Lightspeed Attenuator a new passive preamp

Further comments

The unit has been on for more than 12 hours with music running through it. The sound should be close to settled in.
The unit preserves some edges and flaws in source material that my previous pre homogenized. Listening to Black Cadillac the first few cuts are edgy and close miked. The middle cuts are ballads and are presented with bloom and sweetness. This is how the CD is recorded.
I have to give this thing a real thumbs up. Comes across like an active unit as far as dynamics and frequency extremes.
If diy is not your thing contact Georgehifi and get him to build you one. The sub 400.00 price makes it a steal. Some pay that much for a power cord. This design rocks!
If building your own, a couple tips.
1. Order a dozen graded LDR's and match up at several current points. I used 0.1 ma, 0.5 ma ,and 1.5 ma. This was done with a 15 volt regulated supply and a dropping resistor. Measured the voltage across the resistor to dial in current then measured the other side of the LDR to get resistance. Plotted it out and it is an exponetial decay. It is easy to spot trends and find close matches.
2. Try to find a dual linear pot. Mine are 50K Alps located in a surplus shop. Picjked four by matching end to end, then picked the two that tracked the closest. A log pot will work fine also, but tracking with most is much worse than the linears. A poor tracking pot will negate the LDR matching.
3. A wallwart works great to power. Use the dual regs like the Lightspeed and do not connect the power supply ground to the signal ground. The are optoisolated!!
4. Try diddling with the current limiting resistors. I used 121 ohm instead of George's 100. If you never completely close the pot or open it up this is not critical. My selector is wired up with mute positions so there is no reason to ever close, and the level gets crazy loud before the pots are half open.

Like George mentioned earlier in this thread, the unit seems to work best with low output impedance sources and high impedance amplifiers. My 30+ year old Pioneer 9100's work, but not as well as my SA-8260's. I would guess that transformers and tubes are out. Thnk this is true with any passive attenuator.
Guess Melos knew what they were doing when they used these a decade ago. Too bad their implimentation lead to high failure rates.
Badge, let us know how yours pans out. My pots ate me up too. I still do not understand why they work as they do. Still learn something everyday.

George
 
Ok either im really not getting this or this passive word has been described to me as something different.


passive means caps resistors / volume pot. I have been experimenting with my Alephs. I have 3 devices.

One, a passive Alps pot.
Two, A Buffer
Three, Bosoz Pre-amp In Single Ended / Balanced Mode.

Now to me honestly i have spent a-lot of money on all these devices except the alps pot. Now guess what one sounds the best. ?

The alps pot passively in Single Ended Mode. Now the only problem with this is that the alephs like a little bit of gain my Sony ( 6 or more years old ) has lots of gain so it works really really well.

Right now i am working on a project to build a simple Op-Amp Buffer to work with this.. Simple as in hardly and parts with out ruining the sound. So far it is a hard challenge as I'm trying to do this on my own.
 
jleaman said:
Ok either im really not getting this or this passive word has been described to me as something different.


passive means caps resistors / volume pot. I have been experimenting with my Alephs. I have 3 devices.

One, a passive Alps pot.
Two, A Buffer
Three, Bosoz Pre-amp In Single Ended / Balanced Mode.

Now to me honestly i have spent a-lot of money on all these devices except the alps pot. Now guess what one sounds the best. ?

The alps pot passively in Single Ended Mode.


Any opamp no matter how good, how expensive, how exotic will ever sound as good as a good quality passive pot properly impedance matched. And the only thing that sounds better than a good quality pot, is no pot at all, hence that's why I developed the Lightspeed Attenuator, it is straight in straight out without being corrupted by any form of contacts or switches, it's as though the cd or dac is fed directly to the amp.

Cheers George
 
jleaman said:
If i was to use no pot at all ID have a problem of being deaf :) I guess i could use passive cables with resistors in them Now that would be a idea :)



If you say you could go deaf, that means you have got enough gain, so you can build a Lightspeed. And if your Adelph needs more gain, you could always lower the global or local feedback a little to give more gain.
And when you say use interconnects with a resistor in them, this is exactly what the Lightspeed does, except that resistor can have it's resistance varied acording to the volume you want.
So you can have your cake and eat it too.


Cheers George
 
georgehifi said:




If you say you could go deaf, that means you have got enough gain, so you can build a Lightspeed. And if your Adelph needs more gain, you could always lower the global or local feedback a little to give more gain.
And when you say use interconnects with a resistor in them, this is exactly what the Lightspeed does, except that resistor can have it's resistance varied acording to the volume you want.
So you can have your cake and eat it too.


Cheers George


Ok so can i buy all the parts to build one of these from you :) will it do balanced and Single ended ? Pm me please.
 
jleaman said:



Ok so can i buy all the parts to build one of these from you :) will it do balanced and Single ended ? Pm me please.



Sorry, you can build one yourself, I've given now all the info to do it circuit and parts you need in previous posts, it's not hard, just time consuming.
Panelhead and a few others have done it, or buy a complete one with power pack from me as other are doing that aren't confident in DIY. Stereo single ended only.

