Lightspeed Attenuator a new passive preamp

ptysinn said:
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

Exatctly what I did.

Cheers George
 
Leaving impedance issue to one side, it does come down to the quality of the resistor in the series position. It would then seem that Cadmium Sulphide/Selenium is an audibly great material for resistors. Silonex gave distortion graphs for the LDR , I wonder how many resistor manufacturers have done the same (Resista distortion graph attached)?

It seems that anything that modulates the electrical parameters of a resistor will introduce distortion, so low tempco & mechanical configuration (nude Vishay). I wonder how LDRs fare in these considerations, tempco & mechanical?
 

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Not only does the MkII sound better because of the chemical make up of the LDR's resistance element compared to carbon or metal film/oxide resistors. I have not compared to smd resistors, but I have done Beshlage Vishay and a couple of others, in metal oxide/film.
It still remains that the MkII configuration has a better (lower) off volume, and more importantly it has much more stable/consistent output impedances, were going back over old discussed things here, that were said a year or so ago.
There is no comparison, MkII (ldr series / ldr shunt) kills the MkI (series resistor / ldr shunt) do you guys think I would recall all the MkI's to upgrade them to MkII for nothing? And also have the big hassle matching all 4 ldr's building the MkII when it was so easy to build the MkI's, my profit margins dropped by 20% because to MkII is so expensive to make.

Cheers George
 
George,
I hope all these investigations don't appear ungrateful to your efforts in bringing this unique & great sounding Vol control circuit to attention here. Your generosity in doing this is much appreciated. I have no doubt you have done all these tests before & have proven the MKII is superior to any all combinations.

The spirit of DIY & investigative minds are, I'm afraid, what is in evidence here rather than any doubt about your tests or outcomes.

I am, however, sensitive about the possible commercial repercussions of these investigations! With this in mind I'm happy to go off-line with these posts but I don't know what your thoughts are on this?
 
Tolu said:
but if you use a LDR for series R, why don't you build MKII? I have done these tests also as I wrote some posts above! Even a third LDR in the signal way doesn't disturb the quality. It is the opposite!

Well, my thought was using those unmatched LDR in the series position, be green. Since according to people, you may need to buy 10 or 20 to get 4 matched LDR. I was hoping that if you only find one matched pair of LDR unluckily in your 20 of them, you can use 2 others in the series position setting it at a optimum value for your system. However from George, it seems like even using a fixed LDR in series R position sounds worst than using a variable LDR in series R position. So we have to buy some more to get a 4 matched to make mkII.

Philip
 
The first time I bought 12 pcs which gave me 4 well matched LDRs. The remaining 8 pcs are perfectly used for signal switching. So you have per channel 2 LDR for about 5$ which are far superior in relation to any kind of relay!

So, for about 30 $ you'll get the best sounding preamp (I dislike the word preamp regarding to a vol control; in Germany is a better word for it: Vorstufe ~ pre stage) you can imagine. And I have heard so many of them... :xeye:
 
an attenuator is an attenuator.

A box containing a host of undefined components with selector switches and attenuator knob that is placed between a group of sources and the power amp/s is conventionally referred to as a pre-amp.

If it has active components inside it then it's an active pre-amp. The fact that the gain can only be -ve does not affect the colloquial title.

If the pre-amp has no active components inside then we usually call it a passive pre-amp.

Most of us understand the difference between these.

A gain stage with no gain that presents different impedances at it's input from it's output is a buffer. A buffer inside a preamp box makes the pre-amp the active type but it still has no voltage gain. It probably has current gain as a by product of the differing impedances on its terminals. It's still a pre-amp.
 
All passive poteniometers, Unity gain TVC's, Dact resistive switched pots, are all passive attenuators, passive preamp is not the right name for these devices, They only attenuate, they never preamplify.

As I've said before the Lightspeed Attenuator:

Is at maxvolume unity gain, and it only attenuate from there.

It does not amplifiy so it cannot be called a pre-amp.

It's resistive element is not powered has no voltageor current running through it so it must be considered passive.
Cheers George
 
Potential Divider

Comparing any serial/shunt fixed/LDR combination (MKI / MKII) it would be usefull to post the concrete values of the compared resistances (fixed and LDR-resistance): Serial, shunt and input imp. of the amp.

