onboard buffer for BASS guitar (asking for advice) [documentation]

[jeepe]

[...]
What's the point of all this? The point is while low operating current seems desirable if you go too low you can end-up with distortion problems - the buffer fails to buffer sufficiently. [...]

The op amp based circuits don't suffer as much from the above issues because they have a push-pull output stage. The output stage has a strong drive for both polarities.

thank you very much, George!

as to OpAmps, I only know tl061 as a usable low current (higher noise) one...
you wouldn't use that, would you?
(I have a few Klone buffer circuits at hand, with sockets, I'll definitely try it, only I'm skeptical as to what I'll hear and what not? )

[Jarno]

LM4250, only 20uA or so practical supply current (it can go lower), 10x lower than TL061

[george giblet]

as to Op Amps, I only know tl061 as a usable low current (higher noise) one...

These things are never clear cut as you are trading supply current vs noise and other performance limits.

I can only outline the main points

(In the datasheets) Tabulated currents are higher than graphs as tabulated currents are for full temperature range.

Opamp Current Type Noise Comment
TL061/62 175uA typ. @ 10V, 25C (per amp) JFET 40nV/rtHz ;base-line
LF441/42/44 150uA typ. @ 10V, 25C (per amp) JFET 35nV/rtHz
LM4250 ~50uA (Iset=10uA) BJT 40nV/rtHz ;(noise in NatSemi 2000 datasheet)
MC1776C ~85uA (Iset=10uA) BJT 32nV/rtHz

TLV170/2170 107uA typ. @ 10V, 25C (per amp) FET 22nV/rtHz ;good modern, SMD only
MC34181/82/84 210uA typ. @ 10V, 25C (per amp) BJT 38nV/rtHz ;not that great
MC33178/79 380uA typ. @ 10V, 25C (per amp) BJT 7.4nV/rtHz ;no single version

The LF441 etc was one of the better older op amps with low power and low-ish noise.
The LF442 is used in the HAZ/Fodera/Ken Smith bass preamps.

There's a trade-off between noise and current draw. The MC1776C is similar to the LM4250.
You can see the MC1776C has a higher current draw for the same Iset and that the noise voltage is lower.

The TLV170 is a more modern design and only available in SMD. Slightly more current than the MC1776C,
but still low, and a good trade-off for even lower noise.
TLV170 datasheet does a good job explaining the isolation resistor requirements when driving capacitive loads.
See figure 32, CL=1n => Riso = 250 ohm (pm=45 deg), Riso = 700 ohm (pm=60 deg).
The figure cannot be used for other op amps.
https://www.ti.com/product/TLV170

The MC33178 has no single op amp equivalent. You can see the supply current is quite a bit higher
but the noise is reduced to quite low levels.

For low current op amps be conscious of:
- higher output impedance, less ability to drive capacitive loads.
- require higher resistor in series with the output (Riso) to drive capacitive loads.
Could be 1k or more.
- limited slew-rates
- slower recovery from overloads

If you do you own buffer from transistors then you really should place a resistor in series with the output to ensure it doesn't oscillate. A value of 1k is probably OK but beyond that nominal value it gets a bit technical.

It is possible to add a push-push stage to a DIY design You can use the same trick that the LM4250 uses ie. large resistors on the emitters of the output transistors. The circuit obviously starts to grow and that will push you back to using an opamp :0. A circuit sort of like RG's operating at 100uA to 200uA is in the right zone without going push-pull.

[subbass]

Hi. If all you want is a buffer to begin with, go with a JFET source follower design. Much more forgiving, doesn't require super fancy FETS and can be set for very little consumption. BF245B pig with 100K source R works very well. Around 50ua like that, IIRC. I had two, one per pickup.
It was this:http://effectslayouts.blogspot.com/2015 ... ender.html with 100K source R I tried for low consumption and there were no side effects.
Don't use the pot for a single pickup and take the output from the source cap. Also increase the input cap (100n,220n), because, bass.

Go with drain to out designs(amplifier config) only when you want some boosting, and/or distorting/clipping stage.
IMO, resistor values in the Kreuzer(RIP) scheme were directly taken from Till's, which was for J201. Even if works, it's all guess and luck about consumption and clipping and all that JFET jazz. You really need trimpots to play with JFETS in the amp config and some hair loss lotions. o0

Or listen to master @george giblet here and grab one of the preamp schematics with tone stacks he has drawn over the years. Thanks man.

Having said that, what do I do? I keep going back to playing with the "elegant way" as you put it even after building many opamp preamps. If nothing else, there's that psychological aspect of having a discrete circuit that you make work. You also realize that there's no best buffer or preamp. Everything has a place and time. Keep building and have fun!