can have output voltages that vary wildly from their nominal value.

Quote:
If I measure the output of the MPA pots and they are only 80k for example. Does this mean that I would need to use a 20k resistor inline with the MPA to cover the full range? Would I wire it between the Tip of the jack and the Wiper of the pot?
That is an approach you can try, yes. You might have to experiment to get something you like.
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