Showtec Demux and Strand Mini II

[tom_the_LD]

Hi all,

 

Quick question. I know that some (I believe the older) demux's weren't happy outputting the negative voltage - IIRC it connects the earth pin to the case potentially making the whole case live or something? I also know that they're not very happy running Mini 2 dimmers - something to do with them pulling current down the earth on switch on or pulling a large current on switch on or something.

 

Anyway - I believe both these problems have been fixed in later versions.

 

Now, I've just seen a demux crop up on eBay and my question is - is there any way to tell if it's an older model or a newer model? Are there certain model numbers to look out for where both the above issues have been fixed? I don't want to be buying a demux which doesn't work!

 

Similarly, I've done a quick search but couldn't see anything, I believe there's a way to turn the negative voltage strand dimmers so that they run on positive voltage. Are there any threads/places to look with advice on how to set about doing this?

 

 

Thanks all,

 

 

Tom

[Neil Hampson]

As far as I was aware, the Showtec Demux was always switchable for negative voltage output, so sorry I can't help you there.

In terms of making a Mini II accept a positive voltage, all you need is to have a 6 channel inverter, consiting on a unity gain op-amp and 3 resistsors of equal resistance (100Ohm I think) on each of the input channels. An LM 324(?) is a simple 4 channel Op amp in 14 pin DIL package. you do also need a +15 and -15 Vdc suply for the circuit. I would assume that that is ll that is in the Demux anyway, (That's all is in the older Zero88 Demux48, sold as an add-on board, and used successfully with a Mini II for several shows)

[ianknight]

Hi all,

 

Quick question. I know that some (I believe the older) demux's weren't happy outputting the negative voltage - IIRC it connects the earth pin to the case potentially making the whole case live or something? I also know that they're not very happy running Mini 2 dimmers - something to do with them pulling current down the earth on switch on or pulling a large current on switch on or something.

 

Anyway - I believe both these problems have been fixed in later versions.

 

Now, I've just seen a demux crop up on eBay and my question is - is there any way to tell if it's an older model or a newer model? Are there certain model numbers to look out for where both the above issues have been fixed? I don't want to be buying a demux which doesn't work!

 

Similarly, I've done a quick search but couldn't see anything, I believe there's a way to turn the negative voltage strand dimmers so that they run on positive voltage. Are there any threads/places to look with advice on how to set about doing this?

 

 

Thanks all,

 

 

Tom

The Botex Demux (aka Strand S72 + ShowTec Multi Exchanger) has the basic problem that the outputs are driven directly by the LM324's without buffering - the input stage of the Mini 2 draws more current (albeit milliamps) than these can sustain and so end up killing the LM324's. The other basic problem was that they didn't use a mains toroidal in the early marks and so the negative switch really did nothing except swap the pos & neg rails around (or something completely useless - it's a while since I pointed the problem out to Strand).

 

In all honestly - I wouldn't recommend using the unit with Mini 2's unless you like disassembling things and replacing chips every show (at least on some marks the 324's were socketed) or you can build a buffer board using some chunkier op amps or transistors to let the Mini 2 sink as much current as it wants.