no zero-crossing dissortions in op-amp designs ?!? - diyAudio
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Old 17th February 2005, 12:46 PM   #1
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Default no zero-crossing dissortions in op-amp designs ?!?

Hello!

At the moment Im constructing an active 3-way filter design for my new speaker project.

During calculating process Ive got an idea:

Why not add a DC voltage to the audio signal after the input buffer, so the op-amps in the following stages always work with positive (or negative) amplitudes ?!?

In my opinion this would avoid zero-crossing dissortions!

(e.g. added DC +5V when audio signal range +/- 2,5 V -> chips work between +2,5 and +7,5 V)

The added voltage would be easy filtered by the coupling capacitor at the output .

Do you think this concept makes sense ???

greets
Peter
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Old 17th February 2005, 01:01 PM   #2
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Quote:
Do you think this concept makes sense ???
No. Output devices will still be switching. Only way to achieve what you want is to use a current source at output.
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Old 17th February 2005, 02:11 PM   #3
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Quote:
Originally posted by analog_sa
No. Output devices will still be switching. Only way to achieve what you want is to use a current source at output.
How so? If the opamp only ever has between 2.5V and 7.5V at it's output, how can it be that you would encounter crossover?
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Old 17th February 2005, 02:14 PM   #4
miguel2 is offline miguel2  Portugal
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Cap will be polarised, giving strange noises.

Miguel
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Old 17th February 2005, 02:16 PM   #5
macboy is offline macboy  Canada
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Fundamentally, the switch-over from one output device to the other happens at zero output current, not at zero voltage. There could be a significant voltage on the output at this instant; that doesn't matter (and this will always be the case with a reactive load, such as a filter). Once you realise that the N type output device can only ever source current, and the P type one can only ever sink current, you'll be closer to understanding.
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Old 17th February 2005, 02:57 PM   #6
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I know that Rupert Neve uses this method. The crossover distortion is much less noticeable since it only occurs at high signal levels
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Old 17th February 2005, 02:58 PM   #7
Mr Evil is offline Mr Evil  United Kingdom
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It would work... as long as the op-amp was DC coupled to the next stage so it's always sourcing current to the input impedance of the next stage. A waste of power for little, if any, benefit in my opinion. If you really want a class-A op-amp then build one from discretes.
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Old 17th February 2005, 03:55 PM   #8
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Default Re: no zero-crossing dissortions in op-amp designs ?!?

Quote:
Originally posted by CrazyChipMan
Hello!

At the moment Im constructing an active 3-way filter design for my new speaker project.

During calculating process Ive got an idea:

Why not add a DC voltage to the audio signal after the input buffer, so the op-amps in the following stages always work with positive (or negative) amplitudes ?!?

In my opinion this would avoid zero-crossing dissortions!

(e.g. added DC +5V when audio signal range +/- 2,5 V -> chips work between +2,5 and +7,5 V)

The added voltage would be easy filtered by the coupling capacitor at the output .

Do you think this concept makes sense ???

greets
Peter
any time you couple DC into the input you are asking for noise -- even the best low noise regulators add 50 to 100uV, and off-the-shelf regulators add millivolts -- take a look at TI's website for a comprehensive discussion of single-supply opamp design because it discusses the problem in detail.
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Old 17th February 2005, 04:18 PM   #9
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Consider adding a pull-down resistor (or current) to -V.
This has the effect you desire, running the output NPN
always on. Calculate the peak "sunk" current, and make
the DC bias higher. This is an old trick to force class a
operation.

Best Regards,
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Old 22nd April 2005, 08:28 AM   #10
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You need to do this cautiously, with a distortion analyser attached or you may exchange one distortion for another. OK if your seeking euphonics, bit like eugenics?
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