Lightspeed Attenuator a new passive preamp

[georgehifi]

excetara2, if your going to run a separate dac then chances are the output impedence will be fine on the dac.
The input impedence of the poweramp can be measured with a multimeter at the rca input (poweramp off), if the input is capacitor coupled then it has to be measured inside the amp after the cap. Tell us what power amp you have, I'm sure one of us will know the input impedence, that will save you measuring it.

Cheers George

[excetara2]

Shouldn't be hard to measure it, but if you got it I have a Odyssey Stratos amplifier.

[jleaman]

Here is a dac that i use.. A usb one. But right now im using the buffer from it to work with my cd-player and alephs. Sounds REALLY good now. I like it. I have to spend a few nights listening to it tho.

[georgehifi]

Originally posted by excetara2
Shouldn't be hard to measure it, but if you got it I have a Odyssey Stratos amplifier.

http://72.14.203.104/search?q=cache:...=clnk&cd=3</a>

Your Stratos seams to be a standard 47k input impedence, should be no problem for a diy Lightspeed passive pre, just keep interconects below say one and a halve meters and try to use ones that are of low capacitance < 100pf per foot and all should sound great with your dac driving it all.

Cheers George

[georgehifi]

For those of you who emailed for a look of the internals, here it is, the potted off white block in the centre of the main board houses the 4 x led/ldr packages, the other board is the second regulation 5vdc board, and not in the pic is a regulated 12vdc plug pack.

Cheers George

[Panelhead]

Measured the LDR's again to match up two sets of four. Plot of resistance against current the graph showed a exponetial decay with increasing current. Used 0.1 ma, 0.5 ma, and 1.5 ma.
Does the potting help with thermal tracking?


George

[georgehifi]

No the potting was to try to keep the whole thing secrete, but that has now kinda gone out the window.

[Badge]

I am testing my circuit. I assumed the 100K pots would control the voltage? I am only seeing a very small drop in voltage at full attenuation. In fact, the voltage changes very little through out it's travel.

[georgehifi]

Badge, if you have followed the Lightspeed Attenuator circuit as I,ve posted, with a 5+VDC with the NSL-32SR2S and 100ohm resistors and 1k multiturn calibration pot all will work as stated.
Be carefull that the led side of the NSL-32SR2S is around the right way, the white dot is the negative of the Led and should go to ground, the other leg is naturally positive and goes to the 100k pot via the 100ohm resistors and the calibration pots.
If the NSL-32SR2S are a perfect match, you will not need the 1k calibration pot.
The led takes no punishment at all in these NSL-32SR2S and will blow if wired the wrong way around.
Hope all works out for you, keep me posted.

Cheers George

[Panelhead]

Originally posted by Badge
I am testing my circuit. I assumed the 100K pots would control the voltage? I am only seeing a very small drop in voltage at full attenuation. In fact, the voltage changes very little through out it's travel.

The way that George wired these up is tricky. There is no ground going to the pot. I suspect this is why you are not seeing any voltage drop with pot rotation. Check the resistance as the pot is opened. It should increase. The 5 volts goes to opposides of the stereo pot so that one side decreases when the other "channel" increases.
The pot needs to be measured to make sure when closed, say 7 o'clock the series LDR is seeing a high resistance and the shunt is seeing no resistance.
George wires his is to connect the wiper to the other end. This way if the wiper goes open, the LDR will see maximum attenuation.
I have built mine, got it properly checked up. Now it needs connecting in circuit. It is a Lightspeed clone. Orginally was going to be just an attenuator. But thought what the heck, if people love it as a passive unit, I should at least listen to it as designed by George. Might change my opinion of passive preamps.

George (low fi)