signal routing

[tzenobite]

hi to all! i have ha n00b question... hope you'll help
i need this setup: one input (from a hurdygurdy) goes to as on/off switch then to a looper (jamman express xt) then to a loop monito (vox da5), the input AND the looper output goes to a single line out then to another on/off switch
explain: the first on/off switch is needed to cut the hg from the monitor after i got the loop recorded, the hg+looper mix and the second on/off switch are needed when i play with others
it's possible to put the routing (the two on/off switch and the hg output+looper output mixer) in a single box?
the point is, if i simply join the hg line with the looper output line the loop goes back in the looper input, right? there's a way to avoid that? maybe i need (or i have to make) a mixer of some sort?
sorry for the silly question but i can't figure out a solution...
hope to be clear enough despite i'm italian
thanks in advance!
matteo

[blackieNYC]

Ciao!
Could you post a drawing?
It needs to be available online, and you post the web address of the stored drawing on this forum using the Mona Lisa button up there.

[ElectricDruid]

Yes, it sounds like you need a mixer to combine the hurdy gurdy and the looper output. As you said, if you've already got the hurdy-gurdy connected to the input of the looper, just connecting the hurdy-gurdy and the looper output also connects the looper in and out together.
The mixer will separate these two points in the circuit, since signals won't be able to go from one mixer input to the other (that's the point of the "virtual earth").

HTH,
Tom

[tzenobite]

i think you'll be right! so, i need a mixer... since i need also a couple of switches, is possible to get the parts needed to build a simple two-input mixer so i'll put it into a two-switches stompbox with some wiring inside and a LOT less cables around?

EDIT: will optocouplers do the trick? if the octocoupler has an input and a output it'll insulate a line backward?
hg line > optocoupler > line out jack < looper line

[ElectricDruid]

It might be possible to use an optocoupler like that, but I would guess that it isn't worth it in terms of component count. That's because *just* the opto-coupler won't work - or if it does, it'll generate a lot of distortion.

Instead you'd need some circuit ahead of the "input" side of the optocoupler, to bias the LED and arrange the signal so that it gave a good variation in brightness. If you're going to build such a circuit, you might as well just build a mixer - it'll be simpler.

Also worth noting is that many opto-couplers don't respond fast enough to transmit audio beyond some low bass notes. Some do, but not all by any means.

HTH,
Tom

[tzenobite]

thanks, again. so the best way out is a mixer, but does a 2-in-mixer-in-a-stompbox exists?

edit: here's a simple ascii diagram (i'm so nerd), i need a a/b switch after the hg: while i rec the loop the hg sound goes through the looper to the mixer and then to the main line out, without this switch during the loop recording i'll send the hg signal to the mixer two times

HG--[a/b switch]A--[looper]----[monitor]
                B                        |
                |-----[mixer]------|
                             |
                        line out

[ElectricDruid]

thanks, again. so the best way out is a mixer, but does a 2-in-mixer-in-a-stompbox exists?

It easily can do! Not too hard to build!

edit: here's a simple ascii diagram (i'm so nerd), i need a a/b switch after the hg: while i rec the loop the hg sound goes through the looper to the mixer and then to the main line out, without this switch during the loop recording i'll send the hg signal to the mixer two times

HG--[a/b switch]A--[looper]----[monitor]
                B                        |
                |-----[mixer]------|
                             |
                        line out

Seems reasonable. As you suggested you could put the A/B switch in the same box as the mixer. Then you'd have a box with four jacks:

Input from HG
Output (Send) to Looper
Input (Return) from Looper/monitor
Output from Mixer

You could put a couple of pots on the mixer channels to make it simple to balance the levels, and that'd be it.
The GGG Mini Mixer would do the job admirably:

http://www.generalguitargadgets.com/effects-projects/boostersrouters/mini-mixer/

HTH,
Tom

[tzenobite]

that's it! thank you, i think this will be the best way to go
i'll give it a try and the let you know how it goes
thanks again

edit: the ggg mini mixer (according to the drawing) seems to have all the 4 input wired on a single line before the ic, so the problem of the input-output is still here...
does anyone out there know how to do the trick so i will try to build it myself and live better?

