LIGHTSPEED ATTENUATOR diy

[georgehifi]

Hi guys, if you can hold a soldering iron and solder and have a small amount of electronic knowledge you can make a Lightspeed Attenuator yourself.
Yes I also do a production ready made Lightspeed Attenuator over 800 units world wide now, which was released at the same time as I put up the diy version.
What is a very simple idea I started back in the 1960's, can't be improved on, no matter what the Lightspeed clones are trying to say to make a quick buck.

So lets get started.

To explain a little the 5 photo attachments.

1st: Is the basic led/ldr and how it works in the Lightspeed Attenuator, it mimics a 10kohm series shunt logarithmic potentiometer.

2nd: Is the Lightspeed Attenuator stereo version, complete circuit diagram, note how the signal goes through nothing but one soldered resistor, no sound robbing light contact of a metal wiper on a resistive track in a volume control (potentiometers) in the signal path.

3rd: Is the same but a dual mono, good for room or system imbalances, (for some reason owners of both say the dual mono has the edge sound wise) I believe it's the fact you can precisely centre the image regardless of recording or environment influences.

4th: Is the buffer Nelson Pass designed a few years back for the Lightspeed Attenuator (later to be known as the B1 Buffer) for it to be able to drive low input impedance amps of less than 33kohm.

5th: Is the $5 led tester you can get on Ebay. With this you can quad match your own set of NSL32SR2S(sorted) Silonex led/ldr's you'll need get about 15 pieces to get 2 sets of quad matched led/ldr's for two x stereo Lightspeed Attenuators. So go halves with a buddy and you can have one each.
Match the LDR resistances using a DMM, powering the LED from 1mA to 20mA, give them time as they need a few minutes to warm up.

PS: You can skimp on the cheaper NSL32SR2 but you have to buy almost twice the amount, as their low resistance is not as low and constant, they cannot get to quietest minimum volume level as the NSL32SR2S (sorted) can.



Cheers George

[georgehifi]

Also here are the square wave screen shots for the technically inclined at 10hz 1khz 20khz and 100khz.
The 100khz screen shot is only limited by the scope's test lead capacitance.
To give this kind 100khz square wave frequency response is well over 500khz!!! (Lightspeed'ish)

Cheers George

[Puffin]

George, I built several L/S clones (point to point) years ago following the very loooong thread on Diyaudio. I used one in my system for some years before it developed a fault. Your post has reminded me that I need to have a look at it. I think the thing that possibly stopped me was that there was some concern about the Cadmium in the LDR's. Is this a concern or scaremongering?

Rob.

[danilo]

IF.. ? keen consider these;
http://www.intactaudio.com/atten.html
More than a few rungs up the quality ladder from LED Volume gizmos.

[georgehifi]

George, I built several L/S clones (point to point) years ago following the very loooong thread on Diyaudio. I used one in my system for some years before it developed a fault. Your post has reminded me that I need to have a look at it. I think the thing that possibly stopped me was that there was some concern about the Cadmium in the LDR's. Is this a concern or scaremongering?

Rob.

Can you remember the fault it was doing? Maybe I can give you a hand, I've seen just about everything.
As for the CdS don't worry they are in hermetically sealed packages.

Cheers George

[Puffin]

Can you remember the fault it was doing? Maybe I can give you a hand, I've seen just about everything.
As for the CdS don't worry they are in hermetically sealed packages.

Cheers George

I think one channel went down and I assumed that one or more of the LDR's had died. I will need to dig it out and check it. I have to say that it was one of the best Pre's I had heard. Despite being passive it had certain qualities of an active Pre.

[georgehifi]

I think one channel went down and I assumed that one or more of the LDR's had died. I will need to dig it out and check it. I have to say that it was one of the best Pre's I had heard. Despite being passive it had certain qualities of an active Pre.

If it was just one channel that was playing up then yes it could be a faulty ldr either the series or the shunt one, but if you had a problem with both channels doing odd things, then it will be the 100kohm pot playing up.

PS: What/where do I have to tick to get auto emails to say someone has replied to a thread I've started or posted on????

Cheers George

[Puffin]

Does the 100k pot act as a voltage regulator? If so what is their life span?


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[georgehifi]

Did it, thanks, wait to see if I get any.

As for the front stereo VC pot it acts as a voltage divider in opposite directions to vary power both series and shunt leds of the ldr packages.

It is in effect the series/shunt led brightness controller, but end result is that it becomes the "volume control"

Cheers George

[georgehifi]

Did it, thanks, wait to see if I get any.

As for the front stereo VC pot it acts as a voltage divider in opposite directions to vary power both series and shunt leds of the ldr packages.

It is in effect the series/shunt led brightness controller, but end result is that it becomes the "volume control"

No music signal goes through it as can be seen in the 2nd pic of the first post, if one follows the signal path with the green trace.

Cheers George

[Light Dependant Resistor]

Readers will notice or should notice signal ground is also led cathode ground in this implementation
:stalks:

[georgehifi]

Readers will notice or should notice signal ground is also led cathode ground in this implementation
:stalks:

So sorry guys this was an old circuit, I've amended it in the first post diagram.
The led cathodes go to negative of the power supply, and that negative is not part of chassis ground. So in fact the positive and negative floats, and is not part of the signal or chassis ground. Again very sorry.

Also here is the NSL32SR2S (sorted) data sheet should anyone want it.

Cheers George

[Cobra2]

I have searched..(!) but cannot find any data about how much attenuation the Lightspeed can give (range) ?
And input impedance?

Arne K

[georgehifi]

I have searched..(!) but cannot find any data about how much attenuation the Lightspeed can give (range) ?
And input impedance?

Arne K

In most 99% of systems the attenuation can go right down to a whisper, as the Lightspeed as built cannot go down to absolute zero.

The only time it may be at minimum volume and still be at very low level from the speakers, if the system has very high gain and the speakers are >105db efficient horns.

The Lightspeed mimics a 10kohm logarithmic potentiometer.

Cheers George

[georgehifi]

If you want a full to min range figure from sine wave bench test.

Then with a constant sine wave input of 800mVpp, the full volume output is naturally 800mVpp being unity gain passive. The minimum volume with the same input is under 2mVpp more like 1

Which works out to around 52db-58db of attenuation range.

Hope this helps Cheers George

[Cobra2]

Thanks
Unfortunately, this means that it will be unsuitable for my use
(as are all TVC units I have seen)

Arne K

[georgehifi]

Thanks
Unfortunately, this means that it will be unsuitable for my use
(as are all TVC units I have seen)

Arne K

Wow, this means you either have amps or source with abnormally high gain and or >110db speakers. Or you have a baby sleeping in the same room;)
Cheers George

[Cobra2]

Close...
Altec Model 19, Hypex UcD400HxR, or Yamaha M-4, or Yamaha PC2602M, + various preamps...
Used for analog, digital & TV/video. (24/7)

Arne K

[georgehifi]

Yep big gain there, in these situations the Lightspeed's volume will only get down to back ground (dinner party) levels.

Cheers George

[r100]

why not just add a mute switch ?

[georgehifi]

why not just add a mute switch ?

Yes you could do that, as it wouldn't be in the signal path, just shunt a 1-10ohm resistor to ground with a switch, but I don't find it a problem as any serious listening I do is above background level anyway.

Cheers George