# Opamp preamplifier design question

Discussion in 'Electronic Basics' started by MRW, May 31, 2007.

1. ### MRWGuest

I'm working with these microphones because I have them already:
http://www.knowlesacoustics.com/knowlesacoustics-apps/microphone_productdetail.do?product_id=12

Assuming that the voltage output of the mic at 40dB re 20 uPa is 5
microvolts, would using two opamp stages for a pre-amplifier circuit
be suitable?

I think 5 microvolts is too low, so I'm trying to step up the voltage
to at least 50 mV. That would mean that I would need a gain of 10,000.
But looking thru some of the opamp datasheets, I'm noticing a gain
bandwidth of about 7Mhz for most of the opamps that I've seen so far.
This would mean that the usable bandwidth goes down to 700Hz.

So, I was thinking if I have one amp with a gain of about 400 followed
by another opamp with a gain of 25, then I should be able to step-up
my microphone output voltage to some workable level.

What should my concerns be with using this simple two opamp preamp?

Graham

3. ### John PopelishGuest

MRW wrote:
(snip)
opamps, the one with a gain of 400 will have a bandwidth of
1/16th of the one with a gain of 25. If you make each stage
have an equal gain (100 in this case) you will get the
highest overall bandwidth out of the pair.

4. ### MRWGuest

Oh yeah, you're right. Sorry, I meant to say -40dB re 20 uPa. 40dB
would definitely be a different voltage output.

Thanks!

5. ### MRWGuest

Thanks, John! Is that 1/16th of the gain-bandwidth of the opamp with a
gain of 25? Or is that 1/16th after (gain-bandwidth)/25 ?
How did you get 1/16th by the way?

Thanks!

6. ### John PopelishGuest

400 is 16 times 25. lets say you have 7 MHz GBW opamps.
the one programmed to have a gain of 400 will have a
bandwidth of 7,000,000/400= 17,500 Hz. The one programmed
to have a gain of 25 will have a bandwidth of
7,000,000/25=280,000 (16 times higher than the gain 400
unit). If each of those stages were programmed to have a
gain of 100, each would have a bandwidth of 7,000,000/100=70
kHz.

You can always roll the gain off at 20,000 Hz or some such,
if you want, with a feedback capacitor, but within the audio
band, the extra gain inside the feedback loop will improve
the accuracy of the programmed gain.

7. ### phaetonGuest

If you are just trying to boost the signal, just a single opamp (or
even very small transistor gain stage) will likely be about 100 times
more than you need, depending upon:

1) What voltage you need or prefer to run the preamp from

and

2) If you are building JUST a boost circuit, or if you are going to
follow it with any adjustable filtering, such as a simple RC frequency
limiter or passive tone stack (active baxendall might require the
second opamp stage)

Either way, I can point you to oodles of schematics, but if you prefer
to slog it out yourself, I completely understand.

-phaeton

8. ### MRWGuest

I'd like to know about these other schematics, too. Thanks!

9. ### MRWGuest

This gave me an idea. I'm thinking of using a feedback resistor &
capacitor to limit the maximum voltage amplitude out of the pre-
amplifier. I was thinking of tapping off part of the pre-ampl output
voltage and re-directing it to a comparator. I'm also going to add a
SPDT analog switch in series with the feedback resistor & capacitor
and also with just a feedback resistor.

The basis of the circuit is that if the pre-amp output voltage
surpasses a threshold value. Then the switch enables the resistor &
capacitor branch. But due to the RC time constant, the effects of this
feedback branch won't be fully realized until after the time constant
(would this be considered the attack time?). Are my thoughts correct?

Here is a sketch of what I was thinking: http://i12.tinypic.com/6gx3s7l.jpg

Thanks!

10. ### John PopelishGuest

What is the point of having the capacitor in series with the
resistor? The gain will change with just the resistor.
Also, you can just use the contact to parallel additional
resistance with the one that sets the highest gain. There
is no need to disconnect one resistor and substitute
another. 1 contact saved, and a nasty noise pulse reduced,
during the moment when there is no resistor connected. But
to be practical, there needs to be a lot more effort put
into the switching decision than just comparator. There has
to be some sort of filter that prevents switching on a
single pop, and holds that decision for a reasonable period,
after a loud sound, before it changes its mind during a very
brief silence.

11. ### MRWGuest

Got any suggestions for this?

I was thinking the RC circuit would do this. I thought the capacitor
would store some of the charge and discharge it a certain amount of
time later based on the resistor & capacitor value.

12. ### John PopelishGuest

Are we talking about an audio signal here?

If so, you need some sort of rectifier in the signal going
to the comparator, and the RC filter would modify that
signal, before the comparator makes its decision. In other
words, you need to derive an amplitude signal from the pass
through signal, and process that signal to react to changes
in amplitude. And you might need to obtain that amplitude
signal upstream of the variable gain stage, so that its
decisions will not undo its decisions in an oscillating
pattern.

13. ### EeyoreGuest

You want a signal limiter or compressor.

What you suggest above will sound horrible btw.

Graham

14. ### John PopelishGuest

But potentially, very educational.

15. ### Don BoweyGuest

Consider the reactance of the capacitor at frequencies across the band that
exists at the output of the amp. The capacitor will provide more feedback
at higher frequencies than at lower ones.

Don

16. ### phaetonGuest

Well, perhaps "oodles" wasn't the correct word, but....

First take a look at this:

I don't know what your level of understanding is, but if you get the
amplification circuit. Ignore the stuff about clipping and distortion

Next,

is an old single-transistor booster design with low parts count. I've
built a bunch of variations of this schematic and have been pretty
happy with the results. As you can see, you can also buy a nice kit,
and on the left under "boosters" is an MFT of more circuit examples.

Jack Orman has some circuits there, but here's a link to some kits as
well:

http://www.muzique.com/pcb.htm

But just the schematics are there too if you want to build it without
the kit.

A few simple mods to this:

http://www.aaroncake.net/circuits/fuzz.asp

and you've got a nice op-amp boost. Omit the diodes, change R4 to a
1M linear pot and you'll get an adjustable gain of ~100. See the
first link on how to set the gain range of op-amps. I would recommend
a different opamp though, such as an NE5532 or TL072 for lower noise
than the LM741.

And, since most of these are geared for guitar use, I would change any
input or output cap that is 0.1uF to at least 0.47uf for more bass
response. I highly recommend breadboarding any of these and twiddling
with them until you get the desired result for your application. What
are you recording, anyways?

HTH

-phaeton

17. ### phaetonGuest

Errr... make that an adjustable gain of 0 to 1000. On 9V, expect a
little distortion near the upper range of that, but by the time you
get it turned up past 6 or so you are probably already flogging the
hell out of your mixing console anyways. And also, if you *leave* the
diodes in, you'll have your 'limiter' as well. Snicker Snicker
Snicker.

But seriously (and I'm going to get some stern looks here), depending
upon your application, all the distortion that gets generated from
that could be unimportant or even desireable. Stack more diodes in
series for more headroom (higher amplitude before clipping) and
stagger them (even amount of diodes going one way, odd amount of
diodes going the other way) for a less harsh distortion.

HTH

-phaeton

18. ### MarraGuest

Most mics have a much higher output voltage.

What are you plugging it into?