Lightspeed Attenuator a new passive preamp

[georgehifi]

[QUOTE=DQ; I've just posted the above only to see that in in this case I'd run into another problem, the Lightspeed would not like the 10kohms input of my monitors, right? Then what's the solution, can you spell it for me? Sorry for being such a... hmmm... beginner![/QUOTE]

For these active monitors (not the First Watt) then you will need a buffer after the Lightspeed as well, and you may then be able to omit the CDP buffer, but I doubt it.

Cheers George

[DQ]

Thank you. This makes it rather complicated...

[binspaul]

Hi George,

I found two TI chips than can provide the required ±5V values:

1. Linear Regulators - Standard Voltage Regulators - UA7805 - TI.com
2. Linear Regulators - Standard Voltage Regulators - UA7905 - TI.com

Best regards,
Bins.

[georgehifi]

If your building it as per my instructions you only need the one +5v regulator, not +&-

Cheers George

[binspaul]

Hi George,

I am building the MK II version of light speed. But, the power supply is 15V DC with 400mA rating.

Best regards,
Bins.

[georgehifi]

The MkII is double regulated, it has a +12v regulated linear wall wart (you can use your 15vdc supply) that gets
re-regulated by a +5v regulated supply, which then goes to the 100kohm log pots then to the 100ohm resistors and then to the + led portion of the NSL-32SR2S.
All the 0's of the wall wart, the +5vdc supply and the cathode of the led go together, you can put them to chassis earth, I perfer not too.
Cheers George

[binspaul]

Hi George,

I am going for the TI REF02 for getting the required +5V DC. I have named my project as TEJAS (Which means Radiant in Sanskrit).

Best regards,
Bins.

[udailey]

Binspaul,
This is a fine solution. You should consider using 3 of these in your circuit. If you want your volume to go down low you need to saturate those shunt resistors to get their lowest resistance. This takes about 10mA per LDR. The max output of the REF02 is 10mA so consider using one REF02 per LDR and one for series, or two for series. Either way is fine but both ways will change volume a little differently. If you use one single REF02 you wont have enough output to control the Lightspeed the way it should be. Almost your entire rotation of your control pot will work identically to a higher powered Lightspeed, but when you try to turn the volume down all the way it just wont happen.
However, I dont think it would be the same circuit anymore. There is really nothing wrong with dual regulating down to 5V with a few LM317 or other 3 pin regulator. Its going to be the easiest solution for you.
If you would like to be a bit different and do something else you might try LM4040-10. It will only give out 100mW max and split between the two LDRs sucking the most power at the time this will be fine. You can use more than 5V to control them but you have to be careful not to exceed about 100mW per LDR and its not necessary to use more than 50mW per device to get the lowest resistance. I should say its not necessary to use more than 50mW on a 'typical' device as some of them saturate only with a little higher power since the LDRs are not all identical to each other even though their specs say they are.
Uriah

[binspaul]

Hi Uriah,

If that is the case with REF02, then can I use the PTN78000W (http://focus.ti.com/lit/ds/symlink/ptn78000w.pdf) ? It is having an output current capability of 1.5A.

Please suggest...
Best regards,
Bins.

[udailey]

Hi Bins,
Yeah that will work and probably do a great job. The only thing anyone might not like about it is that its a switching regulator. I have a switching regulator on my first Lightspeed and it DOES make a tiny amount of really high frequency noise that I only hear when my ear it directly on the speaker.
One other thing to consider is that this supply is not stable without a .1A load on it so you might pick a low value high wattage resistor to put across output and ground to load it so you get stable voltage regulation. If your output is 5V then you might use a </=50R resistor of a few watts as a load.
I say if this is something available to you it looks like a great solution.
Also, dont let its switching bother you much. Its at about .5MHz so you are NOT going to hear this at all. Mine was switching about 100 time slower which is definitely in our hearing range.
Uriah