Why do things run on 9V?

[rockhorst]

May well be another very noob question, but I've looked for through most FAQ files and haven't found the answer. Here it goes:

Why do most pedals WORK on 9V batteries/adapters (except for the fact that 9V is a standard)? From very simple to more complicated circuits. My own thoughts so far: the circuit of a pedal determines the current that is drawn from the pedal. The higher the impedance, the shorter battery life, but basically you could run ANYTHING on 9V. But, for instance, in the case of (big) amps it would be very impractical, since the overall current and thus power generated by the battery is very low amd you wouldn't get much sound out of it.
Thought experiment: What would happen if you applied 12 V to a 9 V pedal, provided that all component are designed to handle the extra energy.

Sorry if I made you all laugh your ass off I'm (hopefully) in a process of getting a deeper understanding of pedal/amp design.

2 side questions (I don't like starting new topics that much):
-What determines the speaker cabinet resistance that is matched with your amp? Is that the whole resistance of the amp circuit or only a certain part at the output stage?
-Are there any handy freeware programs for circuit design (as in what would happen with this resistor/cap combination and that type of opamp etc. etc.)?

[H.Manback]

I guess because of a couple of reasons:

• 9V batteries are a standard type (you can buy them at pretty much any store)
• 9V provides (usually) enough amplitude without clipping the signal

The only alternative in commercially available batteries would be types like AA or AAA, but they haven't got enough voltage to provide enough amplitude (the signal can only swing (roughly) half the supply voltage positive or negative), unless you use like 5 or 6.

[Seljer]

batteries: easily available and enough headroom for most guitar related things

for circuit simulation: there are a couple of open source versions of SPICE which works ok for circuit simulation (laying things out isnt that easy though as you have to do it notepad or such).
There are a couple of commercial versions that have got a bunch of features though.
I haven't used the thing in a while though.

[rockhorst]

I'll just restate: APART from the fact that 9V batteries are a standard. Basically the question could be formulated as:

How does the choice of power source affect the design of a circuit? I'd find it hard to believe that you just build a circuit and afterwards supply 9V to it hoping it does what you anticipated...

[H.Manback]

I guess because of a couple of reasons:

• 9V batteries are a standard type (you can buy them at pretty much any store)
• 9V provides (usually) enough amplitude without clipping the signal

The only alternative in commercially available batteries would be types like AA or AAA, but they haven't got enough voltage to provide enough amplitude (the signal can only swing (roughly) half the supply voltage positive or negative), unless you use like 5 or 6.

It's that simple really... It has enough current capacity, and the voltage provides enough headroom for the signal. If a typical guitar signal (or actually amplifier input signal) would be 23V AC peak to peak for example, you would barely have enough headroom with a 24V supply.[/list]

[rockhorst]

Could you elaborate a bit on the term headroom?

[Seljer]

I'll just restate: APART from the fact that 9V batteries are a standard. Basically the question could be formulated as:

How does the choice of power source affect the design of a circuit? I'd find it hard to believe that you just build a circuit and afterwards supply 9V to it hoping it does what you anticipated...

It depends on the circuit. With opamp based circuits you can easily up the voltage if you ever need more headroom (providing the opamps you're using can take it), or converted to a dual polarity power supply and vice versa. Transistors are usually a bit more tricky.

Headroom = the amount you can amplify it without it reaching the upper limit of the power supply (eg: you can't amplify your signal past 9 volts)

[rockhorst]

So if I wanted to amplify my guitar signal with say a simple diode, I'd connect a 9V source and use something like a resistor to taper it down to the required amplification? What would be a typical guitar signal amplitude?

Oh I found a free SPICE program on download.com called B2 Spice. Seems quite nice a tool for frequency analysis of circuits. How would I simulate a guitar input/output in such a program? Just connect a AC voltage source in addition to the 9V?

[Seljer]

I made this little animation to demonstrate having not enough headroom:

Of course this sometimes a good thing providing how its done and that you're a guitarist (eg: tube amps distortion, and there are many distortion pedals that rely on this to get their clipping)


Diodes don't amplify things. Diodes stop any current in one direction and let it flow in the other (providing the voltage is higher than the forward voltage of the diode which is about 0.7V, it depends on the type of diode).
The most simple way to amplify something is probably with a transistor, another option is with an opamp.

as for an roughly emulating of a guitar pickup = try an AC source with a frequency somewhere from 200 to 1200 hz and less than 1 volt amplitude.

[R.G.]

Why do most pedals WORK on 9V batteries/adapters (except for the fact that 9V is a standard)?

Because two (or three, or four) 1.5V cells is not enough and because the first practical supply voltage available in a standard battery is 9V. In the begining of effects, it was common to run simple distortions on 1.5V (one carbon-zinc cell) or 3V (two C-Z cells) and germanium transistors could do that. Silicon makes that much harder. In addition, a guitar signal is 50 to 100mV peak for single coils, up to 500mV for humbuckers, and you just can't amplify those big signals much before you bump into the power supply. And because 6V and 12V batteries are all physically much bigger than 9V batteries, or they were back in the 60's when the form of the modern stompbox was being finalized.

Thought experiment: What would happen if you applied 12 V to a 9 V pedal, provided that all component are designed to handle the extra energy.

It depends entirely on what the circuit is. Some circuits work fine. Some work better in some ways. Some won't work well at all if their internal operation was dependent on the value of the power supply voltage. There can be no general rule here.

-What determines the speaker cabinet resistance that is matched with your amp? Is that the whole resistance of the amp circuit or only a certain part at the output stage?

For solid state amps, having a matched speaker impedance has little or no meaning. Solid state amps are (usually) happy driving any speaker impedance from some minimum up to an open circuit. Too low a speaker impedance may make them exceed their allowable current or power dissipation ratings. There is usually some impedance at which a solid state amp will produce the maximum power output, and that's generally between 8 and 2 ohms, but that's not because of any matching. It's because that happens to be where the amplifier is putting out the maximum possible current for its power supply voltage while not self destructing.

For tube amps, what determines the matched speaker output is the output transformer and tubes. The output tubes have a preferred load for most power out, and any impedance either bigger or smaller produces less. This tube impedance is quite high, and the output transformer transforms that impedance to the external speaker.