Author Topic: Low-noise opamp for battery-powered preamp.  (Read 9052 times)

NewYorkDave

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Low-noise opamp for battery-powered preamp.
« on: February 01, 2006, 07:34:28 PM »
A friend made a request, and I stopped paying attention to what was current in ICs sometime in the mid '90s. :roll:

Can anyone recommend an opamp (single or dual, preferably through-hole) that can provide clean gain of up to 40dB noninverting, from a source impedance of 15K to 20K, powered by a 9V battery? It doesn't have to be a "single supply" opamp since even regular opamps can be run off a single supply by simply biasing in the + input to V/2.

T.I.A.


CJ

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Low-noise opamp for battery-powered preamp.
« Reply #1 on: February 01, 2006, 07:45:34 PM »
741

 :grin:

Actually, I am interested in this also for a field box where I can grab the leather handle and a 57 and be ready for action.
Line out into a cheap portable cassette and I'm happy.

Two 9 volters might provide more gain, make a dual supply circuit.

How many db can I get from +/-  17 volts?
If I can't fix it, I can fix it so nobody else can!
Frank's- www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar- http://tdsl.duncanamps.com/schematics.php

moamps

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Re: Low-noise opamp for battery-powered preamp.
« Reply #2 on: February 01, 2006, 07:57:28 PM »
Quote from: "NewYorkDave"

Can anyone recommend an opamp (single or dual, preferably through-hole) that can provide clean gain of up to 40dB noninverting, from a source impedance of 15K to 20K, powered by a 9V battery?


LM6132 maybe.

CJ

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Low-noise opamp for battery-powered preamp.
« Reply #3 on: February 01, 2006, 08:03:18 PM »
It's almost like which op amp "won't" work here. That would be a shorter list.

Maybe take a green or some other existing low current draw preamp and make it battery powered if you don't want to re-invent the wheel.
Yes, I'm lazy, like every normal person.

SG's Micpre Meta has filled out nicely.
I bet there is something in there you could jump start off of.

like this:

http://www.groupdiy.com/index.php?topic=1221
If I can't fix it, I can fix it so nobody else can!
Frank's- www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar- http://tdsl.duncanamps.com/schematics.php

featherpillow

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Low-noise opamp for battery-powered preamp.
« Reply #4 on: February 01, 2006, 08:35:47 PM »
You don't say what your preferred current draw is, Dave.  I've been toying with a similar thing lately.  LM6132 is OK, but I'd look into the LMH6655 first, because it has less noise.  It's been pretty stable in applications I've tested it with, using supply rails of +/-8vdc and +12vdc with divider.  The 6132 can be kind of *****y sometimes if it doesn't like the PCB layout.  the 6655's noise is about 4.5nv/rt hz.  It's not rail to rail O/P, I don't know if that matters to you or not.  Oh, and it's SOIC package only, so you'll probably want one of those Brown Dog adaptors.

link:
http://www.national.com/pf/LM/LMH6655.html

skipwave

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Low-noise opamp for battery-powered preamp.
« Reply #5 on: February 01, 2006, 10:27:56 PM »
Huh....was thinking of a different circuit.  :oops:
Quote from: PRR
Now, maybe you don't, or shouldn't, grab the ribbon for far-harpsichord, nor the hot condenser for snare-kissing... yet often we do.

PRR

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Low-noise opamp for battery-powered preamp.
« Reply #6 on: February 02, 2006, 12:39:59 AM »
> Dudes, seriously: link

What, that one transistor on yellow paper? It won't be thrilled running on 9V.

TLO81. Lives for 9V supply. Battery lasts long time. Input noise is comparable to your claimed source impedance. Gives 50KHz bandwidth at 40dB. Keep load well above 2K, it is clean enough for jazz. Costs nothing. (People here re-chipping their boards should send them to you free.)

You did not say "for audio", so there are a lot of un-said assumptions. For all we know, this is a video booster.

pstamler

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Low-noise opamp for battery-powered preamp.
« Reply #7 on: February 02, 2006, 02:37:04 AM »
Getting clean amplification of 40dB in an audio bandwidth is tough; taking the example of a TL081, that will have only about 3.5dB of loop gain happening at 20kHz, given the specified gain-bandwidth product of 3MHz. That will give lots of high-frequency distortion.

Look at it this way: in an op-amp circuit you want at least 20dB (10x) of loop gain. If your closed-loop gain is 100x (40dB), and your highest frequency of interest is 20,000, then you need a gain-bandwidth product of 20,000 * 100x * 10x = 20MHz minimum. Preferably more. That's not impossible, but it's difficult in a low-current IC.

Possible work-arounds:

Use an NE-5534 and parallel a couple of batteries.

Use a TL072, both halves, getting 20dB of gain from each. (With one of these, though, that's still pushing it.)

Peace,
Paul

Viitalahde

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Low-noise opamp for battery-powered preamp.
« Reply #8 on: February 02, 2006, 02:57:57 AM »
http://tangentsoft.net/audio/opamps.html

This page has some tests with battery powered headphone amps in mind.

AD823? AD8512?

mhelin

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Low-noise opamp for battery-powered preamp.
« Reply #9 on: February 02, 2006, 04:28:06 AM »
Not an opamp but class-D mic pre circuit:
http://www.oliveaudio.com/index.php?page=20
Mikko


bcarso

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Low-noise opamp for battery-powered preamp.
« Reply #10 on: February 02, 2006, 04:59:03 AM »
I'd use a PNP common-emitter stage or other discrete operating at a couple hundred uA ahead of some ok op amp.  An 081 would be fine for the additional gain, even out to 20kHz, if the first stage gave you 20dB or so to begin with.

