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LDR Switching Questions

1/21/2004 5:12 AM
Omar N.
LDR Switching Questions
Hi all,  
I've built a couple of amps that were either single channel or 4 input, but never with true channel switching. I want to build a two channel amp that switches b/w a dirty and high gain channel with seperate preamp, eq and master volume. The topology will be  
Input > 1st stage > split to each channel, eq, and master volume > sum back at phase inveter > power amp  
It seems that LDRs (Vactec VTL 5C1 or 5C2) are the easiest way to go but I have a couple of questions:  
I was thinking of placing the LDRs where the channels split and where the master volumes sum at the phase inverter to isolate the unused channel so no signal leaking occurs. Is this the most effective way to use LDRs?  
Is there a quick and dirty way to rectify a 6.3 tap to turn on/off the LDRs? And how do you incorporate the footswitch?  
I assume I should calculate the current for 2 LDR's on at a time (not 4) since only 2 will be on the same time?  
1/21/2004 3:53 PM
Jed E.

When using ldr's, specifically the vactec5c1's, you must keep in mind that they only can handle 100v across the resistor section. I use the NEC solid state relays, mouser part number PS-7141-1C, which can handle 400 volts.  
My 2 pesos
1/25/2004 5:00 AM
Jack Koochin

Very cool idea. I can't decipher the little schematic in Mouser for the part.... looks like it's showing a Mosfet device, but doesn't make sense to me the way the device is shown.....  
Anyhow, do you have a data sheet for this device, how much current to drive the LED and so on?  
1/26/2004 3:48 PM
Jed E.

The data sheet can be had here:  
It a photomosfet solid state relay and if memory serves they take between 2 and 15 volts to operate and around 1.2-1.4ma and can handle 400v across the relay terminals. Here's a link to schematic I drew up sometime back  
I got rid of one of the 61.9r resistors and one of the 1.5k resistor and ran them in parallel and have not had any problems. One suggestion though, if you have a high gain amp run the ground shunt from the grid of the last gain stage in order to get rid of some of the natrual high gain hiss crossing over.  
1/26/2004 6:21 PM
Jack Koochin

Thanks for the info - great stuff. Do you notice any switching noise at all? These probably have much faster response than the LDRs, and I noticed in the thread someone mentioning that amp manufacturers use the LDRs because they are slow.  
I take it the output to the power amp goes to the PI coupling cap?  
In my application I think I'd opt for all NO contacts and do a strictly shunt-type of switching, but this type of device should be just dandy for that, too.  
1/27/2004 3:37 PM
Jed E.

I havent noticed any switching noise at all. But, be sure you reference the 6.3v ac (or whatever lov volt ac your going to rectify) to ground via a center tap or 2 100 ohm resistors.
1/22/2004 1:18 AM

Ordinary plain old common garden variety vanilla, white bread Pat Boone relays work just fine too.
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