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Opto-Coupler vs LDR

5/14/2003 4:02 PM
Opto-Coupler vs LDR
Kevin O'Connor is putting aside the use of LDR in is book TUT, for fairly good technical reasons I think, and also about cost.  
Nevertherless, is their a difference in performance between LDR and Opto-Coupler. Aside from cost, does the drawback listed in the TUT are also valid for the opto-coupler?  
Anyone having opinions on the subjet?
5/15/2003 7:19 AM
Stefaan Van Slycken

There is quite a difference in opto-coupler vs. LDR... the most important being that optocouplers need to have correct polarisation... if you use a regular type you can compare it to switching with a transistor, with the only difference that an optocoupler can separate two circuits (the driver circuit for the led doesn't have to be connected to the "driven" circuit).  
Of course there are optocouplers with two antiparallel leds but keep in mind that a led needs about 1.2V before it actually starts working, so i f you drive it with AC there'll be a gap between -1.2 and +1.2V.  
There are also optocouplers with triacs or thyristors (or maybe even two antiparallel transistors i believe) but again you get more or less the same problem as with the two leds.  
Maybe optocouplers *can* be implemented in guitar amp designs, but i think it'll be easier than with LDRs...  
Book Of The Day The Ultimate Tone, Volume III by Kevin O'Connor
Have you ever wondered if there is a better way to build a Bassman, Champ, Plexi, an 800, AC-30, Bulldog or Portaflex? Or you wanted to build an SVT with off-the-shelf parts? How about a master-volume amp that doesn’t change tone with the master setting? Everything you need to know is right here, including: proper grounding techniques, wiring methods, and mechanical considerations. Eighteen chapters cover the “iconic” amps everyone knows and loves, with schematics and layouts for each, along with the technical history of the product. Eyelet-board and chassis-mounted tube socket construction is used throughout, for easy servicing and modding. TUT3 is very accessible even if you cannot fully read a schematic and is a "must have" if you are going to build an amp for your self.

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5/15/2003 1:24 PM

Tanks Stefaan,  
I'm still at the design stage and I cannot make my choice between Opto-Coupler switching circuitry. I found a shematic of the H&K TriAmp, with rely on Opto-C for muting points, and extensive BJT's switching logic circuit.  
As you say, Opto-C have a separate led "drive" circuit form the "driven" line, and I like this idea.  
My other option is the BJT's or JFet's shunt type muting point extensively layout in the TUT. These are part of main circuit though... but I guest that when correctly done, there is no real problem...  
Of course price is a argument agaist Opto-C, but in the long run... And it's less component that a typical BJT's shunt muting point!  
I really don't know what to base my decision on.
5/15/2003 6:23 PM
Rick Erickson

I must be missing something here. What's the difference between an opto-coupler and an LDR?  
Most common opto-couplers I've seen in amplifiers are a package containing an LED and an LDR. The VTL-5C3 is a typical opto-coupler. I don't own any of Kevin's books, maybe I would understand the question better if I did.  
My preference is relay switching. Been using them for many years without problems. Advantages - lower "on" resistance, higher "off" resistance, linear frequency response, able to handle relatively large voltages and currents found in tube preamp circuits, easy to power an led across the relay coil for a simple status indicator. Relays can be very quiet if the circuit is designed properly. Certainly worth considering.  
5/15/2003 7:49 PM

You didn't missed anything Rick, the best way to learn is to ask question, even when you know the answer... or part of it!!!  
"Relays can be very quiet if the circuit is designed properly. Certainly worth considering"  
What are the design proper consideration then? (this one I don't know anything about)
5/15/2003 9:24 PM
stephen conner

An Opto-coupler is normally taken to be an LED shining on a phototransistor. It's not so good for switching/muting audio signals because the transistor only conducts in one direction and can't take more than a few volts in reverse. The proper name for it is 'Opto-isolator'.  
The thing you find in SLOs, Peavey 5150s, etc, which is a LED shining on an LDR, is properly called a 'Vactrol' or something. Folk seem to refer to the whole LED/LDR assembly as a 'LDR'.  
To complicate the issue, you can also get opto-isolators made with an LED, a tiny solar cell, and one or two MOSFETs. These may be usable as replacements for Vactrols, although they have rather a high shunt capacitance.  
Steve C.
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