DC servo...
"No global negative feedback" is not totally exclusive. You can have a 'global loop' exist for DC but not for AC. Why restrict the BW of the AC with an extensive global loop when this is not necessary? ....unless simplicity is your goal.
IMHO, DC servo is the way to go. I don't like deliberately misbalancing the IPS in order to 'balance' the amplifer. The IPS should always be balanced in current. In my balanced output power amplifier, there are two global feedback loops controling the floating reference for each input. But these loops are only for DC offset and have very little effect to audio frequencies. Inside these loops are two seperate amplifiers. One acts as a pre-amplifier and phase splitter (for the BAL output) and the second amplifies the voltage for each output. Each of these amplifers contain an IPS and VAS, with seperate compensation and feedback loops. Essentially they are two seperate direct coupled amplifiers controlled by DC servo amps that sets the value for the floating signal reference GND for each of the two inputs. The output stages are not within any of these feedback loops except the global servo loops. Each OPS has it's own local loops and compensation, and they act soley as pure followers. One output is +15mv and the other is +10mv so the speaker sees 5mV offset. If I used matched components for the DC servo amps I'm sure I can get it closer to 0V, not like 5mV is an issue.
I have never had a DC offset problem from an amp with a properly operating DC servo amplifer. I must be spoiled but I doubt I will ever build an amp without active DC control.
In addition, I was able to play with a certain amount of positive FB via the DC servo global loops to create a slight boost in gain for frequencies in the range of 18Hz - 28Hz. This was a neat little bit I discovered by playing with the filter responce in the servo amps, but although not much program material exists below 28Hz I like the extra gain in the very low end to compensate rolloff of most speakers.
"No global negative feedback" is not totally exclusive. You can have a 'global loop' exist for DC but not for AC. Why restrict the BW of the AC with an extensive global loop when this is not necessary? ....unless simplicity is your goal.
IMHO, DC servo is the way to go. I don't like deliberately misbalancing the IPS in order to 'balance' the amplifer. The IPS should always be balanced in current. In my balanced output power amplifier, there are two global feedback loops controling the floating reference for each input. But these loops are only for DC offset and have very little effect to audio frequencies. Inside these loops are two seperate amplifiers. One acts as a pre-amplifier and phase splitter (for the BAL output) and the second amplifies the voltage for each output. Each of these amplifers contain an IPS and VAS, with seperate compensation and feedback loops. Essentially they are two seperate direct coupled amplifiers controlled by DC servo amps that sets the value for the floating signal reference GND for each of the two inputs. The output stages are not within any of these feedback loops except the global servo loops. Each OPS has it's own local loops and compensation, and they act soley as pure followers. One output is +15mv and the other is +10mv so the speaker sees 5mV offset. If I used matched components for the DC servo amps I'm sure I can get it closer to 0V, not like 5mV is an issue.
I have never had a DC offset problem from an amp with a properly operating DC servo amplifer. I must be spoiled but I doubt I will ever build an amp without active DC control. In addition, I was able to play with a certain amount of positive FB via the DC servo global loops to create a slight boost in gain for frequencies in the range of 18Hz - 28Hz. This was a neat little bit I discovered by playing with the filter responce in the servo amps, but although not much program material exists below 28Hz I like the extra gain in the very low end to compensate rolloff of most speakers.
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