Extremely Low Current Buffer options

[Clint Eastwood]

Could I ask the reason for a JFET buffer of about 120μA working current..??

Because the OP wants long battery life... But if you build this in a jackplug and use phantom power, you can run it at 1ma for example, wich would make it work much better.  With 120ua, large signals will be clipped a bit too easily, you are right.

[pee-j]

would you please outline how to go about it?

To stay on R.G.'s design, for 12V supply just double 470k resistor value (820k to 1M)..
You'll keep the same total current consumption while obtaining the double output swing..

P.S.
A perfectionist should also alter 1M/1.5M ratio a bit (1M ->1.2M) for 2N5088 Emitter biased at exactly +12V half-way but, IMHO, the 500mV "off-set" (6.5V instead of 6.0V) should be a more realistic bias point for symmetrical output swing both due to current source BJT V_CEsat + V_E and finite output impedance..

thanks very much, antonis!

if I could have one further question, it'd be how can we tweak the input resistance?
I'd like to try 500K and even 150/200K
and I know you're good about this, too
see this thread

[antonis]

if I could have one further question, it'd be how can we tweak the input resistance?
I'd like to try 500K and even 150/200K

The easiest way is to tweak 1M5 bias resistor value..
(equivalent input impedance formula is quite complex due to parallel combination of 1M5 bias resistor and current source output impedance(*) times upper n-p-n transistor current gain..)
Take into account the 4n7/1M5 RC time constant for maintaining same input HPF corner frequency..

(*) subject to particular device both Early voltage and h_FE variation..

[amptramp]

The OP wanted to use a low-current design to extend battery life in his guitar.  Low current is always associated with high noise, so there may be better ideas:

He mentioned the Tillman but wanted to limit the current for maximum battery life.  The Tillman is an amplifier where the power supply can be external.  If you put the gate resistor, JFET and source resistor in the guitar, the drain resistor, coupling capacitor and pulldown resistor can go in a separate box with its battery or power supply.  In fact, there is an amplifying cable available now that plugs into a guitar and does this without making any modifications to the guitar.

The 4 - 20 mA current loop interface was designed (and is still used) for chemical processing where the technology is trailing edge and very conservative.  After all, you don't want to make a thousand pounds of almost nylon.  The power supply and the current reader are in a box on the floor and the voltage-to-current converter is in the guitar.  You would have a current at rest of 12 mA and the signal could go ±8 mA from that.  This interface can be made relatively immune to interference since the impedance can be quite low.

Another possibility would be a rechargeable battery in the guitar that would switch off the amplifier and take current into the battery above a set voltage applied to the output.  Then you could put any amplifier you wanted in the guitar.  In fact, you could dispense with the audio output entirely and put a transmitter in the guitar, powered by a rechargeable battery that is recharged by the output jack that now carries no signal.  You could put a standard rotary on-off switch on one of the guitar controls to turn it off or switch between "on" and "recharge".  A standard 19-volt lithium computer battery could also operate a tube stage using a 20EZ7 tube (a 12AX7 with a 20-volt 100 mA heater and different pinouts) with two triode sections for amplification and a cathode follower.  You would have to carve out a lot of room in the guitar for a laptop battery, but it would be an answer.

[amptramp]

I should add one extra design idea: carrier current.  There is DC power going from the box on the floor to the guitar to power the electronics in the guitar and this power supply is used to power a transmitter which feeds signal back from the guitar to the box on the floor.  RF chokes on each end isolate DC power from the RF signal that goes from the transmitter in the guitar to the receiver on the box on the floor.  (I won't call it a stompbox because there is no stompswitch to turn it off.)  The transmitter can be AM or FM and the coax cable will shield everything from radiated emissions and susceptibility.

[antonis]

Are we looking for trouble, Ron..??

[amptramp]

Are we looking for trouble, Ron..??

Just looking for solutions and I bet for every person who asks this question, there are a thousand people who would be interested in an answer.

We do tend to do what has always been done because "that's the way we do things" but in reality, there are a lot of answers to the OP's question.  I think we can all agree that a reduced current amplifier will be noisier - just look at the difference between the TL071 and TL061 series - the TL061 is lower current but higher noise.  External power is the best way to get around that problem.

I'm only looking for trouble from the status quo.