Lightspeed Attenuator a new passive preamp

Heinz, you mention rca conectors contact, look at the massive contact area of those compared to the pinpoint contact of a wiper in a pot, also the pressure of that contact probaly ten fold, even rca's that have been cleaned sound better for some reason, we've all done it, go figure, relays are good, larger contact, heavier pressure, yes they are better than pots, but still not as good as a soldered joint. The pulsed square waves we were using were 1k and 5k, unless you have a extremely fast storage scope magnified on the leading edge at the top corner you wo'nt see it. Just try it, as you say it's cheap enough, you will believe.

Cheers George
 
snip
Heinz, you mention rca conectors contact, look at the massive contact area of those compared to the pinpoint contact of a wiper in a pot, also the pressure of that contact probaly ten fold, even rca's that have been cleaned sound better for some reason, we've all done it, go figure, relays are good, larger contact, heavier pressure, yes they are better than pots, but still not as good as a soldered joint.

George, I agree!

snip
The pulsed square waves we were using were 1k and 5k, unless you have a extremely fast storage scope magnified on the leading edge at the top corner you wo'nt see it. Just try it, as you say it's cheap enough, you will believe.

Ok when I get the time I`ll have a look at this!
Regards
Heinz
 

Terry Demol

Member
2002-04-07 3:12 pm
*
georgehifi said:
I've played with every concievable passive pot, Bournes, Alps Blue Velvet, Alps Black Beauty, Penny&Giles, Dact Switched resistors, and transformers.
Nothing has beat the sound of a series/ shunt LDR arrangment in the attachement, it has no contacts in the signal path, I believe that is the secret.
The only stipulation that all these passive controls including the series/shunt LDR need I believe is,
1: Low source impedence (cd player) <50ohms
2: High input impedence (amplifier) > 100k.

Cheers George

George,

I have done quite a bit of work with LDR's also and like them a lot.

They do however introduce a small amount of H2 which is voltage
dependant. I have not had the chance to measure a spectrum on
them however.

Also LDR's drift (deteriorate) over time, so really, a self
calibration circuit is probably a good idea for very critical
applications.


Cheers,

Terry
 
Also I can not stress enough, that the source (cd player/ Dac ) must be of low impedence <50ohm and decent current drive, this rules out a few of cd players/Dacs with tube output stages.
And the input device (amplifier) impedence > 80k-100k some of my customers say they can hear a difference beween 100k and 300k for the input of the amp with the Lightspeed as the passive.
Also the interconects between the passive LDR and the amp as low capacitance as possible < 100pf per foot.

Cheers George
 
I've attached a photo of what i use for a passive Pre-Amp. I run a few Mini Aleph's Right at the moment i'm using the bozos Pre-amp Built from Twisted Pearl Audio. In some cases i can say that just running my cd player into a volume ( passive Pre-Amp ) and into a set of aleph's It sounds way better more controlled. I have tried DACT switches and didn't like them Over time they fail and sound horrible. BrianGt had a 150$ on in his gain-clone and when i was using it it sounded like HARSH ****! a wasted 150$ on a dact EW. Right now in this cheap YES cheap passive is a peter daniel volume pot it's a Alps. Im happy with it till i get more money to buy a way better one.

Do we really need a pre-amp to run these amps ? I was told we do but i just proved to my self that we don't. NOW if you need lots of output and hi volumes you might need a pre-amp. I have found running the alephs in Se mode they are pretty loud running a passive Pre-Amp.

[IMGDEAD]http://jasesspace.com/images/passive.jpg[/IMGDEAD]
 
Terry Demol said:


George,

I have done quite a bit of work with LDR's also and like them a lot.

They do however introduce a small amount of H2 which is voltage
dependant. I have not had the chance to measure a spectrum on
them however.

Also LDR's drift (deteriorate) over time, so really, a self
calibration circuit is probably a good idea for very critical
applications.


Cheers,

Terry

Hi Terry, by H2, you mean second hamonic I believe, maybe, but a lot better than rectifing the audio signal, and they say second harmonic is plesent to the ear, that's why a lot of people love tubes? I wish I had a distortion analyser, so I could measure this but as far as my ear can tell it's the cleanest thing I have ever heard.
And as far as deterioration goes, my personal unit has now been on 24/7 for two years and is still perfect.
I say to my customers they can leave them on continuosly, just do'nt leave the level control at max or min because then the leds are burning at there max, in my Lightspeed's case this is 80% of the leds max current, for good loud listening levels the control is usually between 12 to 1 oclock on most systems, this is were i say leave it when not listening, and they can also leave it powered 24/7

Cheers George
( I'm sure I'm killing a lot of potential customers who are able to diy, now that my big mouth has started to give out my findings, oh well that's life)
 
Hi George

I've been following this thread with interest as I've been looking at attuenuation, particularly using transformers, for a while now. I think your idea is brilliant, but in all my research over the past couple of years I've not once seen the Lightspeed Attenuator, nor can I seem to find it on the web at least. That being the case, I'm sure you're not missing out on any potential customers here, as I doubt any here have seen your refined product or would pay high-end prices for a finished piece.

