farmosh203

I'm pretty sure pedals are not meant to be used to drive speakers, they are just pre-amps.

Cathbard

The coveted 18W Marshall uses half a 12AX7 into the PI. That's one rich sounding blues amp right there.
But as I said, if all the sound characteristics came from the OT valve guitar amps would have disappeared decades ago along with all the other valve gadgets.

Re using a pedal to drive a speaker - he was taking the piss out of you.

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AcousticMirror

pretty sure what's possible > ignorance.

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farmosh203

Well the pedal runs off 9V, so [(9V/2)/(sqrt(2))]^2/8 ohm = 1.2Watts

Exactly why pedals don't make a great amp (and who knows what the current limit of the output amplifier is in the pedal...).

ethan_hanus

Well, I might be completely wrong, but I think of it the way I think of voltage in a car, even though it's DC. A simple component, like a relay, takes 12 volts to activate, the battery puts out a constant 12 volts, and 400 amps, you would think that 400 amps going through a relay would fry it, but it doesn't, because the relay only pulls what amps it needs. So your voltage may not change, but your amps can change wherever you want them to, depending on how much you need.

So an amp may be getting a whatever milivolt input at whatever miliamp, I believe the purpose of all the circuitry is to change the amount of amps so it puts out a higher wattage, thus giving more volume, while not changing the actual input. I'm prolly wrong, but idc, I tried.

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AcousticMirror

the ot in a pedal can handle at least 10 watts.

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farmosh203

Ok, I hope you're joking now

Warrior47

No they are not, but they can. In fact, if you have a small enough speaker that needs some ridiculously small power to work, you could drive it with just your guitar. Basically, you just need to up the signal a lot so that it can drive a larger speaker if all you want to do is make sound. The tubes and such come in now days more for the quality of sound that they make, as with a couple op amp circuits, you could easily boost the sound enough to drive a cab. It still does require a lot of amplification though. The signal is not very big coming out of a guitar and it needs to be big if you want a loud amp.

The reason that your pedal is over 4 v is that the pedal has a power source in it (the 9 v battery) which is being used to boost the sound. Basically it's being a very small amp that also flavors the sound in some way.

farmosh203

That just confused me... but I think I know what you're trying to say. Basically it's just impedance matching but the amplifier still needs to amplify the signal a little.

farmosh203

So here's a question, if the output of active pickups is never above 1V, why do people do the 18V mod to their EMG's?

mmolteratx

Yup. You're wrong.

A guitar amp is a voltage amplifier. It takes a small AC signal from a pickup (~100mV P-P or so) and multiplies the signal in each stage depending on that stages design. The current output of a guitar's pickup is negligible to the point of not mattering.

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ethan_hanus

Yeah, but it doesn't have to by amplifying the volts, it can just up the amps to pull more current, which if done right wont change your volts at all. Everything in a car runs on 12 volts, headlights, blinkers, radio, AC controls, power windows, power seats, everything. The only thing that changes is the amount of amps being pulled for each certain function, the volts never change. But comparing DC and AC circuits can be a very bad move on my part.

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CECamps

Not true. Speakers require current, not voltage to be driven. A transformer takes a high voltage, low current signal and transforms it into a low voltage, high current signal to drive speakers. The more voltage you can feed the transformer, the more current it can feed the speakers. This is the precise reason why output tubes are required in tube guitar amplifiers.

Because transistors are current amplifiers, transformers are not required. They can drive speakers directly.

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CECamps

The 18v mod increases the supply voltage of the active electronics from 9v to 18v in order to increase the output. But it doesn't mean the guitar is putting out an 18v signal. 18v is the DC supply voltage to the active electronics.

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AcousticMirror

yes. bad move.

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farmosh203

If anyone cares, I measured the output directly out of my guitar, you can get a 2V peak to peak signal coming out of an EMG pickup strumming hard.

littlephil

Because the input voltage of the preamp in an active pickup isn't the same as its output. Running the preamp at 18v changes the way the preamp reacts to the input signal, but it doesn't mean its putting out 18v.

farmosh203

The supply voltage of the active electronics doesn't matter unless the signal is coming close to the positive rail of the power supply. The only reason to increase the voltage supply of an amplifier would be to prevent the signal from clipping if it was coming close. The gain is set by the feedback resistors, not the voltage supply.

farmosh203

I'd like you to explain how running the supply voltage at 18V changes the signal.