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DIY Optical Audio Attenuators?

post #1 of 17
10/24/10 at 12:24pm
Thread Starter 

I am considering either building an existing design or just designing an new optical attenuator.  Probably will use some LDR's but I will do some more research into components for this project.

 

Is anybody else currently building/desiging or built on in the past?

Anybody know of some good solid design to work with or work from?

 

All opinions or suggestions welcome.

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post #2 of 17
10/24/10 at 4:38pm

Some links from diyAudio about LDR-based attenuators:

http://www.diyaudio.com/forums/analog-line-level/80194-lightspeed-attenuator-new-passive-preamp.html

http://www.diyaudio.com/forums/diyaudio-com-articles/168796-poor-serbian-mans-optical-attenuator.html

 

The Lightspeed Attenuator has been discussed a lot on Rock Grotto forums as well, I believe.

 

I haven't any experience with this project, but it looks like an interesting concept and something to try when I have some time in the future biggrin.gif

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post #3 of 17
10/26/10 at 9:01pm

all of my audio is electrical.

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post #4 of 17
10/27/10 at 9:28am

yeah I dont really think its such a hot idea in practice, in theory its great, but the inherent distortion from the process was the deal breaker for me, its quite high. no point striving for high DNR low noise performance in your dac and amp if you are going to put a significant source of error at the front end

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post #5 of 17
10/27/10 at 6:34pm
Thread Starter 

user feedback about units using LDR's and similar designs seem very good in comparison to wiper based pot attenuation.  I am always looking for new projects so I figure this one might be the next project.  I have already started working out a design...

Thanks for the advice.

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post #6 of 17
10/27/10 at 8:30pm

Snake_oil.jpg

 

PHOTONS ARE NOT ELECTRONS!  Screwing with an optical digital signal will either have no effect or cause it to stop working.

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post #7 of 17
10/27/10 at 8:43pm
Quote:
Originally Posted by batphink View Post
PHOTONS ARE NOT ELECTRONS!  Screwing with an optical digital signal will either have no effect or cause it to stop working.


No, no they're not.

 

On the other hand, you might want to google a bit and find out what an optical attenuator is before you go assuming that it's something to rewrite the amplitude information on an optical SP/DIF signal. This is actually a stand-in for the usual volume potentiometer which uses resistors whose value varies depending on how much light is hitting them.

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post #8 of 17
10/27/10 at 10:06pm

So it's an attenuator based on photoresistors?

 

An interesting hack but i was under the impression that photoresistors are noisier than regular resistors.

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post #9 of 17
10/27/10 at 11:16pm
Quote:
Originally Posted by gimble View Post

 

On the other hand, you might want to google a bit and find out what an optical attenuator is before you go assuming that it's something to rewrite the amplitude information on an optical SP/DIF signal. This is actually a stand-in for the usual volume potentiometer which uses resistors whose value varies depending on how much light is hitting them.



Never mess with a digital signal.  Attenuators only work with analog signals.  The most important thing is to have the digital signal as clean as possible.  Using a good cable and blasting the jacks with a can of air will give the best results.  Anything else is a waste of time and money.

 

And it's spelled S/PDIF.

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post #10 of 17
10/28/10 at 12:13am
Quote:
Originally Posted by batphink View Post





Never mess with a digital signal.  Attenuators only work with analog signals.  The most important thing is to have the digital signal as clean as possible.  Using a good cable and blasting the jacks with a can of air will give the best results.  Anything else is a waste of time and money.

 

And it's spelled S/PDIF.


 

I think it's spelled 'moron'.

 

Messing with a digital signal were never mentioned.

As gimble and eric noted, it's attenuation of the analogue signal using photoresistors.

 

It seems like an interesting concept, but I fail to see what advantage it will have over a standard pot.


Edited by limpidglitch - 10/28/10 at 3:38am
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post #11 of 17
10/28/10 at 2:49pm
Thread Starter 

Yes the basic idea is to use photo sensitive components such as LDR's to act as a volume control instead of using a POT.

This is for analog audio signal control...  Many that have worked with similar circuit have reported excellent

results.  I have been speaking with people that have built similar circuits so I am working on a circuit of my own.

I like having a project to work on so I figured this may be my next one...


Edited by ROBSCIX - 10/28/10 at 2:52pm
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post #12 of 17
10/28/10 at 3:39pm

If you look around you'll find some people's reasons for photodiode attenuators being "better" than a standard pot or SA.

The things I'm thinking of were written by the guy that did all of the Melos stuff and in turn the "photentiometer".

 

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post #13 of 17
10/28/10 at 5:23pm
Quote:
Originally Posted by nattonrice View Post

If you look around you'll find some people's reasons for photodiode attenuators being "better" than a standard pot or SA.

The things I'm thinking of were written by the guy that did all of the Melos stuff and in turn the "photentiometer".

 



Well, like i said, it's an interesting hack.

 

I think the relay based attenuators probably give better performance - though the expense and complexity is daunting.

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post #14 of 17
10/28/10 at 9:38pm
Quote:
Originally Posted by ericj View Post

So it's an attenuator based on photoresistors?

 

An interesting hack but i was under the impression that photoresistors are noisier than regular resistors.



indeed they are, this is fact. using even the best implementation you will be looking at around 0.1% distortion at best. reports of the sound are quite good, but I think these are more people with tube amps that are used to these kind of numbers wink.gif just joking. but yeah its cool if numbers and measured performance are meaningless to you, but while sound is the most important thing for me, i'd be lying if I said the umbers didnt matter. I bought the boards for a version of the lightspeed, but decided not to build them as they were destined for a dac/pre and after seeing the data, I didnt want that kind of distortion being built in and therefor unavoidable. digital attenuation at the source is a much cleaner solution IMO


Edited by qusp - 10/28/10 at 9:41pm
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post #15 of 17
10/29/10 at 6:03am
Thread Starter 

The idea is interesting to me.  I may combine the LDR's with a digital volume chip and a few others so I can have 7 seg displays for the level.

I like have projects to do...  I would be lying if I said numbers don't matter to me but the sounds of the unit matters more...

The feedback has been exceptional on these units.

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