Lightspeed Attenuator a new passive preamp

[wapo54001]

Originally Posted by udailey

Wikipedia under just DIODE rather than LED says this:

Temperature measurements
A diode can be used as a temperature measuring device, since the forward voltage drop across the diode depends on temperature, as in a Silicon bandgap temperature sensor. From the Shockley ideal diode equation given above, it appears the voltage has a positive temperature coefficient (at a constant current) but depends on doping concentration and operating temperature (Sze 2007). The temperature coefficient can be negative as in typical thermistors or positive for temperature sense diodes down to about 20 kelvins. Typically, silicon diodes have approximately −2 mV/˚C temperature coefficient at room temperature.

If you control the LED with a stabilized current source, even if the voltage drop across the diode changes, the current through the diode will remain constant since the active control circuitry of the current source will compensate to keep current constant. I wonder if LED light output changes if you keep current constant regardless of voltage drop? Hmmm.

[georgehifi]

Just put a DMM set at high Z, across the cell only, (no need to power the led), and you will get a stable Z reading, then hold the body with just your fingers and you will see the Z change quite a bit.
This is why I have always said to have all 4 units together and then pot them in hard snow ski base wax.
And you can use one of these for matching your LDR's
Cheers George

[udailey]

I use a voltage regulator into a current source into yet another current source with smoothing capacitors left and right and it sounds fantastic. If you blow on the LDR it changes resistance still.

[udailey]

What George does is probably the best solution there. I dont do it because it doesnt bother me, but if I was worried then I would try to get them in wax or hot glue.

[udailey]

Originally Posted by georgehifi

Just put a DMM set at high Z, across the cell only, (no need to power the led), and you will get a stable Z reading, then hold the body with just your fingers and you will see the Z change quite a bit.
This is why I have always said to have all 4 units together and then pot them in hard snow ski base wax.
And you can use one of these for matching your LDR's
Cheers George

I dont recall doing this but George's finding means that the resistive material changes with temp and we already know the LED does that as well. So double edge sword.

[wapo54001]

Originally Posted by udailey

I dont recall doing this but George's finding means that the resistive material changes with temp and we already know the LED does that as well. So double edge sword.

Yikes!

[georgehifi]

Originally Posted by wapo54001

Yikes!

Now you know why I strongly advise against putting the Lightspeed inside an amp, and to pot them in hard wax as well.

Cheers George

[redmaster]

Hi,
A friend of mine built an attenuator based on LDR design.
I noticed that L+R channels are imbalanced.
when the pot knob is turned up, it starts from R channel +5dBFS louder then L channel (minimum),
but when knob turning up, the imbalance gets smaller and smaller, until around -20dbFS (around middle) it's almost perfect, then Left channel (when turning up) became louder and louder - up to -5dBFs, and it's almost perfect when in full 'open' position.
I was told it is just a problem with LDR and it possibly can't be resolved, but I remember reading some audio purist using LDR att. and I don't think they wouldn't notice a 5dB difference between channels, so I presume it's possibly to have it ideal (0.1dB - I can live with, 0.2dB - would be maximum)
Could you please help me - what could cause this sort of problem ?
How to resolve it ?
Is it simply swapping a pair of LDRs or sth else ?
I really appreciate your help, here - thank you in advance
If LDR design is not perfect and it's very difficult to find pair of LDR with 0.1dB difference - could you please advise me, what type of passive att. would be good ? - in a 'human - 99%' price range

[AndrewT]

Buy matched pairs of LDRs or buy the real "Lightspeed" attenuator.

If you read this thread you will see all explained in detail of the why and how.

[redmaster]

Originally Posted by AndrewT

Buy matched pairs of LDRs or buy the real "Lightspeed" attenuator.

If you read this thread you will see all explained in detail of the why and how.

do you claim, that buying a matched pair LDRs off a reliable seller (here's one) would sort it out completely ?
I am thinking about either giving up on this project or using other option (ALPS pot), I am not a real diy'er, so please explain;
thank you