T4B Photocells

[MaxDM]

I think one had a slower response and the other was for peaks

 

[rock soderstrom]

Anyone knows why 2 LDRs in parallel inside the T4B on the GR circuit?

I'm not sure if I understand the question correctly, but you need one LDR for gain reduction and one for the VU meter.

 

[Bo Deadly]

I think one had a slower response and the other was for peaks

That would make sense. If it's true I don't think I've heard that before. Note that, as described in my original posts, CdS cells have a "memory effect" which I believe equates to having two different timing characteristics where the resistance changes slowly but it also has a limited quick change in resistance riding on top of that. So using two cells in parallel would yield a fourth order timing characteristic.

Note that this higher order timing characteristic could be implemented a FET circuit to a great extent by simply using additional capacitors and resistors in the attack release network. In fact, I'm pretty sure I've seen at least one circuit that did that and I wondered why at the time.

 

[dmp]

I bought a bunch of cells and measured them years ago using a DAQ card. There still some data here: DIY T4B, matching EL Panels and Cells and some good discussion

The cells do have some interesting characteristics for attack / release. And the 'grey' cells do have different characteristics than the 'rust' cells. If you can get some of the 'rust' cells (matched) they make a really nice T4B.
I ended up making a pair for an LA-2A in old octal relay boxes that sound really good.

 

[MaxDM]

well, while we're at it, I don't know it it's been mentioned, so just in case, Part of the LA2A sound is the bleed that the EL panel induces into the CDS cells.

There's a metal grid between the EL panel and the CDS cells, to stop the (electrostatic?) bleed.

You can hear the effect when the compression is high

So that's one reason why LED's won't give you the same sound..

 

[Brian Roth]

I skimmed back through this thread and didn't see one factor, which might have some impact. The EL panels I tinkered with have brightness and color which changes with frequency. At. say, 60 Hz, the panel produced a comparatively lower brightness with a green color. As driving frequency increased, the brightness change was quite visible as the color shifted to blue, and then almost white.

Bri

 

[paulcheeba]

Anyone know what photocells would be compatible for the ADL 1500?