Lightspeed Attenuator a new passive preamp

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#1,442
juanitox said:

but the better tweak i have found with my config was to put TWO decoupling caps after the volume pots one for the + and the other for the - i have try 47uf with success
the sound seems to be cleaner than before with tight bass .. ( perhaps it helps in case
of a noisy carbon pots like mine??)


🙂
Click to expand...

Can you please explain that a little bit more detailed? I don't understand where you have put the caps exactly!

What do you mean with "after the volume pots one for the + and the other for the - "?
 
#1,443
😱 hi , this is my modest contribution to the lightspeed , easy to try and to remove..

:angel:
 

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#1,447
Hi Lightspeed fans

I am dithering to build a new version of the LS with a constant series resistant (of high quality) of about 100k and an optocoupler as shunt resistance. After that I will put a Pass buffer circuit.

The reason for that is the wish to remote control the whole thing with a µC and a LTC1655 for the LED part. There will be no matching be necessary anymore because there will be an automatic resistance/voltage measuring at the beginning to set 64 dB steps.

The parts are already in my tinker box, I just need two days off to get started 😎 . I hope this will be 2008:cannotbe:
 
#1,448
Re: Mk 11 Lightspeed

Dr Good Vibe said:
Hi georgehifi, very interested in this thread don't know much at all about Lightspeed what in fact you build? very interested to know if you seel the Mk 11 you mentioned.
Kind regards the Doc
Click to expand...


Hi Dr Good Vibe, once I have sorted, matched built and calibrated the production Lightspeed Attenuator yes I do pot all 4 led/ldr's together, as I have found this keeps them thermally stable over many years of operation and keeps tracking errors to virtually zero.
Also if you shine a powerfull light into the end casings of the ldr's you will find some are not lightproof.

Cheers George
 
#1,453
Dr Good Vibe said:



georgehifi, would love to but because I'm new and I thought a couple of posts were two. I can't.
Great, so I'll just have to wait thanks anyway.
Click to expand...

You have mail, be patient there is 300k of info.
Cheers George



PS To all the new readers please vote for our own Passive Preamp forum with the link below, were at the top now so it may happen soon.
http://www.diyaudio.com/request/
 
#1,455
sorry for the late answers. :cannotbe:

juanitox said:
do i must put the same value for
the caps before the R and the bypass caps of the led
Click to expand...

not really, 100uF on the other side should be just fine.

Tolu said:

Why?
Click to expand...

Why what? 😕

The rationale for strong filtering is to get the cleanest DC at the LED. Any noise at the LED will modulate (->pollute) the signal.

(I guess that should be obvious: after all LED/LDRs are analog opto-couplers of sort...).
 
#1,456
Hi, I've built NP's buffered version of the Lightspeed and have a bit of imbalance between the channels. Would I adjust the values R1 and R2 or even put a 300k trimpot there to adjust the balance?

Thanks to George for an excellent attenuator and to Nelson for the added buffer to allow it to be used with my equipment.

cheers
Rick
 
#1,458
KP,
I've built both the original and the buffered Lightspeed. I used musical fidelity's buffer with the original Lightspeed to drive my F3 clone since it has a low input impedance, I think, of about 9k.

I've found them both very similar sounding. That is they both opened up the highs and seemed to help flesh things out over the full range. I haven't tried without the buffer since my other amps are F4's and need a fair amount of gain to drive them. I've also built Nelson's B1 buffer and I prefer the Lightspeeds with the equipment I'm using.

Hope that helps
 
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