Volume control - digital control with absolute rotary encoder

[Fernando]

I have 2 AMP11 (see http://www.41hz.com/forums/showthrea...d-an-AMP/page2).

I decided to build a volume control. After some research I found the PGA2311 (http://www.ti.com/product/pga2311) that says "Using high performance operational amplifier stages internal to the PGA2311 yields low noise and distortion, while providing the capability to drive 600Ω loads directly without buffering.".

The input source impedance must be less then 2 KOhm ("A source impedance of 600Ω or less is recommended. Source impedances up to 2kΩ will cause minimal degradation of THD+N.").

I think this is enough for input protection of AMP11: "Given VA+ = +5V and VA− = −5V, the maximum input or output voltage range is 7.5Vp-p.".

With the PGA2311 I will have the analog grounds of both AMP11 connected. Unless I use 2 of them.

Some questions:
1- Is 7.5Vp-p enough for avoiding damage to the AMP11?
2- Do I need an input buffer before the PGA2311? I don´t know exactly what are the input sources: CD, DVD, etc.
3- What about the connection of the two AMP11 grounds? Any problem?

For the digital pot I found a nice 10 bit absolute magnetic rotary encoder. I am buiding the mechanical part of the pot.

Both ICs have a very simple serial protocol so any microcontroler can interface them. I will use an AVR
ATmega. I am looking also for a display (maybe 16x2) with serial comm.

The power will be from a little transformer (2x9V).

Who want to comment? Someone working on this?

[kasimir]

Hi Fernando,
Last year i had build a Volumecontrol with a rotary encoder and a 32K digital Pot of Xicor. Have a look to my albums (incl. chematic)

[Fernando]

I don't know where are your albuns.

I don't want an encoder, I am building it.

Encoder.jpg

[tuo]

The schematics are on his blog. Just click the nick on the above message.

[krilli]

Sweet project!

I don't know where are your albuns.

If you click on the username and go to View Profile, there's a View User Albums in there. Here are Kasimir's albums for instance: http://www.41hz.com/forums/album.php?u=1110

7.5V P-P is too much for any of these amps' input. The input is limited to 5VP-P, coinciding with the 5V supply for the input side opamp. AFAIK 4V P-P is the range you want to actually use. There are ways of building a really simple diode-based input protection to clamp the voltage, info for example here: http://www.41hz.com/forums/showthrea...diode-clamping

Re. the grounds of the Amp11's: Should be no problem. I've used two Amp11s powered by the same supply, which is said to possibly induce ground problems. It was OK for me. If you do run into problems, you can easily adjust the grounding scheme with the elimination of one SMD inductor on one amp and a jumper wire between the amp boards. Sorry, I can't find the link but it's an easy mod - if you need it.

Wish I could comment further, but I hope this helps! Good luck!

[Fernando]

Thank you, I saw the album. Nice work but it is not very similar to my project.
I want to build a digital absolute rotary encoder with 1024 positions per 360º (maybe I will use 256).
An ATMEL microcontroller will read the encoder (serial) and will send the command (serial) to a digitally-controlled analog volume control (maybe PGA2311). At the same time it will update the display. I will have a function to set the zero of the encoder always I want, so I don't need to worry with the encoder position when I mount it (with 1024 values it is difficult to mount at the right place).
I don't need to remember the last position because I simply read the current encoder position at start-up.
The first draft of the encoder PCB shows that I could do this with a 22.2mm square PCB with the four 2.5mm mounting holes at 18.2mm from each other. The SMD are 1206 and 1210 for the big caps. Only the TOP SMD are really needed for normal operation. The botom are for programming the fuses (I will not use this) and to drive the LEDs (only for play with the magnets).
Scema.jpg
TOP_pcb lowres.jpg
bottonlowres.jpg
This PCB can be used with austriamicrosystems AS5040 or my favorite AS5140H. Both are magnetic absolute encoders with a very simple communication protocol (3 wires, CS/CLK/SDI).
My problem is that for only one PCB this will be very expensive (more than 100 Euros at eurocircuits).

Anyone interested in this PCB for a group buy?
Can I get a better price in Europe?

[Scratchy]

You might want to see if these guys will ship to Europe.
It's a good price for 3 PCBs.

[samuelsss]

This sounds like a fun project. I'm glad I found www.41hz.com site

[V-bro]

Indeed fun, sadly I haven't got much time to participate right away...

[Fernando]

Thank you for the replies.
I got the fusion PCB service from http://www.seeedstudio.com/depot/services-c-185.html. My PCBs are so little they offered some more than the minimum I asked for.
I like very much the 0805 SMDs but I am working with 1206 because I can see them better (and hope I won't loose so much then I lost with the AMP11). Now I realize I can do a square PCB with around 15mm.
The materials you can see in the photo are non magnetic: plastics, brass, copper.
Where can I ask for machine services to do the mechanics of the encoder? The laser cuting services from seeed are only for flat parts. I have 2 such parts and can use it if I like (I don't) 3mm acrylic. But the axis is not flat.
Now I am designing (with some help) the control board with an ATMEL USB. It's an hard work because I never used Eagle and my work is not hardware related. More one year and I will finish this and the AMP11.
Someone want to participate or help?PCB Enc.jpg