Group Buy - Digital Attenuator (Kevin Gilmore)

[chinsettawong]

Wow.  That's super nice!

[mwl168]

10 hours ago, Kerry said:

Yes.  It's a standard linear pot rather than an encoder, so the volume is set by it's position. 

I have an I2C header if you want a display, but it's not necessary. 

Hi Kerry: 

Not sure if you are familiar with Twisted Pear Audio's offerings but is this similar to the Volumite they offer for their Buffalo DAC?

[JoaMat]

On ‎2016‎-‎10‎-‎15 at 6:45 PM, JoaMat said:

Managed to squeeze the code in ATtiny85. I do think it will be able to control the balanced digital attenuator...

...maybe.

May I jump in and quote myself?

Volumite consists of  ATtiny85, potentiometer and voltage regulator.

At picture above there is a ATtiny on the breadboard, power supply and potentiometer. The microprocessor is programmed and it seems that I get the correct signals to control the digital attenuator.

I don't believe you can use Twisted Pear Audio's codes with Kevin's digital attenuator.

 

[Kerry]

^ Agreed and Congrats!

You'd need to program the ATtiny as well.

For mine, I wanted to ability to control a display and leave the potential for additional integration open.  It's based on the ATMega328P and is roughly equivalent to the Arduino Mini Pro.

[JoaMat]

 

Thank you.

Programming – that’s my problem. I’ve to keep it simple.

With ATMega328 you will have lots of opportunities – motorized pots., OLEDs, remote control etc. etc.. An AMB OLEDuino version would be great. If you just are able to program it. Excited to see what’s coming.

[mtoc]

I have something still can't figure it out, while without any buffers, the input impedance of this attenuator board is constant, but how about output z? We take the 50k version as the example, how does output z change?

[mwl168]

Thanks JoaMat and Kerry.

I figured as much - the Volumite may work but will need to reprogram the ATtiny85. 

[JoaMat]

I used this link for programming the ATTiny with Arduino Uno.

Maybe you have to change the supply voltage on Volumite so it works with the digital attenuator.

Edit:
Consulting MAX4820 datasheet - looks that 3.3V at Volymite should be sufficient??

Edited October 30, 2016 by JoaMat

[mwl168]

Thanks JoaMat. This is an area I have much to learn before I build the digital attenuator.

And yes, you are exactly right about the supply voltage. The Volumite can operate on 2.7VDC  - 5.5VDC. It has an on-board, optional LDO 3.3V regulator which enables use of any 3.5VDC - 20VDC supply.

[Kerry]

One thing to note on my board is that I bypassed the 5V regulator for the out supply going to the digital attenuator.  The attenuator board is pretty power hungry, and that could overwhelm the onboard regulator.

Also, I added a 20K pull-up resistor on the pot's wiper for the larger board since the it's a 100K/3B taper.  I've done some mapping work in code to give a smooth/consistent volume across the full dial (modeled after my encoder).

[Kerry]

Here's the larger controller board up and running

 

The motor works.  I haven't tested the IR yet, but it's pretty straight forward.  I've got a bunch of programming to do to get the IR/motor going.

[JoaMat]

Excellent work you are doing with the controllers, Kerry.

I’m trying to learn how to program the Arduino. Below is an Arduino UNO with an OLED and a rotating encoder. Turning the encoder - volume goes up/down. Encoder knob pushed toggles mute. If rotation while in mute the volume is changed, but takes effect first when mute is off again. Last volume value is stored in EEPROM and is called back next time controller is powered on and it starts with mute on. I don’t know if this really works - I don’t have a digital attenuator.

[Kerry]

If you're able to control the volume on the display, you'll be able to get the digital attenuator to work too:)

[cspirou]

Is there any problem using this board for 4xSE channels as oppose to 2xBAL channels?

[Kerry]

No problem

EDIT:  Yes - agreeing with Kevin's comment below.  They are really two stereo SE controls per board in that you can control each stereo pair independently.

Edited November 1, 2016 by Kerry

[kevin gilmore]

you won't be able to do a balance control  that way, but you could 2 x SE and 2 x SE with different gains

[JoaMat]

Relays clicking.

[Kerry]

Nice 

I still love that you can make your own boards.  I just haven't had the time to even try.

[Pars]

Yes, those look good. Are they 2-layer? You aren't home brewing more than that are you? (Don't even know if that is possible).

[JoaMat]

Yes, it’s 2-layer. I think homemade inner layers is too complicated. Making vias is not that funny either.

My board is a bit smaller than the original, no terminal blocks and one ground plane for the entirely board. Kevin’s board have solid inner ground planes which is preferred, but I can’t do that in my kitchen.

By the way I'm retired, gives some additional time for funny things.

[JoaMat]

Have made some progress programming both Arduino Pro Mini and ATtiny85. My impression is that using 7 relays with 0.5 dB steps is sufficient and the eighth relay mostly contributes to more silent silence.

Opinions from folks that have experience from digital attenuators are welcome.

[mtoc]

Folks told me the ladder/shunt type have only two relays in the signal path... Are the two versions pots from Kevin (the orignal version and the v2) the same design as those designs (two relays in the path)?

[Pars]

I don't believe Kevin has posted a schematic, but I think it is similar to Amb's attenuator. Each relay is shunting a resistor to ground. There could be any combination of relays on at a time (8 maximum), for the 256 steps in the attenuator. I guess it depends on how you define "in the signal path".
See http://www.amb.org/audio/delta1/

Please correct me if this is wrong.

Edited November 21, 2016 by Pars

[cspirou]

I thought he did post a schematic 

 

[JoaMat]

Have been struggling with controller for the digital attenuator. Finally the Arduino Pro Mini works OK, both with rotary encoder and with potentiometer. I’ve also tried the little Attiny85 but it didn’t worked with both relay banks so I’ve given that up.

Power supply is to get an idea of current requirements for the attenuator.

I’ve pops when relays are switching between 7/8, 15/16, 31/32, 63/64 and 127/128 “bit”. At AMB’s forum the pops are discussed and I guess I’ve to live with that??