Lightspeed Attenuator a new passive preamp

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L

Lumba Ogir

Banned
Joined 2005
Gareth,
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.
Click to expand...
JFETs, in their active region are voltage-controlled current regulators, however in their saturated region they act as voltage-controlled resistors.
 
V

vernon.taylor

Member
Joined 2005
A couple of questions for Paul

Hello Paul

I've looked through the many pages of this thread, but I may have missed a couple of things:

1. Do you still have some boards left, please?
2. Regarding multi-amping: would you advise one board to control two stereo sets of LDRs or two boards and piggy-back the control lines? (level adjustment between high and low sections would be done elsewhere).

Thank you Paul, thank you George and everyone else for this interesting thread.

Vernon
 
P

ptysinn

Member
Joined 2008
Hi Ondesx,

If you have tried the TX2575, please let us know the result. One favour I would like to ask you to do is:

1) In your LDR fixed series resistance configuration (I called it Lighspeed Mk1.5 in my previous post), what is your LDR shunt resistence when you do that LDR Series resistance setting at 2K, 5K, 7K, 10K Ohm at the same listening volume level? Since you have a 10k input impedance active speaker, it may affect the sound a little bit.

2) Using George Lightspeed Mk 2 configuration, at the same listening level, what is your LDR Series resistence and the corresponding LDR shunt resistance?

I just what to know your input and output impedance in each case.
It may affect the sound if the i/o impedance is different in both (1) and (2). Best to make it the same to compare to be fair if possible, let say at 10k series setting and find a matched LDR that has around 10K i/o impedance in Lightspeed mk 2 configuration at the same listening level. May be a lot of work though.

Thanks!

Philip
 
P

ptysinn

Member
Joined 2008
More...theoretically if you use Lightspeed Mk 1.5 configuration, ie fixed Series LDR resistance/ variable Shunt LDR, let say at the same listening level, you set 10Kohm series LDR, measured 10Kohm shunt LDR, it should sound the same if you have a Lightspeed Mk 2 with the same measurement at that listening level. I don't know why there is a different in sound in this case.

So I will do, build a Lightspeed mk2, find the best listening level range impedance that sound best to you in your setup, measure the series resistance, then use a left behind umatched LDR as a series resistance to build the Mk 1.5 version and adjust series resistance accordingly you got from mk2. I think you will get the same sound in both case at your best listening level.

If you find that it is not the same sound impression in these setups, maybe someone can explain it, provided everything else (components) is the same.

Philip
 
georgehifi

georgehifi

Member
Joined 2003
lightspeedattenuator.com
ptysinn said:

If you find that it is not the same sound impression in these setups, maybe someone can explain it, provided everything else (components) is the same.

Philip
Click to expand...

I've done this comparison using the same resistance measurements and the MkII sounds better than the MkI, the MkII wins out regardless of which type of series resistor (carbon or metal film) were used in the MkI
All I can say is the MkI the series resistor is either metal film or carbon. In the MkII the series LDR is either Cadmium Sulphide or Cadmium Selenide, these materials are said to be made out of nano partials, I'll leave the rest up to the chemists to explain why the MkII sounds better.

Cheers George
 
P

ptysinn

Member
Joined 2008
Hi George,

But what I am saying is using the LDR also in series configuration but set at a fixed resistance, the same series resistance (eg 3kohm) in mk II at your listening level. So it will be the same Cadmium Sulphide or Cadmium Selenide chemical. With same impedance level at the listening level, it should sound the same, theoretically. Seems like Mr. ondesx finds differently.

Maybe something else is affecting the sound.

Philip
 
mikelm

mikelm

Member
Joined 2003
georgehifi said:
the MkII wins out regardless of which type of series resistor (carbon or metal film) were used in the MkI
Click to expand...

Yeah George, I would not question that, carbon resistors I have heard sound just awful and most metal oxide are also not that great.

When you say metal film do you mean that they are not metal oxide ?

But the resistors we are discussing here are in a totally different league from regular commercial resistors and cost from about $3.50 to over $10 each ! For me when I changed to these better resistor in critical positions it was a sonic revelation.

Another revelation is that LDR's sound just as good and are cheaper ! My personal understanding of this is that it is the excellent quality of LDR's as resistors that make the lightspeed such a great volume control.

But I do not believe they are unique I would guess there are half a dozen other resistors in the same league.

It may be that between these different resistors we would have different preferences between the subtle nuances of each of them but for me they are of similar general quality.

cheers

mike
 
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