LDR transistor gain

[Kipper4]

Is this ok?




Not breadboarded as yet.

[Kipper4]

Why does this little cap (47pf) help smooth the leds transition from dark to light?



I remembered using it before in something. IIRC I saw it on a synth page and tried some values.
10p~100p hmmm.....

[bool]

Yes, ... BUT you should increase the C12 somewhat to cover a more full frequency range; say to 1uF. As it is now, at the lowest "gain" (ie, the LDR full "on"), this circuit will act more as a simple low-pass filter. But that may actually be useful to your quest, Sir.

[Kipper4]

Thanks Bool.

[anotherjim]

Vactrol > C12 path should be to lower end of R18 collector resistor, not the + supply.
Agree C12 may have to be larger - depends on range of LDR and how deep you want it to go (suspect LDR might need some R about 10k in series). C12 @ 470n or 680n might be big enough.

Other Q - Not sure if that 47p can do anything practical. First suspicion is that someone scoped it and saw hf noise and decided to cut it with a cap. Maybe had no practical impact if  controlling an LDR, but could do if the LED was part of a fast opto-coupler.

[Kipper4]

Yer I should have known that. Excellent help.
Thanks Jim.

Stand by for the srpp questions.......

[Kipper4]

I found something else.

Wigglers thread.



https://www.muffwiggler.com/forum/viewtopic.php?t=168810&sid=08c4d1433013487276c782e1e9d3a537

Reply 3 by eat your guitar.

Can I hang my envelope charge cap to cv in?

Maybe a voltage divider.
I'm not sure what cv the circuit is expecting. Assuming it's a positive voltage.

Cheers

Rich

[anotherjim]

In that MW thread the first idea was a swing VCA. I think it worked with negative CV because the emitter got reverse blocked and the base-collector then worked like a diode VCA/gate.

3rd post idea. Positive CV. Control Q base current might discharge an envelope cap too fast without buffering. DC bias shift might be audible.

[Kipper4]

Yeh, replace, in series, in parallel...
Collector-base is really not much different than using the negative feedback loop of an opamp .

Keep it AC coupled so it don't change the DC bias. There's usually 390k-470k setting the DC bias already and LDR range compares better to that than 100R> emitter degeneration resistors.

I saw this today on the breadboard thanks Jim.
I'm not sure I'm gonna get it to play ball within this circuit.
But I'm sure I can find a future use for it.

Still more work to do.

I couldn't get the srpp up and working yet. I need to look over some pm's too.

[Kipper4]

Another idea I've had.
After reading up a bit and watching this video.

I gots to thinking.
Why cant I use a long tailed pair to crossfade/control the signals amplitude.
Shout up if I'm off my head anyways heres my idea.

[EBK]

My first thought is that you're missing proper DC biasing on the base of both those transistors (unless it is me that is missing something).

[Kipper4]

Like this maybe Eric?





http://www.nutsvolts.com/uploads/wygwam/NV_0903_Marston_FIG23.jpg

[robthequiet]

Yep, some ideas too, here, I think....



http://www.8085projects.info/wp-content/uploads/Tremolo%20amplifier%20circuit_thumb.jpg

[EBK]

If you want to use an LTP for this sort of thing, you need one of your inputs to go in through the tail (that resistor that both emitters join at).  Otherwise you just have a differential amp.

Edit: Basically, Rob just showed this. 

[EBK]

In your case, perhaps try using the LDR in a current mirror arrangement as the tail current.  Just a thought.

[Kipper4]

"current mirror arrangement"

as in ltp with current mirror?

my idea was to put a dry signal and a lower volume signal then as the lower signal gets smaller the ltp out will rise in relation. therefore  a swell type effect.

[EBK]

I think I'm completely misunderstanding....
1.  In your schematic, when you say "buffer out", that's your dry signal, right?
2.  And, when say "enveloped signal", is that the output of an envelope follower tracking your input (where the input is the same as your dry signal)?
3.  You want your dry signal to be amplified with a gain that is based on your "enveloped signal"?
4.  Specifically, when envelope goes down, gain goes up?

[robthequiet]

^ EBK

Same question #4, but considering the opportunity to drive down or up, as on a Q-tron. I could see the swell thing happening in an overcranked compressor or a slow gate.  I was reading about tremolo circuits and there was mention about keeping the signal path separate from the control circuit.

In the ltp, if you bootstrap it, can you get a good result from putting the LDR at the base of the tail transistor? The image I posted has the signal going into the tail, but somehow I think that's not the best way.

[EBK]

^ EBK

Same question #4, but considering the opportunity to drive down or up, as on a Q-tron. I could see the swell thing happening in an overcranked compressor or a slow gate. 

Overcranked compressor was my thought as well.  Opportunity to drive down?.... That would be an overcranked expander, but is it useful?  Seems to me it would simulate guitar strings that need to be replaced.

was reading about tremolo circuits and there was mention about keeping the signal path separate from the control circuit.

That sounds like a clock noise concern.

In the ltp, if you bootstrap it, can you get a good result from putting the LDR at the base of the tail transistor?

Well, you're never going to get anything stellar out of it, in my opinion, but it is possible.  Check out the section titled "Long-Tailed-Pair VCAs" on this page:
http://sound.whsites.net/articles/vca-techniques.html

The image I posted has the signal going into the tail, but somehow I think that's not the best way.

You can either have the signal go into the tail or have your control voltage go into the tail. Similar problems either way, I'd think.  Personally, I'm more comfortable designing this sort of thing with op amps or real VCA chips.

[robthequiet]

^ Rereading the thread, seems like RG pretty much covered my question,

That's exactly the chain of thought that once led to "Hey! I'll use TWO bipolars as a diffamp, control the emitter current, and then have a varying differential signal between the two collectors!".  The emitter biased diffamp pair is the basis of all bipolar multipliers, with additional complexity growing to further isolate and enhance the changing gain part from the wildly varying DC conditions.

I guess it pretty much depends on what you want it to do before you build it. I'll go have a read of Rod Elliott's article, thx. And yes, the downward moving Q-tron filter is my least used, unless I'm playing some funky reggae fusion, which I did a couple of times, confessedly.