Lightspeed Attenuator

[DavidMc]

I think I'm right in thinking that Fran was building one of these, was just wondering if the results were any good,it uses a light dependant resistor and varying the amount of light emitted by an LED, right? so their shouldn't be anything else in the way of the signal, did it work out Fran?

[Fran]

Yeah, they sound good but are difficult to match the ldrs. If you're interested, buy the optivol from Greg ball.... I might even have one if you are interested. The gigaworks pots on eBay are absolutely excellent, my current favourites. The likes of an alps blue is Shite in comparison.

I do have a very fancy version of the lightspeed here with shunt power supply etc etc.

Gregs optivol is about 20 euro I think. Give us a shout if you want to try one out....


Fran

[frd1996]

If I may recommend something:

http://www.goldpt.com/

Frd.

[DavidMc]

Thanks for the response guys. I'm thinking about getting a preamp, but before I do that I want to play around with the diy job that's been doing decent service the last while. It's a basic selector with volume control (Basic selector, basic pot, but short silver wiring) The light speed I find intriguing but I wonder why it hasn't been implented on more preamps, is it the ldr matching issue? Have looked at some of the cheap enough stepped attenuators kits coming out of hong kong too, the gold points look like a neat execution of that idea. Having tried out a few preamps, a couple of valve jobs from china a musical fidelity ac3 and my box, I'm just a little wary about committing big bucks for small gains, as in all honesty the differences in my set up weren't that noticeable. I think the biggest difference I ever noticed was hooking up my cd player and squeezebox directly to my audion 300bs. So what am I getting at? Well ideally it seems as little between the source and amps as possible works best, so how little can I get away with, for convenience I need selector and some attenuation possibilty would be great, what creates the least amount of noise between source and amp?
Ta David

[Fran]

First of all on the LDRs: They have a poor enough history - history of unreliability etc. There is only one kind suitable for the lightspeed: they are made by Silonex, part number is something like RS32S or something. Secondly there's the matching. Ideally it needs to be done quickly, very good source voltage and temperature controlled. Then you need to plot each one at a range of voltages (or more correctly currents) and match the linear curves over the range of attenuation. Also, they are pretty variable, so that even ones from within the same batch are fairly wildly different. So the matching is a PITA. FWIW, I believe that the DartZeel gear uses them now.

There is a company in Lithuania or maybe Latvia ("myth" preamp I think) that uses LDRs with a buffer circuit. That guy uses processor control to automatically alter the current so that unmatched LDRs can be used. Georges lightspeed is probably good enough value, but won;t fulfil one of your basic requirements: it has no source switching - although he will build you one if you want. If you want to go a bit cheaper, a guy called Uriah Daily sells matched LDRs and boards to allow you build your own.

The best sounding preamp I have made is the Pass inspired DC coupled B1. Its an easy enough build, sounds bloody good. There are boards available.... but you would need to build it. On the upside, I have one so at least you could hear it if you were really interested.

The problem with just using an attenuator (whether a regular pot, cheap ebay stepped pot or a fancy schmancy goldpoint or elma one) is that the impedance changes over the range of the pot. Most likely thats why going directly into your amps sounded good.

If I was in your situation, what I would be looking to do is get a preamp (I'd probably look to the DCB1 mentioned above) and then modify your audions to remove the pots which you won't need any more. Do your amps hum/buzz if you turn them up full?


Fran

[Fran]

Link to Uriah:

http://www.buildanamp.com/Lighter-Note- ... oductId=15




Fran

[DavidMc]

Thanks Fran, hadn't thought about disconnecting pots on the mono blocs, there is a slight hum discernable on full volume alright. So bypass pot on amp and build a Pass B1...building the case though..."a huge pain in the ass!" :-) The LDR kit looks tempting

[DavidMc]

Bought the LDR kit...:-)

[Fran]

Mad yoke ya....

Tell Uriah I sent you...



In case I forget be very careful soldering the LDRs. Use a crocdile clip or a pliers as a heatsink. They are easily enough damaged by the heat, and can even be "unmatched" by heating. Also, if you throw one of them off by heating, then you really need to rematch all 4....

You'll be fine, just be careful. You know where I am if you need help.

Fran

[Stereo Coffee]

Firstly I am a small manufacturer of LDR passive attenuator preamps.

Matching is really important but not too hard to get right.
You need a DMM ( Digital Multi meter ). Where 2 anodes need current ie a series pair or a shunt pair
provide a trim pot of 2k ohms connected from current source to the pots wiper then each branch
provides current to each anode. You can then set your volume at one level and patiently bring
each LDR's resistance to be the same.

This works very well with our circuit boards.

Cheers / Chris