A new arduino project.. asking help from the arduino geeks in here

Hajj

Hi,
After building my dream sound system (I'd post pics if I knew how to), I am now in the process of building an arduino based control system for my diy sound system, and while I can manage the hardware part of things, my programming skills are rusty to say the least.

I was hoping I could find someone in here who would be up for an interesting and fun project.
It's not the most glamorous or exciting arduino-based project, so bear with me as I describe it a bit. And if you feel like taking part in it though programming or any criticism, please go ahead.
So, here we go:

The system consists of the following independent elements:
1- a turntable
2- an active preamplifier for the turntable (called a phono preamplifier)
3- a passive preamplifier for volume control (more on this later)
4- a power amplifier
5- a pair of speakers

all of these elements are connected with signal wires (what we call RCA cables), expect for the speakers that are connected with simple loose wires.

about the passive preamplifier:

1- This particular piece of equipment is different from the usual volume control (which in 99.9% of amps is a simple potentiometer). It is actually composed of a set of optocoplers (an LED and a photoresistor or LDR enclosed in the same package) whereby you control the LED side's voltage in order to get the photoresistive part of the optocopler to a target resistance.
In their actual passive state in my system, the optocoplers need to be closely matched so that right and left channel exhibit the same resistance at all levels.
Problem is, optocoplers are notorious for drifting with time, and now, 2 years later, my system is heavily imbalanced.

The idea here would be to control the led part through arduino in order to get an accurate tracking on both channels (left and right)
(I chose arduino because it's the microprocessor platform with the widest diy community, I have no religious beliefs when it comes to microprocessors...)
My current plan is to go through a DAC to control the voltage going to the LED part and an ADC to read the resistance on the photoresistance part.
Why the complexity of an ADC and a DAC you might ask? Well it's because these buggers are extremely sensitive to voltage variations at lower power values (where resistance is highest), and that's the reason they are currently fed from a highly regulated voltage power supply
Anyway, the setup would work in two steps:
step 1: calibration
This phase will be done every couple of months or so. The way I see it is working would be for the system to read the resistance of the photoresistance part of one channel at logarithmic voltage increments. These would be stored in a table. and then on the other channel, the system would try to look for these values by varying the voltage on the other channel. Everytime the resistances match up, the value would be stored in the table.
Step 2: Volume control
An encoder would be used to increase/decrease the volume. the encoder would be practically telling the microprocessor to call for the next upper or lower value in the table and drive the LED's with the looked up value.

Other parts of the project would be to control the operation of the system (on/off, delay etc..) through a set of arduino controlled relays and ultimately interface the arduino with a graphic android interface over ethernet.

The applications of the latter part would be much more versatile as it could be installed on any system.

Sorry for the long post (I already had a short post in the introduction section, and I've exhausted my quota for short posts :D )

Hope this pikes the interest of someone in here as I would really love to keep this local.

Regards

Nick

Hajj

Reading my post again, it seems I have indulged in too obscure terminology, and I might also have posted in the wrong forum. The programming or software forum might be a better option..
Mods please do move the thread if you think it should be elsewhere
Anyway, can someone tell me how to post pics?

Joe

Anyway, can someone tell me how to post pics?

Upload them to an image hosting service (like imgur) and link it here using the appropriate tag.

It's okay to keep the thread in this category.

yasamoka

This is very interesting. So basically you want someone to help in your project from the Arduino side?

Hajj

yasamoka wroteThis is very interesting. So basically you want someone to help in your project from the Arduino side?

Thanks
Yes, I need someone experienced in arduino coding. I can provide the technical details and information the programmer could need...

Hajj

This is the main system. Yeah I know, it looks fugly, but sound delicious... The pack of cigs is there to show scale of things
The red boards are the ones for the passive optocopler volume attenuator I was talking about...

Next is the Phono preamp before completion

And finally, my pride and joy, my vacuum tube amplifier

Don't hesitate to ask if you have any questions

xterm

Very clean, love the finishing.

tich

Hmm that sounds a bit overly complicated. It's totally doable though. What's the part number for the opto-isolators you're using? Can't you just replace those every year or so, to eliminate potential drift? Although I see that the isloator's output will also drift with temperature, so you could probably look for closely matched opto-couplers, then hook them up to the same heatsink to minimize potential differences.

Yet again, why not a simple two-channel logarithmic potentiometer?

Also, I see you've invested in a toroidal transformer. That must've cost you a small fortune to ship all the way to Lebanon :)

Hajj

Hi tich,
thanks for chiming in.
there is reason in the insanity of such a setup. allow me to explain.
A regular pot, with its mechanical contact point between the wiper and the resistive surface, will induce distortion at audio frequencies.
This distortion is usually not hears as much as it actually veils some of the details in the music. eliminating the mechanical contact is one of the reasons behind the use of optocoplers.
the second reason is that unless I pay insane amounts for a stepped pot with precision resistors, there's always going to be mistracking between left and right channel resistance in a two-gang pot.

Now that the idea behind the use of optocoplers has been established, we're back to the problems at hand:
the setups I use needs 4 closely matched optos (2 for series resistors and 2 for shunt resistors, since it's the only way to keep a constant impedance load), and in order to get 4 optos to track well, you need to buy around 150-200 of them, at around 2-3 dollars a pop (the ones I use are the Silonex SR series). OUCH!
As you've mentioned, temp related drift can also be a problem, but common heatsinking will minimize that.
So what say you? where do we go from here?

p.s.: the toroid in the pic is only one of 4 transformers used. I have invested much much more in transformers and shipping them than into electronics... the sad part of having to ship iron from abroad.

J4D

Hello Hajj! I can help. :)

Hajj

J4D wroteHello Hajj! I can help. :)

Hi J4D,
Let's get things rolling then :)
Where do we start?