Hallo Zakman35,
Don't remove Ci because the input resistance will be dependent on the output resistance of the pre-amplifier and that may change the output offset voltage of the LM3886 quite a lot.
And, if there is an output offset voltage on the pre-amplifier, that output offset voltage will be multiplied by 33 !!
Don't remove Cc because it prevents noise and - if necessarely - oscillation.
Marc.
Don't remove Ci because the input resistance will be dependent on the output resistance of the pre-amplifier and that may change the output offset voltage of the LM3886 quite a lot.
And, if there is an output offset voltage on the pre-amplifier, that output offset voltage will be multiplied by 33 !!
Don't remove Cc because it prevents noise and - if necessarely - oscillation.
Marc.
Good. Now, what is the AC (noise content) voltage of both DC rails?I measure dc voltage directly on the pins of the chip, both rails are exactly the same (+36.2V, -36.2V)
Don't remove Ci because the input resistance will be dependent on the output resistance of the pre-amplifier and that may change the output offset voltage of the LM3886 quite a lot.
And, if there is an output offset voltage on the pre-amplifier, that output offset voltage will be multiplied by 33 !!
Don't remove Cc because it prevents noise and - if necessarely - oscillation.
I install Ci back again (no changes at offset).
My pre amp (minidsp) is AC coupled at the output, so no dc from there.
I will try tomorrow to install back the Cc cap and report back.
No, No, No, this isn't a solution at all!! (Or I don't understand it)
To trim out a offset voltage of let's say 6 mV, you have adjust the positive rail for a multiple of that. And that reduces your output power and induces distortion much earlier on one flank.
Why not use the way I proposed?
Marc.
To trim out a offset voltage of let's say 6 mV, you have adjust the positive rail for a multiple of that. And that reduces your output power and induces distortion much earlier on one flank.
Why not use the way I proposed?
Marc.
With the LM3886, your solution is probably better.Why not use the way I proposed? Marc.
The output offset should be substantially independent of the supply rail voltages in any opamp style amplifier.
However, a VAS stage without CCS supply/source will change it's Iq when supply voltage changes (both rails changing together), and this changed Iq with push/pull more or less current through the Collector load of the input transistor. This changed collector current will change the offset. This is one of the big disadvantages of a bootstrap feed to the VAS in a P3A (ESP) amplifier.
However, a VAS stage without CCS supply/source will change it's Iq when supply voltage changes (both rails changing together), and this changed Iq with push/pull more or less current through the Collector load of the input transistor. This changed collector current will change the offset. This is one of the big disadvantages of a bootstrap feed to the VAS in a P3A (ESP) amplifier.
Marc, I successfully applied your tip in my DIY studio monitor (with LM4766 - another BJT chipamp).
Previously, due to the low feedback resistance (200R and 3k3) in my design, I needed a large feedback capacitor. There was a slight but audible distortion, which is now gone or at least below my threshold .
Schematic here: http://rdimitrov.info/blog/show.php?entry=Minimalistic%20HIFI%20Bi-Amplified%20Speaker%20System
Thanks!
Previously, due to the low feedback resistance (200R and 3k3) in my design, I needed a large feedback capacitor. There was a slight but audible distortion, which is now gone or at least below my threshold .
Schematic here: http://rdimitrov.info/blog/show.php?entry=Minimalistic%20HIFI%20Bi-Amplified%20Speaker%20System
Thanks!
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