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ulysses · 4th May 2005

I agree that the circuit will perform better without the 100 ohm resistors feeding the amps. They degrade the virtual earth input such that channel input impedance will be affected (slightly) by the number of inputs in use, and to a lesser extent by the impedance of the sources. This will cause a shift in the mix balance when you add or subtract channels. These effects will be small, but this circuit isn't worth implementing unless you're paying attention to small effects. In an 8-channel mixer, the resistor won't be necessary anyway.
You're right though that capacitive cooupling on the outputs is a good idea since the inputs are DC coupled. But I see no reason besides coloration for the output transformer. I suppose if you fed all of the inputs from unbalanced sources and then fed the transformerless output of this circuit into a Symetrix 620 you'd lose 3dB of headroom, but that's a pretty narrow context for requiring differential signal output. The output is still technically "balanced" even when fed by all unbalanced sources. The transformers would be the single most expensive component (except perhaps the chassis) and can be easily omitted.
You're starting to talk about a kit, at some point you're going to have to consider that this circuit needs to be doubled to make it useful. I don't know of many people looking for mono outboard summing. That's easy enough, but it brings up the question of input connections and input switching. You need to be able to configure the inputs for a given mix situation. That is, some tracks will need to be fed to the Left buss, some to the Right buss, and some to both. That means at least a DP3T switch on each input (two DPDT switches seems to make the most sense here). Now you're talking about a control surface (i.e. faceplate) that is electrically close to (but usually physically far away from) the input connectors. It'll also increase the complexity and expense of the chassis.
One other thing - When you're calculating the size of the feedback capacitor, remember that the gain of the amps is dependent on the number of channels actually in use, not just the number of channels you build (unless you include switching that shorts unused inputs).

4th May 2005	#26
ulysses Lives for gear Joined: Mar 2003 Location: Minneapolis and Wiesbaden Posts: 1,452	I agree that the circuit will perform better without the 100 ohm resistors feeding the amps. They degrade the virtual earth input such that channel input impedance will be affected (slightly) by the number of inputs in use, and to a lesser extent by the impedance of the sources. This will cause a shift in the mix balance when you add or subtract channels. These effects will be small, but this circuit isn't worth implementing unless you're paying attention to small effects. In an 8-channel mixer, the resistor won't be necessary anyway. You're right though that capacitive cooupling on the outputs is a good idea since the inputs are DC coupled. But I see no reason besides coloration for the output transformer. I suppose if you fed all of the inputs from unbalanced sources and then fed the transformerless output of this circuit into a Symetrix 620 you'd lose 3dB of headroom, but that's a pretty narrow context for requiring differential signal output. The output is still technically "balanced" even when fed by all unbalanced sources. The transformers would be the single most expensive component (except perhaps the chassis) and can be easily omitted. You're starting to talk about a kit, at some point you're going to have to consider that this circuit needs to be doubled to make it useful. I don't know of many people looking for mono outboard summing. That's easy enough, but it brings up the question of input connections and input switching. You need to be able to configure the inputs for a given mix situation. That is, some tracks will need to be fed to the Left buss, some to the Right buss, and some to both. That means at least a DP3T switch on each input (two DPDT switches seems to make the most sense here). Now you're talking about a control surface (i.e. faceplate) that is electrically close to (but usually physically far away from) the input connectors. It'll also increase the complexity and expense of the chassis. One other thing - When you're calculating the size of the feedback capacitor, remember that the gain of the amps is dependent on the number of channels actually in use, not just the number of channels you build (unless you include switching that shorts unused inputs). __________________ Justin Ulysses Morse Roll Music Systems Minneapolis, MN Put a bottle of juice in your Lunchbox.