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Digitally controlled resistors

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eversince I know about the Midibox project I wish to build a midification for my guitar tube amp. Sounds absurd perhaps, but think of encoders instead of pots and some "digitally controlled resistor" inside the amp.

Of course there are "digital pots", but they have strange resistances and are quite expensive.

I don't know if there are pots with motors (like motorfaders?!), at least I couldn't find them.

So I thought of using a core and a dout in combination with a R2R-digital/analog-converter. The analog out might drive a LED / LDR combination....

Sounds very difficult to get stable and reliable resistance values, and I don't dare to think about timing.

But the pros would be:  

- potential separation

- only resistive component through LDR (nearly)

- resolution of R2R-network can be chosen freely

- part are all low-cost

Any comments on this?

regards, SexyBeast

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  • 1 month later...

You guys should look at using a relay attenuator, Take a look at http://www.recording.org/postt1290.html and you might do a search for the "relay attenuator" on the same forum.  

You can get as many or more resistance "steps" with a relay attenuator as a digital potentiometer, without as much coloration of the signal.

I'm looking at this as a way to eliminate the potentiometer and stepped rotary gain switch from some pre designs I'm working with.

Have fun!


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  • 2 weeks later...

About the digital pots, i have the MIOS code  (SPI interface based on the AOUT module of TK) and board layout for controlling the Analog Devices AD5204 if anyone is interested.

The poblem with replacing pots using this method is noise...

I had a lot of fun experimenting with the "Filth Munster" project and learned a lot about guitar effects and MIOS.

But i'm close to giving up and going back to good ol' pots and 9V batteries. Good shielding and eliminating hum is sometimes hard enough even *without* mixing digital and analog circuitry...


There is a thread with helpfull comments by Duggle about this:


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Thank you for reply!

let us give it a try, even if there is noise....

it sounds too awesome!

can you put the board-layouts and sourcecode somewhere whitout much effort?

I can offer webspace, if you like...

keep it up your filth-munster box! its looking great.

are there any sound-samples available?


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There is a sample of the prototype here:


First you'll hear me turn up the distortion, next the volume. You'll notice the hum at the beginning of the sample from the adapter used to power the Core module. The effect itself is powered by a 9V battery and analog and digital grounds are connected.

I'm now struggling with powering everything by a PSU built into the rack... ground loops and hum cycles ahoy!


The AOUT module was soldered on "3-hole island" vectorboard and follows this circuit :


It's not that hard to build.

I've got a design for a PCB, but haven't verified that. I may put the link here later...

The AOUT driver can be found here: http://compiler.kaustic.net/fm_offline/fm_006/aout_AD5204.inc

You can use it like this:

      ;; update the digipot
      movff      POT_VALUE, MIOS_PARAMETER1      ; value
      movf      POT_NUMBER, W                  ; potnr
      call      AOUT_AD5204

The effects themselves you can find all over the net, some good sites are:





The current build of the FilthMunster firmware is very basic and rough, not very userfriendly but okay to test. It needs some effort before i can post this.

I can't say if i'm going to finish or continue this, it depends on time and other projects going on...

Hope you'll be able to do something with this!


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  • 2 years later...
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  • 7 months later...

another old toppic, but i think i can add thome words to this that might be of interest.

First, LDRs are no good choice when it comes to linearity and exact values.

Of course. This disadvantage comes out strongest when you try to controll a stereo device, have two chanells behave exactly the same, etc.

Big Advantage: They work great, they are reall ohmic resistance (other than fets, otas etc). They do not produce distortion. They can handle rather high voltages (very important for the guitar amp). They are cheap. They come in different ranges. They are quite fast if you keep the led brightness between max and only rather dull. When you turn out the light completly many will wander in resistance for minutes. But they all have a range where they are settlet fast.

Parallel and serial resistors can improve the responce curve for your project.

For a Guitar amp i would go that way.

For a stereo amp i wouldn´t...

Another thing to look at are all the high performance VCAs/VGAs out there. Have a look at the AD or TI-BB chips. Some have excellent audio performance. Some are controlled by analog voltage, some digital, so there would be no need to build a DAC.

The BB Volume controll chips are some of the best out there.

Forget about stuff like the 13700 Ota. Nice for synth projects, but you wouldn´t like your amp after that.

But one thing: Replacing a resistor with an active component can get you in trouble, depending on the ranges and polarities of your signal.

One totally different thing: I play a lot and i wouldn´t want to replace the pots of my amp with encoders! Really! Really! On Stage you´ll hate it many times. Just the grip and responce of the pots, you need to relay on that. On stage less is more!

If you want to load different setups i would think about building a effect unit that has variable attenuation/gain, an eq, some fx loops that can be controlled, can switch and store the amps chanells, etc. Make a nice footcontroller for it that can load your presets, maybe has some pedals. Done.

Then your amp becomes the master soundshaper that you can set up according to your needs. Remember that settings, especially volume and eq, stored or remembered will have to be adjusted for every room you play.

So you could store sounds for different songs, parts etc in the preampbox, and still keep the amp to adjust everything for different playing situations.

And if you have a softwarecrash the unit can be rellay bypassed...

This is only my oppinion on this. Really don´t want to talk you out of anything. Do what you feel like! Modding the amp can be a cool project. But it could destroy the sound and the handling of the amp!

Second thought: A compromise could be to mod the amp, but keep the pots. A second row of encoders would be needed to make settings to be stored. Then you could have a footswitch to select presets OR the pots. A rellay can switch between the variable resistors and the pots.

For a guitaramp this might not be that great, but for some studio equipment it might be cool. Keep the original look and feel that you love for jamming of your vintage gear, and have the option to store settings with your midisequencer.

Have fun!

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