µControlled Volume Control

awasson

For a house warming gift, a friend of mine gave me a set of 6 volume controls and an amplifier interface box of sorts. It's basically a bunch of transformers and potentiometers that can be hooked up to run stereo in 6 rooms with a separate volume control for each room. It's really nice stuff and other than not having 6 seperate rooms to make use of every channel it's going to be pretty cool.

So, I got to thinking: How can I take my obsession,.. er... interest in µControllers and integrate it with the home music experience?

I have a luminex light dimmer that is probably run with a PIC or AVR to slowly increase/decrease the lights and can be preset to a set level so that every time you turn it on it gradually increases to that level. If you tap it twice it goes either on or off. Has anyone done something like that with a volume control?

I just moved and everything is in boxes but I know where 2 SXTech boards are along with my SXKey so... I should be able to start this soon.

Any tips and/or pointers would be great.

Thanks,
Andrew

Guenther Daubach

You might use digital potentiometers, like the DS1803 dual digital potentiometer from Dallas to remote-control the volumes. The DS1803 is controlled via I²C. Although the I²C bus is normally used to control devices on one PCB, or on several PCBs close together, I have used I²C over lengths of a couple of meters w/o problems by using a reduced bit rate, say 10 kBit/s instead of 100 or 400 kbps.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Greetings from Germany,

Günther

Dave Paton

Andrew-

If I understand you correctly, the box you've got does NOT contain potentiometers, but instead uses L-pads. If you can provide more information (model #s, pictures, anything at all) about them I can be more sure, but I suspect you'll kill any digipot you wire into it. The boxen like the one you've described usually use transformers to match impedence on nlong speaker cables and isolated L-pads to control voltage without changing the load on the amplifier. In the course of doing all this however, the L-pads have to be able to dissipate a portion of the load current, and are generally rated at a minimum of 5W. Digipots are meant for the little line level signals, and usually have wiper resistences of 50 ohms or higher, and power ratings measured in microwatts.

-dave

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
This is not a sig. This is a duck. Quack.

awasson

Thanks Gunther and Dave.

Indeed they could be L-PADS. I've been out of the audio loop for 6 years and I've already forgotten basic terminology (and fairly important differences between parts like l-pads and potentiometers) They click as you turn them so they have 11 positions from full off to full on and they have fairly beefy transformers on them as well.

The manufacturer is: Sonance
The model number is: VC51-S

The amplifier interface board where all 6 channels are connected to is a Phoenix Gold one. Model number: SAM100

I'm not sure how I'm going to do it but I'd like to replace the Sonance VC51's with some sort of digital L-PAD.

Andrew

Dave Paton

Andrew-

Yup, you've got L-pads.

Unfortunately, the only ways to get a digital L-pad would be a large series of switched resistors (12 steps = 48 resistors) or build some kind of motor control. If you're really into DIY, I'd suggest a multichannel amplifier, and that you control the line level input to each channel rather than sinking output power. Not only is the line level control option more efficient, it's much higher performance, since those transformers slaughter the frequency response, especially at the low end. If you're a hardcore DIYer, you should check out this thread. Chris and I are working on an LM3886 based amplifier module offline (50ish W at 8 ohms), and I'll be publishing the gerbers and layout once it's all done. There's also a lot of goodies in there on high quality digital volume control, power supplies, esoteric display types, and preamp stuff. Getting a 10ch 50WPC amp together should be possible for under $200 if you scrounge.

-dave

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
This is not a sig. This is a duck. Quack.

awasson

Thanks Dave,
You know I can't argue with that logic and really should have come to that conclusion. Of course the dynamics of the amp get thrashed by big resistors and I'll bet the phase get's slightly twisted as well.

Darn, I was hoping to be able to do something clever to attenuate the output and control it all with my SX's

I'll have a good look at the link you suggested. My Dad will have a fit though. He's a tube guy. 6550's and big Acrosound TO-330's. I just picked up a couple of vintage Dynaco A430's for him. He's going to build a pair of Ultra Linears. He's not going to be impressed with my solid state amps at all

....But, if I can build a 10 channel 50w per channel amp for about $200, I'm in!

