LED display for a guitar signal - keeping the source clean

[BeatleManiac]

Hello!

I'm working on an RGB LED display that will go around the perimeter of my bass amp.  I'm planning to have 2 1/4" TS jacks in parallel, with one wire going from the tip connection to an op amp buffer before going on to the rest of my circuit.  It will be in a plastic enclosure sitting on the top of the amp - not a pedal with a foot switch.

2 questions: 

What do I need to do to the input signal in addition to the buffer to keep from introducing noise into the signal going on to the amp?  There will be no audio processing in my circuit(Other than an MSGEQ7 chip for the FFT 7-band input into the micro controller).  There will be a direct connection from the input jack to the buffer, and I'm planning to use shielded cable for this connection.

Also, should I leave the ground (sleeve) of the 1/4" jacks connected and floating, or connected and tied to the ground of my 5V DC supply for the enclosure? 

Thanks in advance!

Steve Hurd
Floyds Knobs, IN

[Buriedcode]

If I understand you correctly, you'll be using the signal from your bass going into both the amp, and your opamp buffer.  If your opamp buffer uses a single supply, the input would need to be biased at VCC/2 and any noise on that line can go back through the cap and into your amp.  You'll also have the input impedance of both your buffer opamp and the preamp input of your amp in parallel, which could possibly reduce the highs.  I believe most guitar/bass amps have an input impedance of ~1M, and bass amps often have an 'active' input for active pickups - ones that provide a DC bias.  The cap will remove that, but I'm thinking you'll need at least 1M input impedance.

You could stop the 5V supply leaking into your amp by powering your opamp with a bipolar supply - just use a 7660 or something to produce -5V so the first thing the bass sees in that circuit is a high value resistor to ground, a cap and the non-inverting input of the opamp.  But you would still have the bass signal 'outside' the amp, in a shielded box, which may or may not have noisy switching stuff going on.

[BeatleManiac]

Thanks for the reply, Buriedcode.

I do have a 7660, so I can feed +- 5V to the op amp.  I forgot to mention that the bass will not connect directly to the enclosure.  I have a Shure wireless guitar pedal (GLX-D)which connects to an eq pedal(Source Audio).  From there, the signal will enter the enclosure before going on to the amp.  Are you saying I'll need a cap between the guitar signal and the input to the buffer?  I don't have one there presently.  Also, what should I do with the ground connections?

[Buriedcode]

Yes I would add a cap just in case the amp biases the input (unlikely) I suspect the value won't be too critical because you're not going to 'hear' the signal that's going into the EQ, but a value that provides enough low end of the lowest bin in your EQ.  It is best to keep your ground 'clean' but I see no reason not to just connect it all, with a pass-through the enclosure out to your amp,  your circuit inside. 

With that said, what are you using to power this?  Most plug in power supplies are isolated so there probably won't be trouble with ground loops - but they are also noisy - having digital switching transients going on a board, inside a shielded enclosure, where the bass signal is routed (admittedly from just one socket to another) it could pick up noise.  Also many 1/4" sockets are all metal, making the enclosure signal ground, but also some DC sockets are (assuming you're using that for power input).  This would connect the DC supply ground and signal ground, and if the supply isn't isolated there could be ground loops.  Guitar pedals that can accept DC power supplies generally have the enclosure the same as the signal ground and many of those are pretty quiet

Best thing would be to test it.  Bass -> 1/4" socket -> 1/4" socket - patch cable to amp, to give you access to the signal and ground.  Connect a buffer amplifier to it via a switch, and see if you can hear a difference when the circuit is on.  Effectively a 'blank' pedal that is always bypassed, but switches a circuit to 'look at' the signal.

Post a schem of what you have so far.

[BeatleManiac]

Hi Buriedcode,

Sorry it's taken so long to reply.  Life intervened and I also wanted to get the bulk of my circuitry off of the breadboard before I reached out again.  What type and value of cap would you recommend for the coupling between the bass signal and the input to the op amp?  I know ceramics aren't good for this type of application.

Beatlemaniac