Cheers George
 
More thoughts on mine

The one I built is a little different than the Lightspeed. It uses 50k limear pots instead of the 100K log. The pots used track extremely well. This has caused an issue with resulting attenuation taper for the unit.
Initially it was used with a borrowed First Watt F1. The amplifier benefitted significantly when coupled with the Lightspeed clone. Sound had more like, was a little leaner and faster. The sensitivity of this unit is low. The pots were normally opened about 25% of rotation. Drunken listening maybe 40% at most.
Switching to a My_Ref Rev C, the increased sensitivity is a liability. Normal listening is about 10% of rotation. Loud is 15%. Spectral balance is efected also. Sound is leaner, the low end is rolled off. This did not seem to be true with the F1 as the amp. Bit the bottom of the F1 is so fat, it might have not been noticed.
Overall, it is an excellent unit. Just as noted by George, it has a few system matching limitations.
If I can find some 100K log pots that track well, it will be my next project for this unit. Right now this is effectivily an 8K-9K pot. Using a 100K pot should increase the impedance. This may bring the low end back up. My normal preamp has 25K input impdeance.
So overall, it worked better than my usual preamp with one amplifier, and the results reversed with another. I will keep trying to get more out of it. Maybe my system limitations it is exposing, not an issue with the unit.

George
 
ErikdeBest said:


Hi George. What exactly do you mean with the 100k pot increasing the impedance?


This will be long and boring. The pots are not wired up as voltage dividers; input, wiper, ground, but as variable series resistance.
The resistance of the optoisolators goes from infinity, 1 meg, at zero current to say 50 ohms at 20 ma. This decrease is not linear, is very temp dependent, and does drift a little.
With my setup the input voltage is 5 volts, the pot 50K, the series safety resistor 121 ohms. This resistor is there to limit current. If the pot is set to zero ohms, the LED will fry if hit with 5 volts and unlimited current. The LED drops about 2 volts, should be 3, and the 121 ohm resistor drops the other 3, or 25 ma. Actually measures more like 20 ma. This is not an exact science.
With my pot, with the full 50K in series, the measures resistance is 8.5K, with no resistance from the pot in series the resistance across the LDR measures 70 - 80 ohms.
My thinking is that with a 100K in series the resistance of the LDR with increase to 20 - 25K. This may be too much for a passive, but may be perfect. Currently it is a 8K passive. From my graphs the 100K point should be 25K.
Now I just need to locate some 100K dual pots that track together real well.
Right now, the loading my be a little low, all my sources are cap coupled, and the taper is too steep with normal amplifier. The 100K pots will definately increase the impedance of the circuit compared to the 50K used now. And the taper should be better too.
I will try to hunt some pots down this week to see.


George
 
ErikdeBest said:
Hi George

Thank you for the answer! I couldn't understand how the value of the pot could influence the impedance of the optocoupler - but with those I limiting resistors the story becomes clear!

Erik


Erik, that's right, in the Lightspeed as I make it, the value of the pot has no reference at all to the impedence value of the LDR's. The only thing that influences the impedence value of LDR's is the intensity of the LED's which are controled by the pot in the first place.

Cheers George
 
I got mine up and running the past weekend. I usually keep the electronics projects for winter. First, let me thank George for sharing his design. I like the results. Considering I threw together most of it from existing parts, the results are outstanding. My power supply might be a bit much since I have low volume with the pot all the way down? No matter. It has plenty of gain driving a 1.5 meter IC into a 100k ohm load. I briefly tried it with the sub amp/electronic crossover. My initial response was the bass went away, but I still need to do more listening before I confirm that. Pot is a 100k alps blue and channel balance was close enough the multi turn pots were sufficient.
 
jleaman said:
Im lost on the design i don't even know where to start or how to understand the project..

OK let me try to expain this in the simplest way.
Remember when you said hook up the cd players output directly to the poweramp, and solder a resistor in the interconect to attenuate to a fixed level.
This is BASICALLY what the Lightspeed Attenuator does, except the resistor that is soldered in, is a special Light Dependent Resistor (LDR) which according how much light is shone on it by a LED can vary it's resistance, so you can have control over the level of music by controlling the amount of brigtness of the LED by a pot which varies the voltage going to the LED.
Do you see the light. (Lightspeed) , I hope so.

Cheers George
 
jleaman said:
OH CRAP!! NOW I GET IT !! Simular to a photo diode : except a photo diode is either on or off.. :) So basically i only need a cheap pot and a cheap led then. Where can i get a LDR ? Digikey ? Do you have any to spare ?

Any one have any comments on these yet ? How are people liking them ?


It's all there in previous post to build yourself. Remember that it's a series shunt setup, to keep the input/output impedence constant, a bit more complicated than you think,. (wish you luck!)

As for how people like it, not one negative comment yet from any of my 30 odd world wide customers, in fact extremely positive, to the length that some of them are selling their megabuck pre's because they can not warrent keeping them.

Cheers George
 
Easy enough

Diagrams are listed on page four of this thread. The parts are available from Allied Electronics. There are dual pots on ebay.
I like mine a lot also. Still getting it to work better though. Wish I had followed Georgehifi's lead and used a 100K pot to control volume. And I like doing my channels independently, so no nulling circuit is needed.
It works fine in my system, but it does not need any gain. Have used a buffer for many years, first tube, then opamp, then discrete, and back to chip. This is even cleaner than a buffer. Passive.

Good luck,
George


Youmight even consider ordering a real Ligthspeed, delivered looks to run under 400.00.
 
I have had some time to play with this unit. It does everything well except deep bass in my application. Utilizing two outputs, one driving mains amps with 100 K ohm input and Marchand electronic crossover with 25 k ohm input, I still have plenty of gain. The crossover also has gain if needed. For some reason this combination has created a filter and the bass is weak and there is nothing below say 50 hertz IMO. I contacted Phil Marchand about changing the input impedance on the crossover to 100 K ohms. He does not think that will help, but I have ordered the needed resistors to give it a try. Now if I can just restore the last two octaves of bass, I will be one happy DIY'er.