Use an LM386. The LM386 self biases to half the power supply voltage from 4 to 15V supplies, and already has its inputs pulled to ground by internal resistors. You only need to hook up power, ground and the output pin becomes 1/2 the power supply.
It's a much more solid reference voltage than the resistor/resistor/cap in many effects.
Great tip, R.G. It never ocurred to me!
great idea! especially since it`s very low impedance...
That's useful information. Thanks. :)
how much extra current does it use?
Ah - a sharp one! :D
The static current is 4ma.
You can get lower static current with resistors. The trade off is the low impedance. Some circuits really, really need the low impedance on a faked ground, which is what most reference voltage circuits are. I have seen setups that cannot be stabilized right with resistor/cap references. Buffering it with an opamp or other active chip fixes them right up.
yeah prob an awesome idea for stompboxes because they dont require tons of current :D . ive thought about using those in amplifiers before but for something with high current draws those devices have limits. never thought about using them for stompboxes though. awesome idea. thanks. i think im going to do that on my next one.
Great idea R.G.! 8)
Take care,
-Peter
In my tests, the low power JRC versions seem to use less current. As little as half what the Nat Semi versions do. May be worth exploiting.
Hi,
Do you think an unused 1/2 of other dual opamps like TL072 or a 4558 would work as well?
Regards,
Will
...if you hook up a resistive voltage-divider in front of it, and make it a voltage-follower: yes, but not such a low output-impedance as the 386.
R.G.: how about shorting the input-side of the 386 to ground with a cap, so as not to have it floating AC-wise, and thus picking up "noise" ?
Nice... in all of my 386-playing-with, I didn't notice. Thanks for the tip!
-Colin
Quotehow about shorting the input-side of the 386 to ground with a cap, so as not to have it floating AC-wise, and thus picking up "noise" ?
I worried about that before I posted. It's one of those ideas that may be good, may not offer many advantages except in unusual circumstances. Here's why:
The inputs of the 386 are already tied to ground through internal 50K resistors, so the input is not exactly floating. The resistors are not that big, so noise pickup is likely to be small. Certainly noise pickup is going to be as small or smaller than the 386 used as a gain stage with no input, which is acceptable.
The small resistance value also means you need a biggish cap to shunt the input to ground, which negates some of the advantage of the one-chip-solution.
I suspect that you would get almost the same result if you grounded the input, just hard tied it to ground. I *think* that it would work just fine, but I don't have one on a breadboard to try it with right now.
solid and easy...
I always end up using board space or having the divider [say two 10k's and and two 10uf's] creating a little over the board sculpted 'motif'...
A quick tally of parts count/pricing...the two caps alone cost...the low profile and EZ hookup make this the divider I'm trying out on the next one.
This bias trick is cool! I can't wait to try it out on my Flatline. Setting up the bias network on the breadboard was driving me nuts (a short trip in my case! :mrgreen: ) Will this even work with the lowly 386-1s from Rat Shack, or do we need to use 386-3s?
the n1 worked for me. Nice trick RG!
David,
those lm386 1's at Radio Shack are actually lm386 2's. They'll take 12 volts just fine with no problems.
QuoteThe LM386 self biases to half the power supply voltage from 4 to 15V supplies
what about power supplies of the 18v to 40v?
is there a simple, clean and stable way of doing that ?
The best way there would probably be to use an opamp that has low impedance and can take the voltage. An NE5534 will run up to 44V and has a very low output impedance with a current drain of typically 4ma or so. Add a pair of 100K resistors as a divider with a filter cap at the input and there you go. :D
Quotewhat about power supplies of the 18v to 40v?
You could use the same trick. All of National's power amp chips self bias to half the supply, I think. They certainly could be biased there, anyway. The problem is that for the higher voltages, it's no longer cheap to do this.
The LM1875 (http://www.national.com/ds/LM/LM1875.pdf) will go to 50V total, but I think you have to use a couple of resistors and a cap to bias it at half. It's a TO-220 package.
well, pardon my ignorance, but, what is the big advantage over the usual resistors and cap divider - if you you low value resistors the it shouldn't introduce an ippadence problem, right?
