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I just bought a guitar amp that can be powered by a 9v battery or a 9v power supply. The instructions that came with the amp specify that the power supply should deliver 300mA maximum. As far as I understand, the load determines how much current is drawn, so why would it be dangerous to use a power supply with say, 1A? I don't want to fry the amp, but I also don't want to go track down a different supply.

Here's my guess. I think it's probably not safe. I noticed that when I turned the volume up while using a 9v battery, playing a note loudly would cause the amp to die for a moment, which I assume means it's trying to draw more current than the battery can supply. This seems to imply that louder volume equals more current draw from the power source. If I plug in a power supply with a current rating of higher than 300mA, there's the possibility that the amp will attempt to draw in more current than it can handle when playing loudly and fry itself. Is this what's going on?

user3280428
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  • no, it won't fry the amp. – ddacot Sep 04 '14 at 18:55
  • @MattYoung - I read over this, but I wasn't certain if it would apply to a guitar amp. – user3280428 Sep 04 '14 at 18:59
  • @ddacot - I want to be really certain before I try it out. Could you provide some details about what's wrong with my guess? – user3280428 Sep 04 '14 at 18:59
  • "The instructions that came with the amp specify that the power supply should deliver 300mA maximum." Can you provide an exact quote from the manual? It should say that the power supply must be capable of providing at least 300 mA, or that the load will draw a maximum of 300 mA. What you wrote is a very unusual spec. – The Photon Sep 04 '14 at 19:03
  • @ThePhoton - I'll have to post later when I get a chance to quote it directly. I'm certain it says something along the lines of "The power supply should be 9v with maximum 300mA." – user3280428 Sep 04 '14 at 19:10
  • Using a higher rated power supply won't hurt *provided* that it really puts out 9V at 300mA. Unregulated supplies usually deliver higher voltage when operated below their maximum current rating, which might damage your amp. For example, I tested an AC adapter that was labeled "9VDC 1A" and it produced 11V at 300mA. – Bruce Abbott Sep 04 '14 at 19:44
  • @ThePhoton - The manual says this: "Use 9V battery (Danelectro Vintage Battery) or Danelectro DA-1 AC Adaptor. Power adaptor must be center negative, 9 Volts, 300mA maximum." – user3280428 Sep 05 '14 at 00:04
  • OP, please look for Olin Lathrop's example of Johny's interest of eating two apples only, even you provide 2.5 or 3 or 4. – AKR Sep 05 '14 at 03:48
  • @AKR - Sure thing. I understand how the analogy works. Maybe I'm just being over cautious for fear of bricking my amp. I'll go ahead and try it out. – user3280428 Sep 05 '14 at 04:22

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The answer to your primary question is covered completely by Olin Lathrop's answer in Choosing power supply, how to get the voltage and current ratings?

My answer is going to address your second paragraph about the 9V battery. You are correct about surmising the circuit was trying to draw more current than the battery could supply. A typical store-bought 9V battery is actually pretty awful at supplying anything more than a few 10s of milliamps without its voltage sagging significantly below 9V.

You are also correct that increasing the volume on your amp pulls more current from the supply. In its simplest form, a volume knob can be just a variable resistor. As you turn the knob to increase the volume, you're actually decreasing the resistance in series with the audio signal to the speakers (in modern electronics, its not that simple, but good enough for a description here). Less resistance means more current, which means louder sound.

But Ohm's Law is absolute. With the volume turned all the way up, the effective resistance of the load is a (nearly) constant value. And power supply voltage is (nearly) constant as well, so current must stay constant. The current is effectively capped on the upper side no matter how beefy the power supply is.

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