[sdiy] Introduction and a couple of noob questions

Olivier Gillet ol.gillet at gmail.com
Thu Oct 4 14:23:00 CEST 2012

Hi Marc,

Cool to see you here!

> 1) Audio voltage: What is the range of an audio voltage and it's 'polarity' ?

It really depends on the device... The oscillator of a modular
synthesizer is going to be at a fairly "hot" level (say 10V peak to
peak); while consumer audio equipment is more likely to run at less
than a volt peak to peak. http://en.wikipedia.org/wiki/Line_level

The "polarity" of the signal and whether it is referenced to ground or
to a higher voltage (say 1.65V for a media player running at +3.3V) is
irrelevant if you put AC-coupling capacitors at the input stage of
your system.

> I'm expecting it to be +/- a few volts around ground value. But what
> happens when you use an op-amp where the V- is connected to the ground
> (like in the single supply amplifier from forrest mims) ? Is it the
> same ground reference as the audio input ? In that case, what happens
> when the audio voltages is negative ? Can it be transformed by the
> op-amp to a voltage lower than v- ? Or should the ground lead from the
> audio be connected to a virtual ground point ? Somehow I feel I'm
> missing a point there.

In practice, it does not matter because the signals are AC-coupled.
For example, if you want to design a pedal running on 9V with a 4.5V
virtual ground, you will have an AC-coupling cap at your input stage
(see for example the schematics on figure 3 of this:
http://www.eng.yale.edu/ee-labs/morse/compo/sloa058.pdf - which I
recommend reading), so it doesn't matter if the signal fed into your
circuit is referenced at 0V or 4.5V or 1.65V...

> A lot of circuit schematics I see expect a voltage reference of 15v.
> But I also know some of them will work with 9v. How does one know how
> low the voltage can be ?

- If you see that one of the supply rails is used for anything else
than powering op-amps (example: fed through a resistor to an op-amp
summing node to offset a CV ; hooked to one of the pots...), you will
have to recompute some of the resistor values.
- The next thing to look for are headroom/signal levels. If you have a
circuit powered by +/- 12V, it won't handle signals with an amplitude
of more than 24V pp. Many op-amps start misbehaving when the signal is
getting close to the rails, which reduces the maximum amplitude
further. For example with a TL072 your headroom would be more
something like +/- 10.5V.  So reducing the power supply, say from +/-
12V to +/- 8V is going to reduce the amplitude of the signals that can
run through the circuit, and cause it to malfunction if it operates
with large inputs.
- Finally, check for fancy parts that might require high supply voltages!

> +9/-9 as source and all I have is a 9v dc source, is there a way for
> me to generate the +9/-9v from +9/0 ?

Yes, there are DC-DC converter like the MAX7660/LT1044 or LT1054.
Drawbacks of these parts: might introduce high frequency whine if not
used correctly; expensive; somewhat fragile (can be killed by excess
voltage); and limited current output (100mA for a LT1054 and at this
current it delivers less than -Vin).

The other option is to use a "single supply" design (V- to ground; V+
to 9V) for your op-amp, and consider that all your signals are
referenced to 4.5V.

Risks: design is not as straightforward (you can't blindly use a bit
of application schematic made for a split supply design); you will
lose half your headroom; you will lose one important property of
op-amps - power supply rejection (If you are not careful about what
you do, any fluctuation of the supply voltage will affect your
signals, resulting in a noisier output); you will lose the ability of
using chips that requires higher operating voltages (eg: SSM2164
requires at least +/- 4V); and finally you might have to make
excessive use of AC-coupling between the different stages of your
circuit, resulting in degraded performance in the lowest frequencies
if you are not careful about what you do.


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