audio amp with dc output?

I've been away from designing circuits for about a decade, so my knowledge of current components is, well, nonexistent.

I want to make a device that amplifies a signal from a microphone and puts out a dc signal up to 3 volts based on the sound volume. 50mA should suffice. 3 volt supply for the circuit.

This is my first post here. I hope it fits within your guidelines.

Any and all help is greatly appreciated!

Reply to
Charles Rowe
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"Charles Rowe"

** Then you need to acquire some.

Cos we are not here to do that.

** Excellent idea - pal.

You go right ahead and make it.

** Like a square peg does in a round hole.
** Problem is - jerks like you need FAR to much help.

..... Phil

Reply to
Phil Allison

How accurate do you need it? Could be as simple as using a quad OpAmp, like LMV324, two OpAmps for the mic amplifier and one or two for the integrator, with an additional transistor for the 50mA output, or could be a high quality amplifier like this one:

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(I guess Joerg would call it a boutique part) , then digitizing the ouput and feeding it to a DSP for calculating different loudness values, like AWeighted, CWeigthed or the new ITU BS.1770 loudness standard, and then outputting the level with a high resolution current DAC.

Is something bugging you?

--
Frank Buss, fb@frank-buss.de
http://www.frank-buss.de, http://www.it4-systems.de
Reply to
Frank Buss

** Huh ?? That is not my post - f****it.

** Yah - Louie the Fly .

..... Phil

Reply to
Phil Allison

knowledge

First, you can't do this with a three volt supply without a power supply or other boost circuit to give more operating voltage or head room. The minimum operating voltage should be around 9 volts or plus and minus 4.5 volts.

You amplify the microphone output with an op-amp of sufficient gain to get the required three volts of peak audio signal. A level control should be included to adjust the value.

Next you rectify the audio signal with a diode or diodes to detect the peak of the audio wave. A precision rectifier using another op-amp stage would best do this. The output at this point is a DC level representing the peak of the audio wave.

Then you add a resistor and capacitor combination to create an RC time constant to smooth or filter the DC waveform and allow the detected DC level to build and decay at a determined rate.

The output can come from a unity gain amplifier, op-amp with and added emitter follower output stage to give the current gain necessary to supply the required 50mA. The emitter follower stage should be included in the feed back loop of this amplifier.

That is a basic outline of how it could be done. You can look up or search the various amplifier and rectifier components to see actual circuit possibilities.

Reply to
Bob Eld

Charles,

First -- welcome to the group.

Second -- Phil Allison is mentally ill. I'm not trying to be cruel. It's just a fact. So, don't let his irrational responses disturb you. He's quite amusing once you get used to his "style".

Anyway, you can probably do what you need with a decent dual opamp (depending on the microphone) - but the 50mA output current requirement may limit your selection in opamps. Look at Linear Technology for a great selection of devices and some good appnotes. Texas Instruments and Analog Devices are excellent, too. Digikey is a wonderful source for obtaining the parts you need.

The first amp will be a buffer with gain. You haven't said what type of microphone it is. You may need at least 60dB of gain if it's a standard dynamic mic, so if this is the case then you'll probably need mulitple opamp sections to get the level and decent frequency response.

The final opamp section will provide the so-called "ideal rectifier" function (i.e. the ac->dc conversion). It *could* provide some of the gain you need, too.

Do some of the work yourself and try to come up with circuit topology, and then ask more questions.

Bob

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Reply to
BobW

I see your welcome to the group is taking on all its usual dimensions. Yes, your post is right on track, although it also hits the upper end of appropriate posts for sci.electronics.basics (dig up the group charter if you want clarification). Ignore Phil when he's being a crank -- there are either two of him out there, or he's bipolar. When Dr. Jekyll has the ascendancy listen to him, but when he turns into Mr. Hyde just pay him no mind.

You should be able to do this with a 3-volt rail if, as Graham noted, you don't care too much about noise performance and if that 50mA doesn't have to go into too high of an impedance (anything that needs over 3V is right out without a boost supply, and getting all the way up to 3V is at least a minor challenge). If you're driving an analog meter then just about any cruddy old op amp will do; if you're doing something more fancy then you need something -- more fancy.

You could, with a bit of effort and sufficient sacrifices of performance, jam this onto an LM358 (so much for not being familiar with parts!). A rail to rail op-amp would be much easier, although they are more expensive and can have oddball stability and bias-shift issues.

--
Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

TO get proper DC scale from the mic audio you would need a "precision rectifier" which can be derived from a couple of op-amps and minor components. you would of course need a preamp to get the MIC input level up to 3 volts. that would also involve the use of few stages of op-amps to get some gain. You need to know the output level of your mic. some are pre amplified. etc.. from there you can calculate the gains you need in the op-amps. If you require no high quality from this amplified signal you could use a LM324 type chip which employs 4 units.. etc..

