A/B/Y switcher with gain?

[Rodgre]

I'm sketching out an idea for a box to switch between the synth and guitar outputs of my old Roland GR-300 guitar synth. The synth level always seems too low compared to the straight guitar, so I'd like to have some control over the blend between the two.

Here's my initial idea:                                                                  2xSPDT
                                                                                                    >---------
GR-300 SYNTH OUT-----[buffer/preamp]-----[volume control]------>                I
                                                                                                    >        I----[buffer]---OUTPUT
                                                                                                              I
                                                                                                    >---------
GR-300 GTR OUT---------[buffer/preamp]-----[volume control]------>   
                                                                                                    >

My questions are:

1. Do I really need the final buffer circuit as a mixer, or would there be no problem in just combining the two signals as-is?

2. Do I need to tie the open connections on the SPDT switches to ground?

3. By the same token, should the switches be flipped the other way around, so the pole is facing the output, and the throws are coming from the signal inputs? There seems to be logic here the more I think about it. In this case, I should probably tie the unused throws to ground, no?

4. Should I skip the separate buffer/preamps and have one master buffer/preamp at the end (as a mixer) which has enough gain that I can balance the two signals without losing the already-hot-enough guitar level?

5. Did I just answer my own questions?

Also, any thoughts on a simple and quiet buffer/preamp circuit? I'm thinking something like the many FET boost circuits.


Thank you.
Roger

[Rodgre]

So I've tried some experiments and I don't have it happening quite yet.

I've tried scanning my sketch/schematic but the scanner isn't co-operating, so here's an explanation.

I tried a simple A/B SPDT switch, with the output going to a mosfet booster circuit. That works.

Now, the two wrenches in the works are that I want to be able to have an attenuator on one of the inputs to balance the levels between them, plus I want to be able to switch the two inputs to blend in a "Y" config.

So the issue I'm having is that I haven't figured out a way to keep the two signals from interacting in the "Y" configuration, as the pot that balances the level of one input suddenly is in the circuit for both. How can I isolated this? A cap? A resistor? Do I need to build buffer circuits for both signals and then do the switching after the buffers, and mix them with a final buffer/boost circuit?

Thanks
Roger

[Rodgre]

Hmmm.... no one's interested in my boring question. javascript:void(0);

Maybe I should mention that I want to build a negative-ground germanium boost/uglyface with TS808 mods into the switcher.

(If it isn't apparent, I'm saying this with a hearty chuckle, not a grumpy "no one is answering my post!" tone)

Anyway, I'm wondering (to myself, of course ) if maybe I'm over-thinking the whole thing and maybe just a simple 2-channel mixer circuit with kill switches mounted close enough to each other that I can hit them both at the same time to "A/B" them is the way to go.

Again, I assume that the "guitar only" output of the GR-300 is already buffered, so I'm not going to sweat the fact that the straight guitar tone would be always going through circuitry of some sort. I never thought to try the Guitar Only output with the synth powered off. I'll have to try that today...

Roger

[tranceracer]

I'm not to familiar with the GR-300.  But do you want to separately control each output and have the ability to blend them when needed?

This may be a dumb questions but can you program the synth channel to a by pass or mute?  I have a MIDI controller board and the last button on the board dials in the clean setting on the multi effects.  Sorry if I'm off base here.

Also a quick way to see if you need a buffer or 2ch mixer ckt is to use a mono Y cable between synth out and gtr out and see if it sounds OK.  If it does then your schematic should be OK, otherwise you'll likely need a 2ch mixer ckt.  (of which there are many here on this forum)

-bK

[Rodgre]

Ha ha haha... what's this MIDI and programming you speak of?

The GR-300 is circa-1980 analog goodness without as much as a bit nor a byte in it.

I control it with the original G-505 strat-style controller that came with it (essentially like an Ibanez strat with an extra hex pickup and lots of electronics in it...)

Think of it basically as using two guitars simultaneously. Sometimes you want to use one, sometimes the other, and many times, both blended together.

The blending isn't so much of an issue, but my problem is that the synth's output is strangely a lower level than the guitar, itself. I would have figured that it would be line level actually, but it's sort of a 70/30 ratio compared to the guitar's magnetic pickups.

So I just want to make myself a switcher box that does your standard A/B/Y switching, but I want to goose the level of the synth, yet have no weird artifacts by blending the two together.

You know what? In explaining this, I think I just answered my own question: I just need a passive  A/B/Y switcher and I need to simply include a simple hi-fi boost to get the synth's output up to the same level as the guitar.

See, that's why I love this board! We'll always get the answer we need eventually!!!

Roger