squarewave said:
If you want to minimize ground loops you would have to make the inputs balanced. Meaning instead of the ring connecting to ground it would connect to the other input of an "op amp difference amplifier" with 10K input Z maybe. Of course then the jacks would have to be isolated. Balanced inputs could be important. Especially if you want to connect to gear that has it's own wall supply. If there's even a few mV difference, you can imagine you might get a significant current running in/out of one of the inputs. That is probably the primary source of hum and noise in a device like this.
And for guitar, you probably want high impedance. Unfortunately getting both balanced and high impedance can be tricky. If you're ok with 10K inputs, it's a lot easier.
The second most common source of noise would be if high currents are returning on a ground that is also the ground reference for a high gain circuit. In this case, it can pay to have use a separate wire back to the ground at the filter caps in the PS. But you don't really have high current or high gain so I don't think it will matter to much how you do it.
Otherwise your circuit looks fine. You could use a fuse maybe. It's not crystal clear that you need the inverting stages.
The input source is line level signal from FX processor (up to 3 stereo units and one mono/stereo dry signal), so I think 10k is ok.
And, as the matter of fact, they are unbalanced.
All units in the rack are powered from Furman power conditioner (Furman PL-PRO DMC E)
Yep, it's got the 0.25A fuse on live wire, just after the plug. (Sorry, I should update the schematic, cause now it shows just PCB circuit).
Newmarket said:
Is this intended for 'pedal' type guitar fx ? Many of these have their output from an unbuffered pot wiper (like your output stage).
This means the output impedance is changing with output level - meaning you need to be careful with the input pot value - or buffer the input.
For similar reasons I'd recommend making the output stage an active volume control around the opamp so the output comes from the opamp output via just the 100 Ohm 'build out' resistor (and the coupling cap).
Other points:
If space allows I'd say increase the value of the output cap for less LF distortion - if that's a consideration with this mixer.
Might be an idea to put a pull down resistor on the 'outside' of the output cap to minimise pops on plugging in...
And I'd say go for +/-17 max on the voltage rails to give a bit of margin.
Active volume control, you mean something like this? (after C5):
Also they got pull-down resistor after cap as you suggested
I got some free room to install bigger caps, so I'll experiment with values.
abbey road d enfer said:
The most polluted point is the junction of the center-tap and the smoothing caps. You don't want that point to be allowed to pollute the audio. The point that must be connected to chassis is the junction of C119/125, as you have drawn it. Then the ground distribution must obey the ancient adage "ground follows signal", or hierarchical ground. Connecting te input jacks' ground to the main ground is not teh best idea.
Whatever ground switches you have, you'll be facing the problem that grounds will follow multiple paths when all your paraphernalia are connected (unless they have floating connections, which I doubt very much).
Ideally, only one piece of your gear should be earthed, the rest should be powered via non-grounded PSU's (real double-insulation, not a cheater adapter); since I suspect your amp is earthed, and your mixer as well, you must make sure they are powered from the same outlet and not too far from each other.
Yep, they powered from the one power conditioner witch provided AC mains to all devices in the rack.
So do I need to keep each input grounds separately from each other and from PSU ground and C119/C124 junction to the chassis along with ac mains earth tab?
JohnRoberts said:
I may have shared this before but over the decades I have killed many brain cells thinking about human safety around live music equipment.
While a power supply will need a true DC coupled ground return path, audio, especially guitar audio can work with an AC coupled ground. In fact you can size the cap to get effective audio shielding while looking higher impedance at hum frequencies so avoiding loops or conduction.
I experimented with using a GFCI power drop but with the hard ground connection lifted and replaced by a capacitor (IIRC something like a 0.15uF***).
I had a friend who designs and sells guitar pedals (amptweaker) test it out and he said in some cases it reduced hum from powered guitar effects, and never made it worse.
CAVEAT... lifting the safety ground is a no-no and not approved by UL... In combination with a GFCI outlet it should be safe.
JR
PS: My main concern was about musicians getting hurt by energized grounds which is pretty rare but does still happen occasionally.
*** for highest safety the cap should be Y rated to not fail as a short.
This is interesting. Can you please show some example circuits?
PS. Thank you guys for your input!