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Arduino based passive analogue input selection & volume control

post #1 of 123
Thread Starter 
Hi,

originating form the simple question (see below) of how to have a silent, high quality volume control & input selection I have decided to build an passive µController based, arduino compatible preamp input selection & volume control. Stay tuned! PCBs will come in the near future (I really need it).

To give you an image of the whole system I have drawn an image:


An Arduino compatible CPU is driving the whole system. It will have plenty of outputs for LEDs and plenty of inputs for buttons, rotary encoders or even analog potentiometers. The decision to base it on Arduino is to make programming as easy as possible. There is plenty of help out there to programm it to the specific needs.

The basic input selection can switch between a number of balanced or single ended inputs. For each input it is configurable if it is balanced or single ended.
The output can be balanced or single ended – according to your needs.

To convert between single ended and balanced there will be an an single ended to balanced converter. This module will be plugable in some way so that you can choose the technolgy you like (opamps, discrete or whatever).

The volume control will be an plugable module too - since currently there is a hot debate if LDR/chips or stepped attenuators are the way to go.

If needed a balanced to single ended converter can be attached after the volume control. But most probably this be not part of this design.

The beginning:

everybody seems to agree in this forum that using relays for analog input switching and volume control is the absolute best way to achieve the best signal quality.
Ok, sometimes potentiometers are better or have other advantages or disadvantages than relay based attenuators.
But in the end if you do not want to reduce the singal quality as minimal as possible it is the way to go.

But I am looking for digital (CMOS based) chips which do a comparable job. Does anybody now any analog switches and volume controls which achieve and extraordinary signal quality. Since there is no reason to introduce parts which have a higher distortion or THD level than your actual amp.

Any suggestions?
post #2 of 123
TI isn't letting me log in, but there is a digital volume control they offer that works great. Linuxworks might be able to chime in with that part number.

Are you talking about switching an analog signal or a digital one?
post #3 of 123
PGA23xx from TI is more or less the standard answer for your volume section based purely on specs, but why would you want electonic switches for input switching? As you say, almost everyone agree that relays are the best choice? If you are really anal you can find mil-spec, small signal, HF relays which will be even better than the ones used in standard hi-fi gear and they will be miles better than any IC-based switch I have ever heard of.

/U.
post #4 of 123
It terms of volume control, have you consider the Lightspeed Attenuator? This is basically a volume control implemented using light dependent resistors (LDRs). I've ordered a matched set of LDRs from a guy over on diyaudio and intend to use them to build a passive preamp. The approach seems to be generating a big fuss over on that forum so I thought I'd give it a whirl.
post #5 of 123
yeah, I've done a little bit of work with the burr-brown PGA2310 (etc) series chips.

perfboard version:



pcb version (from error401's board, hacked by me):









highly recommended! I love this chip. its dead silent, has no distortion to speak of and works like a charm. I love having smooth fades (like, when I press MUTE on my remote and it slowly sinks to zero and then rises back on unmute).

as for input switching, the analog cmos switches may work ok but relays WILL be better (lower resistance and no capacitance issues to care about).

I've also played around with motor pots and those can be fun, too:



but I like the 'speed' of the PGA solution. you can do neat thinks like assign memories to vol levels and when you change inputs, it can set the vol level to the last-set value for 'that' input, etc.
post #6 of 123
Quote:
Originally Posted by linuxworks View Post
{snip}.......
as for input switching, the analog cmos switches may work ok but relays WILL be better (lower resistance and no capacitance issues to care about).
...{snip}
Can you point me towards a good schematic for this or, ideally, a PCB source.
post #7 of 123
I used an analog cmos switch for spdif switching! works just fine at the 'mhz level' but it IS digital and I'm not sure that analog will pass thru as well.

I don't have a schematic (yet); just these photos showing the board wiring:

DIY: SPDIFmaster v2.0 is complete on Flickr - Photo Sharing!

DIY: spdif digital audio switch 'fabric' board on Flickr - Photo Sharing!