I for one would be happy to pay you for an ldr kit and instructions, I think you're really onto something.

My 2 cents (and my first post!)

Cheers
 

chewrock

Member
2006-05-27 12:11 am
Love My Penny & Giles

I'm sorry, fans. Maybe I'm just not tweako enough to appreciate all this.

My passive preamp is over 20 years old. It has the original Penny & Giles fader in it that I first installed. It is noiseless, completely reliable, and I expect to be buried with it when I die.

About this business of small contact areas becoming rectifiers, I do believe that happens, because of oxidation on the metal. Clean metal doesn't rectify, but oxidized metal does. That's what makes CMOS technology work.

So I use redundant contacts on all of the switches in my passive. Instead of double-pole versions, I use quadruple-pole switches and wire them in parallel pairs.

A French engineer I ran into many years ago pointed out the the junction loss from environmental oxidation of gold contacts was far to low to be of any notice, but that wiring two gold contacts in parallel eliminated the problem. Why? Because the "diodes" are polarized on the fly. If there are two of them, they naturally align themselves, one anode to cathode, the other in reverse! This makes perfect sense, because the oxide layer is so thin that it has no Peak-Inverse-Voltage capability and breaks down. So the one that first polarizes itself protects the other "diode". That is until the opposite direction of voltage "breaks down" the other "diode" which immediately repolarizes itself in the opposite direction.

After all, electricity follows the path of least resistance. In this case, the electricity is burning its own path of least resistance.

Now, about this business of my Penny & Giles being a metal oxide rectifier. Rubbish! The problem with contacts is that the "drop" into place and stay there. The oxide barriers can become polarized as "diodes". But all you have to do to stop my Penny & Giles from being a rectifier is to turn the knob! Yep, as soon as you turn the knob, it smears the oxide barrier into a melange that cannot become polarized.

Sorry, I just can't buy the idea that a melange of plastic and metallic oxides can become polarized. By definition, it is amorphous at that point. Amorphous compounds don't polarize worth a darned. As a chip designer. He will tell you the same thing. Crystals and crystaline oxides; they polarize just fine.

It's like telling me that you can polarize a mud puddle.
 
Vatofale, I have only called them Lightspeed for the last year, and I sell in small quantity here in Australia $500au. through word of mouth.



Your missing the point Chewrock, it's got nothing to do with oxidization of the wiper or the track, rather the contact of the wiper being a feather weight pressure on the track that makes it easy to be slightly disconected when a fast rise time transient comes along, this is what I mean by becoming a diode, it's got nothing to do with oxidization, but that can be bad also if things get that old. Time to get rid of that pot.

Cheers George
 

st2_998

Member
2006-05-22 7:00 pm
Nice topic!
Georgehifi: I don't believe you are losing customers here. I don't think everybody is willing to do the same trial and error research on LDRs you did. Perhaps this could happen only to start a business like you.
Why don't you develop a variant of your design with a servo control? More "natural-minded" people like me could buy the former and the "picky men" the latter. I think you could cut through a thick slice of customers.

As for relays... are the inner contacts gold-plated too? I read about Omron relays. Are they good?

Smooth power supply for LEDs... the same SLAs I'm going to power the outbut buffer on! LED consumption is minimal, I believe.

Mumble... what's the quality of the LDR as a resistor? No point using Dale or Vishay (I don't) in a design and then end up with something not up to the situation.
Bye
 
Ok, here is a ruff of the circuit, just give it a clean +5vdc to the pot and ground the otherside of the supply, as for the Led/Ldr package it looks to me as though the Perkinelmer VTL-5C3 will do the trick you will have to match them up, the ones I use are from a different company that are matched for me, but I still have to match them again, the 1k 25 turn trim pot is for final calibration you only need the one on the strongest channel to match it to the weaker one to get perfect channel tracking. Enjoy

Cheers George
 

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Heinz,I read this paper a while back, I'm sure they are talking about non-linearities in the time (ms) it takes for the LDR to react to the LED brightness, not non-linearities in the sound quality. Who cares if it takes 2 milli seconds to reach the level once you have turn up or down the volume, by the time you have sat down it's at the volume, to be honest the time delay cannot be detected at mid levels, only at low level, but like it said, it is milli seconds, it's like having a built in shock absorber in the volume control.

Cheers George
 
powerbecker said:
Thank`s!
Non linearity is a issue!
Regards
Heinz

If was'nt an issue with you why post it (read above), as for the distortion, I challenge you to hear the difference between .001% and .1% on a system, and that .1% distortion is at 2volts rms, show me a cd player or dac that gives 2vrms, while playing music, it would blow an amp to kingdom come.
Heinz you have yet to hear one of these. Do yourself a favour and listen to one, I'm sure you'll change your views towards them.

Cheers George