the LM chips have a low offset voltage.
It certainly could, but would most likely require modifications to the amp. DC stability would probably be good, thus a servo wouldn't be required.AndrewT said:Could the front end be strapped onto a different power amp?
What DC stability would result?
Would a DC servo be required?
I don't think this can be done. DC output offset is low and the source has the potential to be the main contributor. Any DC offset is amplified by the gain of the amp. 1mV at the input wlll be 20mV at the output with a gain of 20. Blocking DC at the input is the best option. Unfortunately that means using a coupling cap in the signal path.AndrewT said:Could the integrity of the balanced function be preserved if a DC blocking cap were inserted into the NFB loop to reduce DC gain to 1?
The simple solution would be to just put an IC instrumentation amp in front of an amp's input stage. I took this one step further with my design and combined the power output stage with the differential amp portion. As was mentioned recently, an input transformer will passively provide a similar solution with the added advantage of galvanic isolation.AndrewT said:Any better way to extrapolate the inherent advantages of BWRX's creation to a mainstream amplifier?
The LM chips have low DC offset, but poor layout and grounding can easily ruin this. DC coupling definitely requires low to no DC on the source signal. On my design with the inputs shorted to each other, the DC offset on each output is -8mV and -13.3mV. This will vary depending on resistor tolerances, op amp selection, and power op amp selection. With my computer's audio output connected as the source the DC offset increases to 50mV and 100mV.juergenk said:dc is of no concern, the LM chips have a low offset voltage.
Just make shure that there is no "external" dc at the input, which could be amplified.
Upon request, here is the schematic with all part values. These are what I am using on my boards with very good results.
C14, C15, C16, and C17 are optional EMI/RFI filtering caps. I would recommend at least using C16 or C14, C15, and C16 if possible. C14 and C15 should be 5% tolerance or better.
C9 is an optional rail to rail bypass cap. 100V rating minimum.
R13, R14 and R15, R16 may need to be recalculated if you plan on changing the zener voltages.
R17, R18 are optional loading resistors. Be careful of the rated power dissipation if you decide to lower their values. I would recommend using these to help ensure a symmetrical supply voltage and to help discharge the supply caps when the power is turned off. This configuration only has a small turn on click and a small turn off click accompanied by some other rather quiet noises due to the op amp supply rails discharging.
Q1, Q2 can be changed to any suitable NPN, PNP devices. 2N3904 and 2N3906 are cheap and widely available.
IC1 is the dual op amp. I'm using the OPA2134 but other devices may be used. I plan on trying the LM4562 when I get the itch to swap out the 2134.
IC2 is the chip amp. I'm using the LM3875 but other devices may be used as well. If you use a different chip you may change the feedback resistors to change the gain of the chip amp. Be careful with stability issues and lower gain settings.
The gain of this configuration can be changed with only one resistor - R11. 20k gives an overall gain of 20V/V. 10k gives an overall gain of 30V/V. Set your gain based on your source and power supply voltages. The gain formula is on the schematic.