crossfader design

[DanV]

i am trying to design a circuit that has a crossfader which can fade a mono signal between 24 different outputs. i need to learn more about various crossfader design models... any help/suggestion is very appreciated! my other question,, is would it be possible to make something like this that is a passive unpowered design?

thanks!

 

[verbos]

Fade accross 24 outputs? From a voltage? That's lots of steps in a small range.

you could try this...
http://musicsynthesizer.com/Corpse/VCEOM.html

you can chain multiple LM3914 chips together to get more channels. 3 chips could do 24... not passive at all.

Mark

 

[DanV]

interesting chip,
but i may need to come up with something that has 24 outputs and 1 input, ie something that can fade a mono signal between 24 seperate ouputs - not something with one output that selectively fades between 24 seperate inputs.

edit/ i guess this lm394 chip would be handy for either set-up though...

i'm having trouble understanding more about how the dither signal works to drive the lm394 and cause the fade....

have you used this chip before for crossfading??

thanks!

 

[zmix]

Dan,
Do you want to fade one input between 24 outputs one at a time or groups of 24?

You could make a 1 by 24 counter / decoder and use vactrols to ramp the outputs up and down.

-CZ

 

[DanV]

i'd like to fade between them one at a time,,,,

i'm all ears! thanks for your aid.

right now i'm wondering if setting up some sort of shunt relay switch would be easier to implement and less expensive than a full on crossfader, my main reason for wanting a crossfader was simply to avoid clicking and dropouts during the re-routing.

24 vtl5c3's alone is around $150 :shock:

 

[gyraf]

24 vtl5c3's alone is around $150

I don't think you'll find any easy and cheap way around this.

To be able to pan through 24 channels - not only switch between those - you will need to have adjacent channels open-to-a-degree.

What you then would need is 24 soft-edge window detectors/discriminators, set to react at different voltages from e.g. 0-10V, and controlled from a common DC control voltage. Each output voltage controlling one out of 24 VCA's, fed from a common input audio signal.

For cheap consumer quality VCA's, look at e.g. the CA3080 or the dual LM13600. But don't expect high quality audio from these!

Jakob E.

 

[zmix]

Dan,
What is your intended application?
It sounds like a simple 24 position rotary switch might work.

Is this something that you want to work like the Bus outputs on a console, with one source selected to one or many outputs?

-CZ

 

[verbos]

[quote author="DanV"]interesting chip,
but i may need to come up with something that has 24 outputs and 1 input, ie something that can fade a mono signal between 24 seperate ouputs - not something with one output that selectively fades between 24 seperate inputs.

edit/ i guess this lm394 chip would be handy for either set-up though...

i'm having trouble understanding more about how the dither signal works to drive the lm394 and cause the fade....

have you used this chip before for crossfading??

thanks![/quote]

I have used a similar circuit with this chip.

The dither oscillator is a high frequency saw wave. it makes the transistion from one LED to the next a fade insted of a switch. the way it works is that the dither oscillator is mixed with the control voltage. As you approach the boundry threshold this saw wave's tip hits it first, so you start to have the next step on, but only for a small percentage of the time, because it is happening so fast it seems to make an average between the 2 steps, based on how much time is spent in each state.

It's PWM, just like some D to A converters use.In this case you don't need any filtering, because it's triggering LEDs (inside the vactrols) so they add a slew on their own.

I think a switch is a better solution, BTW. 24 channels crossfading from one voltage is too many to control. I have found 16 on one knob sweep to be the most that is practical.

Mark

 

[DanV]

ok, switching sounds like the ticket.
for the thing i'm trying to build clicking is not acceptable.
i don't need the source routed to more than one of the 24 outputs at a time.

thanks for your help everyone!

can anyone suggest a decent form of soft switching?? where clicking is not possible?

 

[PRR]

> mono signal between 24 different outputs. ... clicking is not acceptable.

First, ensure no stray DC at inputs or outputs.

Then.... if you hard-switch in the middle of a wave, it is still going to "click".

You can try zero-crossing detection, which waits for the audio to cross zero. Several chips have it built in, but all are way over-complicated for plain switching duty.

Or you can "fade". But to do this requires a real fader (pot, VCA, or VCR) that has a clean "dim" range between off and on. Mechanical switches can't do that, and it ain't easy in FETs or opto-Rs.

And for 24 outs, it needs to be easy.

Look at the soft-switch chips. SSM2404 is four switches that fade up/down in 5-10 milliSeconds.

http://www.analog.com/UploadedFiles/Data_Sheets/36658935ssm2404.pdf

The obvious thing to do is put one unit in series with each output (power-amp inputs?). The on-resistance is very low, no problem there for 10K or 100K loads. The off resistance is high enough to give 60dB-80dB "off"... on a test rig. However if you have long lines to power amps, the open switch lets all kinds of crap leak into the cable. Test this plan in mock-up before you try it for real. Cures would be op-amp buffers on each output, or a second switch to short the "off" inputs when not in use.

To turn a switch on or off, bring its control pin to +3V or 0V. Simplest would be a 1-pole 24-throw switch, wiper through 1K to +5V, each throw to a switch input with 5K to ground. Either rotary or interlocked pushbutton would force 1-of-24 operation. If multiple outputs at once are not illegal, just use a toggle or push-on/push-off switch for each position.

 

[DanV]

thanks,,, the ssm2404 looks like the hot ticket. :thumb:

by looking at the specs it seems that the turn on/off slope time is controllable to an extent,,, but how???

 

[alk509]

I made something back in college that does just what you want: It takes an assignable MIDI controller (I used my keyboard's ribbon controller) and "pans" an audio signal between 32 outputs - two "banks" of 16 outputs each.

Only problem is... it's a Max patch! :razz:

Peace,
Al.

 

[DanV]

heheh, that's funny alk,
this creation of mine is inspired by a patch i made in synthedit!