Photocells

[pold]

Hi all, I am planning on building a neovibe, the problem is: where to find custom made photocells identical to those on the original univibe? I am really fussy about it. Do you have any idea or suggestions? The aim it would be to create something as close as possible to the Mojo Vibe (in my opinion the best vibe ever made). I know Banzai and Small bear are selling photocells for univibe, but I am just not interested in them, they can't deliver the same thing, that's it, I am pretty sure of it!
Thanx

[Enzo]

Custom made? other than the housings around them, photcells are photocells, you just have to get the one with the values you seek - on and off resistance and changing time. Unlikely they were invented custom for the OEM.

I have made trem roaches with generic CdS cells from Jameco that work fine.
www.jameco.com and search "photocells." They have a selection of them.

What are the criteria you are being "fussy" about? WHich are you not finding in the general parts market?

I know Banzai and Small bear are selling photocells for univibe, but I am just not interested in them, they can't deliver the same thing, that's it, I am pretty sure of it!

So when you tried those, how did they fall short of expectations? How did they let you down. If you didn't try them, how can you possibly know they "can't deliver?" And just what won't they deliver?

There is a lot more to a photocell vibe unit than its photocell. The waveform of the LFO makes a difference on sound, the audio circuits around it make a difference on sound, etc.

[pold]

Hi, thanx for your suggestion, the sound I am looking for is the one you can hear from this mojo vibe demo:
http://www.sweetsound.com/Audio/MKat...tonLuvADog.mp3
All the other demos on the web never get near, and as B. Sweet said, unless you use those photocells you can't get the same thing. Mike Fuller in his website says "we're the only small company who can quite frankly afford to have the correct Glass covered, hermetically sealed photocells Custom-made."
What do you think ?

[Mark Hammer]

When the pedal in question demands a fast response - as in the case of an optical compressor or noise-gate or envelope-controlled filter - photocell properties start to matter, since you can't make them go any faster than they are able to. In the case of Uni-Vibe, clones, however, the response-time parameters of the photocell are moot; there are probably none in current production that are too slow for the mission.

More critical are the following:
1) The resistance range they cover.
2) Their sensitivity, both spectrally and how much resistance change is produced for X amount of illumination.

Unless you found some whack-job LDRs that have a max resistance of 10k or something like that, almost all will be able to provide the needed resistance as long as they can cover the zone between 10k and 100k, and maybe a bit on either side of that.

The quirks of the LDRs can be offset by selecting your LED light source well, adjusting its intensity appropriately, and in the worst-case scenario, sticking a fixed resistor in parallel with the LDRs to shift its resistance range over a bit.

[Jan Willem Bakker]

If there’s something special about the original UniVibe cells in their particular housing there will surely be a logical explanation for it. Usually magic is only in the music that is created by the electronics, not in the electronics themselves.

One thing that comes into my mind is the fact that in the original UniVibe, an incandescent light bulb is used that is made of transparent (not opaque) glass.

Those light bulbs tend to not diffuse their light evenly around them.

So perhaps what the upright housing does, is preventing the light from directly entering into the cell and forcing the light to enter through the reflection of the housing that covers both light bulb and CDS’s

What could be the difference is: 1) less intense light coming in?, but more important 2) a more even distribution of the light between the four cells

Because that’s what probably will not happen if the light bulb shines in directly in the four cells, due to the unevenly spread diffusion of the light

Considering this, it should not be difficult to simulate an equally homogenous distribution of the light over all four cells. If it is of influence to the tone quality of the UniVibe, than it must automatically also be of importance to match all four cells as close as possible.

I’m considering building a UniVibe clone myself, but my major consideration is the specifications of the CDS’s not the housing in itself.

If anyone could help me out on the specs of the cells this I would be very pleased.

[Jan Willem Bakker]

I decided to build a UniVibe clone and bought the necessary parts at Banzai. I’ll keep you informed on my project. I’m going to try to use as much original parts as possible. For instance, I even bought the 2SC539 transistors and modified the PCB layout that I found on the Internet, in order to accept them. I even shortened the board a bit in order to fit on a standard 10cm wide piece of photo sensitive PCB. However, I settled for the Banzai recommended CDS’s. I will however do as much testing with them as possible and will also try to construct the lamp/cell cover in such a way that the bulb will spread its light homogenously over all four cells.

Jan

[Mark Hammer]

The role that the incandescent bulb plays tends to be more one of effective "waveshape". Keep in mind that the light produced is a function of heat. The filament heats up, glows, and then cools down at some rate after power is removed or current is reduced. While it cools down, it still glows. The asymmetry of that illumination/darkening and the manner in which it approaches maximum illumination determines the "shape" of the resulting resistance changes in the LDRs. If you drive an LED with a triangle waveform from an LFO, where the illumination is rather direct and nothing lingers very long, you get a somewhat different "feel".

[Jan Willem Bakker]

Exactly Marc, That’s my approach as well. I’m never sceptic about “the magic of good sound”. However usually there’s a perfectly valid explanation behind it. To me the incandescent bulbs make sense; they produce the saturating effect that you mention. The same goes for tape recorders and valve amplifiers. Never change a winning team, that’s why I’m going to build a clone using the exactly the same components. Even if the transistors are crappy; that’s exactly the reason why I want them. They might contribute to some distortion that is characteristic to the overall result… Who knows.

Even the upright, glass covered CDS’s that Fulltone claims sound better, probably do. But once again there is probably a logical reason behind it, and the only thing that is logically valid, is that in some way, they contribute to a different distribution of the light source. The glass covers will reflect part of the light coming in horizontally, just like the upstanding housing. This should make it very difficult for any direct light of the bulb shining onto the photocell. Indirect light means “less intense” and more evenly spread light. Both might be of influence to the overall sound, perhaps merely because it will force the lamp to shine brighter, and thus enter into saturation (like you described) sooner.

I’m going to build this clone, and will provide feedback on this forum. Luckily, thank substantial pioneer work done by others, there are several schematics and useful tips available.

Jan

[pold]

Hey Jan, before starting building I recommend you to read everything about the Neovibe, you find all sort informations on the forum at diystompboxes.com. Don't be too obsessed by the original univibe, the Mojo Vibe for example sounds even better.

[Jan Willem Bakker]

Maybe once I’m finished with my clone, it will sound even better ;-)

Thanks for the links,

Jan

[pold]

Yes, let us know when you finish, it will be interesting!