Opamp preamplifier design question

You've made some mistake somewhere about the mic output level methinks.

Graham

MRW wrote: (snip)

If you cascade two amplifier stages made with similar 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.

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

Thanks!

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!

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.

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

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

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:

Thanks!

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.

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.

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.

You want a signal limiter or compressor.

What you suggest above will sound horrible btw.

Graham

But potentially, very educational.

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

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 stuff on this page you can build most any rudimentary audio amplification circuit. Ignore the stuff about clipping and distortion for your purposes.

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:

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

A few simple mods to this:

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

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

already:

Most mics have a much higher output voltage.

What are you plugging it into?