Guitar Preamp Current Draw?

[drewl]

Anyone know about how much current a typical acoustic guitar preamp draws?

Got an Epiphone that's draining batteries quickly.
Stuck a new 9v battery in and after about an hour it was down to 8 volts.

Standard preamp with two opamps, five band EQ, I pulled it out and have it on the bench and it's pulling about 10.5ma.

Nothing is getting warm, nothing looks funny or is shorted, the output jack turns off the circuit when nothing is plugged in.

I'm running a signal through it and am going to let it run for a couple hours to see if anything does get warm.

[eschertron]

Then something other than the preamp is drawing down the battery. A standard alkaline 9v has about 550maH capacity. Rechargeables, somewhat less, depending on the construction. But still, tens of hours of life at 10.5ma draw.

[drewl]

Hmmm, interesting.
Thanks.

[mozz]

Depending on age, you could have a few bad electrolytic caps, smd most likely in that case.

[Helmholtz]

Total current draw is 10.5mA. That's neither very low nor very large.
Opamp current demand can be found in the datasheet.

Maybe your "new" battery was stored too long and suffers from some self-discharge (might not show without load).

[drewl]

Possibly.
After an hour of it cranked, voltage dropped to 9.1v, then went back up to 9.3 after being off for an hour with a different, brand new alkaline battery.

The owner did say it was eating batteries, confirmed by the guitar tech.

Aside from the corroded battery spring connectors, I also cleaned up the frayed connections to the pc board and cleaned some slight corrosion they had.

also measured wiring continuity and battery connector continuity to make sure there weren't any power/voltage losses there.

Thanks.

[drewl]

Depending on age, you could have a few bad electrolytic caps, smd most likely in that case.

Oh, I checked resistance of them in circuit to see if they were possibly loading the voltage, but they seemed okay, might just change them while here.

[loudthud]

What are the numbers on the opamps ? Can you post a picture ?

[loudthud]

A damaged opamp can usually be found by taking Ohm readings from the output to each rail, ground and +9V in this case. If they are dual opamps, the output will be pins 1 and 7, 4 will be ground and 8 will be +9V. If they are single opamps, the output will be pin 6, 7 is +9V and 4 is ground. If they are quad opamps, the outputs are on the corners, 1, 7, 8 and 14, +9V is pin 4 and ground is pin 11. Normal outputs are high impedance or diode drops, bad output might measure a few hundred Ohms.

[drewl]

No funny readings on the opamps, voltages look normal.

Voltage dropped .6v after being plugged in for 5 hours.
Preamp stops amplifying at around 6v, so that would be about 30 hrs of use, I guess that's about right.

[TomCarlos]

Is this preamp for an acoustic guitar or electric?

[drewl]

Acoustic

[TomCarlos]

I remember having a Fishman preamp that had was draining quickly. I don't exactly remember how fast or the exact location, but I did find a microscopic short between the ground and circuit. And this was on a NEW preamp!! If you can, take some photos and post them. Look carefully at the ground lead from the battery and trace that out. Typically, it would go to the output jack. From the output jack, the ground would go to the circuit board :through" the switch portion of the output jack. So remove the guitar cable, and measure for voltage between the battery + to the circuit board ground connection. Verify it is "0." By the way, if you disconnect the guitar cable and let the guitar sit overnight, do you have a dead battery the next day?

[drewl]

Oh I checked the switching first
it pulled no current without either 1/4" or XLR jacks inserted.
It has the little micro switch inside the XLR jack, but both are good.

[Greg Robinson]

Are you absolutely certain the owner knows to unplug the guitar *at the guitar end of the cable* between uses? I've had a couple of customers who didn't realise unplugging at the amp end was not equivalent...