Cheers,
Andrew

Guenther Daubach

Andrew,

Dave is right, you can't replace the L-pads by digital potentiometers - the will blow off the socket immediately . You might use then for volume control at the amplifier input side.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Greetings from Germany,

G

awasson

Hey Dave and Gunther,
L-Pads vs digi pots... No argument there... I can almost smell the burning electronics.

I was a huge (I mean HUGE) car audio nut back in the 80's and 90's. Car Audio Nationals, IASCA, etc.. Lots of travelling and crazy audio schemes.

I'm a headroom guy so my car at the time BMW 2002 had Isobarically loaded 15 inch Rockfords driven with a Rodek mono block for each driver. An A/D/S PH15 ran the Dynaudio mid bass, dome mids and Focal Tweets up front. No center channel or rear fill. The car had so much glass it din't need anything in the back. I used an Audio Control 4XS to drive the inputs of the PH15 for the front half of the car and the sub out for my Rodeks. I still have most of the gear.

We got really silly with Alesis rack mount EQ's custom 4rth order electronic Xovers and Pro Audio McCalley and Pro JBL bass drivers but it just got to be too much twiddling of dials and pretty costly too.

I eventually toned it down and put a set of MB Quart 215s in each corner and toned the sub down to one 15 with an a-periodic membrane of sorts. You could say it was a very well done infinite baffle. It worked very well, The car is much more rigid than it was before I reinforced the back with Medite, and fiberglass.

Well it looks like this project has ground to a halt and will have to shift to the pre amp side of things. Dave, I've been trolling your other posts about your amp project but figured I would hold off posting there until I have something usefull to contribute. Great project and I look forward to seeing the amp modules.

Thanks both of you for the input

Andrew

Post Edited (awasson) : 10/3/2005 4:12:25 AM GMT

Dave Paton

Andrew-

Oh man, were those Alesis things MEQ-230s? If they were, I feel sorry for you. They're probably the worst designed pieces of excrement I've ever laid my ears on. The phase interferance and the fader interaction make true tuning impossible without a TEF or SMAART or something like that.

I have a PH15 that will be living in the trunk of my 95 Civic Coupe this winter, and an MTX500D driving a JL Stealthbox that's already in. Components up front are Infinity Reference 6.5"s in the stock locations (mod'd tweter pods, XOs hidden in the doors). It's an addicitve hobby, and while I don't have any aspirations to compete (my house sucks up my time and money), I do plan to stealth everything in really clean, and probably leave all the processing to a DBX DriveRack 260. Right now the only way anyone knows I have a partial system is if they catch the Infinity logo on the tweeter grilles. I like it that way.

As far as the preamp goes, I'd say jump in and experiment. Order some samples of the PGA2310/2311 from TI, drop them into a breadboard, and start messing around with a Stamp. For something like I described above, the topology would go like this:
Single input from the tape monitor out of your existing stereo
Buffer that input with some high quality ultra-low offset opamps
Run it into as many PGA231Xs as you like, with the inputs paralleled left to left and right to right with the other chips, but start with one.
Buffer each output with the same high quality opamps used as the input buffer. BYpass caps ont he power supply and the PGA231Xs should be liberal.

Build an LM386 circuit to test with, and use a small speaker. It's best to blow up a $1 speaker or a $3 amp than a more powerful LM3886 design, if you have the option.

Once the preamp is working well, without DC offset, etc, build a single LM3886 module. Test, verify, fix as required, then repeat as much as you like.

I bet if you ordered samples today and dug up the right power supplies, you could put together a 4 or 6 ch controller over the weekend with some pushbuttons and LEDs for control. If you want to put input switching into the preamp, it's a little more tedious, but not complicated. I'll see about posting some of my older PCBs here later in the week if you're interested.

-dave

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
This is not a sig. This is a duck. Quack.