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Some sound cards need -5 V to have clear sound or to work at all; when -5 V isn't present the sound is raspy or not working. Why?

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    Mind to add what 'raspy' is supposed to mean exactly, also, what computer and what sound cards are you asking about - there is a whole lot of both out there creating an even bigger intersection :)
    – Raffzahn
    Aug 29 at 18:19
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    How on earth is this about retrocomputing
    – pipe
    Aug 30 at 2:32
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    @pipe - have you seen a brand new PAS16 on sale recently?
    – scruss
    Aug 30 at 17:07
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    The question might be salvaged, if reworked to highlight the context of attempting to use a piece of legacy hardware and relate the historical changes in PC power supplies to why the missing -5V is relevant.
    – Retrograde
    Aug 30 at 18:34
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    The edits to salvage this made it worse. A question about "some sound cards" makes it sound like modern computers don't have sound cards.
    – pipe
    Aug 30 at 23:43
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If you take any electronic circuit outside the conditions it was designed for, there are going to be consequences.

It's hard to know the exact reason for this particular case without having a schematic to analyze, but one theory could be that the negative rail was used as negative supply for an operational amplifier.

These are often supplied by both positive and negative supplies and the lack of negative swing might in effect clip half the phase and cause severe distortion of the signal.

But it's speculation, really.

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    Actually, that is the correct answer. Some, but not all sound cards use -5V for operational amplifiers, or for other circuitry/chips such as DAC or volume control or mixer or amplifier, and thus when the negative supply is missing, it is not possible reproduce the audio signal as intended and there will be serious distortion. Some other sound cards had onboard regulators to generate a local -5V supply if they needed it, and it was generated from the -12V provided via ISA bus.
    – Justme
    Aug 29 at 19:21
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    Speculation but rather likely - best that can be done for a vague question.
    – Raffzahn
    Aug 29 at 23:57
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    I'ts not speculation. I am looking an IXW-PAS16NS model right now, and the ISA bus pin for the -5V pin is present on the ISA card edge (unused pins are not present) and it does route on to the circuitry (although hard to see where exactly as this appears to be a PCB with more than 2 layers). People on retro forums also say their PAS16 sounds terrible unless being fed with -5V converter circuitry when a too modern ATX supply which does not natively provide the required -5V supply. If there is a list of ISA sound cards requiring -5V to work properly, the PAS16 will definitely be on that list.
    – Justme
    Aug 30 at 5:20

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