Can I use a VTL5C10 in RG's Neutron Filter?

[bob p]

I've been looking at RG Keen's "Neutron" filter, which uses VTL5C3 vactrols. I'm wondering if the VTL5C10 can be used as a resonable substitute in the circuit, and if so, what mods might be necessary. If anyone has an idea on this, I'd appreciate your input.

TIA.

[Mark Hammer]

Technically speaking, there are a LOT of optoisolators one could hypothetically use in the Neutron/Mutron circuit. The key thing is whether they can be used as straightforward drop-in replacements without needing to change anything else.

Note that there are several properties of the optoisolator to keep in mind:
1) The speed of response of the LDR. Being able to sweep across a given resistance range is not the same thing as how fast the LDR can respond and return to its original dark (or light) resistance. The 5C3 is spec'ed at having a 2.5ms turn-on time, and 35ms turn-off time. The 5C10 is spec'd at a 1ms turn-on time, and 1.5sec turn-off time. So, the 5C10 will certainly sweep upwards as fast as the 5C3 does, but will be sluggish coming back down. That also has implications for the downward sweep setting.

2) The filter frequency is simply a joint function of the LDR + fixed-resistor parallel resistance and the filter caps in the feedback loop of the op-amps. If the LDR resistance range is different, the same resonant frequency can be achieved by tweaking the cap values and the parallel fixed resistor values. The 5C3 is spec'd at a 30k-10M resistance range. The 5C10 is spec'd at 400R-400k.

So, the upshot is that while the 5C10 can be used in a pinch, it will not deliver optimal performance in terms of sweep "feel", and will require some experimentation and changes to cap values (from 1N8 and 2N2 to something larger) and parallel resistors (from 220k to something smaller) in order to achieve the same filter tone.

Bottom line? aim for something else unless you have no other choice. Small Bear Electronics has the original Hamamatsu dual optoisolator, which is the ideal choice. Not that much more $ than a Vactrol.

[bob p]

thanks for your help, Mark. your explanation was clear and concise and provided exactly the information that I was looking for. all things considered, its probably worth just ordering the right parts instead of reworking the circuit.

regarding the original Hamamatsu device, do you know what the model number of that device would be? i looked on the SB site and he has more than one Hamamatsu opto, so i just want to be sure of the part number. i've looked around elsewhere and i couldn't find part numbers for the original device. IIRC they're not mentioned in RG's .pdf on the Neutron, but i could be wrong on that.

thanks again.

[Mark Hammer]

Glad to help out. Re: Hamamatsu - It's gonna be this one: http://www.smallbearelec.com/Detail.bok?no=360