Hi Folks,

I am about to start a very small project which is to add a little pre-amp circuit internally into my acoustic guitar. The guitar already has pickups and a passive tone/volume control, so I want to add the pre-amp just after that existing circuitry and before the output jack.

The pre-amp (which is a very simple well known circuit off the web) runs off a 9 volt battery and will work even if the battery voltage drops to 8 volts ... what I am concerned about is what happens after that as I believe when the battery decays to a certain voltage I will get no signal whatsoever at the output jack.

Now that would mean that whilst playing, if the battery decays I will end up un-plugged ... which is not good ... although others may disagree!

I guess I could simply put a manual SPDT switch on the signal input which would either send the signal into the pre-amp or directly to the output jack, but was wanting to avoid drilling another hole into the guitar body.

So I was wondering if there was a way to replace that manual SPDT with an electronic version that could automatically detect a low voltage condition (maybe fine tunable with a trim pot) and flick some sort of electronic switch that would route the signal direct to the ouput jack?

I have attached the pre-amp circuit I plan to use with some (awkward looking) red lines and text I have added showing what I mean.

I am looking for the most simplest solution possible as I want to keep the component count down to a minimum, plus I am also looking for a solution that does not add to the problem by draining too much battery power.

Any help would be much appreciated.

Thanx

Zig

 

IMO a simpler solution would be to use a slight mod (1 x R, 1 x C) of the preamp so that it works with a wider range of battery voltage; down to, say, 5V.
BTW, is the input fed via a coupling cap?

 

I think you are working on a non-problem. The J201 is a 1ma max transistor and this one is turned down a bit by R2. The last time I looked up a 9V alkaline battery, it had 440 ma-hours. You are never going to play one gig for over 400 hours, so just replace the battery before every gig and every 400 hours of use, whichever comes first.

Trust me. I've done this and have decades of time on my pre-amps.
ps, it won't just quit, it will start clipping. Nasty transistor amp clipping.

 

Alec_T - attached is the circuit I plan to feed into the pre-amp.

#12 - understand what you are saying but was also wanting to do this not primarily for the worry of losing the pre-amp ... but also as a mini-project in itself just to add some fun and a little difference to the whole setup ... and teach myself something along the way ...

 

The drawing in post #1 will load the signal with about 6800 ohms (by going through C1 and R3 and the battery). This will interfere tremendously with the tone and volume controls. You need a double pole switch to disconnect the amplifier. This drawing shows a different way, but Q1 will act as a diode if it's unpowered and mess with the signal, too.

 

Hi #12,

The thumbnail on your last post #5 is exactly what I am wanting to do, but with some form of electronic switch rather than a manual switch ... which was my original question.

I tried to show that awkwardly in my first post with a red line, however I was not suggesting that I just connect the input to the output but was simply showing where I wanted to route the signal via some form of electronic switch ... sorry for any confusion.

 

IMO automatically switching the input to the output when the battery voltage dropped, although doable, could lead to problems. There would be a quite noticeable/annoying shift in signal amplitude at switch-over. Not very impressive during a performance. Personally, I'd use the circuit which #12 attached in post #3 (which is more or less what I was suggesting, though I would have used some higher resistor values to prolong battery life).

 

Sorry. I missed the "electronic switch" part. I tend to glaze over after about 200 words.

ps, the amplifier I posted is not intended for this job. It has way too much gain.

pss, I also hate the "automatic" switch idea. Trying to make it simply fall into the bypass condition when the battery gets low will create a problem of switching back when the battery voltage goes up because it is no longer being used to amplify. Trying to do an active switch point will use more battery power and it will still be switching during the performance. Totally half-baked idea.

 

How about another battery thru a diode switch? UPS!
Better yet an scr or fet.
No more complex than other electronics.
Use current sensing to bring on an indicator when on auxiliary power.
(haven't figured that out yet)

 

Crutschow did this:

 

Where did I miss that?

I'm thinking simple of course.

 

Battery #1 direct to load.

Battery #2 to anode scr, cathode to load.

Scr gate set to fixed voltage. ie. 7volts.

When cathode/load drops, scr fires.

Probably not that simple. Haven't drawn it out.

 

I don't even know if that (post #10) will work at 9 volts, but it's an idea. I have to go to work now. Don't have time to examine the schematic.

 

Folks,

Many thanks for all of your comments.

On reflection, I agree with you that the electronic switch idea, well ... it is a bad idea!

Your input (no pun intended) is valued as I was going off down a track and you pulled me back. I was not thinking about it potentially switching on-and-off continually for a period of time, and was more thinking of it as a one-off event ... but even then, it still would be a bad idea!

I am going to go with the manual switch, but have one last question: would I need to add a diode to the output to prevent the signal re-entering the pre-amp circuit? See attached (amended) image, you know, the one with the red lines!

Please excuse my ignorance if I am way off the mark here ... just learning

Regards

Zig

 

No diode. Just a single-pole change-over switch (1p2w).

 

I already killed that idea in post#5.
This won't disconnect the battery. Do you already have it wired for that?
Doesn't matter. If you have to switch over, you will know the battery is dead and replace it after you finish playing.

 

For anyone following this thread.
This is the 9v UPS idea.

 

Apologies #12 ... just looked at your layout again in post #5 and I can see that it is all about the positioning of that switch, so as alec_t also says ... no diode needed. Thanks for your patience!

Yes, I am planning to disconnect the battery separately using a switched jack input which isn't shown in any of the pics I posted.

Many thanks for all of your help, all settled now.

Best regards

Zig