Arduino Passive Preamp With Remote, Attenuator and Channel Selector

Intro: Arduino Passive Preamp With Remote, Attenuator and Channel Selector

Hello everyone,

Since You supported me that much in my last instructable I was encouraged to improve it more and to share it with You.

In previous build I had cheap preamp with motorized potentiometer, remote and channel selector. Motorized pot had some noise.

After some brainstorming with friend I came to idea to make my preamp from scratch. Arduino pro mini was chosen as "brain" and instead of standard potentiometer we put arduino powered attenuator. Also new preamp will have 4 channel selector, IR receiver for remote control and rotary encoders for "non remote" usage.

This preamp can be used as standalone device.

Skills required for this project:

• basic electronics
• soldering
• arduino programming

STEP 1: Attenuator

I repeat: I am not electrician and my knowledge is questionable, if I wrote something wrong, please correct me. But anyway I will try to explain for people like me what attenuator is.

Wiki definition: "An attenuator is an electronic device that reduces the power of a signal without appreciably distorting its waveform."

As I understood it, it is just set of resistor ladders like this one where you would "slide" and choose how much "steps" your ladders have ( how much resistance you have in series and in parallel ).

I found online logarithmic attenuator calculator which gave me not only values but also schematic for my project.

Logarithmic progression is for audio applications.

Those vales provided here are probably not the best, I just uploaded values I used in my setup.

STEP 2: Channel Selector

I have 4 relays which activate channels. I already had board with relays from old preamp. Relays are activated by optocouplers and optocouplers are activated by arduino. You can use transistors instead of optocouplers.

I uploaded simple schematic how You can use optocoupler to control relay with arduino.

STEP 3: Remote Controller

Since everything is controlled by arduino this was easy task. There is arduino library IRremote and lot of great tutorials on how to use them like this one https://learn.adafruit.com/using-an-infrared-library/hardware-needed.

For controller itself I reused old one. On link above You can learn how to read signal of old remote and re purpose it.

You can find code with explanations further down.

STEP 4: Rotary Encoders

Rotary encoders are like limitless potentiometers, like those on your car's stereo. Actually they don't have anything in common with potentiometers except that you can put button on them and rotate. They contact ground and one of the pins when you rotate them and arduino reads that signal.

Once again, someone else did dirty job for me, I found arduino code for this task online! I will cover that also in some other step.

STEP 5: PCB Layout

And now to put all that together. I used eagle cad for drawing schematic and pcb. You can find image and eagle project.

I decided to separate signal ground from "digital" that is why I have 2 polygons on top layer. I put mass as polygon and marks on top, and everything else is on bottom layer.

You can check attached fritzing breadboard scheme notes if you need more explanation how to connect peripheries to arduino.

STEP 6: Code

I lost full version of code and this is from test phase but remote works, it just needs second decoder configured. I will recover it or rewrite it asap and upload it here.

EDIT: I found and uploaded full version of code.

Remote control

First import IRremote library and initialise it:

#include //import library before including from here

byte RECV_PIN = 11;

IRrecv irrecv(RECV_PIN); //initialization

decode_results results;

Inside setup()

irrecv.enableIRIn(); //enable ir in

inside loop()

if (irrecv.decode(&results)) //this checks if new data arrived
{ //inside check what button was pressed

if (results.value == 0xCC91D36E) { //volume down pressed
volcnt -=1; muxWrite(volcnt);

}

else if (results.value == 0xE936013E) { //volume up pressed

volcnt +=1; muxWrite(volcnt);

}

delay(100); //this is optional it just slows down volume change
irrecv.resume(); //wait for next data

}

Rotary encoders

No additional library is needed for encoders.

Setup

#define ENC_A 8

#define ENC_B 9

#define ENC_PORT PINB

Setup()

pinMode(ENC_A, INPUT);
pinMode(ENC_B, INPUT);

Loop()

tmpdata = read_encoder(); //read new state either -1 left turn, 0 no turn or 1 right turn
if( tmpdata ) { volcnt += tmpdata; muxWrite(volcnt); }

Read_encoder()

I found this function somewhere online

int8_t read_encoder(){
static int8_t enc_states[] = {0,-1,1,0,1,0,0,-1,-1,0,0,1,0,1,-1,0};

static uint8_t old_AB = 0;

old_AB <<= 2; //remember previous state old_AB |= ( ENC_PORT & 0x03 ); //add current state return ( enc_states[( old_AB & 0x0f )]);

}

Rest of code is controlling outputs, just writing LOW or HIGH on pins.

STEP 7: Final Toughts

This is V1.0 and if you have any ideas how to improve it, please share it with me!

I tested it and it works good. Not all steps are equal in attenuation but it is good enough, there are enough attenuation steps to find sweet spot. Only problem is that sometimes there is some noise for millisecond when relay switches on. Maybe I need better relays or something else (any ideas? :)).

For next version I will include relays for channel selection on pcb, but for that job I will need to shrink project since size of my board is on limit for freeware. I even have the idea to redraw all other boards (amp, buffer, PSUs, riaa...) I have now in my hifi on single one to remove all cables inside, but that will be in next instructable.

Price list:

PartQuantityPrice
Total
PCB90cm20.1$
9$ Arduino13.5$
3.5$
Resistors 0.6w 1%340.03$
1$
Relays6 for atennuator + 4 for channels2.5$
25$
Optocouplers100.15$1.5$ connectors
~5$ IR reciver10.7$0.7$ Rotary encoders22.5$5$ Power supply
~10$

TOTAL: ~60$

This price is really rough. All prices are from local store in my city. If You don't need 4 channels or 64 atennuation steps You will get lower price. Respectively you can add more attenuation steps or input channels.

Comments

[lsllll]

Nice project. Wouldn't solid state relays have simplified things a whole bunch? Or ideally just using audio optocouplers for resistance control? I'm in the process of building a passive preamp and opted to go with the optocouplers.

[lsllll]

So, let me reply to myself. I've been looking for a passive preamp since I detected noise on my Nikko. Having looked through the DIYAudio thread on Lighstspeed, I have been planning on creating that preamp, but at the same token, I'm starting to see the benefits of creating something based on discrete resistor values. As such, I'm going to build both and see what I can manage. And I looked for SSRs and I can't find any that'll do doubles for stereo, so perhaps it was actually easier to go with just relays. I'm going with dual-relays for this implementation. Thanks again for the Instructable.

[JonathanP51]

Great project, I'm very new to Arduino and recently learnt how to use MOSFET with analogue write to control a load, could this work for volume instead of resistor ladder.

[AlexandreG78]

Hello, it's okay, your very good priojetoe but I'm having prblema with the code, what it could be since now, thank you very much.

[loretta13]

Looks a great project and just what i am looking for to get me started. I am going to use a TDA1029 signal source switch for selection inputs but needed an idea for volume control and ir control. Will let you know how i get on

[RussoD]

Great project!Code not working

[icsnerdics]

great project! i really like the idea of a passive preamp, i'll try it for sure, btw as you stated steps are not even between them, that's pretty usual with those ladders, you need to have resistors with ideal values, and if i'm not wrong the resistors you used are > 1% tolerance, you'd better go with something more precise, maybe 0,1%

still a great project, and awesome diy pcb (your first project was more messy for sure)

[MCumic10]

Thank You :)

I paired same resistors on left - right channel, so error I get is on attenuation steps. As I stated they are not ideal but it is good enough for listening. Pcb design is mine but someone else made it.

This is definitely not final version so check for changes before making it.

If You have any more ideas how to improve it please post!