This should get you started.

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Reply to
Jamie

Thanks for all the positive responses. The other ones will be ignored.

Here's a bit more info. Fidelity is not an issue. One of my friends has a new puppy and the dog is deaf. Since it's impossible to call him, I wanted to build a small device that would attach to his collar and vibrate in response to noises. All Electronics has a tiny vibrating motor that will operate on 1.5 to 4.5 volts (CAT# DCM-320) . I assumed that 50mA would be more than enough to run the motor; there was no current spec in their online catalog.. I would like to use 2 or 3 AAA batteries so the device is not too heavy or bulky.

I'll look into using op amps. Been a long while since I've done anything like this. Should be fun!

Reply to
Charles Rowe

I don't think this is a good idea, because it will vibrate on all noise, e.g. when the dog is barking, TV etc. Maybe you can use some RF gadget, like used for remote control of garage doors.

50mA sounds like it could be a bit too low for a motor.

Some op amps should do it, if you really want to use a microphone. Maybe instead of linear output you should use just a comparator and a monoflop: If the sound level is higher than some value, it will turn on the motor for some time.

--
Frank Buss, fb@frank-buss.de
http://www.frank-buss.de, http://www.it4-systems.de
Reply to
Frank Buss

I have just the thing - a TL431 adjustable shunt regulator.

The device (in a TO92 case) is actually a comparator with a built in 2.5V reference, of course if you give it enough nfb its quite happy to operate as a linear amplifier. It must have between 1 to 100 mA cathode current to operate, I found it was happy to drive a load as low as 0.2k with a 9 or 12V rail, once you have determined what output operating conditions you want, the nfb and bias is set with a variable resistor to adjust the midpoint voltage ( about 47k VR should do) the control input will always be at 2.5V which suits direct connection to an electret mic very well.

The feedback resistor from cathode to control I/P gives 100% nfb so there isn't much amplification, the trick is to split the nfb resistor and shunt the tap point to GND with a capacitor, the first one of these I made, I used a grotty old 100uF electrolytic laying about on the bench and it worked perfectly, on later builds of the circuit I used better condition electrolytics and found I had problems with the amplifier picking up radio stations - the cur was to put a resistor or even a small choke in series with the nfb shunt capacitor.

Note when setting the O/P operating point, the data sheet says the device cannot pull down lower than 2.5V - in practice most devices will pull down to 2V. Ok so you don't get the full supply rail swing, but its DC coupled and works perfectly with an electret mic.

Reply to
ian field

He was raised by dingoes!

Reply to
ian field

Thanks Ian,

That was the sort of reply I was hoping to see. I'll study that circuit, at first glance it seems perfect for this application.

As you posted, I'm not a newbie- BS Physics 1977, AAS Electrical Engineering Technology 1981. Just out of practice; my job morphed into a sys admin position, not my cup of tea, but the benefits of staying with that employer outweighed other considerations.

Reply to
Charles Rowe

"Charles Rowe" Thanks Ian,

** A fool's question just got a fool's answer.

Poetic justice.

** Believe me - pal , a novice is just what you are.
** Proves the old rule - those who can do & those who can't become their bosses.

Peeeukkeee ....

...... Phil

Reply to
Phil Allison

Try being constructive, Phil, rather than merely showing how insecure you are.

Bob

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== NOTE: I automatically delete all Google Group posts due to uncontrolled 
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Reply to
BobW

Gee Phil, I'm not sure I deserve your valuable time and the benefits of your infinite wisdom. Do you ever post anything that's constructive?

Reply to
Charles Rowe

"Charles Rowe Shithead "

** You don't.

So I did not give you any.

** Posturing fools like you deserve only to be pissed on.

Drop dead - asshole.

...... Phil

Reply to
Phil Allison

Phil is a very disturbed person, Charles. It's best to not expect anything from him except displays of childishness and insecurity.

Bob

--
== NOTE: I automatically delete all Google Group posts due to uncontrolled 
SPAM ==
Reply to
BobW

After reading some of the suggestions and your responses to them, I suspect that you may need a simple 1 transistor mic pre-amp driving a charge pump whose output goes to a 2 transistor schmidt trigger - a 4th transistor may be needed as an emitter follower to drive the load you suggest.

The basic building blocks are neatly described in the Philips EE kit manuals which exist online, they are for 9V but shouldn't be too difficult to re-bias for 3 or 4.5V. Try googling Philips EE, if you have any difficulty let me know and I'll looksee whether I have the exact link saved anywhere. The manual you want is the EE1003/4.

Reply to
ian field

Let me put that in prospective, A "Fools" answer ?

So you answered?

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Reply to
Jamie

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