Without doing this, I think further serial/shunt-discussions are senseless... :)

To get the "same" at every volume step, you need constant input impedance at every step. That is basically only possible with pot. dividers: serial-LDR and shunt-LDR (Georges MKII). If you want exact impedances at every step, there is much work on it, but you don't have to match any LDR, you have to match the resistances (i.e. in the voltage circuit):

- specify the impedance (i.e. 7kOhm) of the whole pot. divider
- specify the input impedance of the used amp
- specify the necessary steps in -dB
- calculate the serial and shunt values for every step with these 3 values
- adjust these pot. dividers at voltage side (stereo: 4 values each step) the way you like (i.e. stepped attentuator...) - measurement at resistance side ...

Regards
Thomas
 
Ex-Moderator R.I.P.
Joined 2005
trafo attenuators may have gain also :D

I know, if theres active curcuits its active

Still, a digital attenuator and also this one, I think I would consider it passive
Unless it also contains a buffer

Its the devellopment, makes old terms seem inadequate
The word "greyzone" is also a modern term

Still have no clew as to what all this is about, other than a a very fine attenuator

Are there any pictures anywhere :)
 
Re: Potential Divider

TJF said:
Comparing any serial/shunt fixed/LDR combination (MKI / MKII) it would be usefull to post the concrete values of the compared resistances (fixed and LDR-resistance): Serial, shunt and input imp. of the amp.

Without doing this, I think further serial/shunt-discussions are senseless... :)

To get the "same" at every volume step, you need constant input impedance at every step. That is basically only possible with pot. dividers: serial-LDR and shunt-LDR (Georges MKII). If you want exact impedances at every step, there is much work on it, but you don't have to match any LDR, you have to match the resistances (i.e. in the voltage circuit):

- specify the impedance (i.e. 7kOhm) of the whole pot. divider
- specify the input impedance of the used amp
- specify the necessary steps in -dB
- calculate the serial and shunt values for every step with these 3 values
- adjust these pot. dividers at voltage side (stereo: 4 values each step) the way you like (i.e. stepped attentuator...) - measurement at resistance side ...

Regards
Thomas
If I understand optocouplers correctly, it seems that the resistance values may not be the same for a fixed voltage or current. So the actual curves need to be matched for a specific board.
 
Hi folks,

The Easter holidays sort of crept up on me without me realising as I have been so busy at work. My lady insists that I am allowed to take time off occasionally! Anyway the Application notes did not get done last weekend as I was on holiday. I hope to get them finished this weekend and get them posted on my website.


I am answering questions from Vernon (post 1991 and a private e-mail) so anyone with a similar requirement can refer to the answer :-

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? I have boards.
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). One VCCS board will control two sets of LDRs but there will be no independance of control for the separate amplifiers .Two VCCS boards with piggy-backed control lines will also sacrifice independance.

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

Vernon

Plus an e-mail from him with more detail :-

Hello Paul

Thanks for your reply. If you saw my thread question it asked whether I would need two
sets of VCCS boards or just one for a 2-way multi-amp system - it would seem I could
easily parallel the control signals for two boards, or if there is enough current, use
one board to drive two sets of LDRs. Thinking about all the possibilities, including RC,
I think I might be better-off with two VCCS boards, two RC receiver boards, and use just
one of the RC emitter boards to control both - this is because I need to pad-down the
low-pass filter by 1dB. I'm currently doing this in software which is of course less
desirable. However, I didn't find/understand whether volume settings are kept after
power-down (I suppose I could keep it powered-up all the time). The volume settings are not kept after power down so you would need to re-adjust your settings at each power up or leave the power on.
I would use the manual
buttons to adjust levels between high-pass and low-pass sections (and balance) and the RC
for volume only. This will work, if you make your high pass and low pass level matching, as well as balance adjustment, with manual push buttons, however, this will be inconvenient if you wish to make fine adjustments from time to time. You could achieve this with the remote control by using two VCCS, three receivers and one transmitter. The transmitter and receiver boards for this project both have two programming links which can be used with multi-board systems to ensure a specific transmitter and receiver work together without affecting a receiver with a different program link. You could have a two-way or three-way switch on your remote transmitter that could select which receivers you wish to activate. The activated receiver would only be wired to the VCCS board terminals that activate the required function for that particular receiver.

So receivers one and two have no links programmed and connect to VCCS one and two. This will be the master volume/balance control, which is active with the transmitter switch at position 1 and it will control both VCCS boards together. Receiver two will have link one programmed and this connects only to the volume up/volume down terminals of the VCCS board controlling your low pass section to allow volume matching of low pass to high pass. You would activate the transmitter link one (switch position 2) to enable this function.

Your thought and advise appreciated and tell me if you would rather I posted the question. For all thread readers, please confine your technical questions to the forum thread so the answers can be of benefit to others. This will also save me a lot of time answering the same questions repeatedly.

Regards
Paul