Need advice for a summing amp

[hknatm]

Hello hknatm



Looking at your circuit, the first question that arises is: do you expect the circuit to be able to drive a normal 8 or 16 ohm loudspeaker? This would be a definite no-no for the TL072, which cannot provide anything like enough current for this. If, by "speaker" you mean powered speakers such as computer speakers or active monitors, then the TL072 would be fine.



The 100 ohm resistor on the output is a good idea as long as the load is a sufficiently high impedance as it decouples the output from capacitive loads.



The biggest issue is the 2.2 ohm feedback resistor. Do you really mean 2.2 ohm, or did you intend 2.2k ohm?



With R1-R4 = 2.2k ohm, the maximum gain with a 2.2 ohm feedback resistor (which incidentally loads the output far too much) is only 1/1000, i.e. each signal input would be attenuated by 60dB. If, on the other hand, the feedback resistor is 2.2k ohm, then the maximum gain on any input is unity, x1.



Creating a virtual earth as you have done in your power supply is OK, but in my opinion it is probably better to make the negative side of your power supply the circuit common or ground, and create a mid-point bias point for the i.c. Whichever way you go, you really need DC blocking capacitors at all inputs and outputs.



You don't say on your circuit what the resistance of the potentiometers is. I think 10k pots were mentioned in the other thread.



In any event, R1-R4 really need to be substantially larger than 2.2k, I suggest 100k ohm. For unity max gain, the feedback resistor would also be 100k. You don't have to worry about the noise contribution of 100k at line level given the tiny input currents of a JFET input op amp.



With all these points (and others) in mind, you arrive at something like the attached circuit. With C1-C4 = 1uF, the nominal corner frequency of the low freq. roll-off is 16Hz. They could be larger, as could C5, if you wish.



The maximum gain on any input is unity. If you need more gain, just increase R5. For example, if R5=330k the gain is 3.3 or 10dB.



The second op amp in the TL072 is stabilised rather than left floating with inputs open circuit.



The power supply has been re-arranged so that a bias voltage for the op amps is generated rather than a virtual earth.



Have a look and ask if you have any questions about it. If you try it and your problems persist, we will need to look at the power supply and layout.



Have fun...



View attachment 843182

Hey Ludus,
The reason i chose 2.2k on both feedback and input resistance is because i dont need any gain. This is a basicly a summing mixer for my synths. I need a balance between 4 inputs. I will be gainin output through my amplifier. 4 3,5mm input jacks and 1 output 3,5mm jack to soundcard.

Thanks for your diagram
All the capacitors are non-polarized? Except C7.

My power supply is a 12v DC basic adapter. As i understand from the circuit i dont need to turn that into +-ground (ac form that i tried to do). Just use it with +- (“-“ is the ground in that case) . Will try that and inform you about it.

I may asked some silly questions. I am newby to electronics. Wanna learn but lots of things to digest

 

[Ludus Tonalis]

Hey Ludus,
The reason i chose 2.2k on both feedback and input resistance is because i dont need any gain.

If you want unity gain, then yes, the feedback resistor and the input resistors all need to have the same value. On your diagram the feedback resistor has a value of only 2.2 ohm, not 2.2k ohm, which accounts for the confusion on this point.

Anyway, 2.2k ohm really is too small. At maximum volume the input impedance seen by the synth is only around 1.8k ohm. They will be much happier with more than this, anything from 10k upwards I suggest.

The input potentiometers need higher input resistors - at 2.2k ohm their law is too compromised. The TL072 output will also be much happier with a larger feedback resistor.

So do yourself and your synths a favour and get a handful of 100k resistors.

All the capacitors are non-polarized? Except C7.

Yes.

My power supply is a 12v DC basic adapter.

Hmm - this only gives you a rail to rail voltage of 12V. This should be just about OK at line level...

This is a basicly a summing mixer for my synths.

I made a similar mixer for my synths many years ago. It has 6 stereo inputs on 1/4" jacks and two stereo outputs. The input potentiometers are high quality miniature pre-set cermets rather than potentiometers with knobs, because I figured that once set correctly, the individual volumes and balance could be controlled better via MIDI. The power supply is a +/- 15V mains stabilised unit in a separate box.