Fender rumble 25 isn't loud anymore

Is your bass active? Might need a battery. Leaving it plugged in when you aren't playing will drain it.

Is your cable known to be good? Might just need a new one.

 

Yes.
You're basically running a 9v out of juice.
Unplug your bass when you aren't playing and you should be good.

 

Absolutely. In my active, batteries can easily last months when plugged in only to play, but if I leave the cable plugged in for just a couple of days, it's new battery time. The amount of leakage current depends on the preamp design and the knob settings; some really naive FET boost circuits can give you a dead short to ground at the bias voltage if you turn the volume all the way down.

Most professional preamp builders are better than that, but even high-end preamps using hand-picked, hand-wired FET boosts will be sending their bias voltage to ground through a 1KOhm resistor. A Class A amplifier (one transistor) with a 1KOhm emitter resistor will, worst-case, be leaking 4.5mA of current while plugged in, which will drain the average 9V in about 48 hours of plugged-in time. Class AB amplifiers using two FETs, each reproducing half the original waveform, are significantly more efficient, but do still bias each FET a little bit to produce a crossover, so even these will eventually kill the battery if you leave them plugged in.

 

Really??? Care to share one of these circuits? I have NEVER seen a commercial circuit like this.

 

Here's one example of what I was seeing in boost schematics when I wrote that. The boost pot, set at minimum, provides effectively zero resistance between the emitter and ground. Now, this is a common-collector amp, not a common-emitter, and the voltage at the collector is set by a 5k resistor. So this wouldn't be a truly dead short through an open gate as I originally thought, but if there's sufficient gate voltage at "silence" to allow current flow through the FET, leakage current would double with the boost off versus at max:



Here is a two-stage JFET that I consider a counterexample:



This is again common-collector, using the transistors to induce oscillation in the supply voltage that feeds the output, but in this design, the transistors are buffered on both collector and emitter sides by a minimum of the sum of Rs and Rd (I'd expect the Rs and Rd resistors to be in the 1-5k range going off related boost circuit diagrams). That makes leakage to ground through any bias in the transistor gates (though I see none here) much smaller and more static. I would think this second circuit, while spec'ing a higher supply voltage, would be more efficient especially when silent.

 

Neither of these circuits are what are used in active basses, neither of these circuits are particularly practical commercial designs either. Neither of these designs present a 1k load to the battery either. Worst, worst worst case for circuit 1 would be a ~5k load. IMO, that's an awful design with either (major design or judgement mistakes) for a whole lot of reasons, this being one of them. This is not at all uncommon in the hobbiest and DIY world, where a full understanding of (real) electronics design is often lacking.

Now before you go off on my comments, I have designed quite a few such commercial products for a variety of companies going back close to 40 years. I'm very familiar with the state of the art for these types of circuits. Most load the 9V battery at ~100uA or less. These are used all the time in the onboard acoustic guitar and bass preamp world.

 

All the techie explanations aside:
beyond my level.....

The easiest way to think about it was put to me this way: Plugging in the instrument cable completes the circuit, just like flipping a power switch on. If the amp is on or off makes no difference.

This of course assumes you really do have active electronics in the bass. If not, there may well be an issue somewhere in the amp or cable. Confirm one way or the other, and try changing the battery first, if you have one.

Two weeks is still solidly within the "infant mortality" window. This means that very often, with new electronics, if there is a component that has a defect, it's likely to fail quickly.

 

I've left my bass plugged in for a week or two and had the battery get low enough that the tone audibly degraded.

I'm nearly certain that is the problem with the OP's situation - the whole point of this thread.

 

I had the same bass and forgot to unplug my cable for about 2 days and came back to find it dead. Always unplug from the bass when not playing.

 

There isn't any collector or emitter common in a fet based circuit. Those source and drain resistances are excessively low, and would contribute towards killing a battery quickly; not a useful circuit except for the battery manufacturers.
Your biasing divider doesn't impose much more than a 9.4M Ohm load on the battery; next to nothing.

 

Your bass battery is running out. Get a new one and everything will be fine.

 

Oh I absolutely defer to an actual commercial circuit designer on this. BTW I mixed up common-collector and common-emitter in the previous post, the above amps are all common-emitters and actually technically common-drain as the transistors are FETs and not BJTs. So I'm by no means an expert myself.

I will note that a very similar FET amp, a voltage buffer instead of a voltage amplifier, is used in the preamp of my TRB, and in fact there are four of these, to individually buffer each coil of the two soapbar pickups. All of them use an E301 current-regulating diode rated at 0.3mA current flow (regardless of supply voltage) tying drain to ground, so if my math is right, open-gate current leakage to ground is 1.2mA just for these voltage buffers (actual amplification is op-amp based). Now that's at peak flow through the buffer amps, so theoretically "silence" should produce 0V at the drain, but again that's ideal and assumes complete silence from each coil, which are only hum-cancelling when the signals from each coil are combined downstream of the buffer amps. The Yamaha TRB preamp circuit is a known power hog, and this is one reason; the 6 op-amps used for general gain, EQ and output impedance buffering, and the IC-based voltage regulator, are others.



The overall point, as basscooker said, is that leaving a preamp plugged in will drain the battery, because amplifier circuits are not ideal and will leak some current even with no signal input. 0.1mA, your figure, will drain a 200mAh 9V in about 2000 hours or 3 months plugged in (a little long not to notice you've done so). That assumes total silence from the pickups (thus minimal current leakage through the amplifiers), which is not guaranteed, and it assumes that a half-drained battery continues to produce acceptable voltage, also not a guarantee.

 

Note that there are a couple of unique features in the Yamaha circuit that you posted, the first is the ability to trim CMMR as part of the noise cancelling, which increases power consumption but does improve noise performance, and the other is that the choice of op-amps are very low current draw (ignoring the OP200GP which is 750uA/section, typically 200uA) so multiply by 6 sections (I don't see the whole schematic, so I'm assuming there are 6 opamp sections) and you have 1.2mA here. The total current draw is <3mA which represents 500mAH/3mA = 160 hours

(500mA is typical for a 9V battery discharged to ~8.25V at a 3mA rate).

This is IMO pretty common but can be significantly less current draw as well. It depends on the skill and intent of the designer.

 

Yes... very helpful to someone who’s been playing for 2 weeks lol

 

Most onboard preamps are good for between 125 and 250 hours of on time. That's between 50 and 100 practice sessions, assuming you remember to unplug the cable from the bass.

 

Leaving my active bass plugged in for two weeks would leave it pretty flat too. Other possibilities, more likely with age but not beyond the realm, are warranty issues at this stage and should be left well alone.

 

Well I'm glad we got that all cleared up..

 

Yes, your bass has an active preamp. And now a dead battery. Put a new battery in it, and from now on, unplug the cord from the bass when you're not playing - it'll last much longer.