Arduino based passive analogue input selection & volume control

[_atari_]

Quote:

Originally Posted by luvdunhill /img/forum/go_quote.gif what about using Vactrol or Silonex LDRs as switches? Also, there are a few commercial attenuators that use reed relays.

The relays are just for switching. For attenuation a PGA4311 will be used. But that is on a different board, so that you can use anything you want.

 

[luvdunhill]

Quote:

Originally Posted by _atari_ /img/forum/go_quote.gif The relays are just for switching.

sure, the question still stands though

 

[_atari_]

Ok,

now I got it right. Hmm - that is an interesting idea. Are those LDRs in light proof cases? Because you will have a lot of them in parallel. How high is the off-resistance and is it enough to not influence other input signals?
I think LDR-base attenuators are a good idea. But I am lacking expereince with them. And there could potentially arise a lot of problems due to heat, input signal strength and so on.
It is enough work to get the whole setup up and working. I will concentrate on this first.
If this is done I could think about an LDR based attenuator.
That is the whole idea of making it modular - you can change any of the components of the preamp for the component you like most.
My budget on time/money/knowledge or personal taste is perhaps not enough for anybody. So I want to leave it open how to implement better methods.
Your idea is very interesting - I do not want to say it is bad. But I do not think I will - currently - find time tinker around with that too.
Or did I get you completely wrong?

 

[luvdunhill]

Quote:

Originally Posted by _atari_ /img/forum/go_quote.gif Ok, now I got it right. Hmm - that is an interesting idea. Are those LDRs in light proof cases?

I'm assuming we're talking about using them as switches now?

Here's one part:

http://www.alliedelec.com/Images/Pro...es_6993010.pdf

 

[_atari_]

I am still concerned about their switch off voltage. according to datasheet this is 2K
If we assume something like 1k for input impedance of the amp - this enhances the inut impedance for one signal from 1 to 3k.
I am concerned about crosstalk between channels
I do not want to ground one channel - is this nice to the input stage??
Lot of questions

 

[linuxworks]

my gut feeling is that ldr's wont be high enough resistance to offer true isolation between input selector channels.

I tested one that I got at a local surplus store. this is the plot I saw:


25ohms @12v
.
.
.
130 @5v
200 @4v
460 @3v
1.7k @2v
100k @1v
inf at 0v

I'd have to check what exactly 'inf' means (very high; but how high, really?).

also, the 'closure' resistance isn't nearly close enough to zero to be a good input switch part (at 5v, though).

and the ldrs aren't that small. you could fit several relays (omron latchers) in the same size as a single sealed ldr module.

but its possibly worth TRYING, at least

who knows, maybe this is good enough for input switching. and its solid state, too, so no parts to bounce or wear out.

 

[_atari_]

But real solid state multiplexers & switches are so darn tempting.

Anyway.

I did some reconsideration:

1. TO220 regulators are real overkills for 100mA - going back to TO92 (should be enough for the 60mA or so.
2. Pansonic TQ relay are easily availlaible via mouser and smaller and come in latching and they are smaller.

So back to square one - expects updates at the weekend. This time with PGA4311 volume control (except above points) nearly finished.

 

[luvdunhill]

Well, I'm just throwing some ideas for you guys to think about out there. I've done my own circuits and research and come to my own conclusions. In this case, I've used LDRs as switches.

Also, I've had my nose in enough commercial gear that making some recommendations puts me in a strange position. For example, what about using CMOS FET switches, both Analog Devices and Siliconix make suitable parts, and there are at least two high end audio vendors doing this.

 

[luvdunhill]

Quote:

Originally Posted by linuxworks /img/forum/go_quote.gif and the ldrs aren't that small. you could fit several relays (omron latchers) in the same size as a single sealed ldr module

huh? They are around 6mm x 6mm, see above datasheet.

 

[_atari_]

As stated above - I have been tempted doing this. Since they are used for test and measurement equipment - so why should the be bad for audio?
Do you have any recommendation.
Thanks for your ideas.
I am really thinking about doing an LDR base attenuator later - after this project. It sounds interesting.
But exactly the personal opinion and preferences are the reason why I want to make it modular.
Its all about choices

 

[_atari_]

Over the weekend I had some time to tinker with the layouts again.
The input selector was too big for me, so I change some things:

1. Using Panasonic TQ4Y latching relays – they are smaller than the Omrons and latching (so less power consumption)
2. removing the possibility to externally power the Relays or get power from the voltage regulator.
3. Changing the voltage regulator 7805L in TO92 package - should be able to provide something like 100mA of power. The circuit will need a constant power of something like 10-20mA.
4. Complete redesign of the board

Without further ado, here it is:
Schematics

Board


I also found some time to start the PGA4311 based volume control:


Very straight forward:
One Input & Output (balanced), an SPI connector for programming of the PGA 4311, some capacitors & regulators for power.

A preliminary board design looks like:


Still some beauty work to do:

1. Checking & optimizing the ground & power planes
2. checking the wiring (especially wire paths & crosses)

Both designs will not draw very much power - so I think it is bearable to use solely film capacitors (with capacitance under 10µF) - what do you think?

 

[jtostenr]

Very nice design

How difficult would it be to include a switchable input attenuator on the selector board? Shouldn't add very many components or increase size much.


Also, what are the dimensions of the board? Max component height?

Jeff

 

[_atari_]

Quote:

Originally Posted by jtostenr /img/forum/go_quote.gif Very nice design How difficult would it be to include a switchable input attenuator on the selector board? Shouldn't add very many components or increase size much.

The input attenuator is designed as a separate board, since some people will not like the PGA4311. It would add a lot of hassle to add it to the board (due to layout) - so I think a separate board is ok (for me).
If you look for a stepped attenuator check Twisted Pear's Joshua Tree attenuator.

Quote:

Originally Posted by jtostenr /img/forum/go_quote.gif Also, what are the dimensions of the board? Max component height? Jeff

The height of the boards is about 2.2 inch. The width of the input selector is roughly 4.2 inch.
The maximum component height is defined by the plugs & capacitors. I think something like 1cm (~0,4 inch) - perhaps a bit more. You should be able to use horizontal connectors if you have more space horizontally than vertically.

Did this answer your questions?

 

[luvdunhill]

I personally prefer putting the input connectors on the board itself. Yes, it increases cost of the board and limits the connectors that can be used (though you could always wire the connectors), but I think it's a huge convenience.

 

[_atari_]

Quote:

Originally Posted by luvdunhill /img/forum/go_quote.gif I personally prefer putting the input connectors on the board itself. Yes, it increases cost of the board and limits the connectors that can be used (though you could always wire the connectors), but I think it's a huge convenience.

You are right.

But I prefer to mount the connectors to the chassis (gives a much more sturdy look) and it gives a bit more flexibility and makes the board size smaller.

If anything is finished I will post the eagle files anyway - so if you do not like any of my design decision (or should iI say compromises) feel free to change it.

Hope that it is OK for you.