Lightspeed Attenuator a new passive preamp

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#1,944
It has been a long time since I posted to this thread. Looks like many more are enjoying the Lightspeed. Here is a link to a picture of my Lightspeed. As you can see, it is built inside a toaster and has a separate power supply. It has remote volume control. The small round knob manually adjusts volume. The lever for lowering toast is now a three stop source selector switch. There are three leds in the black handle that indicate source.

http://i17.photobucket.com/albums/b65/Badge01/002.jpg
 
#1,947
Tubo,
My original basic LS featured a recharchable 9v battery (an isolating switch for battery charging) and it functions perfectly fed from a common cathode valve buffer that has about 200R Zo/p (it never returned!)
A 2 - 4.7uF cap should do the trick - perhaps you can try an Alex Cavalli OTL that has no DC offset and requires no cap at all.
 
#1,949
thanks jh! ill certainly look into that otl option! i don't want to colour the sound of the lightspeed with caps. although tubes has its own sound signature! im glad that it will work...thanks again...

will you be able to point me where to get the working scheme of the otl?


ivan
 
#1,952
Best you look up and bookmark the "cavalliaudio.com" site and then see the section called "OCL" that also contains the links to the" Heretical" series of buffers - it a treasure trove of really useful information and you can easily see the different ccts for your own particular requirements.
My small suggestion is to perhaps look towards some of the current design regulators that can deliver better sound than the traditional CLC, etc power supplies - plenty of these in the "tubes" section of this site and very helpful guys, too.
 
#1,957
If this is a dumb question it's because I'm new to this ...

If I understand properly: These LDRs are good for volume control because they contain no physical contacts/moving parts. The signal doesn't pass through any active semiconductor p-n junctions that might produce rectifying distortion. No capacitors are used which might produce dielectric induced effects either. They aren't overly expensive and are reliable.

As far as I understand, LDRs are semiconductors with ohmic contacts at each end and a resistance that depends on the generation rate of charge carriers due to photo-excitation. Near dark there are few photo-carriers and conduction relies on thermally generated carriers - hence the poor thermal stability at low light. You can pick different semi-conductor materials and optimize parameters to control the spectral response, response times, thermal dependency etc.

By the way, Ohmic contacts to semiconductors aren't always perfect, most likely these realities are the source of the 2nd harmonic distortion that is seen from LDRs.

So my question is why not use JFETs ?

They are very much like LDRs. A semicoductor with two ohmic contacts at each end. There's enough doping to ensure charge carriers are available even though they are not illuminated. To some extent this makes them less thermally sensitive than LDRs. Their resistance is also controllable and more linear than an LDR (not always desired). And they both like to be operated with around 250mV across them for 'best behaviour'.

As far as I can see there's no benefit in the LDR over a JFET.

They are both pigs to match.

They are basically the same thing.
 
#1,958
LDR's have complete electrical isolation between control cct and resistor element.

even with this, some of us think that reducing any noise between LED & resistor ( i.e. LED noise ) is critical to get the best possible sound.

any little noise on the jet gate & the signal will be directly influenced . . . and effectively the signal itself will create that noise.

perhaps you can figure out how to overcome that - and if so, great - you have another option for a volume control 🙂

The other thing about these LDR's is that they seem to be audiophile quality resistors - so I think those us who have already optimised how to get the best out of these devices are not looking for alternatives.

your free to try either or both - but I would recommend you read through the thread and try the LDRs

cheers

mike
 
#1,959
Trouble is, I have to control 6 channels of audio. What I've read of the LDRs for matching has been far from encouraging which is why I'm looking for alternatives. The fallback is a multi-ganged plastic resistance pot. Not ideal.

Has anybody observed noise from the LED ? the response time of the LDRs is very poor, this must filter out a lot of noise.

Has anyone looked at getting naked LDRs and using a single LED to control them all in parallel (which would suit my needs) thereby avoiding the variability in turn-on characteristics of the different diodes of the integrated isolators ?
 
#1,960
Bigun said:

Has anybody observed noise from the LED ? the response time of the LDRs is very poor, this must filter out a lot of noise.

Has anyone looked at getting naked LDRs and using a single LED to control them all in parallel (which would suit my needs) thereby avoiding the variability in turn-on characteristics of the different diodes of the integrated isolators ?
Click to expand...

There is not overall consensus abut LED noise so I can only speak for myself.

The response may be poor but it is not nothing . . . and I have found that anything that can be done to reduce LED noise is very worthwhile - in other words the natural filtration is not enough if you are look for best performance.

For me matching would be easier than naked LDR's but I may be wrong. If you cut the devices in half I guess you would expose the LDR's - good luck for that

mike
 
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