On stereo trems, does one channel always go quiet when you turn down the depth?

[midwayfair]

I had someone ask me about a tap tempo stereo tremolo (using the TAPLFO) recently, and I was trying to explain that one channel will go dead when the depth is down, but then it occurred to me just now ... wouldn't this have to happen with any kind of stereo tremolo? Am I wrong on that? I don't have a stereo setup and it's been a year since I breadboarded the LFO from the Stereo trem (and I can't find the chip for it at the moment), so I can't remember if this was true.

[bcalla]

I wouldn't think so.

I don't know how to design a circuit, so I can't say how it has to work in practice.  I do own a digital stereo tremolo, and here is the description of its options:
"Tremolo is a simple amplitude modulation of the input signal, so it becomes louder and quieter at a regular rate. This function uses a sinusoid from a lookup table to vary the amplitude of the input signal. This LFO can be varied from .17Hz to 43Hz with the pot (MOD1), and pressing the pushbutton on MOD2 changes whether the left and right signals change amplitude together, or alternate, so as one is increasing in volume, the other is decreasing (panning from left to right).  To decrease the amount of effect, mix in some of the original signal with the MIX knob."

I would think you would want depth to affect both channels identically.

[slacker]

I can't see any reason it should, why do you think it has to?
If I was designing a stereo trem I'd do it so the LFO was inverted for the second channel so as one side got louder the other got quieter and vice versa, or have that as an option at least. Depth would dial out the LFO so minimum depth gives straight unaffected signal on both channels.

[midwayfair]

I can't see any reason it should, why do you think it has to?
If I was designing a stereo trem I'd do it so the LFO was inverted for the second channel so as one side got louder the other got quieter and vice versa, or have that as an option at least. Depth dials out the LFO so minimum depth gives straight signal on both channels increasing the depth increases how quiet the channel gets.

Hm. Okay.

Thing is, I know this has been done optically using the TAPLFO - the Catalinbread Stereo Semaphore used that chip. But I've tried everything I can think of to beat two LEDs out of phase with that thing and it never worked. I should probably start a new post, but is there a way to take the PWM from that and make it a normal and inverted signal? (Would it involve an op amp in some way?)

[slacker]

Try this, I think it will work. Using Taylor's Tap Trem as an example http://www.electricdruid.net/images/lfo/TapTremolo.gif add a second 10k resistor from pin 5 to the base of a PNP transistor, collector to ground, emitter to a led and resistor to + 5. Basically duplicate what's already there but with a PNP. That should give an inverted PWM signal so when the NPN is mostly on the PNP will be mostly off and the other way round.
This will give you one LED on and one off at minimum depth, so depending on what the LEDs are controlling you might get one channel silent. One way round that would be to move the depth control somewhere else in the circuit, with the TAPLFO set to max depth.

[midwayfair]

Try this, I think it will work. Using Taylor's Tap Trem as an example http://www.electricdruid.net/images/lfo/TapTremolo.gif add a second 10k resistor from pin 5 to the base of a PNP transistor, collector to ground, emitter to a led and resistor to + 5. Basically duplicate what's already there but with a PNP. That should give an inverted PWM signal so when the NPN is mostly on the PNP will be mostly off and the other way round.

oh crap, really?

I ALWAYS forget to try PNPs when I'm doing anything. This is exactly what I needed.

[PRR]

> stereo tremolo ... explain that one channel will go dead when the depth is down

Why??

> I ALWAYS forget to try PNPs

Oh, foo on practical details.

Imagine a *simple* stereo tremolo, however impractical.

Tremolo is slow cyclic volume change.

We can get that with a volume control, a potentiometer.

Slide-pots are easier to see what's-up.

Hire a demon to slide the knob up/down. "Intensity" is how far he slides; 0-10 is large intensity, 4-6 is small intensity, 5-5 is zero intensity.

Now you want stereo. So the two outputs presumably do "something different". The usual effect is that as Left goes up/down, Right goes down/up.

Use two slide-pots. Mount one "upside down", so that "up" is "less". Tie the knobs together. Now when Left goes up/down, Right goes down/up. The same demon slides up/down same as before.

And when you order zero intensity, he holds both knobs on 5.

Yes, this scheme has problems. The pots wear-out, the demon steals your beer. But the basic operation is the same: with intensity at zero, both outputs should be the same.

[drolo]

Hm. Okay.

Thing is, I know this has been done optically using the TAPLFO - the Catalinbread Stereo Semaphore used that chip. But I've tried everything I can think of to beat two LEDs out of phase with that thing and it never worked. I should probably start a new post, but is there a way to take the PWM from that and make it a normal and inverted signal? (Would it involve an op amp in some way?)
[/quote]

Hi Jon,

I have done this to have an inverted signal from the taplfo:

http://www.diystompboxes.com/smfforum/index.php?topic=103714.0

Actually, if you go with both chanels in my schematic to separate outputs instead of a summing op amp, you have a stereo trem

[midwayfair]

I have done this to have an inverted signal from the taplfo:

http://www.diystompboxes.com/smfforum/index.php?topic=103714.0

Actually, if you go with both chanels in my schematic to separate outputs instead of a summing op amp, you have a stereo trem

Oh right, I forgot yours did that! Good to have a backup plan. I'll test that out when the TAPLF chip gets here. Right now, if the PNP works, then it would be a little more convenient on the layout.

I really need to study op amps more too ... feeding it to the inverting inputs seems obvious now.

[duck_arse]

Nice! I always forget to mess with PNP transistors, but I've seen something like the NPN side of this. Lots of good uses for this, not just with LFOs -- I bet you could use it to trigger opposed envelope responses, too.

he's right, he does always forget.

[midwayfair]

Nice! I always forget to mess with PNP transistors, but I've seen something like the NPN side of this. Lots of good uses for this, not just with LFOs -- I bet you could use it to trigger opposed envelope responses, too.

he's right, he does always forget.

lol, thanks for the reminder.

[midwayfair]

Tested the TAPLFO with out-of phase LEDs just using an NPN and PNP pair. It works perfectly! But it appears that the PNP might have to go in backwards, which is a little strange.

Thanks so much for helping me.

[tubegeek]

he's right, he does always forget.

lol, thanks for the reminder.