I'm building a small mono power amp using the Neurochrome Modulus-86. Its primary purpose is to accept the line-level signal from a digital guitar amp modeler like the Kemper Profiler, or the Line 6 Helix, but I also want to be able to plug a few microphones into it and use it as a sort of P.A. system.
Long story short, I have three balanced line level inputs and three unbalanced signals from mic preamps that I want to mix together at unity gain to feed into the Modulus-86. Normally, the Modulus-86 takes a balanced input, but the designer has supplied me with instructions for bypassing the balanced receiver so I can feed an unbalanced signal to it if I prefer. This gives me two options:
1. I can use a THAT 12XX chip to turn each balanced line input into unbalanced signals that I can easily mix with the mic preamp outputs via an active/virtual earth opamp summing circuit, then feed that into the Modulus-86 by bypassing the balanced receiver (which is itself a THAT 12xx, so I'm just moving that chip to a different place in the chain).
2. I can combine the mic preamps together with a normal unbalanced summing circuit, feed that into a THAT 1510 to turn it into a balanced signal, then mix that with the three balanced inputs by summing the positive-polarity sides, and then summing the negative-polarity sides, with the final result a balanced signal that can be then fed into the Modulus-86.
The primary reason I am interested in the second solution is because it would theoretically keep the balanced inputs intact all the way to the input of the power amp, thus better rejecting noise and interference that might occur inside the chassis. However, my concern is that summing the positive- and negative-polarity sides of the balanced signals independently will cause the differential receiver to reject more than just noise/interference, and incur signal loss.
Should I be concerned with this? Is summing the positive- and negative-polarity sides of the balanced signals this way a bad idea?
Long story short, I have three balanced line level inputs and three unbalanced signals from mic preamps that I want to mix together at unity gain to feed into the Modulus-86. Normally, the Modulus-86 takes a balanced input, but the designer has supplied me with instructions for bypassing the balanced receiver so I can feed an unbalanced signal to it if I prefer. This gives me two options:
1. I can use a THAT 12XX chip to turn each balanced line input into unbalanced signals that I can easily mix with the mic preamp outputs via an active/virtual earth opamp summing circuit, then feed that into the Modulus-86 by bypassing the balanced receiver (which is itself a THAT 12xx, so I'm just moving that chip to a different place in the chain).
2. I can combine the mic preamps together with a normal unbalanced summing circuit, feed that into a THAT 1510 to turn it into a balanced signal, then mix that with the three balanced inputs by summing the positive-polarity sides, and then summing the negative-polarity sides, with the final result a balanced signal that can be then fed into the Modulus-86.
The primary reason I am interested in the second solution is because it would theoretically keep the balanced inputs intact all the way to the input of the power amp, thus better rejecting noise and interference that might occur inside the chassis. However, my concern is that summing the positive- and negative-polarity sides of the balanced signals independently will cause the differential receiver to reject more than just noise/interference, and incur signal loss.
Should I be concerned with this? Is summing the positive- and negative-polarity sides of the balanced signals this way a bad idea?
Balanced line is used to reject interference on interconnects, I don't think there is any need to continue with it inside the enclosure
No faders. I'll adjust the mic volume via their respective preamps, and I'll control the volume of the other line-level devices on those devices themselves. I might add a master volume to the power amp but only if absolutely necessary. The whole thing is mono, no stereo.
Well, the plan is to mount it inside a chassis like this: Blackface-Style Chassis Blank | Synaptic Amplification | Reverb
You'll notice there's no top, that part is covered by the wooden cabinet (I'm planning to stick this into a guitar amp cabinet). So the chassis won't act like a fully-enclosed faraday cage. Is that still okay? If I'm being paranoid, feel free to say so.
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A poor man’s mixer can be made with three 4k7 resistors (use carbon film) tied together at one end as the output and other ends as the input. Assuming your sources can drive 5k loads. Can also be done with 10k resistors or whatever is needed to match your source. I do this all the time to get summed mono from my stereo sources like a phone to play a mono amp speaker combo.
In that case I don't see any problem with or major disadvantage to keeping it balanced.
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Yes,
Convert everything to unbalanced. Process that/those signal/s.
Adjust levels.
Convert back to balanced.
Do that all inside a protecting enclosure and you maximise the interference attenuation.
You could easily do it all balanced with the simple resistors summing junction 3:1 for +ve and 3:1 for negative and results in two outputs +ve and -ve polarities. All at a cost of $1, and I would add probably truer fidelity and less distortion than a bunch of opamps to convert to si glen ended and rhen back to balanced. Remember, this is for mono for a PA application and the sources are mic preamps. They should drive it fine as balanced preamps are by convention, designed to drive 600ohm loads. A 4k7 resistor would be nothing.
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