StephenGiles

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Low-noise opamp for battery-powered preamp.
« Reply #11 on: February 02, 2006, 08:07:54 AM »
.....and what frequency can you hear???
Stephen
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Samuel Groner

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Low-noise opamp for battery-powered preamp.
« Reply #12 on: February 02, 2006, 09:02:29 AM »
Quote
... and what frequency can you hear?

If your question should imply that a gain stage does not need a higher bandwith than 20 kHz I must confess that I very much believe that you are wrong. Re-read pstamler's post in this thread for details.

Samuel

StephenGiles

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Low-noise opamp for battery-powered preamp.
« Reply #13 on: February 02, 2006, 02:11:22 PM »
Does that mean then that in order to get decent amplification of stuff we can actually hear, you need good amplification of stuff we can't hear? :cry:  :cry:  :cry:
Stephen
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NewYorkDave

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Low-noise opamp for battery-powered preamp.
« Reply #14 on: February 02, 2006, 02:33:08 PM »
Sorry, I should have been more specific.

bandwidth: 20Hz-20kHz
Current draw: as low as possible from a +9V supply  :green:
Source impedance: 15K to 20K (as mentioned earlier). This is from a 150-200 ohm mic via a 1:10 input xfmr.
Total electronic gain up to 40dB. I can do this in two stages if I'm using a dual. (20 + 20).

I can handle the circuit, I was just looking for recommendations for the device. I was about to reach for the trusty 5532/5534 and then remembered that I'm dealing with a 9-volt battery here.

Samuel Groner

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Low-noise opamp for battery-powered preamp.
« Reply #15 on: February 02, 2006, 03:10:35 PM »
Quote
Does that mean then that in order to get decent amplification of stuff we can actually hear, you need good amplification of stuff we can't hear?

Well, not necessarily--the amp does not need to have low distortion at 100 kHz but it should have high open-loop gain at 20 kHz such that we have enough feedback at this frequency when closing the loop (which in turn gives low distortion for 20 kHz). "Having high feedback at 20 kHz" implies that the amplifier has a much higher bandwith than 20 kHz.

Samuel

skipwave

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Low-noise opamp for battery-powered preamp.
« Reply #16 on: February 02, 2006, 03:30:47 PM »
blah blah blah...... last time I try to be helpful..... blah blah blah :mad:
Quote from: PRR
Now, maybe you don't, or shouldn't, grab the ribbon for far-harpsichord, nor the hot condenser for snare-kissing... yet often we do.

NewYorkDave

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Low-noise opamp for battery-powered preamp.
« Reply #17 on: February 02, 2006, 03:37:20 PM »
No, dudes. One 9V battery only. Just looking for a suitable opamp. That's it.

bcarso

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Low-noise opamp for battery-powered preamp.
« Reply #18 on: February 02, 2006, 03:57:49 PM »
Although I don't have any specific op amp recommendations, I'd suggest parametrizing the problem to aid selection.

With a maximum peak swing of 4.5V the minimum slew rate for an undistorted 20kHz sinusoid is 0.56V/us.  So, even doubled for a bit of margin this isn't a particularly difficult constraint, although the amp specified should have a spec on SR at the operating voltage as it will typically get worse at lower voltages.

The noise resistance of the input stage should be a reasonable match to the transformed source Z, so e sub n over i sub n should be about 15-20k for a given bipolar input.  For a FET input the i sub n component will be negligible at these impedances so look at the voltage noise in a 15k R: 15.8nV/root Hz.  Thus a garden variety BiFET will give about a 3dB noise figure.

As far as gain bandwidth needs, the hidden variable here is the open-loop distortion of the candidate amp, typically worst at the highest frequency of interest.  Except for crossover distortion, which high feedback doesn't help that much anyway, the stage that will be important is where the big swings are, namely the second gain block if a dual op amp is used.  In any event, if we know the open-loop distortion and gain at 20kHz and close to full output swing we can fairly well determine what the 20dB closed-loop distortion will be.  It also would be good to know the relative amounts at different harmonics---a whole lot of higher order might be a good reason for rejection even if the numbers for total looked good on paper.

At low powers the crossover distortion in the output stages is probably the biggest cause for alarm, and it is not well-quantified, other than to note that some low power amps are absolutely terrible unless run in forced class-A.  For example the LM358 from most vendors is awful, although it cleans up a bit in class A.

Gus

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Low-noise opamp for battery-powered preamp.
« Reply #19 on: February 02, 2006, 04:20:28 PM »
Why not a 2 or 3 transistor stage, fuzz face, neve, RCA hobby transistor manual type.  I think ESP has a simple two transistor one.

If you are using a 1:10 maybe a high Hfe transistor like a mpsa18, 2n5088 etc setup with a 10K C R and a 1K E R for a gain of ten (20Db + 20Db)
different ways to set up the bias.  for max stablity the old rule of thumb was 10 to 20 X the emitter R for the base to ground resistor in the bias string.  However some stuff used a voltage divider network with the "bottom" R bypassed to ground with a cap
one side of the transfomer to the node
 the other to the base
 with a load resistor from the node to the base.
Take that stage to an EF two transistors.


 

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