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I'm reading "A Single Supply Op-Amp Circuit Collection" and in the paragraph about summing (page 9) I read "a single supply voltage is seldom used for audio mixers. Designers will often push an op amp up to, and sometimes beyond its recommended voltage rails to increase dynamic range".

Does this still apply? Do contemporary single-supply op-amps have limited dynamic range for mixing applications? If this is the case for how many channels does summing signal dynamic range need split-supply op-amps?

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  • \$\begingroup\$ You have to be careful talking using the word bandwidth with op-amps. Bandwidth refers to frequency range, dynamic range refers to amplitude range. \$\endgroup\$
    – jalalipop
    Commented Aug 30, 2017 at 11:33
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    \$\begingroup\$ Why is this downvoted? It seems relevant to electronic design, and isn't a homework problem without work shown. \$\endgroup\$
    – Bort
    Commented Aug 30, 2017 at 11:50
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    \$\begingroup\$ Perhaps because there aren't single and dual supply op-amps, just single and dual supply circuits? \$\endgroup\$
    – Finbarr
    Commented Aug 30, 2017 at 12:17
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    \$\begingroup\$ @Finbarr There are single and dual supply op-amps. Sure, all op-amps will have two supply rails, but you can optimize an opamp to work from the negative supply up (or even below the negative supply), which is often called a "single-supply opamp" because they are aimed at simplifying circuits by allowing the design to use only one voltage rail that needs to be regulated. \$\endgroup\$
    – Joren Vaes
    Commented Aug 30, 2017 at 12:19
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    \$\begingroup\$ Some "errors" in circuit design have led to popular sounding circuits, and people listening to those sounds don't know or care whether recommended or maximum voltages are exceeded, so it's possible that exceeding the recommended supply voltage for an op-amp leads to a specific kind of sound that is considered pleasing (or appropriately aggressive or something) to many musicians and producers, etc. I can't cite a specific example circuit, though. \$\endgroup\$
    – Todd Wilcox
    Commented Aug 30, 2017 at 13:52

3 Answers 3

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Why is a single supply voltage is seldom used for audio mixers”

Because in audio circuits the circuits become much simpler when used with a symmetric supply voltage (like +/- 15V). If one were to use a single 30 V supply (for example) then you have to take measures to DC bias the inputs of the opamps to a voltage somewhere "in the middle", with a 30 V supply that would be +15 V.

You could also create a +15 V "AC ground" rail to DC bias all the inputs from but then it is often just a small step to make the power supply deliver +/- 15 V. Then all the inputs can be DC biased to 0 V which is "neat" and "easy". Then the signals will also be centered around 0 V (ground) instead of having +15 V DC added to them.

Designers will often push an op amp up to, and sometimes beyond its recommended voltage rails to increase dynamic range

Does this still apply ?

Yes, in principle it does. Using the maximum supply voltage does indeed give the maximum output voltage swing which is what is needed to reach the largest dynamic range. Feed an opamp with +/- 5 V and even the best opamp is limited to 10 Vpp output voltage swing. Use +/-15 V and you get 30 Vpp output voltage swing.

However, not using the recommended supply voltage (but instead, using the maximum) might result in issues which might not be listed in the datasheet. But usually there should be no issue unless you'd exceed the maximum.

All circuits have a limited dynamic range, depending on the opamp you get more range or you get less.

You cannot say that the number of channels is limited by the dynamic range. As long as you take care that the summed signal (current) is below what the circuit can handle (the output of opamp will not clip) then there is basically no limit on the number of channels.