DIY: spdif digital audio switch 'fabric' board on Flickr - Photo Sharing!
post #8 of 123
Without wanting to hijack the thread, what I'm looking for is a schematic for switching analogue inputs in a preamp (I'm building one of those lightspeed passive preamps).
post #9 of 123
pj: do you just need something that has remote control (like IR) and selects '1 of n' relays to be on?
post #10 of 123
No. Nothing fancy. Say 4 analogue inputs into 1 via a switch.
post #11 of 123
Thread Starter 

Lets get it started!

Thanks for the massive answers.
I have the need to build a digital DAC/preamp. But this parts are for the pasive analogue section only. So expect some volume/input select PCB solution in the very near future. But I have not decided if it will be SMT or traditional through the hole, yet.

Quote:
Originally Posted by PJPro View Post
It terms of volume control, have you consider the Lightspeed Attenuator? This is basically a volume control implemented using light dependent resistors (LDRs). I've ordered a matched set of LDRs from a guy over on diyaudio and intend to use them to build a passive preamp. The approach seems to be generating a big fuss over on that forum so I thought I'd give it a whirl.
Do you have a link for that?
Sounds interesting – though it will be hard to get a clear voltage from an µController (without an ADC) – but perhaps worth a try.

Quote:
Originally Posted by linuxworks View Post
yeah, I've done a little bit of work with the burr-brown PGA2310 (etc) series chips.

[…]

highly recommended! I love this chip. its dead silent, has no distortion to speak of and works like a charm. I love having smooth fades (like, when I press MUTE on my remote and it slowly sinks to zero and then rises back on unmute).

[…]

but I like the 'speed' of the PGA solution. you can do neat thinks like assign memories to vol levels and when you change inputs, it can set the vol level to the last-set value for 'that' input, etc.
Do you know how it compares to the MAX4580-4600 series spec and impression wise?

Quote:
Originally Posted by Nisbeth View Post
PGA23xx from TI is more or less the standard answer for your volume section based purely on specs, but why would you want electonic switches for input switching? As you say, almost everyone agree that relays are the best choice? If you are really anal you can find mil-spec, small signal, HF relays which will be even better than the ones used in standard hi-fi gear and they will be miles better than any IC-based switch I have ever heard of.

/U.
Do you get any part numbers or series as starter?

Since I definitively need an solution it would be no problem to build something that just does not only fit my needs. Any wishes for an analogue, passive input selection & volume control?

I will have some thing like 3-4 analogue inputs, I2C input.
The software will most definitively be arduino compatible (what else currently).
It will have connections for buttons & rotary encoders.

Anything else?
post #12 of 123
Thread Starter 
Some ideas for the inputs/outputs:
It should work balanced or single ended.
With three balanced inputs or 6 single ended inputs – or anything in between.
I have no idea how single input to balanced conversion will happen – but there WILL be a solution
post #13 of 123
Thread Starter 
OK
PGA 2311 seems to be better than the comparable DS1881 (from the datasheets)
THD 0.0002 vs 0.005
Crosstalk -130dB vs -110dB
Noise 2.5µV vs 2.2µV
Interchannel Matching 0.05 vs 0.5dB

So it is decided: PGA2311 with relays.
post #14 of 123
Quote:
Originally Posted by _atari_ View Post
...snip.....
Do you have a link for that?
Sounds interesting – though it will be hard to get a clear voltage from an µController (without an ADC) – but perhaps worth a try.
......snip....
Take a look here. This is the (second) group buy thread and contains links to the original discussion thread.

But, please note my comments on page 4 of the group buy thread. It has been brought to my attention that a complete kit for something called the OptiVol module (from SKA Audio) can be purchased for less than the cost of the LDRs on the group buy. There may be a good reason for this (different quality LDRs?) so I have asked if anyone would care to comment on the SKA Audio offering.
post #15 of 123
Quote:
Originally Posted by _atari_ View Post
I have no idea how single input to balanced conversion will happen – but there WILL be a solution
I have heard great things about the THAT1646 chip

RCA to XLR? It's THAT (1646) easy! - AVS Forum

it seems pretty well respected in the pro audio community. yes, its 'a glorified op amp' (sort of) and so the discrete folks will turn up their noses to it. but the rest of us won't be bothered

there's also matching bal->unbal converter chips from THAT corp, I think. (the THATcorp guys used to be the old dbx company, I believe).
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