Active Pickups low voltage

[Ceredox]

Hey guys (and gals). Recently, I've been starting to design a built-in battery tester for my pickups batteries. I am using a comparator circuit. My problem is that I'm unsure of the point at which the pickup will "die" due to low voltage. I know 9v-27v is their basic operating range but does anyone know how low the voltage goes before the active circuitry shuts off? I'd imagine it's probably not too far below 9v. I'm trying to set the parameters for the circuit but I'm not sure what will be necessary low voltage. On EMG's site, they touch on it a little but mostly just what happens, not necessarily the voltage. Does anyone know?

Thanks!

[52 Bill]

Does anyone know?

Why not you?

Hook up your guitar to a variable voltage source and then reduce the voltage while you listen to the sound. When it starts to sound wrong, measure the voltage at the power supply and then set your comparator to a voltage level a little higher than that.

[Ceredox]

That is a fantastic idea! I have no idea why I didn't consider doing that haha. Thank you for the advice. I'll post what I find.

[J M Fahey]

As you see, we are the "build it and test it yourself" practical kind of guys around here.

That said, real World output from a preamp becomes "not worth the effort" below around + 6 VDC and some pedals incorporate just that, which is simply a temporary test pushbutton, a 3V to 3.9V LED, in series with a resistor and a LED; it becomes real dim or fails to light around 6V .

Easier than the comparator.

[Ceredox]

I know it's a delay, but just for the record I did some experimenting. My EMG 808s went all the way down to 1.5v before the shutoff. The sound noticeably depreciated below 3v. Not bad. I also tried (7-string) Seymour Duncan Blackouts. They went down to 2.5 volts before they shutoff. The sound stayed relatively solid until it hit about 5v. I wish I owned an oscilloscope. I'd love to see them perform at low voltage on a more graphic level. Just a fun FYI. Thanks!

[km6xz]

This questions comes up and as far as I can remember you were the first to actually test it. It is not really a preamp, but an isolation amplifier with little or no gain. It serves a very useful purpose, take the signal that is coupled out of the pickup/tone/volume passive circuit's high impedance and match it to the line out at a low source impedance. That means less noise, and less sensitivity to cable length or capacitance making changes in the tone of the guitar. It prevents different loading when changing amps from changing the guitar output signal characteristics. The active stage can be very simple, a solo FET would be enough to perform that task but probably most use low drop out opamps in a unity gain configuration, which results in a very low output impedance. I would guess that 3 volts would be the practical lower limit without noticeable distortion, but if you want to be safe, and you are using a 9v battery, I would set the low battery light to light at 6.5 to 7.5 volts.

[MarkusBass]

You are asking about two different problems:
- what is the safe voltage for active preamp (buffer),
- what should be the threshold for comparator that tests the quality of the battery that supplies active circiut.
The preamp may work with 4-5 Volts, but imagine that you are performing live and the battery in your guitar is 6 Volts only. It depends how long is the show but I wouldn't start with 9V battery that is actually less then 7 Volts. That's why such threshold is usually set to 7 Volts.

PS: imagine starting live show with 4V battery. This could be the last show in your live because the next time no one is going to hire you .

Mark

[Ceredox]

Haha don't worry I wouldn't start a show with only 4v! I always prefer to keep it above 7v, personally. I was trying to use a led strip with good/fair/poor sections via comparator. After some designing, I realized it wouldn't work because the grounds need to be common yet the reference voltage cannot be from the same battery. Otherwise the led strip wouldn't work correctly. The single LED idea is a very simple and effective way to do it but the LED will drain the batteries much faster than the pickups would - which kind of defeats the purpose. It was just a fun idea I had. Please feel free to correct me if I'm wrong. I've only recently obtained an associates degree in electrical engineering and still have a lot to learn. Thanks for the input!

[MarkusBass]

I'm not sure whether I understand you (most probably not).
Such circuits are commercially available - you can easily buy them in the US. They use so called "micropower comparators". Such a comparator contains voltage reference source (and it's not a strip of LEDs). The voltage reference source uses the same ground as the rest of the circuit. The circuit itself does not drain the batteries because it blinks only few milliseconds in a second (the current consumption is in a range of microamperes). The blinks are still visible because this is how our eyes work. The threshold in such circuits is usually set to 7V for 9V battery operation.

Mark

[Ceredox]

Great! I searched around a bit and didn't find any when I first started doing this, so I had trouble finding a anything. Most of my reason for wanting to build one is because I thought it would be fun to design and experiment with one. I don't really have a huge problem with dead batteries. Unfortunately, I don't have the resources to effectively make any kind of microcircuitry (which would make this 100000X easier). I wonder how they used a common ground but with separate reference voltage. That's interesting. I realize that this would put very little draw on the battery. When I mentioned the single Led drawing a lot, it was actually a misunderstanding of an earlier comment so please disregard that, heh.