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Old 22nd August 2010, 04:11 AM   #1
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Default Need help understanding class B output stage capacitor variations

I'm working on designing a better than average op-amp + transistor class B output stage headphone amplifier. There really isn't that much that is special about it, but I want to the design and math to be solid before I order parts and start to build. My design requirements are: * easy to understand, build, and somewhat small in size, * relatively low power requirements to minimize heat, the electric bill, and to allow for possible battery operation (ruling out class A), * flexible enough to drive lower impedance headphones (not very often) but usually my 600ohm AKG's, and * some flexibility in the design for simple experimentation. My electronics experience level is "noob who knows too much", so please keep answers on that level. My health also sucks so I may not be coherent enough to respond everyday.

The final design will look very similar to this: (I wish I had found this starting out)
http://sound.westhost.com/project113.htm

My question arises from the C3L and C4L capacitors in Figure 1. What value and what configuration leads to optimum performance?

From what I understand, capacitors across the transistor base pins are supposed to help with crossover distortion. But how?

The thing I find confusing is that other similar designs have used other configurations and values...

Headwize shows a single 1nF capacitor across the transistor bases (Figure 11c about half way down the page):
http://gilmore2.chem.northwestern.ed...=opamp_prj.htm
(as opposed to ESP-113's two 100uF's in stacked series paralleled with the diodes)

Like Headwize, this one has a single 22uF across the bases (C6 in the diagram):
http://www.redcircuits.com/Page30.htm

The ESP-113 page mentioned the capacitors being paralleled with the diodes, but this explanation doesn't seem to fit a single capacator across both diode designs... so I'm confused a little more.

What is the math to calculate adequate capacitor size? I'm already familiar with "-3db_freq" = 1 / (2 * PI * R * C). Is this used? If so, does R come from the current mirror resistor?

Thanks for the help.
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Old 1st October 2010, 06:21 PM   #2
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One thing I noticed the other day is that the paralleled capacitors with the diodes is very similar to the capacitors across each diode in a diode bridge to help with on/off noise. Is this similar?

Even though this is a headphone amp, should this be in another forum? Maybe discrete? I'm sure someone has a quick and easy answer for this... somewhere...
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