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    \$\begingroup\$ The amount of channels can be limited by dynamic range. Adding inputds doesn't increase the allowed input maximum level, but does add additional input noise to the system. Hence you lower your dynamic range by 3dB every time you double the amount of inputs. This is ofcourse going by the assumption that the noisefactor of the amplifier is very small. \$\endgroup\$
    – Joren Vaes
    Commented Aug 30, 2017 at 11:50
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    \$\begingroup\$ @JorenVaes Indeed, that is true assuming you keep summing signals using the same value input (V to I) resistors. But then you'd have to limit the maximum input voltages (before summing) as well otherwise the output could clip. In my opinion, to opamp itself does not limit dynamic range when adding more inputs. I think it is more the system itself which determines the noiselevel and therefore dynamic range. \$\endgroup\$
    – Bimpelrekkie
    Commented Aug 30, 2017 at 12:15
  • \$\begingroup\$ Indeed, you would need to limit the signals, but that would require resistors again, so could potentially increase the noise again. That said, I agree that it makes more sense talking about the system itself, as there are too many variables to say it's just the opamp. \$\endgroup\$
    – Joren Vaes
    Commented Aug 30, 2017 at 12:17
  • \$\begingroup\$ What is the difference between powering a circuit from -15/+15 rails, or powering the same circuit from 0/+30 rails, other than where you choose to measure the voltages from? Surely the actual components won't even know whether you're calling the potentials one or the other? \$\endgroup\$
    – hmakholm left over Monica
    Commented Aug 30, 2017 at 21:46
  • \$\begingroup\$ @HenningMakholm Indeed there is no difference except for the choice of ground point. Especially if in the 0/+30V case a "signal ground" at +15 V would be used than the circuits can be identical. Only when multiple circuits are connected together, one must pay attention, instead of using the 0V as a common that +15 V should be used. This is a bit counter intuitive and not what we're used to. Then using +/- 15 V makes things easier to understand. \$\endgroup\$
    – Bimpelrekkie
    Commented Aug 31, 2017 at 5:42
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Because single supply designs suck.

On a dual-supply you can bias your signals around ground. Coupling capacitors may still be used but they only have to deal with the small DC offsets coming from imperfect op-amps, not large deliberate DC offsets.

In a single supply system you must bias your signals around somewhere else. This creates a couple of issues.

  1. It is likely that any fluctuations in the bias voltage will become noise in the signal.
  2. There will almost certainly be a "thud" at power on as coupling capacitors charge to their steady state levels. Unless great care is taken there may also be similar thuds when gain controls are adjusted.
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Audio signals are AC only. Everything below 20 Hz is noise and can be discarded. It is therefore often convenient to design the circuit so that the quiescent level of all the signals is 0. That means using a ± supply.

Pushing any part beyond it specified voltage range is irresponsible design. Since there have been and surely still are irresponsible engineers out there, you can probably find products where this is still done.

Opamp bandwidth has little to do with supply voltage. The bipolar supplies of some audio circuits is for convenience of dealing with signals symmetrically about ground. It has nothing to do with bandwidth.

No, we're not going to Google something to get information that should have been in the question. That's squarely your job.

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  • \$\begingroup\$ There are musical instruments that can produce 8Hz, and 16Hz is relatively commonplace. Your lower limit is at least two octaves too high. \$\endgroup\$
    – user207421
    Commented Aug 30, 2017 at 23:45
  • \$\begingroup\$ @EJP Only if the speakers or floor/seat shakers can reproduce it. A lot of cheap consumer cans are doing pretty good to reach down to 100Hz in free space. General-purpose "pro" speakers are often advertised around 50Hz or so, with subwoofers going (slightly) lower. Also, ~20Hz is roughly the lower limit to hear; anything lower is only felt, but still adds to the experience if it's done well, the equipment supports it, and the building doesn't fall apart from the "earthquake". :-) \$\endgroup\$
    – AaronD
    Commented Aug 31, 2017 at 0:38
  • \$\begingroup\$ @EJP: 20 Hz to 20 kHz is considered "HiFi" audio. This is a industry convention, based on the range of human hearing. Even high end sub-woofers don't usually go all the way down to 20 Hz. 20 Hz to 20 kHz is what the electronics is supposed to be able to handle so that it isn't the limiting factor in a overall system. If you want people to feel (you can't hear them) frequencies below 20 Hz, then you need something beyond a HiFi audio system. That would probably require "shakers" as apposed to speakers for the inaudible low frequencies. \$\endgroup\$
    – Olin Lathrop
    Commented Aug 31, 2017 at 12:30

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