Quote from: Phorhaswhat about power supplies of the 18v to 40v?
http://focus.ti.com/docs/prod/folders/print/tle2426.html
------
1/2 VI Virtual Ground for Analog Systems
Self-Contained 3-terminal TO-226AA Package
Micropower Operation...170 uA Typ, VI = 5 V
Wide VI Range...4 V to 40 V
High Output-Current Capability
Source...20 mA Typ
Sink...20 mA Typ
Excellent Output Regulation
-45 uV Typ at IO = 0 to -10 mA
+15 uV Typ at IO = 0 to +10 mA
Low-Impedance Output...0.0075 Ω Typ
Noise Reduction Pin (D, JG, and P Packages Only)
------
I have used the 3-legged version for my opamp builds, really easy to use, only drawback is that they are a little bit hard to find, but you could always get a few as samples! :wink:
That's what I began to use (R.G. told me about this TLE2426 a couple years ago) in a couple of projects, and I love it.
I only had a few samples I received free from the company so I couldn't use them erywhere, but I'd like to use them all the time.
I'll order some when I go back into full building mode...
Quote from: spongebobhttp://focus.ti.com/docs/prod/folders/print/tle2426.html
------
I have used the 3-legged version for my opamp builds, really easy to use, only drawback is that they are a little bit hard to find
Not that hard, about $1.30 Digikey & Mouser.
buy why use them?
is the 10k/10k & cap network that high impadence?
or does the chip trick (cute...) have a real advantage in another field?
Quotebuy why use them?
is the 10k/10k & cap network that high impadence?
(1) read
Quotehttp://geofex.com/circuits/Biasnet.htm
for some considerations.
(2) Some circuits - especially high gain things with high impedances and high currents flying around - are quite sensitive to noise and un-shunted signal on their references
(3) the 10k/10k/cap setup is a 5K ohm source at DC, and equal to the cap at higher frequencies. Electro caps are not all that good a low impedance source at higher frequencies. An amplifier after the reference makes for a really low impedance reference that prevents the problems in (2) and does not require a high reference current as noted in (1)
(4) The TLE device in particular and the LM386 device to a lesser extent provide superior performance in a smaller board space than the 10k/10k/cap.
uh, old topic here...
i tried this with a 13700, it actually made for more noise than a standard divider buffered with a TL074
does it need some more filtering on the 386, a cap somewhere?
Quote from: cpm on May 12, 2011, 07:07:21 PM
uh, old topic here...
Wow. Nearly seven years!
Quotei tried this with a 13700, it actually made for more noise than a standard divider buffered with a TL074
does it need some more filtering on the 386, a cap somewhere?
Details matter. Could have been a noisy 386; did you have the 386 jumpered for lowest gain? That would make it more noisy. TL074 probably IS quieter than an LM386 overall, but can source/sink less current. I'm guessing (because I haven't looked at one of these in a long time) that you could use a single resistor and a cap after the 386 to make it quieter. But check out the other stuff first.
Well since it's been resurrected - If you're gonna go with an 8 DIL format just to get half supply (a bit chunky for the intended purpose), I'd be inclined to spend the extra 60p for one of these...
http://focus.ti.com/lit/ds/symlink/tle2426.pdf
upto 40v
less quiescent current
less noise
and also availabe as a 3 pin TO-226 package (or DIL-08)
0.0075 Ω output impedance (!!!)
Not a particular high current output drive ability, but for most stompboxes it ought to be fine.
My only complaint about the TI rail splitter is that they're harder to find. If you can find them, they're great. I always liked the the TO92 version myself.
The LM386 is useful as a target of opportunity.
Quote from: R.G. on May 13, 2011, 10:22:04 AM
My only complaint about the TI rail splitter is that they're harder to find. If you can find them, they're great. I always liked the the TO92 version myself.
The LM386 is useful as a target of opportunity.
Agreed....incidentally, most small audio amp ICs, sit at 1/2 VCC & therefore can be brought into play like this , eg TD7052A etc - so if you want to go this way, have a look in you parts box for any audio amp IC you might have lying spare.