[tzenobite]

BREAKING IDEA!
what if i put a preamp somewhere? i got one from another project... the preamp actually has no "return", right? i may put this way:

hg ->[A/B]-A->[looper]->-|->[monitor]
           |                          V
           B->[preamp.]---->X->[on/off]->[line out]

[ElectricDruid]

If you do it like that, you're doing some kind of passive mix of the pre-amp output and the looper output. It might work, but it will depend a lot on the looper output (which is an unknown). Passive mixing can be done, but active mixers have a lot of advantages. Do a search and read a bit about those two.

One reason to use a full mixer circuit is that it won't care what you plug into it. It'll provide a sensible mix of the two signal every time. That's extremely useful. A simpler circuit might work with particular equipment, but there'll be a time you need to plug something else into it to get up and running, and that'll be the time you discover it doesn't work properly and the levels are all wrong.

Tom

[tzenobite]

you're right, but according to this pdf the ggg minimixer does a passive mix of all the inputs BEFORE the ic, right?

http://www.generalguitargadgets.com/pdf/ggg_mixer_sc.pdf

edit: according to the digitech website, the jamman looper specs are:

LEFT(MONO)/RIGHT INPUTS
Connector Type: 1⁄4" Unbalanced (Tip-Sleeve)
Input Impedance: 550 kΩ Mono/1.1 MΩ Stereo
Unity Gain Input to Output: Loop Level knob set to 12 o'clock position
Max Input Level at Unity: +4 dBu DFS
Left and Right Audio Paths: Each channel maintains analog dry path
Loop Level knob set to 12 o'clock position
LEFT(MONO)/RIGHT OUTPUTS
Connector Type: 1⁄4" Unbalanced (Tip-Sleeve)/Balanced (Tip-Ring-Sleeve)
Output Impedance: 100 Ω Unbalanced / 200 Ω Balanced
Playback Unity Gain: Loop Level knob set to 12 o'clock position
Maximum Output Level: +4 dBu DFS
PERFORMANCE
Frequency Response: +0/-3 dB 20-20 kHz
Signal to Noise Ratio: > -100 dB

[slacker]

No it's not a passive mix, I can see how the way it's drawn makes it look like that though. the resistors R5, R6, R7 and R8 are all connected to pin 2 of the opamp (ICa), this is set up as an active mixer. In this configuration all four inputs are completely independent, so you can connect the input to one and the output of the looper to another and the looper output won't feedback to the input.

[anotherjim]

It looks like passive mixing, but it isn't. An op-amp based "virtual earth" mixer joins the inputs (each one has a resistor in series) together at the -input of the op-amp. The amplifier reacts via the feedback resistor (R9) from its output to the same -input by moving the output voltage up or down to keep that -input ALWAYS at the same DC voltage as on the +input. The result of this magic is there is zero AC signal on the -input. It is a virtual ground or earth. Therefore the different inputs do not interact, they each see virtual ground and not each other.

The down sides to this are...
The amplifier has to be inverting - this may be bad in some circumstances. The GGG mixer corrects this with another op-amp that inverts it back again.
The input impedance can't be super high. GGG has 100k (those input resistors). 100k is fine for a lot of things, but some pickups won't like it without being given their own pre-amp.

[tzenobite]

ot: anotherjim, the moment i saw the red dragon i knew where are you from
i'm a huge rugby fan and i saw the red dragons beats poor italy way too many times...italians's fault, of course :-(

[tzenobite]

what about this? is from another kit i found on ebay...

[anotherjim]

That one has pre-amplifiers on each input, which you don't really need and it will have more circuit noise than the GGG one.

We have a liking for the Italian rugby team because they always work so hard. Often don't get the result they deserve. Then again, Wales have been too good at "clutching defeat from the jaws of victory" lately.

[tzenobite]

i trust you, i'll go with the ggg project (since i'll pay 22 $ for shipping costs only i think i'll try by myself..)

(i love the rugby as a sport, i'm not a "fan", even if sometimes i feel bad when italy loses it's more because simply the team who win a rugby game is (almost) always the one who has better played...)

[tzenobite]

(it's time to bed, almost...)

in the ggg website's schematics i found also the micromixer: http://www.generalguitargadgets.com/pdf/ggg_micro_mixer_sc.pdf
doest it works as i need to?

[Tony Forestiere]

Maybe this?

[tzenobite]

this is just like the ggg MINImixer we were talking about before
i was asking for an opinion about the MICROmixer from ggg, it has only 2 inputs but i can't understand if it has virtual ground too like the minimixer