Lightspeed Attenuator a new passive preamp

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#2,041
georgehifi said:
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
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soongsc Read carefully last paragraph,
Cheers George
 
#2,042
Minimising the signal path

Hi George,

That's half right, when you add a buffer you narrow the playing field between the different passives, still the Lightspeed wins out, but the margin has narrowed, "remember what I keep saying the best buffer is NO buffer" The sound is only as good as the weakest link in the system. Take out one of the weak links and you have broadened the playing field.
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Wouldn't it be nice to drive the speakers directly from a dac's current output with volume done in the digital. No problem the a current output dac would love to see an 8 ohm speaker load trouble is the speaker would have to be a 1000db odd efficiency.
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I am almost there but unfortunately the real world places limits on what you can leave out.

The Altmann DAC, that I am currently using, has a current to voltage converter followed by a unity gain output stage buffer. I am planning on deleting the on-board buffer and re-jigging the current to voltage conversion stage when time allows. This will simplify the path from the DAC chip current output nodes.

I feed the DAC, or my phono stage if I chose to listen to vinyl, straight into a “Lightspeed” MKII then into ONE high power, DC coupled IXYS IXTH 20N50D depletion mosfet, in the source follower configuration, operating in class A. The source follower is current source loaded, with another IXTH 20N50D, on split rail supplies to allow DC coupling to the loudspeaker load. The mosfet source follower drives my parallel-wired, open baffle line source arrays, which have an efficiency of 105 dB per watt at one metre.

Because the eight Visaton B200 line source array drivers in each array are wired in parallel, for a 0.75 ohm load, very little voltage swing is required to go very loud. I like to listen at realistic volume levels, when the mood takes me, and the arrays are basically just idling at these programme levels and their distortion contribution is very low. The IXYS depletion mosfet provides all the current required to motivate the line arrays in my listening room. This is a very transparent minimalist arrangement that complements the musical abilities of the “Lightspeed” very well, and crucially, it sounds good at all listening levels.

Regards
Paul
 
#2,043
Paul,
I love the simplicity of what you are doing - it was one of the reasons I was attracted to Susan Parkers Zeus but unfortunately it requires some transformers to realise this design.

I have just searched & found your thread on Audiocircles about this circuit so rather than asking any questions here (& possibly repeating what was asked over there) I will ask my Qs there.

Thanks Paul

One other use for odd LDRs might be their use as a high quality resistor in the feedback role of GFB circuits (also allows adjusting & varying the feedback)?
 
#2,049
ptysinn said:



Cool! I will use the unmatched LDR to build a switch for selecting input.

What current did you feed those switch LDR when turn it on?
Did you just disconnect the power when switching it off? Any crosstalk problem?

Regards,

Philip
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@Philip and Thomas

Signal switching:
One LDR in signal path. One reed relais after LDR to ground.

if source is active, full 2.5V to LDR (gives around 30 Ohm) and reed relais open

if source is off, 0V to LDR, and reed relais shortens signal path to ground.
 
#2,050
For those who are interested in the "comparison" of different LS flavours...
First, as stated several times by GeorgeHiFi, the MKII version won all comparison with both MKI and MK1.5. Whatever the resistors used the worse are SMC.

I couldn't measure impedance values easily and the only achievable data was the resistor value for series before inserting the module in the audio system.

All the parameters are better with the MKII version, sound stage, transparency, resolution, harmonic structure of the instruments and voices, etc...

I definitely don't explain why, but in my system the MKII version was the large winner.

For the P&S LS OPC271, I asked to these guys several months ago and the answered that they used LDRs in series and shunt, then it's also a... MkII version ! After my request, they offered both single-ended and balanced versions. The price falls slowly and I hope it will be available at a decent price in a couple of months... If everybody send a mail to ask a better price, this will probably boost the fall...
 
#2,051
Lightspeed remote control

Hi folks,

It is taking me longer to get the website revision sorted than I thought. Never enough time in the day. However I am nearly there and the Lightspeed remote control application notes will be included when I post the updated website.

I found a problem with the transmitter board when I began assembling my own unit. The PCB manufacturer cut them to the wrong width and this means a very sloppy fit in the extrusion I designed the board for. They are sending replacements, correctly cut, by the end of the week. I will be sending all the boards out as soon as the replacements arrive so you should get them around the time the application notes are on line. I will be starting the module build and getting these out as well.

Regards
Paul
 
#2,052
ondesx said:
For those who are interested in the "comparison" of different LS flavours...
First, as stated several times by GeorgeHiFi, the MKII version won all comparison with both MKI and MK1.5. Whatever the resistors used the worse are SMC.
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Could tell us what resistors you used for your experiments - many thanks - mike
 
#2,058
Yep, Mikelm,
This would be good news all round if the LDR resistors sound better than the bulk foils - good for the pocket & good for sonics.

Only way to find out is do the corollary of the above experiment - replace a bulk foil R with a LDR set to give the same R & listen.

Who's volunteering? Mikelm, I think you have some of these bulk foils in specific places in a SKA amp, no? Do you have any spare Optivols? Would it be difficult to do this test?
 
#2,060
Hi George, agreed MkII seems to be the winner soundwise for vol control.

What I'm talking about is using an LDR in another place in a circuit where high quality resistor is a bonus, such as feedback resistor. So replacing an existing bulk foil R with a LDR in this position & reporting on sound.

The opposite of what Ondesx did!
 
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