Onboard Preamp Circuit Power Questions

[DaKraut]

I am planning on designing an onboard jfet preamp for my guitar, but want to have a design that when the 9 volt battery runs out, the signal would just bypass the circuit. Is there any way to do this, or is it just too complicated/impossible.

I am looking at this circuit, and am also wondering if the j201 could be used instead of the suggested transistor

http://www.marshallschematics.com/buffer.shtml

[Taylor]

Designing something to automatically bypass when the battery is dead would be pretty tough, not just electrically, but conceptually. What is "dead"? 8 volts? 7? 2? 0?

Instead, you can do what is common in basses with onboard preamps - have an active/passive switch. Useful also when playing into a fuzz face/fuzz factory/etc. which expects a guitar pickup impedance to sound right. The switch just bypasses the preamp like you would an effect. You'll need to put a 250k or so volume pot before the preamp, otherwise you'd need separate volumes for active and passive mode.

[petemoore]

  I put a bypass jumper across the top side of a DPDT pushpull potSw. [the two ''topside throws'']
  I connected the pickup to the left pole.
  I connected the volume pot input to the right pole.
  A stratoblaster went across the remaining two throws, at the bottom of the switch, opposite the bypass jumper.
  When in boost mode: Pickup>LPole>L bottom throw>circuit input>circuit output>R bottom throw>Rpole>volume control>out.
  When in bypass mode: Pickup> Lpole, topLthrow, topRthrow, Rpole>volume control>out.
  Works fine, I dropped the gain to 12% or 20% boost so it didn't superblast the speakers when playing clean/worked well with fuzzes.
 

[Gus]

Look in the gallery

http://www.aronnelson.com/gallery/main.php/v/gus/9pp.GIF.html?g2_imageViewsIndex=1

Something to look at.  Yes it opens big but it might be worth studying.

[panterafanatic]

Why not just get a 5 volt relay? SmallBear sells one with a footprint of a 14/16 pin IC. Run a series resistor. Once the battery cannot supply the current for it, the relay switches back, and then bypasses the buffer.

The only problem is that you'd have to design a relay driver if you wanted to bypass it manually as well.

[Gurner]

Why not just get a 5 volt relay? SmallBear sells one with a footprint of a 14/16 pin IC. Run a series resistor. Once the battery cannot supply the current for it, the relay switches back, and then bypasses the buffer.

The only problem is that you'd have to design a relay driver if you wanted to bypass it manually as well.

Except the relay will drain the battery quicker - so you have a peverse scenario where the 'solution' put in place to detect a dying battery, makes the battery die (vs the drain for a basic jfet preamp). Moreover, since a jfet preamp has very little drain ...you're putting in circuitry to detect something that happens once in a blue moon.

Sometimes there's a yearn to over-engineer (& I'm guilty of that m'lud), but for this situation I much prefer this setup....

Battery starts dying -> an ear detects this  -> half a second later a finger toggles out the jfet preamp   ....job done (& it's full of win  )

[petemoore]

  9v circuit...and link specifies:
   low-Vdss FET if you know what you're doing.
  See Runoff Groove, Fetzer Valve, includes a study of various Jfet's @9v, and related testing/selection circuit.

[DaKraut]

I have thought about the idea of using a phantom power design, and do like the idea alot, the same system could also power a guitar with say emg actives.

The only opposition is you would need to keep around a box with a power supply in it, with the onboard battery you can use any setup as long as its your guitar, much more freedom. Although you could make a buffer cable and as long as you have that use it for any guitars, but i would like an onboard buffer on my main guitar.

[Gus]

What else you can do with the phantom is use an external power supply box with say AA batteries.

The box would have a TRS to bass and TS to amp jacks and you could even clip it to the guitar/bass strap.  With the extra room you could mount a battery monitor that would light an LED when the battery was getting low to indicate to flip a switch on the bass to bypass the active electronics

Here are some links

http://www.discovercircuits.com/DJ-Circuits/9vbatmon1.htm

http://www.discovercircuits.com/DJ-Circuits/voltst1.htm

http://psn.quake.net/eco8/battery_monitor.html

search for "voltage level monitoring ICs"  etc.

[DaKraut]

I look at those circuits and (because I have practically no experience with making circuits) wonder why something similar can't be used to just shut off the circuit when the voltage drops below a certain point.